Hi... I need your help. My cousin lives in a townhouse and there are two thermostats... one downstairs and one upstairs. They are both older manual thermostats. She wants me to replace them with digital ones so she can set the heat and air automatically. The problem is, I don't know why she has two now? I took them apart. The one downstairs has no levers on it for FAN or Heat or Air... just a dial and a level to move for setting the temp. Inside, there were three wires, of which only two were connected... the White and Red wire were connected... the blue wire was not. The one upstairs has levers for Fan (on, off, auto) and Heat or Off or Cool. It has more wires... white, red and yellow. Does anyone know why there are two??? Any can I replace them with standard digital thermostats? Thank you for your help.

 

Does the house have completely separate heating & cooling sytems, like maybe hot water heat and a ducted cooling system?

 

No... it has forced hot air through ducts and the air conditioning comes through the same ducts.

 

Before you can do anything you need to find out what the deal is on the 2 stats. Maybe the downstairs one controls a damper? Maybe it's in series with the one upstairs? Maybe the house had heat only at one time and was upgraded and they neglected to remove the downstairs stat? Maybe someone has wired the system so the upstairs stat controls the cooling but the downstairs one controls the heat? What letter terminals are in use on each thermostat? You need to get the 2 stats figured out first.

 

I can tell you when she bought the townhouse, she was told the stat downstairs controlled the heat and the one upstairs controls the A/C. The townhouse was built this way.

 

That's right upstairs controls the air and down the heat. You'll have to replace both t'stats. This is a common set up for townhouses. No special wiring involved. Upstairs is R & W and downstairs is R, Y, & G.

 

New one on me, although it makes sense. Do you power the downstairs stat from B terminal on the upstairs stat or do you use an isolation relay in the furnace powered by O terminal on the upstairs stat? Somehow there has to be an interlock, otherwise you could have heating/cooling on at the same time.

 

So... you're saying just buy two identical digital stats... hook them up using the exact wires that are there now... and it will work fine? They don't need to be any particular kind?

 

doogster- That is correct, just hook up the wiring the exact same way.

Big John- Usually the t'stats are not interlocked in anyway what so ever. The t'stat is the isolation. Think about this...the old mechanical auto change over t'stats used two separate bulbs for heating and cooling, but the same power source.

 

Not sure I follow you. His upstairs t-stat has a switching subbase, but the downstairs one does not. If you set the upstairs one to cool and as low as it will go and the downstairs one high as it will go, you'll bring on the heating and cooling at the same time unless you wire in an interlock. I've haven't done this hookup before but it seems simple. Either have B terminal on the upstairs stat power R terminal on the downstairs stat or wire O terminal to a cooling relay in the furnace. The cooling relay would have a NC set of contacts that would break the W leg at the furnace when energized. Another variation is B terminal could energize a heat relay in the furnace. The heat relay could break Y and make W at the furnace or break Y and make R to the heating thermostat. I think I like the O terminal idea the best because, as long as the upstairs subbase switch is not set for cooling, the system defaults to heat. The only way I see two auto change over stats working is you would have to shut the downstairs one to off in the Summer and the upstairs one to off in the Winter.