Intertherm MH heater

[rguy1010]

Aloha-

I'm new here but I looks like the place for good info ....

I have a Intertherm mod # MMHA-070AAHR-01 ser # MMH9401-05872

When it gets real cold out the furnace blower fan starts up for a second or two and then quits. The flame never goes out, but the blower fan just starts and restarts. The combustion fans runs all the time also. I replaced the blower fan motor a couple of years ago, and the ignitor several times.
I bought new limit switch (626279) and thermal fan switch (626337). I have switched both of these in and out one at a time between the old and the new to double check it. I have bypassed the upper aux. switch and re-attached it with no difference. With advise of my local HVAC guy I replaced the HSI control (626421) (not just the circuit board) with the original part, not an aftermarket.... The old controller was never flashing a code either.
The gas valve seems to work perfectly- even though the fan switch is bypassed, the thermostat shuts the furnace on and off within limits so it must be OK. I have checked the orifice and burner nozzle for spiders etc...
Now I have the fan switch (thermal) by-passed so I could get heat (-18 here), but the fan has to run all the time now. The ON-OFF-FAN switch seems to work fine.
There seems to be no problems with the wiring w/rodents or anything.

What have I missed, and how can I fix it ????

Thanks, Ron

[Robert]

Hi Ron,

Some of the furnaces during that time would have a metal shield welded or riveted in place above the fan switch to block air from hitting it directly and cooling it down quicker.

Yours may or may not be one of those. You can look down into the furnace and see if you see signs of where there may have been a shield. Maybe see it down at bottom where it broke off and fell.

Also, sometimes they installed fan switch and limit switch in wrong holes. IF you see a shield, be sure it is over the fan switch.


With all you have done, other than that, I would say it is time for an hvac tech.


Thanks,
Robert

[rguy1010]

Had the furnace since new, so I know that no one has switched their locations... According to the wiring diagram and the position of the wires they appear to be in the correct positions. Will check about shield later today... Didn't find a extra part or anything in the bottom of the combustion chamber...

Wouldn't the shield be below the switch to prevent direct heat transfer ????

I took the part # off the old parts and replaced them in the corresponding holes. Doesn't appear to be gigantic, great differences in the heat range either... (F115-30f, 140f).

I used to (in another life ) used to repair space heaters and home appliances. This looks like a big gas clothes dryer, or a small space heater... So any tech hints would help... I feel like I am missing something that is quite obvious to someone that works on these daily....

Now getting about a foot of snow on top of the frigid temps- I would say that all the techs are unavailable after the holidays....

HELP !!!!!

Thanks, Ron

[Robert]

Hi Ron,

Nothing is quite obvious when you are not there to see and test the circuits in person. This type thing happens a lot and there are several reasons and/or different reasons it can be.

Test the entire heat and blower cycle circuit, every component and every wire connection and every inch of every wire.

Could be a bad connection, loose wire, bad wire , etc. You first check the common causes and then the less common.


There will be an interruption in the circuit somewhere and once found, determine why it is being interrupted there.

Thanks,
Robert