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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8383
Experience:  Furnace, A/C and heat pump specialist.
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AC won't turn on. Not getting 24V signal to contactor . Unit

Customer Question

AC won't turn on. Not getting 24V signal to contactor . Unit runs if contacts are manually engaged.
JA: I'll do all I can to help. Tell me a bit more about what's going on so we can help you best.
Customer: Thermostat was replaced last yr by daughters BF, it worked fine with heat. He & old thermostat r gone
JA: Sometimes HVAC issues that that you think will be really complicated end up being easy to fix. The HVAC Technician I'm going to connect you with knows all the tricks and shortcuts. Is there anything else the HVAC Technician should be aware of?
Customer: It is Lennox gas furnace with heat pump for AC outside, thermostat is Honeywell rth111
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the HVAC Technician about your situation and then connect you two.
Submitted: 1 year ago.
Category: HVAC
Expert:  Sean replied 1 year ago.
Sounds like a wire at thermostat may be not hooked up correctly. Should be a easy fix.Can you check at thermostat and tell me if anything is hooked to the Y terminal.
Customer: replied 1 year ago.
Yellow wire
Customer: replied 1 year ago.
There is a slider switch on back of thermostat that has 2 positions, gas/oil or electric/heat pump. The fur ands is gas and AC is a heat pump. Where should that b set?( I've tried both & it didn't make any diff)
Customer: replied 1 year ago.
Stupid auto spell! Furnace, not fur ands......
Expert:  Sean replied 1 year ago.
Alright. I seen you said this is a heat pump system also ?
Customer: replied 1 year ago.
Lennox HS23-261-1P unit is outside
Expert:  Sean replied 1 year ago.
Do you have access to a voltage meter?
Customer: replied 1 year ago.
Sure, how else would I know I'm not getting 24V to the coil?
Expert:  Sean replied 1 year ago.
I just wanted to make sure before we go into it further.First things first I'm almost 100%sure the thermostat you have is not a heat pump ready stat.....So at the contactor at AC. You have 2wires at coil that then go down to wire nuts which is thermostat wire that runs to the furnace....... I can't just tell you colors because some installers do it differently but I would like you to.check for 24v at furnace.
Customer: replied 1 year ago.
Nope, nothing there, only place I have 24VAC is the R terminal. The coil runs between T & W
Customer: replied 1 year ago.
This is on the PCB on the furnace unit
Customer: replied 1 year ago.
There is also nothing connected to the Y terminal on the furnace PCB
Expert:  Sean replied 1 year ago.
I'm gonna opt out right now see if someone can grab this I'm out and limited what I can do over phone I'll be home in about an hr and if no one else grabbed the.question I'll.jump back on.
Expert:  airheatman replied 1 year ago.

Your daughter's BF has installed the incorrect thermostat for your heat pump.The RTH111 is not compatible. You will need to get a two stage cool and single stage heat thermostat designed to work with our heat pump.See link below

Customer: replied 1 year ago.
Manual that came with it says it can't b used on a heat pump with auxiliary B/U heat, it does say it can't b used with a heat pump. I went thru Honeywells website to help me choose the correct non-programmable, DIY thermostat and ended up with the RTH111 as one of the choices......😳
Customer: replied 1 year ago.
this cooling unit is a 2 stage system?
Expert:  airheatman replied 1 year ago.

OK lets backup just for a minute. The original question is a bit misleading. You stated that you have gas heat with a heat pump outdoor unit.If that were the case, you would have 2 stage heat. (Heat pump is first stage and furnace is second stage or backup heat) The RTH111 would not be compatible.

However as I read your previous conversation with other expert you provided model on outdoor unit.

The HS23-261-P is NOT a HEAT PUMP (Heat pump would be an HP23-261).

So with this figured out that means the switch on back needs to be set to AC instead of Heat PUMP.

WE should have wires as follows:


R with jumper to RC--------------R




Nothing at thermostat------------C-------------------------C.

Verify this and let me know if it works