HVAC - Zone Control Help

[TWG1572]

I'm hoping we have a few resident heating experts that may be able to help me with diagnosing a recent change in the behavior of my zone heating system.

For the last five years we've owned the house, it's been very good at holding setpoints in each of the zones. But for the last couple weeks, our main floor zone has been holding about 2 degrees higher than the set point. The thermostat knows it, and I've confirmed the thermostat is reading accurately with my IR thermometer. I'm trying to figure where the extra heat is coming from. My second floor zone is fine.

We have an LP furnace, which is set up with two zones. The main floor and second floor. The main floor also includes registers in the basement, which the prior owner added later. So we are looking at maybe 1,000 sqft on the second floor zone and 2,600 on the first/basement zone. Based on what I've read, this is probably a bit unbalanced for a zone system. I do have a bypass damper, which runs open when only the second floor is going.

I plan on troubleshooting this tonight, but I'm sitting at work thinking about what I need to do. At this point, about all I can think of is that maybe my first floor zone damper is stuck partially open? That would mean I'd be getting heat in the first zone when the second zone is running, but the first should not be.

Are there any other failure points I should troubleshoot? It seems like the thermostat is OK, it's saying it's too warm and not calling for heat. But I'm getting it anyway.

 

[ironman_gq]

I'd say stuck damper, what kind of return system is there? Do they draw from throughout the house or just pull return air from the basement/ main floor and are they also zoned?

 

[TWG1572]

Returns are not zoned. They pull from throughout the house.

 

[Pop_Rivit]

If it always has been working correctly in the past, your most likely culprit is the damper. That's the first place to check.

Originally Posted By: TWG1572
Returns are not zoned. They pull from throughout the house.


That's not correct in a properly designed zoned system. If a home is compartmentalized (lots of rooms rather than an open floor plan) some of the returns may need to be dampered. If the system is not correctly designed parts of the house will become overpressurized and push heated air into the areas that aren't calling for heat, but are still pulling return air. By damping the returns, warm air can be kept and circulated where needed without getting pushed to parts of the house that are set at a lower temperature.

The return should be open to whatever zone(s) that are being heated or cooled, and dampered for the zone(s) that are not being heated or cooled.

 

[TWG1572]

Originally Posted By: Pop_Rivit
That's not correct in a properly designed zoned system.


I don't disagree. There are quite a few things I would have done differently had I been around when it was designed, including putting the basement on a third zone. But the prior owner did it, so I've got to live with what I have. The majority of the basement is drywalled, so there is no way to retrofit anything without a costly mess.

 

[TWG1572]

Well, problem found. In Aprilaire dampers, there is a flexible band that goes from the motor shaft to the damper assembly. As the shaft rotates, the band rotates around it and pulls on the assembly to actuate the damper. That band is broken. Here's hoping it's something that I can buy a replacement for. Fingers crossed.