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Cycling manifold pressure?

HarnessedPower
HarnessedPower Member Posts: 10
in Gas Heating
Good morning, folks…

I’ve got a question regarding a gas valve in a 50kbtu Mr Heater Maxx unit heater (on natural gas). It’s been driving me crazy since we finished the install a few weeks ago. The gas side of things was done by a local plumber and that all looks good. In the interest of transparency, I will relate the entire story:

The furnace ignites and seems to run okay… except that it will randomly (and I mean randomly) lose flame, close the gas valve, wait 15 seconds, go through the ignition sequence, light back up, and continue on like nothing ever happened. Sometimes it’s once in a day, sometimes it’s 3-4x an hour. It never sets a flash code. 

Through testing I’ve found that the high limit, rollout switch, inducer pressure switch, and electrical side of the gas valve will all set a flash code if I “trip” them to kill the burner while it’s running. So it shouldn’t be those. 

Here’s the problem. Once install was completed, I never realized that the plumber never checked the manifold pressure out of the gas valve as recommended. That was a miscommunication and my own fault. I have no shortage of testing equipment from working on propane-fired municipal asphalt hot patch trailers for the last 10 years, so I hooked up my manometer. The manifold pressure spec is 4”w.c. at my altitude. 

The manifold pressure cycles from 3.8” to 4.9” over the course of 12-15 seconds. It slowly climbs to 4.9” and then drops like a rock back down to 3.7-3.8”. I can actually hear a VERY brief change in the sound of the burners when this happens. And about every 70-120 cycles of this (yes, I stood on a ladder and counted) the pressure drops to 1.5” and the burner flame will stutter and go out. Then it will wait 15 seconds, trial for ignition, and relight. 

I always thought manifold pressure should
be more or less steady, or am I all wet?

Inlet nat gas pressure is rock-steady at 9”. The furnace is fed directly from the meter through 50’ of 3/4” black iron pipe. 

So….is it worth taking the regulator spring out and checked it? This unit came with a propane conversion kit and instructions on the changeover (though I have verified that the nat gas orifices and spring in it currently) so I know it can be opened and removed…or should I just buy a new gas valve and be done with this one? Mr Heater will not send me one as they are “out of stock and have no timeline on receiving any at this time.” SupplyHouse has a direct replacement, however. 

Any advice or thoughts would be welcome. 

Thanks and have a great Friday!

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,477
    Sounds like a bad valve. The outlet pressure should be rock steady as well and your inlet pressure is fine

    Maybe some pipe dope or a pipe chip got in the valve?
    HarnessedPower
  • HarnessedPower
    HarnessedPower Member Posts: 10
    That’s what I was thinking too. I suppose when I get home from work today I’ll pull it out and at least take the parts detailed in the conversion kit out and see what I can see. But for my own sanity’s sake, I will just see about ordering a new valve. Thanks!

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