Heil H9MVX60 gas valve pressure ?

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Old 01-01-19, 02:29 PM
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Heil H9MVX60 gas valve pressure ?

I have a Heil H9MVX060 which is a 2-stage 95% gas furnace setup for natural gas. It has the Honeywell VR8205Q2795 gas valve.

I was checking return and discharge temps and only saw a 32 deg rise on high which is on the low side of the acceptable range (30-60 deg) so before checking fan speed, it's variable, I decided to check gas manifold pressure.

On high fire I measured 2.0 in/w.c. (spec is 3.5). I tried adjusting the high adjuster and the manifold pressure didn't change. Ran it all the way in and about 1 turn from the all the way out and observed no change in manifold pressure. I also adjusted the lo adjuster when the furnace was on hi fire to make sure I was mistaking the hi/lo adjusters, the lo adjuster did not effect the manifold pressure when the furnace was on hi fire.

On low fire I measured 2.0 in/w.c. (spec is 1.7). I tried adjusting the low adjuster and I could get the manifold pressure to go up to 3.2 in/w.c. by running the adjuster all the way in but couldn't get it any lower than 2.0 in/w.c..

I ran it on hi/lo using the test mode on my 2-stage Honeywell t-stat and verified by measuring 24VAC on the W1 and W2 wire at the control board. I have the furnace control board dip switch set for using a 2-stage t-stat.

I measured 24VAC on the gas valve to verify it was getting commanded from the control board. When on low the blue wire had 24VAC referenced to ground when on hi both the blue and brown wires had 24VAC referenced to ground.

I checked inlet gas pressure and it was 7.4 in/w.c., in the middle of spec (4.5-14).

I removed the adjustment screws and they look fine, they aren't damaged in any way and nothing foreign appears to be in the gas valve. The two screws cannot be swapped since one is left hand threads and the other is right hand threads.

The furnace is well grounded from the electrical supply, Verified all ground connections, inside the electrical box, at the electrical box and at the burner. The gas valve has a 3 wire connection, hi/lo/ground.

I have very little history with this particular unit, not sure about past attempts to "solve" any perceived problems of the P.O. so I'm open to the possibility that someone thought they were doing the P.O. a favor by mis-adjusting this furnace. There is no sign that this furnace was ever converted to run on propane and then converted back, no L.P. switch, no conversion sticker, etc. Other than the measured manifold pressure is low and temp rise barely meeting spec the furnace is working properly.

On lo fire 2.0 is pretty close to 1.7 but I'd really like to get manifold pressure on hi adjusted up closer to spec of 3.5. Adjusting the pressure on a gas valve with a manometer is pretty straight forward, never had one that wouldn't changed when the adjusters were moved, any thoughts?
 

Last edited by seafur; 01-01-19 at 05:10 PM.
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Old 01-01-19, 07:06 PM
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It sounds like you have a bad gas valve. Regulator's shot.

the pressure should change as you adjust it.
 
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Old 02-18-19, 11:27 AM
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I replaced the gas valve and I was able to adjust the new gas valve to the exact manufacturer specs of 1.7/3.5.
 
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