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Shouldnt a dual stage furnace save you money on natural gas?

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Shouldnt a dual stage furnace save you money on natural gas?

I know there is another dedicated HVAC thread. But I think that thread is for more important issues than this, and for helping people repair or get help with failing units. Hopefully I can explain well enough below.

So first cold snap with my new furnace installed Apr 2021. Went from a 100K single stage 96% efficiency to an 80K dual stage 97% efficiency. Tons of research and quotes done, spent a lot of time to make sure I was doing what I assumed was right. Contracter that installed did a lot of preliminary checks and work with me as well.

To add, I also have a dual zoned system in a bi-level, Total sq. footage with both floors is 2645 according to the real estate sheet

Highest Natural gas usage since Ive lived in the house for 3 years. Even compared to Feb 2021 cold snap that lasted longer and was colder. Used 6 GJ more in natural gas than my old single stage furnace when it was extremely cold Feb 2021

And yes I know a lot of factors contribute to NG usage. but the lifestyle of the house hasnt changed. Using the same settings on the ecobees, run times, etc Sure I adjust things with the 2 Ecobees, but I adjust to help with comfort and savings. IN one day I have 6 different comfort settings.

I thought these things were supposed to save a person money using single stage to keep the temps level in the house, or to only use single stage for those slight adjustments in heat as needed.

I called the installer to see if it maybe over firing or something as well, and he is coming to check.

I understand fluctuations, but to me I should be using less gas when its cold, not more. And if it was close to the same usage, sure. But this is alot of usage. In the cold weather, yes I assume dual stage will be used way more. But even so, it is a smaller furnace to.

Went from a 100K to an 80K furnace as well. And while the furnace kept up, it wasnt running 24/7 to keep to the cold either like an undersized furnace would. But as well, it wasnt short cycling or anything like that. Temperture rise in the morning of a 4 degree setback was around 2 hours, sometimes 2.5 when it was really cold, -35.

AS well, with my zone panel, I can adjust how long my single stage stays on. I know many on here say single stage runs for 15 minutes, than of course goes to dual stage when it cant reach the temp you want. I have mine set for 30 minutes. Its adjustable from 7 minutes to 47 minutes.

Curious on the use of dual stage and the length of time single stages can run. Always read different opinions on different sites. And of course comparing dual stage to single stage. Some sites saying dual stage really doesnt save you as much money as having single stage in extreme winter temperatures. Many on this site have said how they have their single stage run "all day" How can it run all day on single stage if after 15 minutes and the set temp isnt reach, dual stage runs. Or does that mean that only single stage is used during the day when it calls for heat, not that their single stage runs ALL DAY. To me all day would be your furnace runs on the first stage for, well 8 hours straight,

Single stage - 100% all the time

Dual stage - 70%, then 100% when needed

Since dual stages can be controlled by the furnace board or the Thermostat as well.

Does less gas get used to have single stage run a shorter time (say 10 min) before going into dual stage, which would make the total run time shorter. Or have the single stage run longer before going into second stage, which would make the total run time longer

Basically, for NG use, is it better

- Use 70% for a longer time, and 100% for less time, but taking a longer time to reach the set temp. (1 hr as an example)

Or

- Use 70% for a shorter time, and 100% for more time, but taking a shorter time to reach the set temp. (45 minutes as an example)

I realize for a regular mild winter day, that dual stage probably comes out on top since it would just use the first stage only to keep your house temp level.

But what about -30 and beyond, usually a house takes longer than 10 minutes to reach your set temp, so then it will use dual stage anyway, making the run time longer. Is that when single stage is better, starts off full blast and warms up the home faster, and then shuts off sooner? The house will loose the same amount of heat after the heat isnt on. So the next time it cycles, single stage or dual stage, that wont change.

This is for correctly sized furnaces. Comparing the same size furnaces, 80K vs 80K (as an example)

Are there online calculators that can be used, or some simple math Im just not seeing right now.

As well, so many sites say this. "By using a longer, low output, heat cycle in Low Output mode, 2-Stage Furnaces are able to minimize coarse temperature swings within your home."

