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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22541
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Modine PAH90AF Unit heater on natural gas. When I

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I have a Modine PAH90AF Unit heater on natural gas. When I turn up the thermostat the fan will run but the burners will not ignite. I'm thinking flame sensor but i can't even see the darn thing let alone get to it.

Billy :

Hello, bu motor running do you mean the exhaust fan?

Hello, by motor running do you mean the exhaust fan?

sorry chat hung up so let me know if you got this

Customer: replied 3 years ago.

The blower motor is running that would normally push warm air instead of coldEmbarassed

Thank you,

so the exhaust motor that blows the exhaust out chimney is NOT running?

Customer: replied 3 years ago.

No it is running as well. It starts up first then the fan blower motor starts. Just no burner ignition

Do you know what the presure switch is? Size of hockey puck..has rubber tube attached and 2 wires?

Customer: replied 3 years ago.

Yes - I can see that

we need to check the rubber tube for cracks or holes...while tube off we want to use a paper clip or small nail and poke out the fitting it attaches to on heater.

We also need to make sure there is no blockage in chimney.

I am betting that air switch )pressure switch) is not closing..can you check this now?

Customer: replied 3 years ago.

How can I check if it is not closing? will it be visible?

You would need a meter to make sure.

We can also do a hands on test if your game?

Customer: replied 3 years ago.

I just pulled the hose off on the vent side not the pressure switch side and the plastic fitting pulled right out of the vent pipe. The end appears to be rusted off. Can that be sealed back up or does that vent piece need to b replaced?

You can try to rebuild...I have seen epoxy used and high temp caulk (for wood stoves and hi temp projects)

If that is not possible then a new part is needed. The bad fitting is likely what is impeding operation.

The first thing needed is for the pressure switch to close when exhaust kicks on.

As a test...remove the 2 wires from pressure switch...start unit...AFTER the exhaust starts touch and hold the 2 wires together metal to metal.

Tell me if ignitor starts

Customer: replied 3 years ago.

OK - be back to you in a couple minutes -

standing by

Customer: replied 3 years ago.

You are a genius! I jumped the wires and the ignitor came on. I think the piece that broke must have a jam nut on the inside of the vent as well as on the outside. is that little plastic nipple available as a service part?

I do not know if the actual part/fitting is available,

You may have to contact the mfg or "fabricate" your own.

Here is Modine contact

You are now on the right track!


Customer: replied 3 years ago.

Does it have to be sealed up pretty tight in order to detect the pressure or is that switch pulling vacuum?

The switch is being pulled in by the air flow created by the exhaust fan.

This is a failsafe to make sure the draft fan is running before it lets gas out.

You can move the fitting also. You can buy the brass fitting at any plumbing store/home improvement....drill a hole and thread the fitting in...attach rubber tube..patch old hole with epoxy.

The main concern is to have a way to verify the exhaust fa runs.

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