Coleman 7956-856 burner won't light

Questions about repairs and parts for Coleman furnaces, air conditioners and heat pumps for manufactured homes. Click here for Coleman parts.

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Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Mon Feb 24, 2014 6:37 am

Our Coleman 7956-856 natural gas furnace has an intermittent burner lighting problem.

The thermostat clicks, the furnace clicks but burner does not light. Pilot is always lit. We had a tech out maybe six weeks ago for this problem, he said the valve orifice had spider junk in it, cleaned it and it has been fine until late last week. This tech also did a quick once-over of the furnace and said everything seemed good.

If I turn the power off at the furnace, wait a minute, and turn it back on, usually the burner will ignite and do a complete normal heating cycle. A multitester shows 24v at the gas valve when the burner is lit, but I have not measured it with the thermostat 'on' and the burner unlit.

Before I tear into it or call another tech, does this sound like a problem with the valve itself?


DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Mon Feb 24, 2014 8:36 am

Sounds like good power at the valve when burning. Only other thing you might try is to hang a cheapy, snap action, heat only thermostat with about 10' of wire on it for a few days. You don't have to mount it permanently, drape the wire over a door in the hallway. If the furnace runs fine with no faults it could be either the wiring to or the thermostat. If the problem continues, replace the control valve. Another thing you might check is gas pressure in and out of the control valve. You will need a manometer or gauge that reads in inches of water column. Hard to troubleshoot intermittent problems....they never show up when the service guy is there.

Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Mon Feb 24, 2014 12:19 pm

Will try those, thanks.

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Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Mon Feb 24, 2014 5:18 pm

Had a chance at having a service guy come today so I took it.

He was very good, checked the gas pressure, voltage everywhere, airflow everywhere and just plain visual troubleshooting. He measured the the pressure at 5.4wc and watched the valve open irregularly, if that's a proper word. Voltage at the valve was kinda sketchy.

He replaced the gas valve and got steady voltage with pressure at 3.49wc and we cycled the furnace many times to see what would happen. Not a fault in the last few hours.

I definitely paid a markup on that valve, but watching him have a ton of trouble trying to get the old one off the bracket made me feel like I made the right choice. That, and the promise of reliable heat.

I've learned a bunch from reading this forum. After reading many posts here, I self-diagnosed it as a bad valve but didn't really want to believe that. That's why I came back to finish this thread just in case it could help someone in the future.

DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Mon Feb 24, 2014 6:28 pm

Good deal!!


Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Tue Mar 18, 2014 7:39 pm

I came back here to poke around and my post is still the first on the list so I thought I'd update the story.

Replacing the gas valve, although bad, didn't fix it. On subsequent visits the A-coil was cleaned to be sure it wasn't so dirty it was blocking airflow and overheating, and the final frustrating thing (to the service guy) was a relay in the switch box. I was on the phone with the tech throwing around the idea to replace the relay since we had no other ideas. I asked if he needed any numbers or diagrams off the existing relay before he headed our way, and when I touched the top of the box with a screwdriver the burner ignited. At that point we decided we'd found the problem.

He wound up coming over the next morning and swapping the relay at no charge because he was as frustrated as we were and we'd already paid for four service calls.

That $20 relay cost me almost a thousand but we've had consistent heat since then, so just maybe it was worth it.

DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Tue Mar 18, 2014 10:09 pm

Does your furnace have the relay box with the 2 rocker switches on it accessed through the front panel on the furnace?

Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Sun May 04, 2014 8:19 am

Probably doesn't matter two months later, but yes it does.

DaleM
Posts: 385
Joined: Wed Apr 07, 2010 10:07 am

Mon May 05, 2014 10:11 pm

Kevets, the reason I asked that question about the relay box is because many don't know how to troubleshoot a 2 wire t-stat system. 2 wires sound simple but it really complicates things in the low voltage control system. Here's why:

To start off with you MUST use a heat ONLY thermostat. A dual purpose thermostat can be used but will ALWAYS have to be set in the HEAT mode. That relay box translates what the thermostat is telling it to do by recognizing the contacts closing for the heat cycles and the thermostat contacts opening for the air conditioning cycles. There are relays in the relay control box that come into play depending on whether you have that rocker switch set to heat or cool. If the WRONG relay is installed the box, heat and air conditioning, will not function properly. There is an oddball 5 terminal relay in that box that will give most techs trouble simply because they do not understand the 2 wire thermostat system. Many guys are looking for 24 volts to make things work. HOWEVER on the 2 wire system the lack of voltage makes the air conditioning work right. The wrong relay in that box will also cause problems with the heating low voltage circuitry. I hope you finally have things figured out.

Kevets
Posts: 8
Joined: Sun May 31, 2009 5:30 pm

Tue May 06, 2014 7:27 pm

I have a Honeywell RTH2310B, a programmable heat/cool thermostat which can be jumpered to support a two-wire system.

Just today I flipped the switch on the relay box to 'cool' and while the flame indicator is on on the thermostat display, the condenser outside was running and the furnace was pushing cool air through the house. Looks kinda confusing showing the heat icon but it works. I did hear the relay click though, it's dang loud compared to the old one.

I think I got the basis of your post, but at this point since we've had reliable heat on demand since the end of February and the cooling system seems to work as of today, I really don't care. I'd just like to enjoy springtime and worry a little less about the air system.

And the guy who replaced the relay was the one who suggested it on a previous day and lives in a manufactured home with a similar system, so I'm guessing he knows a little bit about this style unit.

Thanks for the posts, though. All 'free' help is greatly appreciated whether it seems relevant or not.

(I just had to go look, the relay that was replaced has the number 184-50114-402A for those trying to follow along.)

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