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#1
10-29-2012, 11:42 AM
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Slow reacting forced air gas furnace
When the thermostat calls for heat it takes quite a while before the furnace kicks on. Tenant says around 30 minutes. I waited about 10 minutes when testing it yesterday and it didn't kick on. It was running when I checked it today so it does come on but with a long delay. Any ideas why the gas valve appears to be lazy. Older furnace with a gas pilot light.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13 
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#2
10-29-2012, 01:09 PM
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Not much you can do with those devices.
Either the thermocouple has deteriorated and is not tripping the main gas valve in a timely manner, or the gas valve itself is history. The thermocouple is the least costly to replace if you want to take a shot.
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#3
10-29-2012, 01:55 PM
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Is the pilot adjusted properly?
Maybe the thermocouple is just skirting the fringe of opening temperature?
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On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST 1983 300SD - 305000 1984 Toyota Landcruiser - 190000 1994 GMC Jimmy - 203000  https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif
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#4
10-29-2012, 01:59 PM
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Quote:
If the pilot flame is too small, it might take 30 minutes for the thermocouple to heat to the required temperature.
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#5
10-29-2012, 04:12 PM
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Ok, took things apart and looked at it. I think it probably is the thermocouple. I thought I had remembered that it was the kind of furnace which had a pilot which kept a thermocouple hot all the time and the gas valve just sent out the burner gas when called to but it's not. It's the kind with the small pilot light which gets bigger when the thermostat calls for heat and heats up the thermocouple just with that bigger flame and not all the time. I'll get another thermocouple, swap it in and see if it fixes it.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#6
10-29-2012, 05:08 PM
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The plot thickens. What I thought was a thermocouple wasn't. It was a small copper tube carrying gas from the gas valve to the pilot for the secondary pilot. It's an older Carrier furnace. It has a manual shut off valve first in sequence, then another valve, then a third valve. The third valve has the copper gas line that looks like a thermocouple line. The pilot comes off the shut off valve (not the electric gas valve) and has it's own little shut off valve. What I now think is the thermocouple is two flat pieces of metal which are heated by the flame from the secondary copper pipe. At the base of the pilot/thermocouple fixture are two wires which appear to be attached to the two flat pieces of metal (maybe 3 flat pieces initially which become 2 pieces where the flame hits them). So I guess the next step is to see if this odd style of thermocouple is still available. Anyone have a clue?
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#7
10-29-2012, 05:39 PM
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__________________
1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#8
10-29-2012, 06:21 PM
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Is it a forced draft unit? Is the draft forcing part operating correctly? Nevermind, read the pdf, no it is not.
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Remember, Safety Third! '99 E300 Turbodiesel, '82 300TD, 1996 12V Cummins Turbo, '94 Neoplan - Detroit 6V92TA Last edited by cullennewsom; 10-29-2012 at 06:34 PM.
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