But most systems only stay on the first stage 10-15 minutes, and if more heat is needed go to 2nd stage. How can you make your 1st stage run longer all day. Some sites say your furnace should be running 80% of the day in 1st stage. But how can the first stage run all day to maintain the heat. Doesn't that temperature get reached eventually. And if not, then wont the furnace just go to 2nd stage since it cant reach that set temp?

So after all that, am I costing myself more money on Natural Gas having the 1st stage run longer than 15 minutes, instead of having the second stage kick in sooner?

As well, my Ecobees cant control the staging, because my zone panel is the brains in between my 2 Ecobees and the furnace. So I cant do things like reverse staging, or have it kick into 2nd stage right away when it knows the temperature increase it needs will be longer than 15 minutes.
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I think the big advantage of a dual stage is improved comfort. Your furnace is quieter in 1st stage, and it keeps the temps more level, rather than swinging from cold to hot with a single stage unit.
Since your house stays at the set temp more regularly, that means increased thermal transfer from your house to the environment, but I doubt that would make a big difference.
Adding to that, since your fan is running longer, it's circulating warm air to the cold areas of your house, adding to heat loss from your house.

With a properly setup system, you could tell it to always run on 1st stage, unless there temperature difference is too high, then it would engage 2nd stage to catch up.
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engineered wrote: I think the big advantage of a dual stage is improved comfort. Your furnace is quieter in 1st stage, and it keeps the temps more level, rather than swinging from cold to hot with a single stage unit.
Since your house stays at the set temp more regularly, that means increased thermal transfer from your house to the environment, but I doubt that would make a big difference.
Adding to that, since your fan is running longer, it's circulating warm air to the cold areas of your house, adding to heat loss from your house.

With a properly setup system, you could tell it to always run on 1st stage, unless there temperature difference is too high, then it would engage 2nd stage to catch up.
Some say a modulating furnace provides more comfort which starts on high and goes to low, some say a 2 stage is more for comfort which starts on low and goes to high. They're all snake oil salesman who can talk the talk and you don't know who to believe.
My mid-efficient single stage furnace starts at 21C, does a cycle, shuts off and the thermostat is still at 21C and never moved through the entire cycle so I can't see where all these temperature fluctuations these salesman are talking about.
A HVAC system is theoretically designed, sized and installed so the temperature in each rooms drops/increases the same amount as all the other rooms. If it doesn't then someone screwed up and didn't do a proper job.
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pootza wrote: Some say a modulating furnace provides more comfort which starts on high and goes to low, some say a 2 stage is more for comfort which starts on low and goes to high. They're all snake oil salesman who can talk the talk and you don't know who to believe.
My mid-efficient single stage furnace starts at 21C, does a cycle, shuts off and the thermostat is still at 21C and never moved through the entire cycle so I can't see where all these temperature fluctuations these salesman are talking about.
A HVAC system is theoretically designed, sized and installed so the temperature in each rooms drops/increases the same amount as all the other rooms. If it doesn't then someone screwed up and didn't do a proper job.
Theoretical is one thing, real world is another. Most home have crappy, unbalanced duct work.
The free (other than electricity costs) way to improve comfort is to run your fan 24/7. That will improve comfort around the house.
From experience upgrading from a mid-eff furnace to a 2-stage in my previous house, I can confirm that it was more quiet and felt more comfortable. That said, my current house is a mid-eff that I'll keep until it goes or there's great rebates. I do have a Mars Azure ECM motor I plan to put in it, to have it run more quietly when on 24/7 and reduce electricity costs.

Even what you were told about a modulating starting high and going to low makes some sense, since it would heat up quickly but reduce output to avoid over heating. Similar to an oven I suppose. Though I've never heard that.

How does your tstat know when to run if the temperature never varies?

Mine varies by 0.6C from off to on at the bedroom sensor that it usually runs on, and that's with the fan on 24/7. At my tstat it varies buy up to 0.9C. It's much worse in the areas closer to the furnace.
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engineered wrote: Theoretical is one thing, real world is another. Most home have crappy, unbalanced duct work.
The free (other than electricity costs) way to improve comfort is to run your fan 24/7. That will improve comfort around the house.
From experience upgrading from a mid-eff furnace to a 2-stage in my previous house, I can confirm that it was more quiet and felt more comfortable. That said, my current house is a mid-eff that I'll keep until it goes or there's great rebates. I do have a Mars Azure ECM motor I plan to put in it, to have it run more quietly when on 24/7 and reduce electricity costs.

