Where shouldn't thermostats be?

Replies to This Discussion

Permalink Reply by David Fay on February 13, 2011 at 10:37am

OK, let's think about this. There is some cool air coming through the door at the right but none whatsoever through the door on the left. This looks like it might be an interior wall, with maybe a closet entrance on the right. So let's assume it is.

Above the thermostat is a vent. If this picture was taken in the winter (a big if), that might be a return duct for a forced air system. I'm guessing return duct rather than supply duct because the wall below it is so cool. I would think a supply duct would warm up the wall rather than cool it. But it could be a supply duct with a lot of cold air leaking around it from below the floor. But let's assume it is a return duct.

Now why do we think the thermostat is confused? The thermostat should be located in the coolest part of the room, farthest from the supply ducts. Otherwise, the thermostat will be responding to the supply ducts and not to the overall temperature in the room. I've heard from HVAC technicians that they try to locate the return duct near the thermostat. That makes sense to me because that's where the coolest air in the room will be. So, if that is a return duct above the thermostat, I don't see why the thermostat should be confused.

On the other hand, if that is a supply duct above the thermostat, then I can understand why the thermostat is confused. It is getting cold signals from the cold air in the wall around the ductwork and hot signals whenever the hot air comes out of the supply duct. That would make for pretty crazy temperature swings and a lot of unnecessary furnace cycling.

So, which is it? Winter or summer? Exterior wall or interior? Supply duct or return?

Permalink Reply by Guy DuBois on February 13, 2011 at 11:39am

You are looking at a kneewall. I've never seen a  residential supply of that size around here.  I applaud you for pointing out that I should have made other specific distinctions,among them seasonal.  This is in the Southeast (mixed-humid climate). Without that being clear on the images, come summertime this will be even more confusing.

Inside of the door to the right is the AHU, all open attic behind that wall. Return chases are often thermal bypasses that span from attic to garage or vented crawl/basement. Looking at the thermal image I still maintain that the poor thermostat is helping to act as a conductor attached to the wall assembly and doesn't know what the actual air temperature in the room is.

 I retract any barbs thrust at the HVAC installer. Thanks for giving me pause for review before my next post David!

Permalink Reply by Ed Minch on March 25, 2013 at 8:25am

There are 2 potential reasons why theis return area could be cold - air leakage and low R-value.

Air leakage - note the center of the grill is not cold, just the edges.  This tells me that cold air is coming into the room from around the unsealed sides of the return at the wall.  I don't think the problem is air leakage

Low R-value - note the even temperature of the wall.  The return duct is located right behind that wall and the insulator may not have interpreted that the wall was a part of the envelope and missed it.

My guess is an uninsulated wall.  Depending on the temperature difference, the thermostat can definitely be affected by this cold surface that it is mounted on.

Ed Minch

Permalink Reply by Guy DuBois on March 25, 2013 at 9:22am

Man, you found an old one Ed! This is from two years ago. I'm not even sure if I would argue this one the same way today! From what I recall, yes you are correct in that a return duct runs up that kneewall, leaving no space for proper insulation.

Permalink Reply by Ed Minch on March 25, 2013 at 10:46am

Guy

Just cruising through the archives and didn't see the 2011 - if a house doctor poses an opinion and no one is there to here, does he really have an opinion?