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gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5000
Experience:  51 years of experience install design and repair of all makes and models of AC
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Our customer has a MMHB-070ABHR-01 Intertherm furnace. The

Customer Question

Our customer has a MMHB-070ABHR-01 Intertherm furnace. The furnace fails and kills the gas valve and 24 & 110 volts stay on to the Ing. control. If you just prop open the sight window the furnace stays on. The flue and in let look OK the gas pressure is 3.5". We did find the ohms on the glow coil was 125 ohms and the book says it should be 40-75 ohms. Any ideas? Thanks Chuck Bramhall Thermal Comfort Air, Manhattan, Ks
Submitted: 6 years ago.
Category: HVAC
Expert:  gotoman for ac replied 6 years ago.

The vent on these furnaces are strange, There is one vent on the inside for venting furnace. The space between it and outside vent wall is supply air for furnace. (combustion air) If this vent gets blocked it will snuff out flame. Check for blockage and also check to make sure vent motor or wheel is in good shape. Rus

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Customer: replied 6 years ago.
The blower is OK we took it out and ran it on the bench. We also look at the flue from the top and used a mirror from the bottom and look up as best as we could while the blower was out.
Expert:  gotoman for ac replied 6 years ago.
The inside vent some time will get holes in it or slip out of bottom. The vents the blower pushes air thru will stop up also. Also on a few furnaces they put a damper door just above combustion air motor . The dampers get loose. I have also had to drop my gas pressure down to 3 inches . I have had this problem on several occasions. Seems like it is a different problem every time. Rus
Customer: replied 6 years ago.
Do you think any thing about the glow coil having the wrong resistance have any bearing. I assume they use the coil to light the burner as well as prove it is on. I could not find a vent damper and all of the openings looked open whan we had the blower out. I shined a light in from the blower out let into the combustion chamber. Where are you talking about the inside vent that may have holes? Thanks again for getting back with me.
Expert:  gotoman for ac replied 6 years ago.
If the hot surface igniter. Glow and lites flame you are ok. with that. Two times I ended up changeing heat exchanger because of the intake air chamber was leaking. It leaks between the chambers on the furnace itself. If no restrictions are found pon intake pipe. That is where your trouble is located. Rus