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Heating, Air Conditioning, Fridge, HVAC/Warehouse gas heater-jfr-340 switch relay


QUESTION: Hi Jim, I am trying to help some people get a heater working before the winter gets here. It is a gas warehouse heater suspended from the ceiling. Someone has disconnected the wires to the relay switch.  It has a transformer with a black and white wire as power in. On the out side it has a red and green wire. The heater has a temperature sensor on the heat chamber with two wires to be connected somewhere. There is two wires coming from the gas valve. The blower motor has two wires coming from it. There is no thermostat on this heater.

Basically we want to be able to turn the power on and the heater come on. The thermostat is not necessary.

I understand a little about it. I know the black & white wire on transformer is for the power in. The rest of the wires I am not sure how they should be connected. I have heard that on this particular relay switch there is a high and low power side.

I hope you can help me get all the wires connected to right terminal or wire.

Unit heater
Unit heater  
ANSWER: Hi Don, the relay you are referring to is DPDT relay with 24V coil. There are many ways it could be wired. If you can get me the brand and model number off the heater that would be a big help.

Just to make sure we are talking about the same thing, does it look like the one in my picture?

Let me know, JIM

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QUESTION: Hi Jim, thanks for your reply. Yes the heater looks like the picture you sent. It is a Reznor Model No. US-75-F. I have tested the transformer and it is working. I have tested the fan and it runs good. I hooked the red wire from the transformer to a wire on the relay and the relay seems to be working as well. It looks like the transformer and the relay are not original to this heater. But, my friend who owns the business where this heater came from tells me it was a working heater when they took it down. Some how while it was on the floor in a storage room some of the wires got disconnected.

ANSWER: Hello Don, I found the manual for your heater. If the heater was made before 2008 it has a standing pilot. We will need to know for sure before we can proceed. Go to and look for Reznor 75F. Download the installation manual. It will help you find the pilot light.

I don't know how old this unit is, so I think you should have a Pro come out and inspect the heat exchanger with a camera.

---------- FOLLOW-UP ----------

QUESTION: Hi Jim, this heater was definitely made before 2008. It definitely has a standing pilot light.

Don, with the standing pilot it makes it a little easier. I would wire the fan to the high side of the transformer. Wire the red wire on the transformer to the sensor ( hi-limit switch). The other side of the sensor to the gas valve. The other wire on the gas valve will wire back to green wire on the transformer.

No need to use the relay because you are not using a thermostat. Depending on what kind of transformer you have the green wire may have to be attached to the to the unit.

Let me know how this works out, JIM  

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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