Hello,
My knowledge of HVAC is very limited, and I need help please.

This is the description of my heating system that I have:

Heat System: Forced Air
Heat Fuel: Natural Gas

Cool System: Central A/C
Cool Fuel: Electric

It's line voltage (as far as I can tell... ) (120VAC, with 2 wires coming into the thermostat)

I have an ugly looking thermostat on my wall (non-digital). It has a knob for heating/cooling/off and a knob for the temperature adjustment. There are air ducts going through the apartment, and when I select either heating or cooling then hot or cold air blows out of the ducts.

I'm looking for a new thermostat (preferably a digital one), it does NOT have to be programmable, and touch screen would be nice but not necessary.

The only types of thermostats that I've found in home depot/lowes/grainger are either
-low-voltage digital ones
-line-voltage digital heating only
-line-voltage non-digital heating/cooling.

Like I said I would like a heating/cooling digital one... maybe I'm not understanding something correctly...

Any help/advice would be greatly appreciated.
Thanks,
-Max

Have you checked the voltage to your current stat with a meter? I'd be surprised to find 120 vac, most are 24 vdc.

The reason why I think it's a 120VAC thermometer is when I turned off the power, removed the existing thermometer and I see 2 wires coming out of the wall, I turn the power back on, took my DMM and checked the two wires, the voltage between them is 120V. So I was surprised as well...

You would find more stats on the market rated for 24 vdc. You can add a transformer to that 120 vac and drop it down to 24 vdc.

A transformer or a relay?
And also, I have limited space inside the junction box, so I'm not sure if I would have enough room to install the relay and still wire the thermostat.

I don't see how you can control both heat and cooling with only 2 wires at any voltage. Maybe you need to take a closer look at the thermostat. If you know little of HVAC, you might browse the sticky at the top of the forum. Northern Heat has done a good job explaining the basics. If you have what you think you have, you may have to try a HVAC supply. Many of the line voltage thermostats are meant for electric heat.

Thanks for the response. I don't know much about HVAC, but I know enough about electricity (I'm an electrical engineer, and like to do a lot of "electrician" things on my own).

I was surprised too that it had only 2 wires, and it had options for both heating and cooling. The thing is I have the "heating/cooling" unit in my apartment (it's in a closet next to the thermostat), so when I get home I'll take a look at the make/model, etc. post it here, maybe that'll help you guys.

I'll also take a closer look to the wiring on the thermostat itself (it shouldn't be too complex).



The only thing that I can think of is that maybe the apartment complex controls seasonally when the heating switch controls heat and the cooling switch controls the cold air -- I lived in another apartment complex where on the wall the thermostat had heating/cooling options, but in the summer if I tried to turn on the heating side of it, it wouldn't work (I think it would blow room temperature air, or maybe even cold air... ), and in the winter when I tried to turn on the AC it would blow room temp air or even hot air, so the switch was sort of deceiving... it would still only blow the air that the building was supplying.

I just moved, and didn't really have a chance to explore what is really going on... so maybe my initial thought of when I turn on the cooling side of it that it blows cold air was wrong, maybe it just blew room temperature air, and when I turn on the heating switch it blows hot air (this I know for sure), since it's heating season.

If that were the case (I'll check that tonight), any ideas on what kind of thermostats I can get then (since it is rated for 120V)?

Ok, I checked everything:
The boiler is a Bohn 124ADV-4, 115V 1 phase 60Hz. There was nothing else legible on the nameplate, the boiler is about 30 years old (and they told me it could die anytime in the next 3-5 years)

The thermostat is a Johnson Control (electric room thermostat)
Model T-7262-1 49-19086
Serial C68

I attached a rough sketch of the wiring diagram inside the thermostat (this is what was drawn on the inside cover, I wasn't able to take a good picture of it, so I just sketched it.

http://i164.photobucket.com/albums/u31/mmaaxx2000/Sketch.jpg

About the sketch: The 1 and 2 are the leads on the outside of the thermostat, that's what I connect the two wires coming out of the wall to, I have no idea what that neutral wire is doing... there is no third connection point.

