Nest Thermostat isolation relay?

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Old 12-12-20, 04:00 PM
J
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Nest Thermostat isolation relay?

I've had the nest thermostat for a year now and haven't had a good experience. I have an oil burner with a two wire system and was assured by a nest rep that the Nest would work no problem with this setup. I ran a red white and common from my peerless boiler (replacing the 2 wire from the old boiler with the new 3 wire to the nest. The aquastat also ties in at these terminals as well with a red and white wire.) Since I've had it I get E74 errors randomly. The thermostat heats up the house and then shuts off. It says under equipment detected no power to Rh. After troubleshooting with a Nest rep today he determined either I need to purchase an isolation relay if I'm doing it myself (which I intend to if thats what is needed) or contact a Nest Pro. The purpose of the isolation relay is to communicate an analog boiler to a digital thermostat.

I'm just confused bc im hearing different answers. One will tell me i need the isolation relay. Another will tell me the Taco board (Sr501-4) on the oil burner is capable of doing what the isolation relay would do. Im also confused about the wiring diagram. I have plenty of experience with outlets and lights but transformers and relays are not my specialty so any help is appreciated. Ive attached pictures of the Taco board, the relay I purchased and its diagram.


 

Last edited by PJmax; 12-12-20 at 05:38 PM. Reason: resized pics
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Old 12-12-20, 05:40 PM
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Let's see if we can get you running here.

What is the relay used for ?
Usually the thermostat and the aquastat are not connected to the same point.
I'm guessing the three wire with the blue on C goes to the nest...... correct ?
Two wires go to the aquastat. What are the terminals labeled where those two wires connect ?
 
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Old 12-12-20, 05:54 PM
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I see I missed you....... study this.

I'm assuming the red and white wires that come from the aquastat are labeled TT and are connected to R and W at the thermostat connection. That's wrong. Move the red and white wires from the aquastat to the relay as shown in the diagram. Your lucky your system is still working as two transformers don't like to be connected together. That's why your nest was complaining.


The SR501 does have an isolation relay built in.
 
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Old 12-12-20, 07:47 PM
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Yes the red and white doorbell wire coming from the aquastat are on the same R and W terminals as the 3 wire R,W,Common from the nest. That is how the company installed the oil burner. All I did was remove the black and white 14g wire(old thermostat) from those terminals and replaced is with the new 3wire from the nest
 
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Old 12-12-20, 08:01 PM
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Ok..... now you're going to move the aquastat wires to the isolation relay.
 
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Old 12-12-20, 09:31 PM
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Ok so you're saying based on the diagram.......... from the aquastat , red to the "6 Normally Open" and white to the "5 Common"? As I drew in here

 

Last edited by James King; 12-12-20 at 10:46 PM.
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Old 12-12-20, 10:59 PM
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That is correct. That relay is there specifically to control the aquastat.
There is no power on that relay so it cannot hurt anything,

When the stat calls for heat.... both relays will close. One for the circ and one for the boiler.
 
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Old 12-13-20, 03:11 PM
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Well that worked for a couple hours and now I'm getting the same no power to Rh error again. Any suggestions?
 
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Old 12-13-20, 03:16 PM
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Your picture shows using red as R, white as W and blue as C.
Is that how you have it attached at the nest ?
Do you have a meter..... if yes..... check from R to C at the relay.
You should see 24vAC. If you do..... you may have a defective sub base for your nest.
 
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Old 12-13-20, 04:29 PM
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Correct from nest to taco board is R to R, W to W and C to C. Just looking at the board now I'm noticing the green power light isn't on. Dont know if that has anything to do with the boiler or the low voltage
 
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Old 12-13-20, 04:51 PM
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That board may be damaged. Check the fuse.
 
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Old 12-14-20, 07:39 AM
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My Taco board came with a resistor installed between C and W. I left it in place when using my Nest Thermostat E.

http://media.blueridgecompany.com/do...o501-EXP-4.pdf

 
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Old 12-14-20, 12:34 PM
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It didn't come with the resistor installed..... it would have been added during install. The resistor is needed when there are only two wires from the stat to the relay. The resistor connects to C and W and allows extra current to flow thru the stat. increasing charging rate.

With a three wire connection.... the resistor won't help nor is it needed.
 
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Old 12-14-20, 12:54 PM
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Have you measured between R C at the sub base for the Nest?
Geo 🇺🇸
 
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Old 12-14-20, 01:55 PM
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The green LED should always be on, indicating that power is connected and the solid-state fuse is good.
 
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Old 12-14-20, 03:53 PM
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Yea when the nest said no power to RH i checked the board and the green light wasn't on. Then a few hours later it just randomly turns on and starts heating and the green light is on and the nest works.
 
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Old 12-14-20, 05:03 PM
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That's pretty strange. It could be a loose AC connection. Check the incoming power wiring.
Could be a loose fuse in the holder. Take it out and put it back in. Make sure it fits snugly.
 
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Old 12-15-20, 11:04 AM
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I second all of PJ's comments. Check for a loose fuse or connection at ac input. Everything else is wired up properly now.

Its amazing that your system worked at all with the TT on aquastat to RW on the Taco relay; as PJ said, two transformers connected like that dont play well. TT and R terminals are always powered (in my experience). Dry switches/end switches (like that on the bottom right of your Taco relay board) are not and often labeled XX.

It might be possibly that the RW/TT miswiring did damage your relay board, aquastat, or nest. My gut says it would be the relay board or the nest. If it were me, I would check the transformer on the relay with a meter and then swap in a dumb stat that doesn't need common. If it works, then I would swap back in the nest and test. If it acts up, I would consider consider a new nest (actually I would get a different smart thermostat but that's another discussion) or an external power supply for the nest to rule out the relay board being the issue.

good luck
 
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Old 12-15-20, 11:48 AM
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Hi ,is this Green LED you are referring to on the Taco control? If so the only reason it would be out is that you are loosing power to that control, ie loose connections, bad solder joint on the board, pull the board out and have a look, there is also a replaceable ice cube relay on that board, pull it out and reseat it although I doubt that’s the issue.
Geo 🇺🇸
 
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Old 12-19-20, 02:39 PM
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Slomin's repairman came today. It turns out is was the pressure trol. First we tested the low water cut off which had power, then he saw that there was no power leaving the pressure trol. He bypassed that and everything fired up. He replaced the clogged pressure trol with a new one and everything worked. I guess it was a combination of electric and plumbing. Thanks everyone for their input and hopefully this thread can help someone in the future.
 
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Old 12-19-20, 08:41 PM
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Update: got the error again. New technician said bc the oil burner is turning on by jumping it(connecting the white and red) thats its most likely the Nest backplate
 
 

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