Intermittent Heat Operation Evcon DGAT070BDC

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ared947
Posts: 6
Joined: Thu Jan 20, 2011 11:07 am

Coleman Evcon Model DGAT070BDC

All AC operations function correctly.

For Heat:
Furnace turns on when stat calls for heat. Bar glows inside fire box, but the flame does not ignite and the main blower does not turn on. It will just keep running like this with no heat.
While checking for loose wires on the Robertshaw 100-00812-45 I inadvertently crossed the TH and VALVE terminals and the flame came on. The main blower started and the unit worked just fine. It seems to malfunction only when the ambient temperature falls below a certain point (65 degrees).
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Sticky valve. It may or may not last through the winter. See this with units that sit a long time without being used, as in older used homes that sit for years vacant then get sold.
ared947
Posts: 6
Joined: Thu Jan 20, 2011 11:07 am

Thanks for the reply. What needs to be replaced to make the furnace work reliably? This is my primary home so it is in constant use. Tough getting up at 3AM when the unit is running and no heat comes on.
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Replace the Robertshaw gas valve. Be sure there's not a web in the oriface first. Have seen a few cases where a web in the oriface is flopping around a bit and it acts like a check valve. Sometimes the unit works, sometimes it doesn't. Once you are sure there's is no obstruction in there and it still continues to operate intermittently then replace the gas valve.
ared947
Posts: 6
Joined: Thu Jan 20, 2011 11:07 am

I am concerned because the valve is a bit pricey. I can get the unit to turn on by crossing the TH and VALVE terminals on the ignition control block. I haven't thought to check for voltage at the gas valve terminal, but I will do so. Just wondering if it could be a faulty ignition control.
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

The wires on the gas valve usually don't make a difference. It's 24 volts AC going to it. You can monitor that voltage while the wires are on the terminals. Put a multitester on it and check it while it is cycling up. If there are 24 volts there and the valve isn't opening then it's bad or going bad. You changing them may be causing some kind of movement in the wiring internally in the valve assembly, thus the vavle works when you switch wires.
ared947
Posts: 6
Joined: Thu Jan 20, 2011 11:07 am

I don't physically switch the wires, I cross the TH and VALVE terminals on the ignition control block with a pair of needle nose pliers and the unit turns on. I do this when it hangs up. It has been in "working mode" and I put a multimeter on it and initially there is no voltage at the valve for a few seconds and then the voltage comes on and it fires up with a good strong click all by itself. I don't know what is on the ignition control that passes the 24 volts to the valve, but it seems like a relay or switch is not closing reliably. Again, it happens only when the thermostat is set low and the room temp gets to about 65 or less. Thanks for the responses.
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

I've only had one instance where a board failed intermittently. A relay on the board worked then didn't work, then worked, then didn't work. It was that way for several days. Finally showed itself while I was standing in front of it. That was a tough one to track down. Tap on the small black boxes on the board when it fails next time. Those are "mini" relays and control several different functions on the furnace.
ared947
Posts: 6
Joined: Thu Jan 20, 2011 11:07 am

Well, the unit has been working reliably for the last few weeks. However, it is acting up again. I am thinking it may be the sensor that is on the blower(?) assembly that sends a signal back to the ignition control block indicating a heat source. I am guessing that this is a thermocouple or similar device. I put a meter on the RS terminal of the ignition control module and at start up there is 115 volts. When all is working the voltage on the RS terminal jumps to 140. At all times when the unit turns on for heat the igniter glows red hot. This is a Coleman/Evcon DGAT070BDC furnace. Can the sensor be replaced or do I have to change the entire assembly. I doesn't seem like there is much else involved aside from the "glow bar" and the sensor.
DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Have a closer look at the igniter during full burn operation. Is it glowing red from being energized or just from being in the flame. You could test the leads going to it, there shouldn't be any power to it after the gas valve opens and the sensor proves flame.
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