Any ideas?

That looks to me like what I suggested, a thermostat meant to control an electric base board heater. You would run a power feed to it, connecting the black wire to terminal #1. Then run another cable to the heater connecting its black wire to terminal #2. Connect the neutrals together and the grounds together. Thus power would flow to the heater when the thermostat closed the contacts between the terminals.

I guess you could make such a thermostat control the cooling too if power went to a double throw relay with the heat connected to the NOC contact and the cooling connected to the NCC contact. There would need to be additional switches to keep the system from always having one or the other on. Every May and September, somebody throws a master switch switching all the heat off and cool on, and then back? Somehow, in my original answer, I never thought of that.

Bottom line is that somehow the system works with your single throw thermostat. Therefore, it should still work with a nicer heat only thermostat matching the voltage and power rating of your system.

I anybody is distressed by my explanation, they can suggest a different scheme or make comments on who might have wired something like that up. My explanation fits what you told us.

Thanks for your reply.

You are correct about the neutral, I just realized that it wasn't going through the thermostat, but it was inside the junction box just connected together.

"Every May and September, somebody throws a master switch switching all the heat off and cool on, and then back?" Yep, as far as I know, that's the way it was in my old apartment complex, I just moved into this one, but as far as I can tell it's going to be the same way...

I might be slightly distressed by your answer. I thought about using a heat only thermostat (I actually found one) that is rated for 120V.

---The thing is, let's say we're in heating season (winter time), and the heat only thermostat is set at 72F, and in the apartment now it's 65F, then the thermostat would know to close the circuit and connect terminal 1 to terminal 2, and the hot air furnace would turn on, and the apartment would get warmer and all is good, once the indoor temperature reaches 72F the thermostat would open the circuit and the heating would turn off.

---Now let's assume we use the same heat only thermostat, but now in cooling season (summer time). The temperature in the apartment is 78F, but the thermostat is set at 68F. Since this is a heat only thermostat, it would not close the contact between 1 and 2 to turn on the central AC to produce the cold air, it would think that since the temperature inside is warmer than the thermostat is set at, no point to turn it on. The only way around this would be to set the thermostat at 90F during heating season, but then the central AC would be blowing ALL the time, and not shut off, do you see what I'm saying?

Thanks again for your help, but it looks like I still need more help...

-Max

EDIT: But yes, I agree all I need is a simple digital SPST thermostat rated for line-voltage. That's where the problem comes in, everything that I found rated for 120V is only manual, and not digital. I did find ONE Honeywell that is heat-only rated for 120V digital, but so far that was the only one...

EDIT EDIT: My other option is to get a transformer or a relay for 120V to 24V, and get a simple 24V thermostat and connect the heating and cooling leads together to make it work, but I would need to explore this idea further, and take a closer look at the wiring diagrams of the 24V thermostats. The other problem with that is that I have very limited space in the junction box to put the transformer in there... I could put it in the closet where the boiler/hot air furnace is, but then I would need to set the wires somewhere to not get heated... It just boggles my mind that there is such a limited selection of line-voltage thermostats...

''and the cooling connected to the NCC contact.''

''but the thermostat is set at 68F. Since this is a heat only thermostat, it would not close the contact between 1 and 2 to turn on the central AC to produce the cold air,''

It is a simple thing to use a relay to reverse the action of a switch. Wire the power from the thermostat to the coil of the double throw relay. When the contacts are open because the temperature is above the set 68, the relay switches the common to the NCC allowing power to flow to the cooling. When the room cools off, the contacts close, energize the relay coil and open the NCC shutting the cooling off.

I agree the lack of line voltage thermostats is surprising. As common and as expensive as electric base board heat is, you would think many people would want programmable thermostats.

If the relay I suggest works your system is in your heater, you might only need to add a transformer and replace the existing relay with one with a 24 volt coil.

labman thank you very much for your help.

This is more work than I anticipated at first, but def. worth the effort, I'll give it a shot when I have a little more time, and I'll post back with the results (and/or questions if I have any along the way).

Again, thanks a lot and your help is really appreciated.