Even what you were told about a modulating starting high and going to low makes some sense, since it would heat up quickly but reduce output to avoid over heating. Similar to an oven I suppose. Though I've never heard that.

How does your tstat know when to run if the temperature never varies?

Mine varies by 0.6C from off to on at the bedroom sensor that it usually runs on, and that's with the fan on 24/7. At my tstat it varies buy up to 0.9C. It's much worse in the areas closer to the furnace.
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As with anything it's all what is in your mind. Some people change to a higher efficient furnace and they say theirs is louder now. I use to live with a Lennox Pulse when they came out in the 90's and it sounded like a refrigerator ... but got used to the different sound and never heard it after awhile just like I never heard the trains go by after while living close to the tracks.
Moving air is not more comfortable in the heating season IMHO as like in the summer moving air sucks the moisture out of your skin faster.
My thermostat has to sense a difference in temperature or else it wouldn't turn on and off. It probably fluctuates the same as yours by going from 21.2C to 21.8C which wouldn't register a change on the stat.
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i dunno about saving gas, because last few years temperature fluctuated so much, but I can definitely say my 2 stage is more comfortable then when we had the old furnace, i hated how it just give burst of hot hot air, and then just stops and fires up again when temperature drops. Whereas the new 2 stage, yes, it runs much more often (almost constantly), but at lower fan speed and heat. I can't hear the furnace running when I'm sleeping, so its money well spent, also the cost difference not as bad as you may think depends on the brand
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WikkiWikki wrote: Shouldnt a dual stage furnace save you money on natural gas?
" the DOE test procedure consistently shows a reduction in energy consumption by about 3% compared to single-stage furnaces at the same AFUE level. In contrast, the 2006 AHRAE test procedure shows almost no difference in the total energy consumption at the same efficiency level. The reason is that although two-stage furnaces operate longer at the reduced mode, this is offset by lower fuel input rate at this mode."

https://www.aceee.org/files/proceedings ... aper16.pdf

So this was a 2006 test and the conclusion was the NG use goes down and the electricity goes up but this was before the variable speed motors.

So if a 3% savings ... your gas bill would be $145.00 instead of $150.00. Would you actually notice the $5 savings?
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engineered wrote: I think the big advantage of a dual stage is improved comfort. Your furnace is quieter in 1st stage, and it keeps the temps more level, rather than swinging from cold to hot with a single stage unit.
Since your house stays at the set temp more regularly, that means increased thermal transfer from your house to the environment, but I doubt that would make a big difference.
Adding to that, since your fan is running longer, it's circulating warm air to the cold areas of your house, adding to heat loss from your house.

With a properly setup system, you could tell it to always run on 1st stage, unless there temperature difference is too high, then it would engage 2nd stage to catch up.
This is what Im trying to wrap my head around, or I am missing something so obvious.. When people say they have their system run on first stage all the time, like your example above. How do they do that, with their Smart Thermostat? I know I lose out on some smart advantages to my Ecobee with my zone panel being the middle man. So my going to 2nd stage is literally timed manually right now.

Because what I have read on here that most people their first stage is set by the furnace to run for 10-15 minutes tops, then it goes to 2nd stage.

But then as well as I explained, is it better to have longer runs with 1st stage, or shorter runs with 1st stage and then use 2nd stage for the end, shortening your run times.

