Reduce 28vac to 24vac on thermostat controller?

Electronics
#1

I have a Wyze thermostat that is apparently sensitive to over voltage :man_shrugging: It was working for a while, then it started popping the fuse on the AC controller. After some back and forth with Wyze, they say it’s because my transformer is outputting around 28vac, and the thermostat can’t tolerate much more or less than 24vac. Don’t know why it worked for a while before this issue unless my AC voltage in my house fluctuated a little (wouldn’t be surprised).

Here’s my thermostat controller, a White Rodgers Intell-Ingnition. Any way to reduce the AC voltage without swapping the transformer?

#2

Start by checking that both 120 V legs of your power are the same voltage. A poor neutral bond to the utility can cause one leg to go up, while the other goes down. This can be a bad situation for a range of reason, and can damage a range of electronics.

Do you have any lights that get brighter when you turn on the microwave, or vacuum cleaner? That would be another sign of a weak neutral bond.

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#3

That’s a piss poor design. Most HVAC transformers put about 28 volts out. I dont know that I buy the fuse popping because of that either.

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#4

Yep….I second Tim……piss poor design.

Ugh. You’ll need a DC-DC converter, after the transformer, that can provide enough juice to keep your contactors etc going.

Is it only popping when turning on the AC? I.e. if you leave it on fan only does it pop the fuse? If so then you can swap the contactor in the condenser. I had one acting up last week and the only thing I could find was a slightly low resistance on the contactor coil.

If it were me….I’d ditch the wyze……it seems less wyze for financial and pain in the ass factors.

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#5

Thermostats use AC power. Or at least that’s what their onboard power supply expects as an input. I suppose the power supply could be completely bypassed but at that point just get a different thermostat that’s designed better. :thinking:

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#6

Agreed, but apparently it’s a thing with a lot of smart home thermostats (Honeywell supposedly being an exception)

#7

@lukeiamyourfather is right, it’s AC voltage, and it’s popping as soon as I plug in the thermostat. Since it’s WiFi, it’s always drawing power, albeit minimal when there’s no activity.

@TBJK what about adding a “resistor” to the line? AKA an additional 25’ of copper to the c-wire.

#8

First,

I think they are using the voltage as a cover story to block coverage of a legitimate failure.

Second, a resistor is going to be a variable voltage drop.

How about two strings of about 5 or 6 suitably rated rectifier diodes, in parallel, opposite directions? The goal isn’t to block or rectify current, so one string conducts each direction, but each string drops the voltage about 3V when passing forward voltage, and has little dependency on current.

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#9

Sounds like these companies have a lot to learn. Honeywell has been around for a long time. They make components for many other major manufacturers.

You might be better off trying to find a better ratio transformer suited to your voltage. Not sure if you could use maybe a variable inductor or similar to drop the voltage. Your transformer is likely a 40va transformer, which would put your max current draw 1.2-1.6 amps. What size fuse is blowing?

#10

I know you’ve done a lot of back and forth with Wyze - but I’m going to bet you have a different problem than the 28VAC. If you’re blowing the fuse in the White Rodgers Intell-Ignition, it is protecting an overcurrent condition like a short, which would mean the thermostat is already toast.

I would look for poor connections, loose wires, and broken wire insulation… but I’m guessing you’ve already done that. I just don’t think the 28VAC is enough to cause problems.

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#11

My bad. Sorry guys guess I wasn’t clear headed when I responded.

@lukeiamyourfather good catch.

AC voltage limiting is slightly more involved than dumping in components….

As @TBJK stated you can hunt for either a transformer with a more tightly bound output or potentially put together a clipping circuit with two zener diodes and resistor……but the clipping circuit is not a path I’d travel down as it likely wouldn’t be UL listed so if something goes south insurance wouldn’t cover it.

I’d try to isolate the thermostat itself as causing the blown fuse prior to investing time/effort……

  1. power down the furnace
  2. label and disconnect all thermostat & condenser control wires in the furnace….the little ones
  3. put wire nuts on each wire individually to keep them from shorting
  4. in the furnace re-connect the R (usually red) and C (usually blue), from the wire bundle to the thermostat, to their corresponding screw terminals.
  5. connect the green wire in the thermostat wire bundle to the “G” screw terminal
  6. power the furnace up.
  7. see if it blows the fuse.
  8. if you haven’t blown the fuse yet, switch the fan from “auto” to “on”
  9. if that doesn’t blow the fuse, I’d think the thermostat is ok and you should look at your condenser contactor and the wires going to it.
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#12

As happens sometimes, this project got sidetracked. Dove back in today, and found the culprit. Let’s see if they replace it:

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