Thermostat is not lit up and won't turn heating on
The thermostat was not lit up and was missing batteries in the battery compartment. It has worked fine for two years without batteries; don't know what they are for. I put two AA batteries in and now the themostat displays but it won't turn the heater on.
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Remove the cover from the thermostat and check for voltage (usually 24VAC) on the red "R" wire. It comes from the transformer at or in the furnace. Place one probe on the screw marked "R" and the other probe to any of the other wires coming into the thermostat. Depending on how old and the type of system you have, you may have a black wire (Common or neutral), green (fan), yellow (cooling call) and white (heating call). Heat pumps and staged system will have more wires and other colors. If there is power at the thermostat, check that all connections are tight and you have a fresh battery installed if it has one. If it still does not work, replace the thermostst. If there is no power at the thermostat, check for power at the furnace. There should be a disconnect switch near the furnace, it may or may not be fused. Check with a voltmeter to see that you have power at the disconnect first and that any fuses are good. If it is ok, move on to the transformer at the furnace. Check the "Line" side of the transformer for 120VAC power and the "Load" side of the transformer for 24VAC power. If it is ok, and you have no power at the thermostat, the wires to the thermostat are bad or you have a loose connect at one end or the other.
Check the exhaust port of the furnace. Wasps like to build nests in there and it will not allow the furnace to ignite if blocked. You have a gas detector and it is turned on I assume? I forgot this many times.
Heat pumps require specific heat pump thermostat. If you are trying to install a thermostat that is generic forget it. A heat pump is a diff animal than a central unit. Get the thermostat that matches your system. A generic won't cut it.
Your Thermostat should wire as Red to Rc/Rh (if Rc/Rh are not connected you need to put a jumper between them; White to W; Green to G; and Yellow to Y on your thermostat. These will be the only wire connections that you need to connect from the unit to the thermostat. However, if your thermostat is not battery operated and is programmable then you will have to run on wire form the transformer in the units C to the Bl on your thermostat.
If you replace the battery in time, the furnace should still work. You didn't mention what type thermostat you have, but the battery is located inside the thermostat housing. Most can be accessed my opening the flip down cover of the front of the thermostat (this where you generally program it). Inside you will either see instructions for replacing the battery or you will see another small compartment door with a battery icon on it or it says "Battery". If none of that appears as stated, the batter will be behind the thermostat housing. It's generally 1 AA Battery.
Sounds like you have a heatpump. Is that correct? When it gets under 35F or so the heatpump just isn't able to keep up with the heat demand and will require help from a backup source such as electric strip heaters. The way the thermostat works is that the heatpump will work until room temperature gets 2F to 4F below what you have the thermostat set for, the Auxillary heat will kick in to help out. I think I am answering your question, please let me know if I have misread your problem and I'll try to come up with something else.