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justplumducky 03-27-2013 08:29 AM

Coleman Evcon Furnace, Igniter Glows but no flame
Model DGAT070BDC

Has booster motor, ignition module HS-780 RobertShaw, no control board listed in parts diagram.

Booster Fan comes on, igniter comes on, but goes out (no flame), 2 more unsuccessful tries but no flame. No more ignition attempts, then booster fan just keeps running & running (no shutdown) until main power switch (system switch per your diagram) is turned off. Turn switch back on, problem repeats exactly.

Are these the possible causes? : Ignition module, manifold pressure too low (don't have a manometer), no voltage to gas valve (loose wire). Dirt blockage (spider nest?) in gas valve, Other possibilities?

Thx for any help.

yuri 03-27-2013 08:35 AM

no voltage to the gas valve. if the igniter glows then the safety controls are OK, should have 24 volts to it after the igniter warms up for about 15-30 seconds. no gas pressure from the meter? do you have a gas water heater and does it work. if it does not work then call the gas co. spider webs in the orifice sometimes but that is usually a Fall problem as it takes them all Summer to build them and I assume it was running recently.

justplumducky 03-27-2013 08:42 AM

Only one question Yuri - what took you so long? :thumbsup: (just finished posting and got on the phone - then email came in notifying me of your response).

Thank so much for your reply. Will be heading up to this property this morning.
It was running recently, yes. I'll check the voltage.

yuri 03-27-2013 04:40 PM

1 Attachment(s)
well I guess you were justplumlucky and not ducky.:laughing:

justplumducky 03-27-2013 04:41 PM

Oh, just saw your post with the image, LOL . Can I use that for my avitar or whatever it's called?

Yes, it was running recently, and igniter was performing as you said.

I went to this job this morning and turned furnace on. 3 ignition failures, then booster fan continued to run, and run, until I shut system switch off. Turned it back on, 3 more attempts, same results. On third try I was going to check voltage In (to TH terminal and Out ("Valve" terminal on module - to gas valve), of HS-780 RobertShaw module. Before I could, flame came on (first attempt), and has been working ever since.

25.2 vac consistently at the gas valve terminals.

I watched a video not too long ago, where a residential, horizontally mounted furnace (garage attic I believe) had to be tapped on side of the gas valve each time it cycled on, in order to get flame. Could the problem with this Coleman Evcon be the same... sticking mechanism (solenoid?) in gas valve?

yuri 03-27-2013 05:20 PM

could be the gas valve sticking OR the circuit board/ignition control is sticking and not sending power to the valve. only way to know is to hook up your meter with aligator clips to the valve and see if there is voltage or not and if there is and you bang it then case solved. I would say 90-95% of the time it is the board as valves rarely stick. A seasoned Pro like myself and others will recycle the unit 25 times in a row and the odds are 95% of the time the problem will show up. If the furnace runs 25X a day and sticks once a day then the odds are it will show up when I recycle it that many times. Sticking parts will always get worse in a week or 2 so it should act up more. Yah sure use my daffy duck. :laughing:

Google ducky and hit images and go down a ways and he should show up and save as a jpeg if you cannot get it off my post. I just right clicked him and could save him from my post as a jpeg.

justplumducky 03-27-2013 05:36 PM

Cool beans on cycling it 25 times. I never got a chance to check the voltage at valve, while it wasn't coming on/flame on I mean (well, I did get the chance but I waited too long - then it started working and consistently flame came on). Been working ever since.

Going back in about a week to check up on it again. In the mean time, I told him to tap the valve if it stuck again and call me. Next time I'll check voltage present or not, and if so, bang on the valve.

Thx for ducky! Have two parked URL's: and Both are parked and for sale, but maybe I'll do something with them one day, dunno - and use ducky! :thumbsup:

Thx for your help.

yuri 03-27-2013 05:47 PM

Yur Welcome.

justplumducky 03-27-2013 09:53 PM

I get a call from this guy again tonight (you said it would get worse, Yuri) and he tells me he has has ignition failure again (same symptoms (igniter on for several seconds - maybe 15 sec's - but no flame, igniter goes out. no flame. Booster fan keeps running though and won't shut off until system power switch is turned off.

Told him to shut sys switch off immediately and I'll be up immediately to hook up meter first thing upon arrival (to gas valve terminals), then we'll turn it on again and check for voltage at valve. Turn sys on, igniter glows normally, but then it goes out, and at no time was there any more than about .444 vac. during that sequence. I cycled it multiple times again in attempt to duplicate the problem in front of the homeowner. Duplicated it again 3 or 4 times at least (in between successful ignition attempts that is).

During some of those attempts, I hooked meter up the incoming power on the ignition module, at TH terminal and GND. terminal.

Attachment 68353
This is not the actual pic of the module, so the numbers in the pic that follow the "HS-780" are not correct.

Cycled it on again two or three times. Input vac remained at 26v during at least two failed ignition (flame) attempts (no voltage at gas valve) and dropped to about 25v during successful attempts.

So I'm thinking you're going to tell me this is an ignition module (Robert Shaw HS-780) "on its way out." There is no other control board on this furnace. Only two full size relays (as opposed to the smaller relays found on an IFC control board), these full size relays being located inside the same box where the transformer is located, which is the box immediately next to (and connected to) the system switch housing. Only other controls are the Fan and Limit switches mounted into the cabinet face, and the auxiliary overheat switch (resettable) on top of the cabinet next to the base of the blower.

ryandeen 03-27-2013 10:25 PM

Yuri sound like a real pro. You always know the answer. I am sure you are a HVAC tech. :)

yuri 03-28-2013 08:05 AM

Yep. One of those relays is for the booster/ventor fan and pulled in by the tstat so as long as the tstat is calling then that fan runs. the other relay should B 4 the blower fan and yeah it looks like the ignition control is failing.

justplumducky 03-28-2013 08:12 AM

Ok Yuri, thx and may the Gods of.... (well, whatever Gods you prefer...HVAC Gods?) shine down upon you forever more! (Sorry, that's all I could think of this morning).

yuri 03-28-2013 08:22 AM

U will do VERY well as a HVAC tech because unlike most of the distracted bums I get for apprentices you think logically and really want to understand how the system works rather than just change parts and collect a paycheck so continue what you do and you will do very well. :thumbsup:If the valve was draining the voltage it usually would blow a fuse or the transfomer/XFRMR so it looks 2 me like the ignition control is failing. feel free to ask questions here as other guys like you read them and can learn.:yes:

justplumducky 03-28-2013 08:28 AM

Thx for those comments, especially the explanation about the gas valve potential of occasionally shorting out the voltage - I'll change the ignition module (soon as I can get the part) and post back the results.

Also, my apologies about you possibly choosing the "HVAC Gods" (my prior comment). You obviously don't need 'em :laughing:

yuri 03-28-2013 08:49 AM

actually I do occassionally. I get a couple calls a year that make NO sense whatsoever, drainage/flow problems with high efficiency furnaces so that is where the real techs get seperated from the parts changers.

Good Luck with your endeavors.:)

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