I do agree that my furnace is quiter. I did decibel tests with an app before the install and after and Im 10 decibels lower. And my current fan has varying speeds. The very low speed when its just the fan set to ON, then the fan increase when it starts 1st stage, and then the fan when it hits second stage.
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pootza wrote: Some say a modulating furnace provides more comfort which starts on high and goes to low, some say a 2 stage is more for comfort which starts on low and goes to high. They're all snake oil salesman who can talk the talk and you don't know who to believe.
My mid-efficient single stage furnace starts at 21C, does a cycle, shuts off and the thermostat is still at 21C and never moved through the entire cycle so I can't see where all these temperature fluctuations these salesman are talking about.
A HVAC system is theoretically designed, sized and installed so the temperature in each rooms drops/increases the same amount as all the other rooms. If it doesn't then someone screwed up and didn't do a proper job.
We all know, well should now, there is no perfection in HVAC. I know this, there is no perfect ducting, perfect distance, perfect scenario. Can there be, yes. If you build the house yourself and make sure everything in HVAC is done to perfection.

But for 99% of homeowners, that is not and will never be the situation. From cold rooms above a garage, to a far bedroom being cold but a close bedroom being hot. Can these thing be fixed, yes. To a degree, but perfection, never.

I myself, have come to terms with that in my house. In the last three years I have learned more and done more to my HVAC then any other house combined. Basically because of the zoning and the obvious over sizing of the furnace.

But what I am trying to figure out that after all this work of a a better furnace, ducts worked on, a 5" media filter, went down 20K in size, and shown proven measurements of better static pressure, air flow, etc. That my usage is 6 Gj higher than my old setup of a single stage, one speed fan. While I wasnt expecting a huge drop in NG use, I was expecting a decrease of something, not a massive increase.

Thats why I put, does running it longer on single stage actually contradict the savings?

I know I have 3 things that use NG in the winter. My furnace, my tankless water heater, and my garage furnace. But my garage furnace, the tempertaure I set it at is the same, and in fact, I actually increased my garage roof insualtion to R50. So that use should be lower as well, in theory.

So all this extra NG use is coming from where?
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engineered wrote: Theoretical is one thing, real world is another. Most home have crappy, unbalanced duct work.
The free (other than electricity costs) way to improve comfort is to run your fan 24/7. That will improve comfort around the house.
From experience upgrading from a mid-eff furnace to a 2-stage in my previous house, I can confirm that it was more quiet and felt more comfortable. That said, my current house is a mid-eff that I'll keep until it goes or there's great rebates. I do have a Mars Azure ECM motor I plan to put in it, to have it run more quietly when on 24/7 and reduce electricity costs.

Even what you were told about a modulating starting high and going to low makes some sense, since it would heat up quickly but reduce output to avoid over heating. Similar to an oven I suppose. Though I've never heard that.

How does your tstat know when to run if the temperature never varies?

Mine varies by 0.6C from off to on at the bedroom sensor that it usually runs on, and that's with the fan on 24/7. At my tstat it varies buy up to 0.9C. It's much worse in the areas closer to the furnace.
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Yes the "theoritical" thing is just that. Unless you are an HVAC expert and build from scratch, no house will have perfect ducting. We all wish that wasnt the case, but its true.


My 24/7 fan runs change with the weather. Right now with over night lows of -5 or so, I dont have the fan run during the night at all. Just during the day I have it set to on from 6 AM to 10 PM. But when the temps drop, the fan is on 24/7

I do this, so I save money (in theory) on NG, since the air is circulated, hot spots warm cold spots, and vice versa. But it doesnt seem to be the case.

And I have done a lot to "save money" I dont have my thermostats kick in until it reaches .6 degrees of an offset. Lowest is .3 I tried .8 but it was just enough of a chill before the furnace kicked in, to be comfortable.

Saying that, I had that same setup with my single stage.

With the amount of sensors in my house, and each comfort setting using varying sensors for that time of day, Im not seeing anything but a big jump in NG use.

This is what I cant figure out, I have done all I can to make this house more energey efficient and as well, streamlined for better use. And its like Im taking 2 steps back.

I realize zoning is a different beast. But I had the same zoning with my single stage as well.
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How can I tell my furnace is single or dual stage unit. It’s 21 years older old, builder’s grade, Lennox Elite, most likely single due to builder’s and age.
Last edited by teoconca on Jan 27th, 2022 7:29 pm, edited 1 time in total.
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pootza wrote: " the DOE test procedure consistently shows a reduction in energy consumption by about 3% compared to single-stage furnaces at the same AFUE level. In contrast, the 2006 AHRAE test procedure shows almost no difference in the total energy consumption at the same efficiency level. The reason is that although two-stage furnaces operate longer at the reduced mode, this is offset by lower fuel input rate at this mode."

https://www.aceee.org/files/proceedings ... aper16.pdf

So this was a 2006 test and the conclusion was the NG use goes down and the electricity goes up but this was before the variable speed motors.

