Thermostat on return duct - correct placement?

[kaiserdom]

Hi,

I suspect that my furnace/AC thermostat should re-located, but I would like to hear from you if this would resolve my issue.

The issue:
When my (electric) furnace runs, the thermostat changes by at least 2 to 3 degrees. Then it heats (in winter) or cools (in summer) to the correct temperature, at which point the furnace (and blower) stop. After about 1 minute, the thermostat reads 2 or 3 degrees more (or less depending on the season). If the difference is more than 2 degrees, the furnace and the blower starts again. The correct temperature reading comes back within 1 minute to the true room temperature (before actual heat or cold air hits the ducts). It all stops. Then 2 minutes later, it all starts over again.

7 years ago, when the house was built, I called the HVAC guy and he told me not to worry, all I had to do is leave the fan/blower on (continuously running). He said it would allow better temperature exchange and healthier air. I was concerned that running the fan 24/7 would wear it out, but he said that is not probable and many clients of his do this.

So I've been running my furnace fan for the past 7 years. Some days, when we open the windows, obviously I turn everything off.

The thermostat location:
My furnace is in the basement close to the middle of the house. I have the ground floor and the floor with the rooms.

The thermostat is on the ground floor, in the hallway (no light, no door drafts, no kitchen heat or washroom walls). It is at 5' from the floor on the drywall.

Behind the drywall is the furnace air return. So the front of the thermostat is visible on the wall (as it should be), but the back of it is exposed to the air return. The furnace itself is just about underneath the location.

What I suspect:
I suspect that the heat generated by the furnace (after it stops and the blower stops) raises in the ducts (especially the return) and the thermostat increases the temperature reading.

Some questions 

1. I know the thermostat must be close to the return air duct, but is it good practice to mount it on the wall that has the return?
2. Do you beleive the temperature reading (different when fan blowing) is due to the fact that the furnace is right underneath?
3. Is it possible the thermostat is defective?
4. Any suggestions so that I can have the fan/blower run only when the furnace is heating/AC cooling?
5. Should I re-locate my thermostat a foot to the right/left so it is not in the air return?

Thank you for your thoughts and suggestions.

 

[sda]

 I called the HVAC guy and he told me not to worry, all I had to do is leave the fan/blower on (continuously running).

 
Yeah, he's an idiot.
 
You've covered the basics of why the problem could be occurring, so unless you have access to accurate thermometers and want to spend a bunch of time doing a science experiment, just move the stat.   Preferably mount it on an interior wall that has a sealed stud space behind it, i.e. drywall on both sides.   It wouldn't hurt to get a new stat either.

 

[ano]

There is actually an AC spec that specifies how many times per hour an AC can start. Exceed that and your warranty won't be covered for failure. Same would hold true for a heat pump heater.
 
In any case, move the thermostat if it is receiving any cold/hot air from the HVAC unit. 
 
Second it IS possible you have a defective or misadjusted thermostat as well.  Most have a "compensator" that can move the thermostat in the direction of the heat/AC.  If you don't think temp feedback is causing your problem, its very very likely this. Get out the instructions and look for "compensator" settings and reduce them. 
 
It also sounds like you need a new HVAC guy that knows what he is doing. :angry2:

 

[LarrylLix]

If you have $40-100 per month to waste on your fan running continuously leave the 'stat where it is.
 
An alternative may be to get a fan cycler in place to reduce your energy consumption and still keep fringe areas dry and evenly temperatured. Some 'stats have them built in.

 

[Del91574]

Look for a T-stat that offers hysteresis unless you plan on moving the unit.

 

[kaiserdom]

Thank you everyone for your help at resolving this issue.
 
The thermostat has very little configurable elements (hysteresis, cyclical fan, etc.) so I decided to take multiple temperature samples at various events (fan on/off, temp reading when heat is started, temp reading on wall, temp when heat stops, etc.).
 
I detached the t-stat from the wall (so I would not get any air from the inside wall) to do my tests.  I used an infrared (gun-like) temperature reader.  Over a couple hours, I took temperature readings from the t-stat display (target and actual), the temperature on the wall where the t-stat was, the temp on the facing wall.  And noted all the numbers (trying to find unusual patterns).
 
After about an hour, I said to myself: "why not get the temperature inside the wall?"  So I took a couple of reading in the hole where the t-stat was.  Well that was THE answer!
 
The bloody HVAC installer had mounted the t-stat on the wall with the heated air duct, not the air return!  What a Dumb@$$  :angry2:
 
I kept on taking readings to make sure there where no other apparent issues.
 
I later moved my t-stat 16" right on the wall (where the interior studs are the return air), patched the wall, plugged the hole for the wire and now, I finally have a correct setup.
 
With all that being said and done, I am now tempted to replace my t-stat with Nest.  I'll continue my reading on Nest and Lyric, etc.
 
Thank you once again everyone.

 

[LarrylLix]

The compensators in some of these stats are done with a small heating element and require fairly static air to function properly too.
 
Best of luck!