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gargurlie · Oct 10, 2008, 10:56 AM

I recently moved into a home that was built in 1978 with many updates, however I am not sure about the furnace. The thermostat is an old mercury therm that is a round dial on the wall. I opened it up to see it has two wires, I believe they were red and black, but it's been a few weeks. I am unable to find any kind of manual or paperwork on the furnace but it's gas, that I do know! I found information online where someone had switched out their old Honeywell round dial therm for a digital. This doesn't have a brand on it, just says day/night, if that's a brand? Does not look like the Honeywell. Then I found a site that says not all therms are compatible with all furnaces. Does anyone know if I can switch out this dinosaur for basic digital or programmable therm?

hvac1000 · Oct 10, 2008, 02:06 PM

First all furnaces by law have a model number tag or information. Usually that info is located on the front of the furnace wall behind the door.
Next the thermostat you are talking about is still made by Honeywell and it does a good job but you can use a digital thermostat as long as it is battery operated since you only have two wires and no common.

Here is the Honeywell main page. Under home products category pick from the various analog or digital thermostats and read up on what is offered. Then you can make your decission. Now there are many different companys that make thermostats so look around on a Google search to find them and gain more info.

Default - Honeywell Environmental And Combustion Controls

KISS · Oct 10, 2008, 05:40 PM

With two wires just about anything can work, but you may be able to control the fan from the t-stat.

Some stats need power from the furnace. This is the common terminal.

Some digital stats will be battery powered or can use a common terminal.

If you can easily run a cable, your options open up for now and when/if you upgrade to a new furnace.

The number of conductors required tend to increase with more energy efficient options.

Missouri Bound · Oct 10, 2008, 08:22 PM

If you have only two wires you are limited as to what your thermostat will do. You won't be able to select fan operation from the stat. You need a heat only thermostat which is the cheapest to purchase. And if you get a programmable thermostat you may want a battery backup, to not lose the programming in the event of a power failure. Check the White Rogers thermostat site for an economical thermostat to take care of your needs. Honeywell is very good, but ofter overpriced because of the name.

gargurlie · Oct 13, 2008, 08:30 AM

Thank you all for your input. I was at my local home improvement center this weekend reading the packaging on all the thermostats they carried. Was confused by all the 1 stage or 2 stage heating/cooling, heat only. Was wondering also why some had betteries and some didn't. This has now been cleared up for me. The house heats quickly, so I don't think I need to worried about the fan only option not being available. Thank again. I feel more confident about choosing something that will work for me.