So if a 3% savings ... your gas bill would be $145.00 instead of $150.00. Would you actually notice the $5 savings?
I realize cost can vary. Different service charges, price of gas goes up or down, etc, etc. That part, is a variable. Expecially with inflation

But its the actual usage increase of 6 GJ is what Im trying to figure out. I checked online bills from current all the way back to Feb 2020. Took me about 10 minutes, but basically wrote down my Electrical usage that billing cycle and Natural Gas Use

The coldest month since I have moved into this house, was Feb 2021. That was with the old single stage furnace. 15.94 GJ was used that billing cycle. And that month it was colder for longer, and we hit come -40 nights as well.

Since then it hasnt been that cold until Dec 2021. And it was cold, but wasnt -40 cold. Well -35 cold. And really when its that cold, its just cold. But as well, the cold snap didnt last as long

The NG use with the new furnace was 21.91 GJ Which is almost a 6 GJ difference.

All things equal, new furnace, 1% higher efficiency, 20K lower in BTU, and dual stage. My usage should be either less by even a little, or at least close to the same. But higher? Thats what this post was for, how can it be higher. What am I missing, or doing wrong thats making this usage so much higher? Hell even it was just a bit higher, that would be fine. Christmas time, more activity, more peope going in and out, people home more because of no school.. Sure that I could get.

But 6 GJ is a lot of extra usage.

Saying that, the entire city was up in arms as well, because some people have the highest bill they have every had in 10 years living in the same house, with locked in rates, and their NG use was through the roof But yet their neighbours, bill was basically what they expected.

Unless the city is pulling some scam and jacking up certain people NG use to make some money. I highly doubt it, yet no one that has these high bills can get an explanation of "how"
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Is wind chill factored in to your current and historical numbers? Could it be a lot windier? Where are the weather stats coming from? Ecobee? I thought, and may well be wrong, that Ecobee changed the service where they collected data. Might have happened a few months to a year before I bought my Ecobee. I bought it around December 2020. I can't remember for sure what exactly they did or didn't do.

Your old furnace had a AC motor, the new furnace a DC motor. The old motor consumed more electricity which was converted to heat energy. Did you run that one and the new one 24/7 ? Big difference in the consumption if you did as the extra low speed of the DC motor is much more energy efficient than running the AC motor at constant run speed (high speed). That will add up to something in extra natural gas consumption.

Some furnace companies provide the input and output BTU of the furnaces they sell. Of the very limited number I've seen, the energy efficiency on both stages worked out exactly the same. Interestingly on one furnace (2007 model), rated at 95.5% when I did the math it worked out to 97% on both stages.

In the case of a very old home I would expect it more more likely have restrictive ductwork and then the efficiency would drop somewhat in the second stage as the heat rise goes closer to the limits and more heat is lost out the exhaust pipe. I have no idea in that case what % decrease it would be. Same deal with the single stage though.

Longer run times means less cycling. When the furnace cycles it preheats the heat exchanger and for about 30 seconds before that, the exhaust inducer fan runs. So short run times contribute to heat loss. Longer should be a bit more energy efficient.

You used 6 GJ more? What was your total consumption for the 30 day period? I just sent my meter reading in today and I consumed as much in the last 30 days as in did in the 60 days previous. The last 30 daays has been much colder for me than last year.
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rogerrabbit168 wrote: i dunno about saving gas, because last few years temperature fluctuated so much, but I can definitely say my 2 stage is more comfortable then when we had the old furnace, i hated how it just give burst of hot hot air, and then just stops and fires up again when temperature drops. Whereas the new 2 stage, yes, it runs much more often (almost constantly), but at lower fan speed and heat. I can't hear the furnace running when I'm sleeping, so its money well spent, also the cost difference not as bad as you may think depends on the brand
Im happy with my furnace, the installer, the install, everything. As you said, I like how its quieter.

