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No power to rh wire

jf2
Community Member

I've checked that the wires are all snug.  Bought 3rd generation learning thermostat, but I get the error message that no power is detected.

86 REPLIES 86

laurentbourg
Bronze
Bronze

Do you own a multimeter by any chance?

Kdoll
Community Member

Similar issue with no power to Rh wire. Have a C wire. AC ran fine during the summer. 

Reinstalled my old Honeywell thermostat and the heat runs. Still no solution for the Nest though.

This seems like a Nest misconfiguration issue, next time you try to install your Nest again and run into issues please reach out on this thread

this is how the configuration looks. 

Screen Shot 2021-12-02 at 10.20.34 AM.png 

Also came across a suggestion to try swapping the Rh and W1 wires but that did nothing. On some other threads I saw that some systems need an additional transformer element added to the heating system but i do not have enough knowledge to implement and doubt most HVAC professionals do either as it seems nobody knows a proper solution. This is the original configuration before I had a professional add a C wire for me.

Screen Shot 2021-12-02 at 10.27.19 AM.png

What kind of hvac system do you have, is it one forced air electric or gas furnace that provides both heating and cooling?

If that’s the case, then try disconnecting the Rh wire (just keep the Rc one in) and check if your Nest now works properly. 

I have a split system with an outdoor AC unit and a gas boiler in the basement. These are the connections I could find inside my boiler https://imgur.com/or23cuu and https://imgur.com/DWpeBOu 

The gas boiler provides hydronic heating? (Radiators)

I think it's still forced air? no radiators. comes out the same vents that the AC does

Yes it should be, I am just making sure. Try as previously mentioned to remove Rh on the Nest and see if it all works well. But since your Nest is not installed anymore, you can verify this assumption using your current Honeywell thermostat by simply disconnecting R. 

I'll give that a try!

My assumption is the following: Your Rc red wire has power but your Rh wire does not, your Nest thermostat has electronics that do not jump Rc and Rh when it detects that both wires are connected to the wire base, hence failing to heat but working fine for cooling. Your current Honeywell thermostat works well because it is internally physically jumped by an R-slider tab or other mechanism (I don’t see a jumper wire on your picture). 

Just disconnected my Rh wire from Honeywell and the heat is running. So hypothetically if I have the Rc connected in my Nest it should jump the heating as well?

Correct

The air doesn't seem particularly warm. Could it be kicking on the AC still instead?

It might be just starting the fan (the fan gets activated by the G terminal) but not the actual heat, is your gas furnace running?

I believe so. Maybe it's the location of the vent that I'm checking. Will try again. Thanks for walking me through this

No prob. You don’t own a multimeter by any chance? Would make things easier. 

I don't. Thought about picking one up but don't know how to use it.

Trivial to use for various use cases and always good to have around the house, you can get one for under $10 online. Did you manage to validate whether the heat + fan is actually running or just the fan?

I can't tell anymore. Keep testing it with and without the R wire attached. It's definitely warm with the R wire, but running the system again to see if it's not use blowing out the rest of the hot air when I unplug the R wire.

Kdoll
Community Member

After running it for a while, it does seem like it is only heat when the R wire is plugged in.

Ok. Suggest you a get super cheap multimeter online so that we can diagnose further over the weekend,  like I said it's always good to have one. Search of "axfer 2000" on Amazon, it's $7.99 one day delivery and will do the job.

Ordered! Will follow up later. Thanks!

Kdoll
Community Member

Multimeter received! Let me know what I should check when you get a chance! Thanks!

Ok great. Turn the dial of the multimeter to the right on 200 (the blue number in the AC section with the symbol ~ ).  Reduce the temperature of your thermostat a bit to make sure it is not going to call for heat while you test. Then put one probe on the screw of R and the other probe on W. It does not matter which probe goes where. It should read around in 20V range if it has power. Then put one probe on Rc and the other one on Y and that also read around 20V range if it has power. 

Just checking in

Thanks for following up! Haven’t had a chance to try it yet but will tonight. My Honeywell is one of those thermostats where I take the face off and the whole thing stops. Will testing the screws still work without the system running?

Yes you should test with a live system, the furnace should be on so that voltage is there but the furnace should not be running, so what you described should work

You might find this video handy https://youtu.be/6fHveU0Qv_s

Thanks for that! Very helpful. Testing the same way i get R to W 25.9 and Rc to Y 23.3. Also FYI i did not use the C wire that I previously had installed for the Nest. I just taped it up with some electrical wire and instead used batteries for the Honeywell. Not sure if that makes a difference or not. I think my neighbors didn't use a C wire in their Nest and had no problems. I had a C wire installed to avoid the power issues that I read about when no C wire is present.

Ok great, can you remove the electrical tape on your C and measure voltage between Rc and C and also between R and C?

R to C 25.9

Rc to C 23.3

Ok so this proves you have power on both Rs and that your C is wired properly. It is possible that you don’t have enough current going through R but it is unlikely because it should recognize power on Rh even if insufficient. I think it’s is either a Nest wire plate failure and need a replacement or you potentially did not insert into Rh. If you have time now, suggest we give the Nest install another shot and go from there. 

Since the Rh shows up on screen would that mean it's inserted properly? I definitely remember rejiggering the wires to make sure it was in right. I did read in another thread somewhere that someone got a new wire plate but it didn't help with a similar if not the same issue. 

A while back, someone had suggested switching the Rh and W1 wires in case there was accidental swapping during the install of the heat but that didn't work. Another person suggested testing the Rh and Rc terminals to see if the transformers were in/out of phase? I also found this wiring diagram somewhere. Not sure if I have to go through all thatOpening to trying anything you recommend!

I don’t think you have an out of phase situation. If you did, you would see a negative reading. When you put the red probe on Rh and Rc and do your measurements with the other wires, the voltage always reads positive, correct? One thing I am not totally clear about is whether you actually have 2 different 24V transformers or just one in your furnace for feeding each R. 

can you send me a pic of where the C connects in the furnace as well as the Rs?

Yep always positive. This is what my overall system looks in my basement https://imgur.com/lO6NoeO

Here are close ups of the connections https://imgur.com/or23cuu and https://imgur.com/DWpeBOu but I haven't been able to trace how these connect to my thermostat. I believe the C wire is only connected to the air handler in my ceiling. Does not run down to the basement to the boiler.

 

Can you send me the model number of your furnace?