AskCodeMan

What is the required clearance between thermostate and ac air return?

Hi Richard,

There is no clearance requirement other than not "in the return plenum".

For the thermostat to work and control the system properly, the thermostat must be in the return air flow path, that is what allows the thermostat to sense the temperature of the air in the conditioned space and regulate when to cycle the air conditioning system on or off.

There are many locations for the thermostat to be located which can cause the system not to cool or heat the conditioned space properly or evenly, but the location of the thermostat is up the the HVAC engineer who designed the system and laid the components out.

I know one house where the thermostat was mounted right above the return air grille/return air filter, and where the majority of the house did not heat or cool as desired or intended. The reason was that the return air grille was located below and 3 feet horizontally from a supply register in the dining room. The system would cycle on, the cold air from that supply would be drawn past the thermostat on its way to the return (located approximately 10 feet diagonally from the return), satisfy the thermostat in a short time, shutting the system down.

The thermostat was relocated about 45 feet away from the return to a hallway where return air which more closely replicated the average temperature in the rest of the house would circulate past the thermostat, cycling the system off only after properly conditioning the air in the rest of the house.

The air conditioning system then worked properly, with the temperature of the air in all of the rooms being at the set temperature +/- 1 degree (a 2 degree temperature difference) from the warmest area to the coolest area, not something which one felt, but which could be measured with digital thermometers while walking around measuring for the difference in temperature. Before relocating the thermostat the temperature difference was 10 degrees.

If the location 'just looks wrong', it might be wrong. However, further checking will determine how closely consistent the temperatures are throughout the conditioned space, and if close, then the thermostat is doing its job properly. An example of "close" might be setting a thermostat to 75 degrees and expecting no room or area to be higher than 76-77 degrees and no room or area to be lower than 73-74, making a 73-77 spread (+/- 2 degrees, or a 4 degree spread) the greatest spread you would want. Even a 4 degree spread seems a bit much as you could walk into one room and feel cold at 73 degrees, then walk into another room only to feel warm at 77 degrees.

I am sure there are is an ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) Standard which specifies a recommended or maximum temperature spread, however, I do not have the ASHRAE manual so I cannot check for one.

Hopefully the above information helps.

Codeman