[SzLT]

Recently my utility company was running a campaign to replace standard thermostats with Smart Thermostats. Since the cost was nothing more than postage, I enrolled.

Well, as luck would have it my furnace went out about the same time the new thermostat arrived, so I had the repairman install it.

A couple of weeks later, we went from a cold spell, to a heat spell. I set my new thermostat to cooling and got nothing. Calling up the outfit that did the install resulted in telling me that my Mastertrol MM-3's bad and they needed to replace it (4 zone TrueZONE HZ432). The cost quoted was north of $1.1K.

Since Covid, I've had a run of bad luck, water softener died, furnace died, roof needs replacement (that's the big one), etc. I balked at the quoted price and now I'm looking into doing the work myself.

The issue is is that I'm a complete newbie with HVAC. So I'm looking for advice.

I'm not sure how the tech determined the board was bad. It worked fine before any of this and works with heat. The Cooling LED never lights up though, and the fan remains dormant, but I'd rather replace the unit with something from eBay (if I can find it) than shell out over a grand to have someone swap boards. If the swap is necessary, I'll try to do it myself unless convinced otherwise. I'm not even sure I need the board they suggested. I have a 3 zone system with natural gas... AC & Heat.

 

[57TinkerMan]

Don't start buying things yet. It's probable that the installer didn't touch anything in the panel, he just connected the new furnace and thermostat. Take a picture of the low voltage wiring in the furnace, the new thermostat wiring and the zone control panel wiring and post them here. Pic below of a Mastertrol panel to help visualize what you've got. Which zone got the new stat? Do you have a volt meter?

 

[user_12345a]

A fuse or transformer may have blown when changing the t-stat.

I would not assume the board is bad.

 

[57TinkerMan]

The heat works so I don't think it's a fuse or transformer. I'm thinking Zone 1 got the new stat and doesn't have separate O/B terminals.

 

[user_12345a]

i missed that. yes, that could do it.

can wire a switch between r and o - turn on in cooling season.

 

[SzLT]

Here's a picture of the stat and controller wiring.

I have a Fluke T5-1000 tester to check voltages, etc.

 

[SzLT]

Okay, I got a message from the stat manufacturer:

Thanks for the pictures. I see the problem. Your Trol-A-Temp control panel uses switching wires to tell the panel whether it should be cooling or heating. Your original thermostat would have had separate O (O1 on the panel) and B (B1 on the panel) wires installed. (O for cooling, B for heating).

Sensi only has one terminal for that switching capability: O/B.

If you want it to work in Cool Mode, you need to put your orange wire in O/B and then configure your thermostat for AC1, GA1 (if you have a gas furnace, EL1 if you have an electric furnace), and "O."

The downside is that any time you switch seasons, you'll have to switch that O/B wire on Sensi and your configuration. (When you want O/B to activate in Heat Mode, configure from "O" to "B.")

There is a way you can install a relay at the panel to combine those wires into one wire so you're not constantly switching it, but the downside is that since the panel is older, the transformer may not be able to support this. You'd have to refer to a contractor or call the manufacturer to confirm if this is a viable option.

Hope that explains it more. I've attached the relay diagram to this email for you to check out and show to a contractor/manufacturer. If you have any other questions, let me know.

 

[user_12345a]

There's another way around it, putting a double throw switch getting power from terminal 2 and connecting one output to O and the other to B -> flip seasonally to switch between heating and cooling.

 

[57TinkerMan]

For $135.OO plus tax, install it and be done. Use your "free" stat on one of your other zones. If money is really that tight, install the old stat if you still have it.

https://www.supplyhouse.com/White-R...rmostat-24-Volt-or-Millivolt-system-5774000-p


https://s3.amazonaws.com/s3.supplyhouse.com/product_files/White%20Rodgers%20-%201F95-1277%20-%20Install%20Instructions.pdf

 

[user_12345a]

Does that stat actually support using O and B at the same time?

the honeywells don't.

 

[57TinkerMan]

I pretty sure it does, take a look at the 2nd link (installation instructions). Clearly shows separate O/B terminals.

 

[surferdude2]

A Honeywell T8602C will also work.

 

[surferdude2]

Also the Honeywell TH2210DH1000 will work and the price is right... 40 bucks from lots of sources.

Sources

 

[user_12345a]

The stat is usually programmed for either reversing valve in heating mode or cooling, not both and may only energize O/B terminals unless set to heatpump. It's important to double check. The terminal can be there but it doesn't use it by default.

The 2220dh appears to be heatpump only and will energize Y during a normal heating call, W2/aux for supplemental heat.

T8602C is old, looks like a chronotherm 4, discontinued a long time ago.

 

[surferdude2]

I think Honeywell is a little ambiguous since they say the TH2210DH1000 is suitable for conventional and heat pump applications but if you read further they say it was designed for heat pumps only.

"The newly designed horizontal PRO 2000 programmable digital thermostat provides electronic control of 24 Vac conventional and heat pump systems or 750 mV heating systems.

Note: This thermostat is designed for heat pump applications only."

They seem to give in the first paragraph and take it away with a note. :wink2:

I have used the old faithful T8602 but it was 25 years ago. New ones can still be had if you look hard... even with extended warranty. Their main problem now is that they look old school. lain:

I hated zone panels and the oft needed bypass ducts and dampers and always hated to bid one.

 

[SzLT]

Just to wrap things up, I reinstalled my original t-stat. Now everything works again. Thanks for all the help!