We are using 2.1.1, all thermostats are in basement, our HVAC installer is using Taco HVAC controls. The issue is when we hook up common from his system our remote temperature sensor starts fluctuating 5-6 degrees randomly. When we are on battery alone it works fine. We took a voltage reading and his system is giving us more than 24v, something like 28v we are getting. Has anyone encounter a situation like this before? Any help would be awesome.

The 24VAC transformers are only NOMINALLY 24 volts. It is not, at all unusual to see HVAC transformers measuring much higher than 24. However, I typically see them in the 26 to 27 range. Just sayin'

I'd love to know the answer to this. Had a similar situation and had to do a little science project. if there are not good solutions I'll let you know but really it might not be a good solution for a client 

Can you temporarily connect your own 24V power supply and try to pinpoint the problem to a specific device/system?

You say battery, so that's DC. I sort of expect usual thermostats to run on AC. Of course, control types probably run on DC. I'm just saying, I suppose, that I don't know a lot of details about this.

That said, if it's DC, how much ripple is there? If the supply can output, say, 200 mA, when no current is drawn, the supply will be at a high DC voltage with zero ripple. When you draw near the total available amperage, its voltage will drop and ripple will appear. That means that this function might be affected by how much current is drawn for other functions.

 in my case the problem was the 24V AC coming from the controller. I was getting common from a radiant heat controller.

Ernie the thermostats take AC when powering them not by battery. you can connect a common wire from your HVAC/radiant heat and power the thermostats so they do not drain battery.

I had to connect a relay in between and power the thermostat from another power supply. I am guessing I could probably regulate the power from the controller some how. It must be fluctuating creating the jump in temperature from the remote sensor.  I really would love to know a solution to this

I can't yet understand the wiring. For instance, "common" usually is one wire, with the other considered "hot," but you've got 24 VAC and also a common. What does common mean here?

It makes total sense that you need a relay to switch between battery and AC. Otherwise the AC and the battery supplies would be damaged.

The experimenter could take the 24 volts AC, put it through a diode bridge to rectify it to pulsating DC, filter it, then put it through a regulator and adjust its voltage so that it provides a slight charge to the battery at all times, then leave the AC connected this way to the battery. No switching over at all. The big IF here is IF the battery is not degraded by a trickle charge. bcf1963 should have some GREAT input here.

A problem with doing that is the time involved in engineering a one-time solution that will undoubtedly void the warranty on some or all of the interconnected items. This fact separates the DIYer from the professional. The DIYer can dick with it all he wants and will take responsibility for it, though never silently, when the heater quits during a snowstorm. A pro has to install something available, with the backing of warranties, that he can walk away from knowing that it will continue to work. The pro might customize, but not usually at the level of power supply construction, more often at the level of appearance or clever combination of available goods.

Which brings me to... if the DC voltage is 24 volts, try to locate a product that's a 24 volt DC supply with its own built-in charging supply. An off-the-shelf item that supplies the proper DC voltage (at the proper current) might not void any warranties and give you DC from a battery that doesn't drain the battery when 120 VAC is available.

A caution about current, though -- be sure you understand the load. I once blew several relays in a lighting system because they were meant to be pulsed by an intentionally crappy 16 volt DC supply that dropped to 10 volts after a quarter second and could only power one relay at a time. It was supposed to work that way. I replaced it with a beefy supply that held steady at 16 volts, and holding down a lighting button would blow a relay.

are you using cat5 for the temp sensor wire?

in my instance yes.

that is your problem.. It will NOT work properly on cat5. dont ask me why but it wont.

You must run solid core wire.

pick up a spool of hvac wire and retro it in.. i promise it wont stop fluctuating until you do

to be more precise I have it running on both and it fluctuates on both. I forgot the majority of them are running on solid hvac wire. The install is a year old at this point and working by powering them with their own AC power supplies with a HAI 29A00-1. I would love to get rid of that science project though. reliability going forward is something I am concerned about. 

this is a solution I would be looking to do. You can power the thermostats with 24 volts from the thermostat wire by connecting the common wire from your heating/ cooling source. In my instance its a radiant heating controller. That controller is not putting out a consistant 24V. I would not be using the battery at all. When the thermostat detects the 24V it powers off that and not the battery. Would this work?

I find that to be an odd comment as the rest of what you propose is not that at all.


Two problems I've had with this nomenclature from the beginning are that
~~"common" is very often used to mean the ground or negative connection
~~you can't just connect one wire, so does "common" mean two wires of some sort? I mean, is "common" the name you give to the two wires with 24 VAC on them?


More details, please. How do you switch from "that" to battery? How do you keep the battery charged?

The idea I proposed was to use the 24 volts to keep the battery charged, then run everything off the battery. Varying 24ish volts won't matter if the charging circuit is done correctly (bcf1963 could help, I might be able to help). You'd need a buck converter to make a voltage higher than 24 volts to charge the battery.

well I would like to take parts of your solution.




in this instance common is a ground. by connecting the common wire to any thermostat that will accept it you can power the thermostat off the 24v coming to the thermostat over the heating and cooling wires. When a thermostat detects this voltage it does not use the battery at all. the problem in this scenario is that the common wire(ground) and remote temperature sensor ground on the control 4 thermostat share a connection. So both wires are connected to the same terminal. The fluctating 24v from the heat controller now affects the remote temperature sensor so it does not read a correct temperature. I probably will just leave well enough alone.