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I have a Ruud Furnance/Heat Pump system. Heat Pump is

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UPLA-031JAZ. Air Handler is UBHA-17J11NFAA...
I have a Ruud Furnance/Heat Pump system. Heat Pump is UPLA-031JAZ. Air Handler is UBHA-17J11NFAA. Original thermostat is Rheem part number 41-20804-15. The thermostat will consistently read 5-10 degrees warmer than the air in the house really is. I even installed an Ecobee and a Honeywell Lyric T5 with the same results. The air temp is 5-10 degrees cooler than the stat will read. When I put my hand on the stat it's warm. When I take the stat off the wall the wires and wiring cradle are warm. What causes this heat and how can I eliminate it?
Submitted: 4 months ago.Category: HVAC
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Answered in 4 minutes by:
3/20/2018
HVAC Technician: airheatman, HVAC Technician replied 4 months ago
airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 9,213
Experience: Furnace, A/C and heat pump specialist.
Verified

If the stat is reading higher than room temp there has to be something generating the heat.It can be coming from attic air down wall .Remove thermostat and stci a thermometer in the whole in wall and see if the air is hotter than room temp.If so, use foam insulation to seal whole in wall where wires come through.

Make sure there is no light or heat source near thermostat also.I had one once that installer mounted too close to a wall scone and was giving it a false reading.

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Customer reply replied 4 months ago
I took my Fluke with a temperature probe wire and stuck it through the drywall into the cavity. It reads 1 degree F cooler than the hallway. There are no lights, switches, appliances or anything like that on or near or around it. Behind this wall is a staircase to the upstairs.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

When you say,the thermostat is reading warmer than room temp in the house, are you taking the room temp right beside the thermostat with the Fluke?

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Customer reply replied 4 months ago
I have a thermometer in the main part of the main floor. It reads 64 degrees. I move it to the hall and it reads 64 degrees. I take the fluke and the thermometer attachment and it reads 64 degrees in the main part of the house and the hallway. I stick the attachment (which is a high temperature thermometer wire probe) through the hole where the wires come out of the wall and it reads 63 degrees. But, the thermostat (Rheem, Ecobee and Honeywell Lyric T5) all read between 72 and 75 depending on the device. I take the device off the wall and let things cool off. When I put them back on the wall and hook things up again, the temperature on the device reads correctly. After being connected for a few minutes, the temperature spikes.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

Sounds very strange.Three different thermostats all read high but the fluke reads correctly in hallway. ONly thing I could imagine is something to do with the load through the thermostat control wires. Touch the Fluke probe against the wires and see if it also climbs

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Customer reply replied 4 months ago
The wires read about three degrees above room temperature right after I take the stat off the wall. They read ambient temp when things have cooled off. There's no difference in temp across the different wires.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

If you have an amp meter clamp around wires while thermostat is connected but everything off and see if there is any load on the wires.

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Customer reply replied 4 months ago
I'll have to get a clamp accessory for that test. I'll get one later tonight.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

Not thinking of any reason for those wires to be warm unless there is a load on them

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Customer reply replied 4 months ago
Okay. I'll let you know what I find out. So many things miswired on this system. Thanks for your patience as we get it sorted out. I replaced the main breaker cuz it quit holding itself closed. And the slot next to it was melted from another breaker failure. It looks like this system has had issues for a while.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

Just a thought.Is the thermostat wire 18 gauge (not thin phone wire)?

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Customer reply replied 4 months ago
It's proper 18 gauge thermostat wire. That's one of the first things I checked when we were trying to determine which wire went to the heat pump vs the stat.
HVAC Technician: airheatman, HVAC Technician replied 4 months ago

Sure would like to know what we determine in the end.

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Customer reply replied 4 months ago
Okay. I've taken an AC voltage detector and checked each wire at the stat for voltage higher than 24. None of them are carrying anything higher than 24 volts.
Customer reply replied 4 months ago
I think I found the problem. The heat strips will occasionally run without the blower turning on. The heat will then come up the return air duct and exit right under the thermostat.
HVAC Technician: airheatman, HVAC Technician replied 3 months ago

Let,sget that issue corrected(replace sequencer or whatever is causing strips without blower) and see if issue goes away.That one definitely sounds far fetched.

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