Problem with MGHA

[yooperdirt]

Help!

I light the pilot...no problem...flip on thermostate and furnace fires...no problem...the house heats up fine a couple of times but then something triggers the pilot/gas valve to go off.

Tested the thermocouple and it clicks at 2 minutes. That seems long but it works.
Tested the transfomer and it reads 27 volts at output
Fan switch works but delays by about 3 minutes after thermostate calls for heat and after burners shuts down. But not 100 percent sure is shuts off evertime consistantly at 3 minutes

The furnace sometimes works for up to 3 or 4 hours at times before the gas pilot goes out...and then other times it lasts just one heat cycle.

I also tried boosting up the pilot flame a bit to eliminate draft issues. Wind doesn't seem to play a role.

I have not ruled out the fan switch but....if the fan switch fails to shut off after house reaches temperature and keeps running is there any reason why the gas valve would call to close??? The gas valve should only close if the termocouple senses no flame, right?

How do you test the gas valve??

Any ideas?

[Robert]

Hi Possibilities:

bad thermocouple
bad gas valve
pilot flame not covering top 3/8" to 1/2" of thermocouple tip
pilot tubing and/or pilot burner/orifice not completely clear
gas valve losing call for heat before thermostat is satisfied

All you can DIY test of gas valve is whether it gets 24vac at GV terminals on a call for heat and the 24vac remains until no call for heat when t-stat is satisfied.



Thanks,
Robert

[yooperdirt]

It turns out that my heat exchanger is cracked wide open. I thought about welding it but I don't want to take any chances with carbon monoxide leaking in.

I noticed a new heat exchanger for this furnace costs as much as a new replacement furnace
Does anyone have an extra one for cheap???

If I replace the MGHA-056 with the newer M1 series will there be any problems with the chimney and roof jack size or will everything need to be replaced???

I also noticed when I took the top portion of the roof jack off that the inner chimney tube (exhaust vent) was rusted at the top and about 1/2 inch of the tube had coroded away. I treated it with rust neutralizer and muffler paint and it still seals but....

Can the inner tube be replaced without removing the outer intake tube and roof flashing???

Thanks in advanced,

Mark

[Robert]

Hi Mark,

Don't want to try and repair a heat exchanger ever, so wise choice there. New furnace will need new roof jack.

Cannot separate and replace just inner or outer portions. Would need a new one anyway with a new furnace.


Glad you found the crack and nobody was harmed.



Take care and best wishes,
Robert

[yooperdirt]

One more question....do you know if the coil box is interchangeable between these models???

Thanks Again

[Robert]

Will depend on the dimensions of the furnaces. I don't know right off. Just measure what you have and do the same for the new one.



Thanks,
Robert