A/C thermostat issue?

[MTPockets]

Do I have a bad thermostat? We have two A/C units, each with a separate thermostat. I can set the bedroom at any temp and the A/C will work as set. The living area unit works good when it's hotter outside than my thermostat setting. If I set temp at 78, the A/C will shut off at 78. But, if Ouside temp is say 74 and inside is 78, the A/C will come on and run longer. At times it will not stop until it gets to 76, even though I have it set at 78. ????

 

[Bob&Patty]

Thermostats are not accurate to a exact setting. The thermostat's will cool or heat to the inside temp. The outside temp is almost a mute point. Your car/truck does not care what the outside temp is, only what the in car temp sensor says it is..

 

[MTPockets]

Thermostats are not accurate to a exact setting. The thermostat's will cool or heat to the inside temp. The outside temp is almost a mute point. Your car/truck does not care what the outside temp is, only what the in car temp sensor says it is..

Although I would tend to agree, mine consistently cools to the inside setting when it's hotter outside. Then when it's cooler outside, only the living area A/C will cool the inside lower than the setting. The bedroom A/C does not. I've experimented by turning on only the living area A/C to see if it shows the same behavior and it does. Maybe I'll buy a different thermostat and see what happens...

 

[danemayer]

Terry,

Are you checking the temp at which the A/C shuts off by cycling the thermostat to OFF and pressing the + or - button?

If you have the Dometic Single Zone LCD, I'm not sure if there are any substitutes.

 

[Bob&Patty]

Let clarify my statement. They should be within 1-2* + or - of the setting.

 

[MTPockets]

Terry,

Are you checking the temp at which the A/C shuts off by cycling the thermostat to OFF and pressing the + or - button?

If you have the Dometic Single Zone LCD, I'm not sure if there are any substitutes.

. I'm not changing the set temp on the thermostat.
The inside temp is checked on a separate digital thermometer we have on the wall. The digital thermostat controls the furnace and one A/C. I set the A/C temp at 78; it will cool to 78 as displayed on our thermometer. When it's below 78 outside (as displayed on our outdoor thermometer), the A/C will cool the inside to 76 or 75. (As displayed on our thermometer) it also feels colder, so when that happens I usually just turn it off. If there is a built in tolerance I guess it's OK, but still curious why it seems more accurate when it's hotter outside.

 

[danemayer]

It sounds like your thermostat and your digital thermometer are maybe sometimes getting different readings of temperature inside the coach.

Next time you see the problem, cycle the thermostat to OFF and check its ambient temperature reading by pressing the + or - button. If your set point is 78, it'll probably read 78, even if the other thermometer says 76.

If that's the case, then the question is why the thermometer is getting a different ambient temperature at times. Maybe it needs to be replaced, rather than the thermostat.

 

[Dave_Burrow]

2 questions...
First How close is the inside digital thermometer to the thermostat? If they are more than a few inches apart it is very possible that there is a couple of degrees difference in the air around each of them. It is also possible that if they are well separated the air conditioner may cool the digital thermometer to a lower temperature than the thermostat in the same amount of time. This depends on how much cool air flow comes out near the thermometer and how much comes out near the thermostat. Our thermostat is near our door and our weather station with an indoor temperature reading is at the rear of the coach. It usually reads a cooler temperature than the thermostat because there are no cool air registers near the thermostat. In real hot conditions we get more heat in the back of our camper due to heat loss through the windows. At these times the weather station and the thermostat read closer together.

Second...What kind of thermostat do you have? Our Dometic multi zone CCC2 thermostat (the size and shape of an index card) has a push button on it that will display the inside temperature that it is currently seeing. Normally it displays the set point temperature but it will display the local observed temperature. Maybe your thermostat has the same feature or it may just run the blower for a while after the compressor cuts off. That may be enough to allow the extra air over the cold coils to drop the inside temperature a degree or so if heat loss from the camper is slow as it would be in 74 degree weather.

 

[MTPockets]

it may just run the blower for a while after the compressor cuts off. That may be enough to allow the extra air over the cold coils to drop the inside temperature a degree or so if heat loss from the camper is slow as it would be in 74 degree weather.

I hadn't thought of this, but the heat loss when it's cooler outside would allow the inside temp to drop.