Hot Surface Ignitor/Gas Valve Sequencing


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Old 01-31-09, 08:36 PM
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Unhappy Hot Surface Ignitor/Gas Valve Sequencing

I am trying to diagnose an occasional ignition failure on a Goodman GMS80903BNA gas furnace (three tries and then a timeout) installed in an unheated attic. The furnace is more likely to fail on colder mornings (we live in Massachusetts just south of the New Hampshire border so it gets reasonably cold). The hot surface ignitor seems to work fine. This furnace uses a fast opening White Rodgers gas valve (36G22Y). I was checking out the operation of the gas valve so I connected a digital voltmeter to it as well as a dial-type gas pressure gauge. From the time I hear the relay on the control board click until I see voltage applied to the gas valve and until I see pressure on the output boss of the gas valve there is a slight delay. Itís hard to be precise but Iíd say itís a bit less than a second.

More interesting to me was the fact that the hot surface ignitor shuts off before the gas valve closes. I thought I had found the source of my problems but then I checked the basement furnace, a Goodman GMS95. They both use the same White Rodgers fan control board (50A55-289) and the same gas valve (36G22Y). To my surprise, the same thing happened: the ignitor shuts down about two seconds before the gas valve closes. Considering that the gas valve is only open maybe four seconds Iím really baffled by this. It seems like this would certainly contribute to ignition difficulties. However, the basement furnace never has a problem lighting up.

My questions are: How quickly should a fast-open gas valve open? Is it basically instantaneous or is there some delay? Is it normal that the hot surface ignitor shuts down before the gas valve closes? If so, why? Is there any way this can be changed? I see no adjustments on the control board.

By the way, these furnaces have been converted to propane. I noticed that the pressure regulator adjustment has been cranked all the way in on both gas valves. The output pressure is about 10Ē water column, which seems OK.
 
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Old 02-01-09, 01:30 AM
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The normal operation of a HSI system powers up the HSI for thirty seconds to a minute or so to give it a chance to get hot.

Then the gas valve turns on and the HSI remains on another second or two to give the gas a chance to light. The main burner then stays lit until either

1) the thermostat shuts off the furnace

2) the flame sensing circuit doesn't detect the main burner lit for a period of 2-4 seconds or so.

It's not clear from your description what is happening. The burners should stay lit until the furnace is shut off.

You describe both furnaces as shutting off the gas valve a couple of seconds after the ignitor shuts off. I don't understand that description.


Again, normal operation is for the burners to light and stay lit until the thermostat shuts off the furnace. If the burners shut off after two seconds, that might well be a failure of the ignition module to detect that the burners lit properly.


So you need to be more clear about what's happening.
 
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Old 02-02-09, 08:14 AM
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More Details

Sorry for the confusion. Hereís a more complete sequence of events:

1) Thermostat calls for heat
2) Draft inducer fan turns on and runs for maybe a minute
3) Hot surface ignitor turns on and stays yellow-orange color for 20-30 seconds
4) Relay on fan control board clicks
5) Gas valve opens a fraction of a second later (verified by dial pressure gauge)
6) If flame is established, ignitor powers down after a few seconds, gas valve stays open, and furnace runs until thermostat is satisfied
7) If flame is not established (either because itís the attic furnace with the ignition problem or if I intentionally block the gas orifice of the basement furnace nearest the ignitor so it cannot ignite) then the ignitor powers down a second or two before the gas valve closes. The gas valve is only open maybe four seconds. So for the last two seconds or so (half the time itís open) the ignitor has shut down.

Both furnaces exhibit this same sequence of events and Iím really baffled by it.

Also, for what itís worth, the basement furnace is mounted vertically so the burner ports are horizontal. The ignitor is on one side and the flame rod is on the other. The attic furnace (the one with the problem) is mounted horizontally so the burner ports are arranged vertically. The ignitor is on the top and the flame sensor rod is on the bottom. When I watch the burner fire up the flame spreads pretty quickly from the lit burner port to its neighbors so I donít think this is a problem but just thought Iíd mention it.

Going back to basics, to build a fire one needs an ignition source (got that with the hot surface ignitor), a fuel source (verified that by gas pressure gauge), and oxygen. My skin isnít blue so there seems to be oxygen in the air. My concern is whether the HSI is on long enough to ignite the gas. Why would it power down before the gas valve shuts off?

