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one forced air furnace - 2 separate control zones (thermostats)

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  • Nov 18th, 2020 2:53 pm
Newbie
Sep 9, 2020
21 posts
1 upvote

one forced air furnace - 2 separate control zones (thermostats)

im a little curious on what it takes to accomplish something like this.

ive seen 2 stage furnaces but thats just for low and high instead of on/off. what im thinking of is one furnace but would like to have one thermostat control at top floor and another thermostat controlling basement/main level for both heat and cooling.

does anybody have this setup at home? any concerns or things to consider with this kind of setup? what kind of balancing/load should be thought of?

thanks.
15 replies
Deal Addict
Oct 19, 2020
1047 posts
677 upvotes
GTA
It needs to be done perfectly or not at all. Ductwork has to be designed for it from the ground up - retrofit may cause major problems particularly if the different zones don't have dedicated trunk lines and ducts in general are undersized.

2-stage is required at a minimum for the system to be reasonably efficient so heating/cooling can be kept on low when only one zone is on.

A properly done duct system should keep the above ground floors within 1c or less of eachother. For cooling, maybe 2c.
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Sr. Member
May 17, 2006
605 posts
191 upvotes
xoyo123 wrote: im a little curious on what it takes to accomplish something like this.

ive seen 2 stage furnaces but thats just for low and high instead of on/off. what im thinking of is one furnace but would like to have one thermostat control at top floor and another thermostat controlling basement/main level for both heat and cooling.

does anybody have this setup at home? any concerns or things to consider with this kind of setup? what kind of balancing/load should be thought of?

thanks.
I had a 3-zone setup at a previous house out east. I converted from electric heat, and designed the system from the start with separate zones in mind. The air handler (on top of the heat source) is specially designed to open/close the trunk that's calling for heat/cooling from the thermostat in the zone. It wasn't cheap, but it was TOTALLY worth it.

I had each floor on a separate thermostat. The most beneficial times were when the upstairs called for cooling in the summer, and heating the basement in the winter late at night when everyone else was asleep.

I don't know what the cost was separately, though. Work with a pro/big name/referral as this is something that not everyone has experience in. In my case, it was Enbridge themselves who did the install out east.

Good luck.
Deal Expert
Aug 22, 2011
41800 posts
30054 upvotes
Center of Universe
xoyo123 wrote: im a little curious on what it takes to accomplish something like this.

ive seen 2 stage furnaces but thats just for low and high instead of on/off. what im thinking of is one furnace but would like to have one thermostat control at top floor and another thermostat controlling basement/main level for both heat and cooling.

does anybody have this setup at home? any concerns or things to consider with this kind of setup? what kind of balancing/load should be thought of?

thanks.
No, you can't setup two tstats to operate one single HVAC system.
Deal Addict
Feb 10, 2006
2652 posts
587 upvotes
Kitchener
Yes it's possible but if you want it done PROPERLY it is a large undertaking. Your house, and duct work should be reviewed and likely modified to work with the zones. Don't by in to the hype that a tstat can achieve the desired effects- ecobee example. It's a good stat but will not dial in zones properly.
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Deal Expert
Aug 22, 2011
41800 posts
30054 upvotes
Center of Universe
Double_J wrote: Yes it's possible but if you want it done PROPERLY it is a large undertaking. Your house, and duct work should be reviewed and likely modified to work with the zones. Don't by in to the hype that a tstat can achieve the desired effects- ecobee example. It's a good stat but will not dial in zones properly.
How exactly does 2 tstats operate with only 1 HVAC?
Banned
Jun 4, 2020
1634 posts
1750 upvotes
Clarington, ON
vkizzle wrote: How exactly does 2 tstats operate with only 1 HVAC?
Both t-stat would generate a call for heat or cool. This would be couples with some sort of damper that would open for the zone that is looking to be served.

Generally this is how a large building - office, school, etc. - would operate, albeit with a larger rooftop unit.
Deal Addict
Sep 8, 2017
4695 posts
4968 upvotes
GTA
We have a single furnace, a Nest thermostat on the ground floor, and a secondary Nest temperature sensor on the second floor.

