Checking Voltage on Thermostat. Explained!
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- Опубликовано: 8 фев 2025
- This is a step by step explanatory video on how to test the voltage on your thermostat using your multimeter. Troubleshooting your thermostat, wiring, voltage starts with being able to determine if your thermostat is responding to the set... setting such as cool, heat, or blower motor fan.
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This was quite helpful to check power in thermostat wiring--not so much a thermostat. Turns out mine was reading 3v and i had a blown fuse on the furnace board. replaced it and everything is perfect. Thanks!
Great explanation. My mistake was that I thought 24V thermostat voltage was DC. But it's AC. Finally everything is making sense.
never to old to lean new tricks
Sorry my friend, but if you have 24 volts from R (red) to any other wire, that means there is NO call for anything. You’re simply reading voltage across the contacts in this video. So long as the equipment has power and the transformer is ok, you’ll always have 24v sitting on R only. The thermostat face plate is what switches that 24v from R to all other terminals depending on the call.
The thermostat is a simple N/O relay
Facts. He never checked a thermostat in this video. He was essentially checking contacts across the board and the wiring
Yes you have to create a jumper in order to test if the t stat is doing what it is suppose to. But you would have to have a fault somewhere first. For example the heat not working. You jump r to w and if heat turns on then it is your tstat if it still doesn’t turn on then your t stat is likely not the problem.
I got 26 volt on w and y but not g
@@jonschmitz50 Is this a heat pump? If not, you shouldn't have 24V on w & y at the same time. If this is a basic furnace, 24V between w & common means it's calling for heat. Also, if it's a furnace, there will be no 24V on G during a heat call. The thermostat only switches 24V to G during a cooling call. In heating, the furnace switches the fan on.
Thank you great job... You saved me $100 service call thank you
Thanks for info because of it got the problem figured out
AS AN HVAC/R CERTIFIED TECHNICIAN, IF YOU CHECK FROM R TO G AND IT HAS 24 VOLTS THEN ITS DEFINITELY NOT CALLING SIR
Difference in potential
Excellent video no nonsense just business with every step included for the homeowner
Awesome help. Thank you
These tests are good of you want to check if your transformer is working, it is bad then your tstat will not have the 24 volts.
Thanks for this. Great explanation.
Thanks so much for this clear and precise information. Really appreciate this help!
ß'
Thanks, helped me solve my problem! I knew I bought a multimeter for a reason.
Thanks for your video it’s great you post these. Was hoping for help, my problem is I’m not getting 24 volts at the thermostat. I tested my thermostat for continuity using ohm setting, and got a no resistance reading. I checked the out door compressor and was not getting 24 volts there either. I have power going to the contact and if I press the plunger in the contact fan and condenser turn on. I’m wondering if this problem was caused by the drain line from the return being clogged and system such down. I cleaned the drain line and got the water out but when I turned the system back on there was a spark from the board in the handler. This board has no lights or error codes it’s an older model unit although I had it replaced with a new one. The handler blower is not activating. I do have power phase power coming in, but I don’t know how to check for the low voltage or voltage going to the blower. I truly would like any suggestions you may have. Trying not to get hit with having to call the HVAC folks as things are a bit tight nowadays. Kind regards, Gil
Thank you! Very helpful 😃
First video I got when googling how to check the voltages! I was getting an error for the power on the Nest Pro saying power might not be on. So I went and tried to figure out why. Came across this video to figure out what it's supposed to be. Long story short. I apparently hit the kill switch in the crawlspace for the heater so even if the power was on, nothing was going to the thermostat. But I learned how to check what it's supposed to be and a basic intro to the multi-meter! Thanks a ton!
Thanks for saying you hit the switch. I went looking for mine after I couldn't find any voltage. Good Internet:)
hello, thanks for the video. what if I'm getting readings at 31? thanks
If you connect red to green the fan comes on. Yellow to red cooling cycle initiates. If you can read potential across the two lines then it would be shorting the circuit to connect the two wires. Am i mistaken?