But in any case, the basement furnace works fine with this situation while the attic furnace does not. I just can't seem to identify anything different between the two furnaces. Any suggestions where else to look?
 
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Old 02-02-09, 08:57 AM
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The operation of the furnace through your step six is normal.


After the ignitor shuts off, the ignition module has a few seconds to detect the presence of the burner flame at the flame sensor. If it does, the burner remains lit. If it doesn't the burner is shut off.

When your attic furnace isn't lighting properly, what blink codes are being signaled by the ignition control module?
 
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Old 02-02-09, 10:44 AM
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I have not checked that for some time. But earlier when I did I was getting one flash, furnace lockout due to an excessive number of ignition retries. I expect it would still show that LED code.
 
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Old 02-02-09, 11:33 AM
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The instruction manual for an ignition control module similar to the one you have is here:

http://www.white-rodgers.com/wrdhom/.../0037-6426.pdf



Sounds like the ignition module is not verifying that the main burner flame has ignited. Fairly commonly that is caused by a flame sensor that has become covered with oxides and needs to be cleaned.

The flame sensor is usually located on the opposite side of the burner compartment from the hot surface ignitor, and is commonly a thin round rod sticking up into the burner flames with a thin wire going back to the ignition module.

Try removing the flame rod and cleaning it with a wire brush or abrasive pad --- it doesn't take a lot. Reinstall and give it a try.
 
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Old 02-02-09, 03:23 PM
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My guess is that if you caught the furnace shutting off early, the ignition module would be giving a rapid green LED flash. After it cycles repeatedly and fails to ignite, it goes into lockout and given you the one red flash.

See page 6 of the instruction manual.

A troubleshooting ladder is given on page 7.
 
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Old 02-02-09, 03:56 PM
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This is not a failure to detect a flame--there is no flame to detect. Occasionally the burner simply does not fire up. It doesn't fire up and then lose the flame. It just doesn't fire up.

The troubleshooting ladder you provided a link to does not help me. My control board only has a red LED. The control board you reference has yellow, green and red LED's.
 
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Old 02-02-09, 05:08 PM
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You have gas, and you have ignitor that glows - but no fire. It could be your ignitor is not getting hot enough. You describe it as yellow to orange. Should be more like brilliant pale yellow. But not to split hairs over descriptions - Ohms test the two wires going to the HSI, with it disconnected. Should be under 100. Often new ones I have found can be as low as 28 and up to about 50.
 
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Old 02-02-09, 05:15 PM
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Originally Posted by BikerBill View Post
7) If flame is not established (either because itís the attic furnace with the ignition problem or if I intentionally block the gas orifice of the basement furnace nearest the ignitor so it cannot ignite) then the ignitor powers down a second or two before the gas valve closes. The gas valve is only open maybe four seconds. So for the last two seconds or so (half the time itís open) the ignitor has shut down.

OK --- I thought the flame was established but was sometimes shutting off. But instead you are getting no flame sometimes ---- presumably this happens repeatedly in order to get the lockout code.

ecman's suggested test will determine whether the relay that opens the gas valve to turn on the gas is burned out. I doubt that's the case or the valve would never open.

The test I would suggest would be to measure the voltage being applied to the two contacts on the gas valve. That should register 24 VAC whenever the gas valve should be open. If you are failing to get voltage when the gas valve should be open, it's pointing to a defect in the ignition module unless some limit or pressure switch is opening.

So tell us what you are finding on those items.
 
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Old 02-02-09, 05:35 PM
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I guess it's still not clear to me---- is gas coming out of the valve and simply not igniting off the hot surface ignitor?

If that's the case I'd start by measuring the voltage across the ignitor.


Also, I think the is an instruction manual for more or less the right ignition control module.

http://www.white-rodgers.com/wrdhom/.../0037-6265.pdf

On page 6 under "Heat Mode" the module varies how long the ignitor has to heat up. Perhaps the module isn't leaving it on long enough before turning on the gas and doing the ignition trial.

Sorry if I'm a little slow here.
 
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Old 02-02-09, 06:33 PM
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Originally Posted by SeattlePioneer View Post
ecman's suggested test will determine whether the relay that opens the gas valve to turn on the gas is burned out. I doubt that's the case or the valve would never open.
Not my suggestion. I'm suggesting that since he has the gas valve open, as verified by output side dial gauge test he did, that there is a good chance his HSI is simply weak, and shuts down, after his gas does not ignite.