I was just expecting a savings of some sort. Hell even if I paid more because of inflation and Carbon tax, fine. But be nice to see the actual NG used be lower than the old system.

And Im comparing cold months to cold months to. I know that the milder it is the less NG you use. So I expected my December bill to be higher than November.

What I wasnt expecting was the highest bill ever receieved since living here
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fergy wrote: Is wind chill factored in to your current and historical numbers? Could it be a lot windier? Where are the weather stats coming from? Ecobee? I thought, and may well be wrong, that Ecobee changed the service where they collected data. Might have happened a few months to a year before I bought my Ecobee. I bought it around December 2020. I can't remember for sure what exactly they did or didn't do.

Your old furnace had a AC motor, the new furnace a DC motor. The old motor consumed more electricity which was converted to heat energy. Did you run that one and the new one 24/7 ? Big difference in the consumption if you did as the extra low speed of the DC motor is much more energy efficient than running the AC motor at constant run speed (high speed). That will add up to something in extra natural gas consumption.

Some furnace companies provide the input and output BTU of the furnaces they sell. Of the very limited number I've seen, the energy efficiency on both stages worked out exactly the same. Interestingly on one furnace (2007 model), rated at 95.5% when I did the math it worked out to 97% on both stages.

In the case of a very old home I would expect it more more likely have restrictive ductwork and then the efficiency would drop somewhat in the second stage as the heat rise goes closer to the limits and more heat is lost out the exhaust pipe. I have no idea in that case what % decrease it would be. Same deal with the single stage though.

Longer run times means less cycling. When the furnace cycles it preheats the heat exchanger and for about 30 seconds before that, the exhaust inducer fan runs. So short run times contribute to heat loss. Longer should be a bit more energy efficient.

You used 6 GJ more? What was your total consumption for the 30 day period? I just sent my meter reading in today and I consumed as much in the last 30 days as in did in the 60 days previous. The last 30 daays has been much colder for me than last year.
Ive been going through a lot o info trying to figure this out. Besides getting a white board out with strings making it look like a mafia ring sting board to track stuff.

All that you mentioned I know are all factors of things that can affect NG use

The motor is out of the loop, because thats electrical use. And Im not trying to track that down. Decemeber being the Christmas light month electricla usage will be higher anyway. Saying that, I did right down that information as well going back to Feb 2020.

And since my 24/7 fan schedule varies with the temperature at night, that one will be tricky, or basically impossible to compare apples to apples.

The closet apples to apples comparison for NG use, is me going back to the last cold snap of 2021 with the old furnace. Which was Feb 2021. The last cold snap for the new furnace was Dec 2021.

Im not comparing Dec 2020 to Dec 2021, because last Decmeber wasnt cold. Last December bill I used 11.89 GJ, this year was 21.91 GJ. The time frames for the bills are always the same.

But Feb 2021, the NG use was 15.94 GJ compared to Dec 2021 of 21.91 GJ Comparing the two coldest months of old furnace to new furnace.

Variables aside, of extar people in the house because of Christmas, more hot water from taps, dishwasher run more, etc, etc. I can see that increasing NG use to. But 6 GJ is a lot of extra use.

Really, it may be impossible to say why the huge jump. Maybe all that combined above does make up for that much gas.

But what I want to find out, is it the way I am running the furnace a cause of this big jump.

Is running the 1st stage longer actually counter productive?

Zoning aside, my zone setups are the same, yes Ive tweaked things, to the better, moved a vent, which actually helped with better heatr and air flow. But it seems Im going backward in efficiency, not forwards. Thre replacement furnace has helped with air flow at the farthest vents a lot as well.

Hell Id be happy if it was even equal. Just stumped on how doing all this wort equals more NG usage. Going back all the way back to Feb 2020, since that as far back online it goes. This is the highest amount of gas I have ever used. Feb 2020 was 18.21 GJ was the second highest that I can find. And that was before I understood and fixed all this dual zone stuff.