You are correct, when you put the meter on the 2 lines you are shorting it with the meter. Think of the meter as a light-bulb. If you connect the light-bulb to 2 lines you are shorting it to get the lightbulb to turn on. The thing is, the short with the meter or the light-bulb is a very high resistance short, so it doesn't blow the fuse. Think of dropping two ends of a garden hose in a raging river. Nothing happens. Now put one end of the hose upstream in the river. You will get "current" in the hose.
What if voltage is somewhat high at 29.4 volts?
My red to green is only 18 volts, but my red to everything else is 27 volts. Also, my fan sometimes doesn't come on. What could be the problem?
Would you have 24 volts to each wire when that respective component is off?
No. 20-28 AC comes in to the thermostat and the thermostat sends the voltage to each thing it wants on
Thank you!
Can I test these wires if they are not connected
Hi Sam I'm wondering how to get the voltage lower than the existing coming into the thermostat, I'm getting 32/36.8V and supposed to have not more than 28v, that's probably the big reason and problem why I'm getting error E104 with Nest Pro 3rd Generation thermostat and can't getting done cause is receiving more voltage than required, all thermostat different brands work flawlessly but Nest Pro not getting through said problem with G wire overcurrent, I do have an new 18/3 ac cabling with all connections Y/W/G/C/R just replaced but the problem with Nest still persist, I'm not an electrician but I do have the basic skills cause I'm an Networking IT guy and I'm guessing with a quick lesson or explanation I'll get this done. Thnx
What if I am getting 24v AC across green, yellow, and white, even with the thermostat off? My blower is not working. I feel like there is a short, but I don't know what having voltage across all three signal cables means.
Thanks for the video =)
Make sure you check it with your right hand.
Are you saying this to refute the "Left Hand Rule" more in regards to flipping switches on a breaker panel? If so, this says it all imo www.canada-training-group.ca/mind-your-safety/32/where-do-you-stand-on-the-left-hand-rule
I have 21v out of my blower/green wire...where should i go to further inspect ? Thankyou
What was your solution? I have 19v.
@@emeltea33 check the voltage directly from the transformer in your furnace.
@@dannyl6507 Thanks, I did, no degradation. Guy came out and replaced the motor, now it works. Said it was electronic motor controller or some such.
I just bought the ecobee thermostat and it does not turn on. The seller told me that sometimes is the voltage. Can I please get some help with this. I just moved in and idk if I have to change the wires and stuff
I’m only getting 24v to the red and white wires only .. and fan constantly runs when tstat off
What size wire I want for a thermostat
Wait, so your thermostat is removed from the sub base. So there's no way red is connecting and sending power to any of those terminals.
You don't have 24 volts going to green white or yellow. You have zero volts going to those
What you're reading is the difference in potential between red and common. Between red and green you have 24 volt difference in potential. All you reading is your hot and the other side is going back to common.
I thought the same he is only reading red, if there was power being sent to green or yellow or white if you put one end to red and the other to any of the other colors you will reading zero not 24.
Your correct. This guy is going to do nothing but confuse people.
Correct. The meter is measuring the difference between the two leads. Right now there is 24v at red but there is 0v to the other terminals because voltage is not being sent to the other terminals. It reads 24 volts because that is the difference between the two. A thermostat is basically a temperature controlled switch. If the thermostat were sending 24v to let’s say the G terminal or our blower motor, we would measure 0 volts across R and G because we would be reading across the same line.
I came to the comments to see if my suspicions were correct about the way he was measuring the voltage. I was seriously getting confused.
That’s what I thought. That’s why he paused when he saw 24v on heating and cooling 😂😂. So the thermostat is basically traffic police. It grabs 24v from the red and depending on how you set the thermostat (heat/cooling) it makes that connection with the other terminals. You’d have to manually jump the red incoming 24v with whatever terminal you want to check. Respect to the man though for trying to teach ppl and taking the time out
You are wrong. Yes you have power but red sends power through. If you have 24v between red and green then it's not sending voltage through. Plus the damn tstat isn't even connected.
