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I removed the old mercury switched thermostat and replaced…

Hi, I removed the old...
Hi, I removed the old mercury switched thermostat and replaced it with a new Honeywell TH4110D programmable thermostat. I only require the heat function to work. There were only two wires red and white. I hooked up the red wire to the R contact and left the jumper connected to Rc. I hooked up the white wire to the C contact. I did not change the switch in the back of the thermostat it is on gas or oil. I have a Lennox mid-efficiency gas furnace. I call for heat and nothing happens. Everything was working fine before I changed the thermostat.Do I still need to jump a wire from the C contact to the W contact?Thanks Harry
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Answered in 1 minute by:
10/15/2017
Rick
Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 24,193
Experience: 40+ yrs. experience as a licensed oil & gas technician.
Verified

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

Red to R White to W, jumper can stay to RC. No jumper to C. Is this a wi fi thermostat?

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Customer reply replied 8 months ago
No it is not.
Customer reply replied 8 months ago
let's try and sort it out without the phone call

The call prompt is automatic I have no control over it. We can continue here. No wi fi = no C connection. Red to R White to W only

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Customer reply replied 8 months ago
that is correct. Currently I have it set on heat, and hold at 80 degrees. The furnace should fire right away correct?

It shoud respond if you don't have a broken thermostat wire. Whether it fires up depends on whether the furnace is working properly or not.

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Customer reply replied 8 months ago
furnance was working perfect before with the old thermostat. Can I pull the thermostat off while the red wire is hot. I can put a volt meter on the red and white wires to check for voltage.

Voltage is irrelvant. This is a switch leg. If you're careful you can do this without turning the power off to the furnace but it's best to turn it off. Since you only have 2 wires worst case the furnace turns on and off if you touch the 2 together.

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Customer reply replied 8 months ago
I just tried to call for heat again. I can hear the click inside the thermostat. I will pull the thermostat cover off and check the two contacts with a volt meter. Looking for continuity. I really thought this was a no brainer. sigh...Thanks so far.

Ok

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Customer reply replied 8 months ago
Ok. I touched the red and white wire together furnace kicked in. Can I take the jumper from the R and Rc terminals out? Seems like a contact problem in the terminal block.

The R to RC jumper is irrelevant. It's there if you have AC. You can remove it if you like.

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Customer reply replied 8 months ago
Well I pulled the jumper out. I touched the white to red wire again, furnace fired. I then secured both wires back into the terminal block and then use a paperclip to jump across the screws in the terminal block, furnace fired. I snapped the thermostat back into place and called for heat nothing. Looks like a defective thermostat or are there setting in the thermostat that needs to be done? When I read under installer set up but the default settings are for gas oil.

Nothing special for you to do. Yes looks like a defective thermostat

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Customer reply replied 8 months ago
Awesome. Thanks so much.

You're welcome

Rick
Rick, HVAC Supervisor
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