Any idea how to tell if a thermostat for my gas furnace needs replaced?

I just bought an old house and the furnace has been here over 20 years. I can't afford to replace it this year. Any ideas on how to tell if the thermostat is working or needs replaced?Any idea how to tell if a thermostat for my gas furnace needs replaced?O.K. First off,,, a mercury thermostat is nothing more then an on/off switch to bring on the furnace, it works off of changing temperature warping a piece of bi metal Thu expansion and contraction,,, temp goes up, the mercury goes one way,,, temp goes down mercury goes the other way... depends on what position the heat/cool switch is in for the mercury to send power either to the furnace or the air conditioning... Second,,, since this is just a switch, it can still operate and not control at the temp you set it at if it is out of level or the bi metal is worn out ,,, you may have it set for 70 degrees and it will control at 65 degrees or 75 degrees,,,so don't compare with a another thermometer unless all you want to know is if it is controlling at the set point or not,, set the switch at heat and turn the dial to the highest setting and see if the furnace starts..third,, even if you turn it up and the switch is set to heat,, it could be the furnace that is not working and the thermostat is still doing its job,,,, I believe you need to call a qualified tech.. There should be a fan switch on the thermostat also,, this switch should say fan / auto set it to fan and see if the fan starts,, it it does we know the thermostat is getting power, in this case it probably will be a furnace problem,,,good luckAny idea how to tell if a thermostat for my gas furnace needs replaced?Does it work? would be the first question. If not, have someone that knows what there doing look at it.Any idea how to tell if a thermostat for my gas furnace needs replaced?The thermostat on a furnace that old operates by opening and closing the gas valve directly, it operates on a 24 volt system and should open and close within four degrees of the set point. (if your stat is set at 68 then it should turn on at 64 and off at 72) you can solve this by replacing it with a digital thermostat. You can do this yourself by turning the power off to the furnace and replacing the wires following this code

Red = power

White - neutral or heat

Yellow - cooling

Green - Fan

Orange - reversing valve on a heat pumpAny idea how to tell if a thermostat for my gas furnace needs replaced?digital or mercury filled?

digital will click a relay

mercury will show a arc when you move the dial back and forthAny idea how to tell if a thermostat for my gas furnace needs replaced?thermostats will last for the life of the furnace and beyond. Most old ones like yours are just wound bi-metallic strips (a coil made of two different metals that expand and different rates). You can check it by placing a thermometer along side the thermostat and seeing if the furnace kicks on at the temperature at which it is set. It should continue to heat about 2 or 3 degrees above this set point before shutting off. If it does not kick on the contacts that the coil pushes together might be dirty or corroded.Any idea how to tell if a thermostat for my gas furnace needs replaced?First of all. we're talking about gas and that's nothing to go cheap on. in the end it could cost you everything and more! I'm a fair hand with heaters and the Government hired and flew me back in the 70's to install gas hot water heaters and heaters for the people in Johnstown Pa. when they had those three damns break and the water flooded out the town. I was a young teenager back then so when I say gas is something not to reckon with, you can pretty much take that to the bank. The fact that your system is 20 years old, there's a great chance that you should replace some of your system to be safe and have an experience tech to check it for you. In the mean time, you can get an external thermometer and see if it reads the same as your wall stat. This will tell you if your thermostat is still working. PS..A service call is cheaper than replacing your burnt down home or worst. even if money is tight. good luck and keep warm, :)Any idea how to tell if a thermostat for my gas furnace needs replaced?Raise the thermostat one degree above room temp.If you hear the furnace ignite OK if you can't hear it go to the basement and listen at the furnace if on OK.(room temp is the bottom indicator on the thermostat)Make sure all electrical switches are on that pertain to the furnace. also make sure you check the clean out at the base of the chimney.Remove any debris bird nest etc.normally mercury switches last they are sealed so the mercury makes the contact.