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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: 24,846
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 11 months ago
No it is not.
Customer reply replied 11 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 11 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 11 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 11 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 11 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 11 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 11 months ago
Awesome. Thanks so much.

You're welcome

Rick
Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 24,846
Experience: 40+ yrs. experience as a licensed oil & gas technician.
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