I was wondering why nobody else called him out on that. What a stupid mistake. I guess the people who are here trying to learn from him wouldn't know any better anyways.
All you’re doing is checking the voltage from R to ground. This isn’t actually telling anybody anything besides that the transformer is sending 24v to the thermostat. If the stat was bad you’d have to check at the furnace/air handler at the terminals to see if the thermostat is sending 24v to Y,G,W etc
How?
Bring ur meter to your circuit board attached somewhere in front of the blower cabinet, should contain a schematic of what kind of contacts your furnace should have. "Normally open, normally closed" .
I'm glad someone said this. A multimeter only checks the difference in voltage between the two probes, not how much "voltage" there is.
I had a new HVAC system installed last year and a honeywell stat that has 4 wire running to it.(no C wire). The system when in A/C mode will set there and run until I turn the stat up, but I have to turn it up 3 or 4 degrees or the temperature reading will jump a degree or 2 to match where I set it to. I know sounds crazy. It will also jump down and match within a degree or 2. SO... first I called honeywell thinking this must be a stat issue. They had me test the voltage readings of the 4 wires running to the stat. The 24v red was fine but one of the wires read lower like 9v and seemed to vary. Honeywell blamed the HVAC system as the problem. So I called the HVAC tech out and after thinking it was the wires running to the stat he checking the voltage readings at the unit with the stat wires removed, he got the same readings ( one circuit was low and varied) eliminating the wires to the stat as the issue. He then said it was an ECM module/motor.He did a warranty replacement of the ECM and said it's fixed. For some reason this issue takes a few hours before it acts up. Then the longer it runs the worse it gets. He finished the warranty repair and said even though the voltage is still low in one circuit the system was working fine, but the next day it did the same thing. I replaced the Honeywell stat again and it does the same thing also. Any ideas what I should do?
If only one wire is reading low, see if you have an extra wire and swap that color from the air handler to wherever it’s going, I didn’t remember reading what I read, but either swap it to the outdoor unit or swap it to the thermostat depending on which wires Has the bad wire
Thank you for the great video. I have only two wires coming to my thermostat, connected to R and W. The voltage measures 77 V-AC. The thermostat is rated (like most other thermostats) at 24 AC and it has no issues. What puzzles me is how can you get voltage between a wire that is supposed to have 24 AC (attached to terminal R) and the wire going to terminal W, which I believe, is supposed to complete the circuit of the zone valve, thereby turning the boiler on?! I thought one can measure the 24 V line with the other probe attached to a common wire/terminal only!
Your comments are appreciated in advance.
Zack
But it looks like you have a crossover, what happens if you have individual RC and RH wires
Thank you great job
How do I turn power off to the wires going to the thermostat. The breaker does nothing.
You’re probably shutting off the outdoor unit it’s a separate circuit for the air handler.
Thanks man
Based on basic electrical, that's just a connector which is connected to nothing but 1 live wire at R. The rest are just grounded so they show the R's voltage... 😅
Sam, can u please give me some suggestions what cuase my ac to turn off i check all the voltage on my thermostat all good but for some reason I have my ac running it turn if itself won't turn on until I reset the power to the unit and it start working again for lil bit it do the same thing????
Thanks buddy
What if from red24v to yellow'cooling its 13v..... but C G W all have 24 ?
Same here I'm at 10v and my cooling is not kicking on.
My a.c R to Y is reading 23.1 volts...... help!!
I get about 24v on all except the C-wire which reads about 12v. Why might that be?! Thanks.
Did you resolve the 12v at C issue?
Thanks
Thanks for the info. Check my thermostat and I'm not getting any power to it.
What if I got 19 voltage instead of 25v?
Thank you Sam for sharing knowledge with us. My thermostat is blank and after seen your video all the wires get 24 volts. Doesn’t have a float switch.pushed the contractor and starts. Is the thermostat bad? Thanks
Did you ever find an answer to this problem. My AC is having the same issue.
@@adagiotexas sorry for the late response. What’s happening to your ac?
