[serker]

I'm very confused, so the question may be long and confusing as well, I endeavor to be precise. Please remember, this question is about thermostats.

I have a three story house, levels are named B, 1, and 2.

There are two A/C units, and one heating unit.

Main A/C handles B and 1, Secondary A/C handles level 2. Yes, there are two "outside" units on the side of my house--they are separate units (i.e. they don't share freon)

The heating unit is a hot water baseboard recirculator, which splits hot water from it's boiler to all three floors.

Level B has a round thermostat that controls heat. It's a mercury thermostat.

Level 1 has two thermostats right next to each other. One is Rectangular and controls Main A/C, the other is the same round thermostat as on level B, and controls heat. They're both mercury thermostats.

Level 2 has two thermostats right next to each other. Both are Rectangular, mercury, different brands; one controls heat, and one controls the Secondary A/C.

Confused yet? I'm confused, but here are more notes:

None of these thermostats have HEAT/OFF/COOL on them, since they're all separate units.
The A/C units do not have a FAN/AUTO switch on them. That's all controlled in the A/C unit evidently.

Here's the question:

I want a programmable thermostat on each level, to replace all of them. That means I need a simple two wire digital programmable for level B. That's easy.

Here's the hard one:

I want a programmable thermostat on Level 2 and Level 3, that controls both heat and A/c on the level, all in one unit. I have two wires from the existing thermostats (each), meaning it's a simple on/off circuit. And I already know they're all 24v.

The issue is, I need the thermostats to have separate 24v connections--no common ground--otherwise power could get back through the system and (not that I'm concerned about blowing anything) cause the A/C or Heat to turn on.

Heh... just for comic relief:

The idiots at Lowe's say they don't sell anything like this, because it can't be done. I have to use separate thermostats. They've also never heard of an A/C thermostat that only uses two wires, so I'd have to contact an HVAC supply to have them trace down my "serious fire hazard" problem with the thermostats in my house.:rolleyes:

My local HVAC supply people want to sell me a $450 thermostat for each floor. It's digital, backlit, touch screened, is programmable from my laptop, and requires a alarm pair and monthly monitoring fee.:rolleyes:

Please tell me the truth is in the middle, and what kind of thermostat I'm looking for... is it a multi-stage, stage 2, etc... or... what wire colors am I looking for on Grainger's site? (They always list B, W, Y, O, etc... but some have O2, and Y2... and now you know why I'm confused... )

[labman]

A thermostat primer, plus see the sticky on thermostats at the top of this forum.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

Most thermostats could easily handle one heating and one cooling zone plus fan on. You can't control the cooling in 2 different areas independently with one thermostat. The guys at Lowe's are right about that. Even a blind hog finds an acorn sometimes. Your plan of 3 thermostats should work. Since you have to keep a second thermostat, you might look into adding zone valves and controlling the heat separately too. You should also be able to cut back one transformer for each thermostat.

Some of the details will depend on your current wiring. You will need at least 4 wires from the boiler to the thermostat controlling the heat if you are going to have the fan on option. 5, if the thermostat requires a connection to the common for it power. An A/C thermostat could be made to work on 2 wires by jumpering the Y & G terminals. When calling for cooling, thermostats switch RC to both the Y & G terminals. However, I think some electronic ones power themselves through the W terminal. This sticky may be more useful than the people you have been talking to, to identify which thermostats will work for you, https://www.askmehelpdesk.com/electr...als-37098.html

[NorthernHeat]

A new programmable thermostat will have Rc=+24 volts from the cooling transformer and Rh=+24 V from the boiler. Just remove the jumper. W is for heat, Y is for cooling G is for fan on. By t-stats that run on batteries and don't connect a common (ground) Now about the cooling stat only using 2 wires, an installer had to rig that to get it to work, in most cases the stat sends 24 V to Y and G when there is a call for cooling, to start the condensing unit outside and the G to start the fan inside. If the furnaces are quite modern the fan will turn on with only a call for cooling on Y, but more than likely not.