But honestly, the secondary sensor doesn't save any energy. The system still heats/cools the entire house. You would need some kind of valve in the ducting to divert air flow from upstairs to downstairs, or somewhere in between. Like mentioned in post #3, but that all depends on how the ducting is currently designed. Otherwise the ducting would need significant rework.
Deal Expert
Aug 22, 2011
41800 posts
30054 upvotes
Center of Universe
KevinM56081 wrote: Both t-stat would generate a call for heat or cool. This would be couples with some sort of damper that would open for the zone that is looking to be served.

Generally this is how a large building - office, school, etc. - would operate, albeit with a larger rooftop unit.
You're assuming they read the same temp.
Which tstat gets priority?
Deal Addict
Dec 5, 2009
2789 posts
3289 upvotes
Looked into this (and so did our GC for his own home). We wanted to split our house into up/down zones. He didn’t think it was worth the extra cost to do this in his own home so we took his advice and stuck with baseboard heaters for the basement. We are already spending 17k for a new furnace, a/c, and new duct work. Installing dual zones properly would probably balloon our costs up to 25k.
Deal Addict
User avatar
Oct 11, 2005
2379 posts
4561 upvotes
Airdrie, AB
I had a 2-zone system in my last home (one thermostat on main floor, another thermostat on the second floor), with a single furnace/AC. I loved it. The HVAC contractor used by the home-builder had to provision/design the system for this from the outset during construction. I don't remember details (I've since moved), but you obviously couldn't set one thermostat to heat while the other cooled; all you could do is set (for example) the upper floor to be warmer or cooler than the main floor. In the furnace room, there was a module with blinky lights that opened/closed dampers to the different plenums to divert more/less air to the respective zone. I don't remember whether the thermostats were special and coordinated their communications/control or not. But ultimately this is stuff that HVAC companies do/design, so they'd know what you need, and whether it's feasible for your home.

This was a great system, versus the previous home I had, which was equipped with 2 furnaces and 2 AC units -- one for each floor. It cost quite a bit more to run that home for heating and especially cooling. The two-system configuration isn't bad if you have the space for it, and don't mind the higher operating costs. One benefit (although maybe not worth the cost) is having a bit of redundancy, so that if one system dies in the middle of winter, your whole home isn't frozen... although I've personally never had this happen.
Deal Addict
Feb 10, 2006
2652 posts
587 upvotes
Kitchener
vkizzle wrote: You're assuming they read the same temp.
Which tstat gets priority?
He described the dampening system correctly. If I have my basement set to 20 and there is a call for heat and damper will direct air flow to that portion of the house. If more than one zone has a call for heat the damper will allow air flow to both sections.

The damper continues to adjust based on the temperatures in the different zones. But it's critical that ductwork is designed properly. If it isn't, not only will it function poorly but it can be harder on the equipment.
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Banned
Jun 4, 2020
1634 posts
1750 upvotes
Clarington, ON
vkizzle wrote: You're assuming they read the same temp.
Which tstat gets priority?
T stats are in parallel for furnace. Either or can call for heat. It's the direction they give to damper(s) that is important.
Deal Addict
Nov 8, 2005
3457 posts
3341 upvotes
I have a 3 tiered system, a thermostat on each floor. It's not a traditional furnace system though. A 100k btu gas condensing 95% efficiency hot water tank pumps hot water into the coils of an air handler that has a single blower that can blow to 3 different venting systems to each floor. It's a very efficient system for heating and cooling but it's costly. The hot water tank is stupidly expensive, and the vents are flexible 6" high velocity vents. You can't just retrofit this kind of system. All in all I like the benefits of being able to pump heat or cool air to one floor by itself but overall I'd rather have just a standard system.
Newbie
Sep 9, 2020
21 posts
1 upvote
does anyone have a schematic or an hvac drawing to help with this?
for those that have/had this in your homes can you explain your setup and sizing (what blowers, handlers, venting etc.. that were used)?
Deal Addict
Feb 10, 2006
2652 posts
587 upvotes
Kitchener
xoyo123 wrote: does anyone have a schematic or an hvac drawing to help with this?
for those that have/had this in your homes can you explain your setup and sizing (what blowers, handlers, venting etc.. that were used)?
Look up some videos on YouTube. It would give a better visual than a schematic.
Tired of renting from Reliance? This might be useful reliance-home-comfort-2464779/2/#p34473179

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