Hi, I'm looking for help troubleshooting Modine HD 75AH0134 garage heater. I've owned the house for the past 10 years, and never had it running. I have no idea if it ever ran before. It has LP connected with internal gas valve converted to LP. I get no gas on the outlet but there is 10psi on the inlet side during ignition cycle. Ignition element heats up to glowing red but no gas is released to burn. I replaced the valve with brand new unit and same behavior. Also new transformer, no change. Exhaust fan and vacuum swith work, main blower and delay switch work. Rollout switches also work, and bypassing them also doesn't fix it. There is no visible damage or heat discolorations anywhere. I also installed new digital thermostat The only thing that i noticed is that despite transformer outputting 24v AC, voltage drop exists on some connections like for example on rollout switches and exhaust pressure switch. At these I measure 14v not 24v. Is this normal or a sign of something wrong and if so, what could be causing this voltage drop?
Modine heater not turning gass on
- Thread starter tad_b
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Firebrick43
Well-known member
First there should NOT be 10 psi on the inlet side. There should only be 11” of water column which is around .75 psi.
Measure amp draw on the igniter element. The gas valve has to see close to 3 amps for it to open.
They start to go bad but still glow somewhat sometimes.
It’s probably the second most common problem after a completely broken element.
Gas valves usually don’t go bad.
Measure amp draw on the igniter element. The gas valve has to see close to 3 amps for it to open.
They start to go bad but still glow somewhat sometimes.
It’s probably the second most common problem after a completely broken element.
Gas valves usually don’t go bad.
Firebrick43
Well-known member
Please make sure you don’t actually have 10 psi to the heater first.
That is the pressure the first stage regulator drops it to. If someone didn’t install a second stage to drop it to 11” of water column things could end up very badly.
That is the pressure the first stage regulator drops it to. If someone didn’t install a second stage to drop it to 11” of water column things could end up very badly.
Ok, I did put new igniter element in. No change in behavior. Same thing, everything operating up to ignition, no gas flow, no ignition, 5 attempts and fault with 6 flashes. 
What could be the 14v on switches, is that normal? Transformer outputs actually 26.8v
What could be the 14v on switches, is that normal? Transformer outputs actually 26.8v
That's interesting, thank you for pointing this out. Gas valve manual calls out need for 10psi at inlet, with manual tuning screw where the LP conversion spring is. I'll investigate more.
Firebrick43
Well-known member
On page 12 of the Modine HD manual it says inlet pressure needs to be 11-14 inches of water column.
Maybe the previous owner put the wrong valve in and you replaced with the same?
Maybe the previous owner put the wrong valve in and you replaced with the same?