Throwing all variables in the mix for almost 2 years of bills, the only major change that eats up the most NG has been my furnace, and suddenly I have the most usage ever. Thats the problem I am having, WHY

And the kick in the face is, Id be fine if I was keeping the house tropical. Ya know, walking around in shorts and a Tshirt, temperature the same 24/7. But I have things set to save money. Night time set backs, using the windows/sun for extra heat, keeping the fan cycling to promote air circulation, ceiling fans, etc, etc. Ecobee sensors in every room. Im not new to how to save money on NG. There is no news article I can read that I didnt know that trick already

And it all seems for naught
Last edited by WikkiWikki on Jan 27th, 2022 8:06 pm, edited 1 time in total.
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WikkiWikki wrote: Ive been going through a lot o info trying to figure this out. Besides getting a white board out with strings making it look like a mafia ring sting board to track stuff.

All that you mentioned I know are all factors of things that can affect NG use

The motor is out of the loop, because thats electrical use. And Im not trying to track that down. Decemeber being the Christmas light month electricla usage will be higher anyway. Saying that, I did right down that information as well going back to Feb 2020.

And since my 24/7 fan schedule varies with the temperature at night, that one will be tricky, or basically impossible to compare apples to apples.

The closet apples to apples comparison for NG use, is me going back to the last cold snap of 2021 with the old furnace. Which was Feb 2021. The last cold snap for the new furnace was Dec 2021.

Im not comparing Dec 2020 to Dec 2021, because last Decmeber wasnt cold. Last December bill I used 11.89 GJ, this year was 21.91 GJ. The time frames for the bills are always the same.

But Feb 2021, the NG use was 15.94 GJ compared to Dec 2021 of 21.91 GJ Comparing the two coldest months of old furnace to new furnace.

Variables aside, of extar people in the house because of Christmas, more hot water from taps, dishwasher run more, etc, etc. I can see that increasing NG use to. But 6 GJ is a lot of extra use.

Really, it may be impossible to say why the huge jump. Maybe all that combined above does make up for that much gas.

But what I want to find out, is it the way I am running the furnace a cause of this big jump.

Is running the 1st stage longer actually counter productive?

Zoning aside, my zone setups are the same, yes Ive tweaked things, to the better, moved a vent, which actually helped with better heatr and air flow. But it seems Im going backward in efficiency, not forwards. Thre replacement furnace has helped with air flow at the farthest vents a lot as well.

Hell Id be happy if it was even equal. Just stumped on how doing all this wort equals more NG usage
As I mentioned, the energy efficiency of any furnace I checked was exactly the same when running either first stage or second. It's impossible to work out the thermal losses caused by air movement in your home. If there's warmer air blowing by windows then more heat loss. That's why I use those air register heat deflectors. Particularly when the duct is sitting under a drape. I don't want to be heating up the window rather than the room.

I think you're discounting the extra consumption in gas caused by the high efficiency motor though. They even mention that on the US government website or maybe it was the Canadian. It wont be huge but it's real. I suspect though it's a bunch of stuff adding up to the extra consumption. I'll bet number 1 is the weather. I think something must be off with the weather stats.

If the furnace where at fault the extra consumption would only be coming out the exhaust pipe. If the furnace gas pressure is off and the mixture changes I believe it affects the CO2 output. Easily detected by the furnace tech.
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fergy wrote: As I mentioned, the energy efficiency of any furnace I checked was exactly the same when running either first stage or second. It's impossible to work out the thermal losses caused by air movement in your home. If there's warmer air blowing by windows then more heat loss. That's why I use those air register heat deflectors. Particularly when the duct is sitting under a drape. I don't want to be heating up the window rather than the room.

I think you're discounting the extra consumption in gas caused by the high efficiency motor though. They even mention that on the US government website or maybe it was the Canadian. It wont be huge but it's real. I suspect though it's a bunch of stuff adding up to the extra consumption. I'll bet number 1 is the weather. I think something must be off with the weather stats.

If the furnace where at fault the extra consumption would only be coming out the exhaust pipe. If the furnace gas pressure is off and the mixture changes I believe it affects the CO2 output. Easily detected by the furnace tech.
Very true, factors to consider I will quite honestly never be able to find out. Can they, I assume if I worked hard enough. But lets face it, doing this wont change the bill. Or change how I do things either, because, well, I cant think of anything more I can do to my house to make it more streamlined, or how much cheaper I can be to not keep the house warm.

