HVAC thermostat question

[CRAWFW]

So I installed a Honeywell T9 yesterday and had a blank display after install. Removed thermostat is a battery operated Aprilaire 8446. I ended up reinstalling the Aprilaire until I had time to troubleshoot some potential issues.
Everything I read is saying the line in voltage is usually 24 volts sometimes 120v but that’s rare. I just tested mine and it’s 10.5 volts. So is this an option on some systems? Our system is Bosch and only about 2 years old. Just wondering if I have a bad transformer or it only puts out 10.5 volts being a kind of unusual system (Bosch).
The reinstalled Aprilaire works after putting it back in.

[TampaJim]

We have a Honeywell thermostat on a Trane XL20i. Wanted to upgrade to a "smarter" controller, not an option, ours apparently is beyond the capabilities.

Here's a link to a walk-thorough to check your compatibility, maybe it will help you.
https://www.honeywellhome.com/us/en/...#compatibility

[XingEyeballs]

If the previous stat was battery powered and the new one is 24V only with battery back up it’s possible that there’s no common/neutral wire (not “necessary” for battery stat and not uncommon not to have the wire there.) Verify what you have wired.
R- 24V Hot
Y1- Cooling First Stage
Y2- Cooling Second Stage
W1- Heat First Stage
W2- Heat Second Stage or emergency heat
C- Common/Neural
may be an O/B wire if heat pump, not sure what you have.
wires at the stat should match in the furnace/air handler

[bhjr.]

Just installed one of those. Look VERY carefully at ALL of the terminal markings. Some of the lettering for the terminals is very obscure compared to some of the terminals that are marked very boldly.

[CRAWFW]

So there are 6 wires.
Red suppose to be 24vac but reading is 10.5vac
Yellow
White
Green
dk blue- common
Orange which was placed in the O/B terminal.
All the wires were hooked up the same on the new thermostat
Looking at the transformer in the unit, it says it’s 208/220vac to 24 vac. Wondering if I have a bad transformer.
There is a note on the terminal block with the W and says Do NOT connect wire to W for heat pump.
I believe I pulled that wire out and it still didn’t work.

[CRAWFW]

I’m going to check the voltage again but I’m certain I had my meter set right.
The 10.5 vac is strange.

[CRAWFW]

Just installed one of those. Look VERY carefully at ALL of the terminal markings. Some of the lettering for the terminals is very obscure compared to some of the terminals that are marked very boldly.

So do you have a heat pump? And if so did you hook up the white wire?

[bhjr.]

Yes. White wire is your heat if I remember correctly. I don't do this professionally, but it's not rocket science either. The only problem I had was the reversing valve for heat/cool. Just an oversight by me because the lettering was so small for that terminal. The thermostat itself requires no secondary voltage(24 ac) as it's powered by the battery. Instructions are kind of vague, read them carefully. What colors of wires do you have. Hopefully they are standard.

[CRAWFW]

Yes. White wire is your heat if I remember correctly. I don't do this professionally, but it's not rocket science either. The only problem I had was the reversing valve for heat/cool. Just an oversight by me because the lettering was so small for that terminal. The thermostat itself requires no secondary voltage(24 ac) as it's powered by the battery. Instructions are kind of vague, read them carefully. What colors of wires do you have. Hopefully they are standard.

I listed the wires a few posts up. My thermostat instructions didn’t say anything about a battery and I didn’t want to pull the thing
apart to check. Lol
It certainly doesn’t have a door for batteries and didn’t come with any in the box.
Are you sure yours is the T9 smart thermostat? Maybe I overlooked a battery?

[KYRANGERMAN]

We had to replace our AC recently so we went ahead and replaced our 32 year old furnace too. Ran to HD and purchased a Sensi Smart Thermostat and had them install it while they were replacing everything. Works like a charm. Plus it's pretty neat to tell Alexa to set my thermostat to a certain temp.

[BroWhoop]

You probably don't have a common wire. Did the Aprilaire run off of a battery? If so no common wire. The wire may be run just not connected, maybe either end. You must have 5 wires at a minimum. The thermostat has to have a common to make 24 vac or a battery. Typical; red is 24 vac, green fan, yellow cool, white heat, depending on the installer common could be black, blue or brown. This is a conventional 1 stage wiring. 2 stg and heat pumps have more wires. If the Aprilaire works you have 24 vac.

[CRAWFW]

You probably don't have a common wire. Did the Aprilaire run off of a battery? If so no common wire. The wire may be run just not connected, maybe either end. You must have 5 wires at a minimum. The thermostat has to have a common to make 24 vac or a battery. Typical; red is 24 vac, green fan, yellow cool, white heat, depending on the installer common could be black, blue or brown. This is a conventional 1 stage wiring. 2 stg and heat pumps have more wires. If the Aprilaire works you have 24 vac.

The Aprilaire did have a common but not sure if the furnace side is hooked up. I know the red wire has about 3 volts as soon as I put the + probe of the multi meter on it and then goes to 10.5 once I touch the common. I’m not sure why it jumps to 3 volts without having the neg probe touching anything. I’m certainly not a electrician but I thought I knew enough to test the power to common voltage. Apparently I do not. I’ll stick to DC and call the installer to have them look at things. I thought about buying a new step down transformer but it could be a board sending the wrong voltage to the transformer.

[XingEyeballs]

The Aprilaire did have a common but not sure if the furnace side is hooked up. I know the red wire has about 3 volts as soon as I put the + probe of the multi meter on it and then goes to 10.5 once I touch the common. I’m not sure why it jumps to 3 volts without having the neg probe touching anything. I’m certainly not a electrician but I thought I knew enough to test the power to common voltage. Apparently I do not. I’ll stick to DC and call the installer to have them look at things. I thought about buying a new step down transformer but it could be a board sending the wrong voltage to the transformer.

Your voltage is going to be effected by what you are checking it to (ground/common/etc)… as far as the bad transformer I would rule that out because the unit wouldn’t run and work with your other stat either if it wasn’t getting proper voltage. What’s hard to troubleshoot without being there is a thermostat is just a series of switches and many homes the stat is running itself off battery power because there’s not enough wires ran to the stat to run a power/common loop. Also, some of the stats now need or come pre-wired with jumpers, I’ve seen them missing right out of the box new.

[CRAWFW]

Yeah the T 9 has a switch for the red wire(s).
If it’s a one red wire system, you move the switch to that position and it jumps the two together internally rather than having a visible jumper.
Maybe that switch in the wiring/backing plate is broke or something. Who knows.
I have the experts coming out Wednesday. They thought the 10.5 vac was strange as well. Maybe my meter is garbage. I don’t really use it for AC but it’s not a cheap meter.