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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22627
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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how would I use a tjernlund uc1 with a rheem 42v75f

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how would I use a tjernlund uc1 with a rheem 42v75f

My name is XXXXX XXXXX I will try to help you today.

Are you familiar with the gas pressure hex fitting on the bottom of the gas valve?

Customer: replied 4 years ago.

No but I'm sure i can find it. I am trying to help a friend at his house but am not there right now. If you can tell me if this is possible and the parts I will need... I know we can make it happen.


There is a hex fitting on the bottom of the gas valve. This port is live when the gas valve opens.

So we put a fitting in that port and feed a pressure switch. You have 115v from another source going through this pressure switch to the motor.

The pressure switch is open until the gas valve opnes which pushes on the pressure switch with gas pressure. This in turn closes the switch and sends power to fan motor.

So it is the pressure of the gas out the service port but that ends up starting the operation. need this pressure switch to work with the fan motor or the UC1 controller. If it is just for a water heater I would not use the controller and would just rely on the pressure switch.

Let me know if you need further clarification


Customer: replied 4 years ago.

Thanks Billy...Do you know a specific pressure switch that will work? I would appreciate it.

Regards, Mike

Is this a natural gas or propane system?

Customer: replied 4 years ago.

natural gas

The switch closes with 2.85" water column.

The gas valve for natural gas will be supplying 3.5" w.c. to the burner.

You can use the kit shown HERE along with the UC1.

Once you see the switch and and how it closes when the gas valve opens it makes more sense.

The draft motor sits waiting....the water heater kicks on as usual...the gas pressure on the leaving side of the burner feeds the burner while the pressure port closes the switch...which sends power to draft motor.

The manufacturer site has some detailed drawings and schematics as well.

BillyHvac and 3 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

It worked out fine and we were able to test it, so we know it works. Thanks.