I still am getting the tech to come in, he is going to check some things and about over firing.

I thought going to dual stage decreased your NG use, not increased it? Isnt that one of the selling features?

I like the idea of deflectors, but also not deflecting the air help with window fog due to the air circulation on the windows. And since that part hasnt changed, would that part even be considered for the extra use, since the old furnace was used the same way

I did post in antoher thread about some weird moisture on my stucco for the furnace exhaust Ive never seen before. The tech came and looked as well.

Colder weather though has been considered, and while it was cold, why do I have the highest usage ever in 3 years. Its been cold here before and the usage was never that high

Or its just a perfect scenario of more people, more hot water being used, more doors being opened at Christmas than in February because of more company. I guess that all adds up. But is that enought to make is be 6 GJ of use?
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You can't compare the consumption from two different time periods unless you have the Heating Degree Days history for your municipality which you can probably google and get. Compare two months with the same number of degree days and compare the usage on your gas bill.
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OP, from my days of research when I replaced the furnace (2013) a dual stage won't reduce gas usage but will massively improve comfort. This is due to operation in 1st stage is not as efficient as 2nd stage. The efficiency figure they quote is measured at max heat output and ideal conditions, which cannot be achieved in real life operation.

Based on your description (2nd stage engages after a fixed runtime in 1st stage) I presume your thermostat is single-stage. This is not ideal and may contribute to increased gas usage. Get a proper 2-stage thermostat and connect it such so it controls staging instead. Talk to your contractor about a Honeywell 2-stage thermostat

You want the lowest possible heat output in 1st stage for comfort, and if that causes the furnace to run all day in colder temperatures that's exactly what you should aim for. 2nd stage only engages when heat loss exceeds what 1st stage can deliver. In my case I went down from 75K to 60K modulating (but used with a 2-stage thermostat) due to 1st stage output was lower than the equivalent 2-stage model.

And other people here are right, you need to compare the Heating Degree Days for your area otherwise comparison won't be accurate

Edit: Forgot the most important aspect - did you do a proper heat loss analysis and sized your new furnace accordingly? Even reducing from 100K to 80K might still leave you with an oversized furnace. Easiest way to tell (and if you have a proper 2 stage thermostat) is on the coldest day of the year (windy and at night) your furnace should CONTINUOUSLY cycle between 1st and 2nd stages. The more it stays in 2nd stage, the closer you are to the furnace output matching the max heat loss
Last edited by wally_walrus on Jan 27th, 2022 9:54 pm, edited 1 time in total.
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pootza wrote: You can't compare the consumption from two different time periods unless you have the Heating Degree Days history for your municipality which you can probably google and get. Compare two months with the same number of degree days and compare the usage on your gas bill.
What are Heating Degree days?

While true I am not comnapring the exact days of a cold snap, although I could. Im just pulling a general conclusion of. Feb 2021 was a longer and colder "cold snap" compared to Dec 2021. And the change between those 2 scenarios si the different furnace

Yes I know there are so many contributing factors that did affect the over NG usage during those times. But seeing your highest NG usage ever in 3 years, and in those 3 years, we have had more than 2 "cold snaps" when you replace your furnace with something in theory is supposed to be more energey efficient, and its the opposite effect, makes a person wonder, wheres the increase from

And as well, as I previously posted, the entire city has been on the citys facebook page with the same results. Not only the highest utility bill in 8-10 years of the same house, the highest NG usage as well in those years, and for sure we have had colder and longer cold snaps than Dec 2021

And while the one person with lhas this "highest ever" NG use, the next door neighbour, saw barely an increase at all in NG use. Many are saying, something isnt adding up.

Hell even my friend at work, who has kids at home, has the same walk out style house, same age built, said his bill didnt see an increase of NG more than normal, while mine was way higher

Is this all offfical "apples to apples" testing, of course not. Just a lot of people,are scartching their heads this last bill and saying, whats going on

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