Given that the thermostat is completely disconnected, the work you’re doing here has nothing at all to do with the thermostat… the only thing you’re testing here is if the furnace control board is outputting voltage on the R wire.
what state are you in ???
I got 25v on red and green but the rest is lower voltage, i tried to install a smart thermostat and i have all the 5 wires. No luck!!!
Very informative! Thanks!
I have the same meter can you do more videos about how to operate
There are many out there already. Not much you'll find that isn't already there.
No conprende.. my tstat only has voltage going to the individual things when theyre on or active.. if i switch from ac to heat i only have 1 or 2 volts going for the ac and viseversa.. everything works.. was trying to install a buck regulator and hardwire the tstat to stop having to use batteries.. and i was drawing off the 25v just fine then switched to heat and no mo 24 vo...
You're cutting power to parts of the circuit you are turning off, just like a switch is supposed to do.
I don’t need a multi meter to know I don’t have power coming in because my thermostat is completely blank…
You know your stuff !!
CL 210 Klein is it a good meter to work with
Not much for the DIY'er to complain about with pretty much everything Klein makes for the kind of electrical they do.
👍👍👍
Sir how does it works...how it controls fan motor speed in 3 speeds...my single phase fcu motor has 3 wires....low ,medium,and high...
Without thermostat ,I just tried to connect live(230v) wire to low speed winding...no power trip...
But if I connect live wire to medium and high speed windings.power is getting trip...what is the reason sir... without thermostat how I can run the motor in high speed?pls help me
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Hey Sam, I just bought a Nest thermostat and our reads at 27-28. It says the Nest works with "Most 24v systems". Do you think this is okay to install in ours? Thanks!
It will be fine 👍
Just to expand a little, voltage is pretty hard to control perfectly. When it says 24V system it actually means something around 22V-30V. Don't quote me on those numbers, but in general a system designed for X volts will work with voltage near or close to that number.
This makes no sense… You’re only suppose to have 24 on the red wire only when that thermostat is off the wall… U got 24 to the Y, G, W that means everything is calling… The thermostat Heat, Cool modes is what sends 24 to the W for Heating or sends 24 to Y/G for Cooling
So I don’t know why u have 24 on all the terminals with the TT face off smh
Exactly...a very misleading video😖
Voltage to the heating circuit and the cooling circuit?
Yes, the thermostat shorts the wires when you hear the click sound and thats what gets the heating or cooling circuits to activate.
Wrong...that voltage is from the 'furnace board' and does not involve the thermostat, which is pulled of its base plate at that point
this all makes perfect sense,but what happens if you touch the Rh wire onto the white wire? I thought the heater would come on.
I have no valtage between any wires to Red but everything works as it should. Any reason？My multimeter works when measures battery and I have 4 wires without common wire.
When thermostat is calling you will lose voltage depending on what's calling. When satisfied you will have all your voltages.
U r just verifying you have 24 volts from transformer. If it was calling you would have 0 volts between R and the other terminals. Potential Difference dude
I have only two wires going to my old thermostat. When I measure the voltage across them it is 48 volts. Is this normal?
24 volts on each Instead of the two hot wires at 120 volts each (240 volt circuit), stepped down to 24 volts ×2/wire (48 volts) coming from your breaker. If you have to ask though, it sounds like you're having a problem regardless?
Will my heat pump work if I use a thermostat with out emergency heat and will that be why I do not have 24 volts at contact in heatpump
Very helpful!!
Rule achiever how do you test by AC24 votes
👍
Youve just checked R 3 times.
This guy doesn't know what he's doing. red is 24v power to the stat, using a volt meter is showing you potential difference. If you check between red (constant 24v) and a line that's energized (green for fan, yellow for cool white for heat) it will show 0 if on, because there's no difference. If it's showing you 26 volts it just means you still have power at your red wire.
He's checking from red to green and it's showing 26 volts, he's saying the thermostat is energizing the blower motor, he's wrong.
No lo ok o
All you did was just check the red wire voltage.... guy doesnt understand electrical circuitry
MUL-TIE-METRE
Thanks