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

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.

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.

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.

WikkiWikki wrote: ↑ Shouldnt a dual stage furnace save you money on natural gas?

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.

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.

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.

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?

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

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.

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

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.

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.