He did a pretty good job explaining a thermostat. Think of it as a light switch which turns the heat on and off, it's just activated by heat is all. That little metal coil in the thermostat is bi-metal, which means it reacts to heat and cool by expanding and contracting which in turn causes the mercury bubble to sway one way or the other. Then the mercury inside the bubble rolls to one side or the other and makes a low voltage (24V) contact to send the power back to the furnace to turn it on.
These older, round, Honeywell mercury thermostats were very good thermostats. Since they're doing away with mercury T-stats now, Honeywell has a new round one out which replaces this one,
but it's a piece of JUNK and you shouldn't buy it. The new one sans mercury now is a snap action T-Stat which basically has a set of contacts in it not dissimilar to the points in old distributors, with the same problem of, sparking when the contacts close leaves a small residue on the contacts which build up and can block the proper function of it over time.
The have some very good digital T-Stats out now. The best T-Stats to buy for your heating system are neither the bottom of the line T-Stats, nor the top of the line T-Stats. If you want a long lasting, reliable T-Stat today, do not get one of those top of the line T-Stats that have a Computer board in it. Well, I guess all digital T-Stats have a board, but some are overdone. With so much electronics in them that they may look attractive and be impressive, but are not long lasting or reliable. Some of the T-Stats out there now, have to have batteries in them or they will not function the furnace! Bad choice. The top of the line T-Stats, think about like this...the more that
can go wrong, will go wrong.
Honeywell is probably the best T-Stats out there across the board. There are some others which are good but not many. Carrier makes some good T-Stats (Some bad!), IAQ is a good T-Stat. But like most companies, everyone has some poor products. Honeywell's bottom of the line T-Stats are no good, nor their absolute "Best" ones. The best one going for Honeywell is their middle of the road T-Stat This one:
The Model #RTH7500D (Which is the one that I have in my home. It's only 40 bucks and very probably the most reliable programmable T-Stat that they make. Here it is:
http://www.homedepot.com/s/honeywell%20thermostats?NCNI-5
The touch screen T-Stats that are on the market today have a high cool factor, and in terms of reliability, are hit and miss. There are some decent ones out there and some impressive junk. The one above that Kathi posted is actually a pretty darn good T-Stat. But it does have a few more bells and whistles than is really needed in a T-Stat.
T-Stats to stay away from...
This is a very popular T-Stat. They're all over the place (prolly because they're cheap). I take a LOT of these down as they fail often and are pure junk. These ones also:
Junk.
The best middle of the road T-Stat that your contractor can bring you is this one:
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