HVAC Dual Zone Heating

Hello everyone,

I have a two level home with a single HVAC and thermostat downstairs. The upstairs gets too hot and I want to turn it into a two zone system. I currently have an AEON multi sensor upstairs that prevents the heat from turning on if it’s too hot upstairs. This of course makes the downstairs a little bit on the cooler side but we prefer this in the interim.

I was thinking with a temperature control upstairs and downstairs, that I could simply close a valve preventing hot air from reaching all of the upstairs vents when the temperature is sufficient upstairs. I wouldn’t need a valve downstairs since there would never be a case where the upstairs would be colder than it is downstairs. The HVAC guy we had over said we’d also need a bypass valve in case the pressure in the system gets too high it can recirculate that air and protect the system.

So is this plausible? Any better ideas?

Edit: Here is an example of using Zwave to control a damper to a single room. I would just want to use Zwave similarly to control a damper to the entire upstairs. http://community.ziy.io/ziy-automated-hvac-damper/

I have 6 zones. I do have a bypass damper. I automated the bypass using an arduino because the gravity version didn’t work appropriately. And yes, it does work. All three levels (including basement) maintain an appropriate temperature.

Have you looked at this? The app is pretty easy to configure but you will need some additional hardware. Dependent on how many vents you have this may be cheaper or more expensive than a 2-zone damper.

I’m still not convinced these things can prevent excessive back pressure to the furnace. I think the OP was smart to get a tech involved.

I researched HVAC systems off and on for a few months in prep for replacing our air conditioning. The opinion of the author (who is well respected in the industry) from the post below believes a by-pass duct should never be used. Don’t shoot the messenger…:

In my old house I had a damper that adjust flow between floors. In Summer more cold went up (since cold goes down) and in winter adjusted for the lower floor. It helped even out the temp quite a bit, and I only had to adjust it twice a year. I never used an automated control, but for the most part it did a good job evening out the temp.

I like the idea of Keenect, but back pressure is a huge risk. The author has done a lot to help with that (using external sensors he built) but it does not fix an off balanced system.

Do you have an intake vent on both floors? I know that really helps with evening out the temps.

Thanks for the link to the article about the bypass duct. I’m wondering if I could go into the attic and find a valve that would reduce the airflow before reaching the actual vents. I guess these are more questions for my HVAC guy. But I’ll keep looking into this bypass duct and whether we really need it.

I noticed we have one of those summer/winter valves too. I moved it to the winter mode as it was on summer mode. Maybe that will help, but can someone explain why it would help?

I do have an intake on both floors. Upstairs it’s on the floor and downstairs it’s in the ceiling. I tried leaving the HVAC fan on all day and I don’t know if that really helped except cresting a constant noise throughout the house.

It directs the majority of the air to either upstairs or downstairs.

Warm air rises, so you should direct winter heating to the lower floor.

Cool air descends, so you should direct summer cooling upstairs.

If you have Cool air coming into lower floor, it will hit the thermostat well before the upstairs is at the right temp.

System is not perfect, in my old house maybe a 2-3 degree difference, good enough. Running the fan helps, my old thermostat had Circulate, which ran like 15 minutes every hour.

My new house is a true dual zone with double the AC/Heater. I like this since I just turn off the lower levels when we go to bed, and the upstairs units keep us kozy. Still have a basement level that is colder (by 5-10 degrees!) which I plan on automating the fan through remote sensors and nest-manager (feature in development).

Thank you for the explanation. So in Winter mode it’ll direct more air downstairs since hest will inevitably rise and heat the upstairs thus keeping less of a difference in temperature between the upstairs and downstairs. But how does it do this? That vent just seems to go directly outside. I don’t get how it is controlling where the bulk of air is sent.

I hope that isn’t going outside, because it looks like it is on your supply side. A lot of systems do have a fresh air intake from outside, but it is connected to the return side of the furnace.

Your vent may be different then may set up. Is it on the intake?

How I tested mine, was turn on the fan and went around to different vents with a piece of ribbon and observe the flow. Then I turned the vent the other way and repeated. Mine was not marked so I had to figure it out, it was not super scientific but got the job done.

I don’t think that duct is leading outside… That would certainly be shocking. It’s probably going into the wall and up. Or perhaps it’s part of a bypass that feeds directly into the return.

Here is a photo of where the duct leads to. You guys be the judge. 8)

What in the…

Uh… so assuming that duct is connected to the supply-side of your furnace, that makes absolutely no sense to me (I’m assuming that only because I don’t think your furnace is installed upside down, and typically the supply ducts come out of the top).

You certainly have a non-standard installation. I’d reach out to a local HVAC service company and ask them to explain it to you. A properly-designed HVAC system in a properly-insulated house should not have temperature differential the issues you’re reporting anyway. So I’d start with a full system check, and then perhaps move to smart vents and/or a smart damper (with bypass).

yeah WTF. Is your air filter at the top of the unit or by the floor?

I think what you have is a “combustion air duct”. Your furnace needs a certain amount of air to burn gas/fuel and if your utility room does not have sufficient ventilation this air needs to come from elsewhere (usually outside if you don’t have a direct vent power connection). If there is not enough air the furnace does not burn efficiently and there is risk of CO build up. Because the furnace only runs in the winter you can close it in the summer. The way your system is built is not very energy efficient and probably a couple of years old.

Google “combustion air duct supply plenum” to find more info.

This duct has no impact on cold/hot air distribution in your home.

Combustion air either runs directly to the furnace or to the general vicinity. His runs straight to what looks like the supply plenum.

Yes, that’s an “inefficient” and old way to connect combustion air. By feeding into the supply duct you actively suck in air and you can use a smaller diameter vent.

I am not sure if the manual summer shut off is still up to building code because if you forget to open it when the furnace runs it can lead to dangerous CO build up…

I recommend checking with the local building department (usually free) or a HVAC professional.

You can’t feed into the supply. It’s the pressurized side. Maybe you’re thinking the return side where there is low pressure to “suck” air in?