HC-125 low flame

Old 11-28-11, 04:14 PM
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HC-125 low flame

Hi. Just found site & am hoping someone can give me a "sanity check". My house has an old Hydrotherm HC-125 gas boiler furnace feeding cast iron radiators. (Actually I have 2 furnaces; HC-100 for upstairs) Not sure how old they are but at least +30Yrs.

The issue is that sometimes when the thermostat calls for ignition, the main burners only have a partial flame. i.e sometimes only a few ports are lit with a miniscule flame or all are lit with a 1/4-1/2" high flame.

I believe the standing pilot generator is giving out & the gas valve (White-rodgers 36C03U-333) is on its last legs I'm thinking that because even though the pilot is lit and all flames are blue, I can tap the side of the valve and flames come on full force. I found out yesterday that if I turn the knob from ON to pilot then back to ON the flames also come on as they should.

Just for "giggles" I put my cheap voltmeter across the generator terminals after the system had sat idle (cold) for about 1 1/2 hours and got 420mV. (750 mV system). The other furnace which has a different gas valve 575mV. After full flame has been on for about 10 minutes the reading on the HC-125 is about 150mV.

I'm thinking the mV value should be higher and maybe something is sticking in the gas valve. Anyone have any thoughts?

Old 11-28-11, 05:16 PM
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Sounds like you may have a spider nest in the burner orifice, start by checking that before you go any further
Old 11-28-11, 05:19 PM
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Even though the flames are blue, the odds are that the pilot burner is dirty and needs to be cleaned.

You can try shutting off the pilot flame and using canned air (which is flammable), compressed air or a straw to squirt air into the throat of the pilot burner. Pretty often that will restore normal service.

The correct thing to do is to disassemble and clean the pilot burner and pilot orifice. I'd do that if the cheap & dirty method doesn't restore normal service.

Cleaning the pilot burner is normal maintenance that is probably past due for doing in any case.

If that doesn't work, the replacing the pilot generator would be worthwhile.

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