I have had to replace HSI's for that very reason. It is when the HSI is just starting to burn out or crack, but has not fully done it yet, and can cause it to not glow quite hot enough and bright enough to set off the gas, every time.

A gas valve will indeed open with a bad HSI. And if there is no flame within about 4 seconds, the gas valve and HSI will shut down, and try agin until lockout.
 
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Old 02-02-09, 06:38 PM
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Originally Posted by BikerBill View Post
3) Hot surface ignitor turns on and stays yellow-orange color for 20-30 seconds
SP,

That would be plenty of time for the HSI to get hot. If this indeed happens all the time (including times it never fires off), I doubt the module is prematurely shutting down the HSI.
 
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Old 02-02-09, 07:58 PM
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OK, here's some additional information. When the gas valve is actuated I measure 25 volts across it which seems totally normal. I checked the resistance of the HSI and got 38.5 ohms. This is a silicon carbide ignitor. The values of 28 to 50 ohms that you mention were those for silicon carbide or silicon nitride ignitors? Mine seems to be right in the middle of the range you mention. Any other suggestions?
 
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Old 02-03-09, 12:13 AM
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I'd also measure the voltage across the HSI while it's lit. It's always possible a relay is making a poor electrical connection and supplying significantly less voltage than it should.

If it's 120 VAC though then changing out the ignitor is the likely problem.
 
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Old 02-03-09, 05:22 PM
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IF current to ignitor per SP, then I'd want to satisfy myself that even though I saw I had gas pressure (per your dial instrument test), I'd want to actually smell the stuff with my nose, to be sure some stagnate air is not in there.

I'd turn off the furnace, then turn off the inline gas valve, and then undo the 1/2 inch pipe union, and see if I have good gas there when I crack back open the inline gas valve momentarily. If not fresh gas smelling, I'd flush it a number of seconds with furnace switch off first/no sparks. Then if I smelled gas, and new ignitor was good and had current to gas valve to MV terminals -I'd rap on the gas valve to see if that jostles it to fire up.

If there was good gas at the valve, and the gas valve was getting 24 volts, and the ignitor was getting the volts it should, I'd also take a look at condition of where gas leaves where it does by the HSI, in case that somehow is plugged up in that area.

And if the HSI was not original, I'd want to research the looks of it, so that exact position of it is correct with it's width and length and how it is mounted, to be able to fire off the gas.
 
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Old 02-03-09, 07:08 PM
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OK, I got out my digital voltmeter, my gas pressure gauge, and my watch with a second hand. Here's what I found.

The ignitor voltage is 120 volts on the money. I had my wife check the ignitor color and she said it was orange. I already reported the resistance of it to be 38.5 ohms.

Time from draft inducer fan turn on to voltage to ignitor = 20 secs
Time from ignitor voltage to gas valve click = 7 secs
Time from gas valve click to pressure at output boss on gas valve = 1 sec
Time from gas pressure at output boss to ignitor voltage shut down = 1 sec
Time from ignitor voltage shut down to gas valve shut off = 2 secs
Then on to post-purge and shut down.

I think I basically have a race condition: will the gas reach the ignitor before the ignitor shuts down. The time overlap of gas at the output of the gas valve to ignitor shut down is only about a second. Since there's some finite transit time from gas at the output boss of the valve to gas at the burner tube that has the ignitor in actuality there is less time than that. Sometimes it works and sometimes it doesn't.

This is supposed to be a fast-open gas valve. It seems a little lazy to me but I don't know how quickly a fast-open valve really works. How quickly should it open? And can anyone explain why on earth the ingitor shuts down two seconds before the gas valve? That really seems to be the fundamental problem here.

I checked the area around the ignitor and there are no spider webs, dust, wasps nests, etc. The HSI is the original (the furnace is only two and a half years old). There is a single mounting hole for the ignitor and no positional adjustment is possible. It seems to be properly positioned.

If the controller board timings are correct and/or cannot be changed then I'm leaning towards getting a new gas valve. I'm assuming the gas valves are not field-repairable--correct?

Any other suggestions on what to check or measure?
 
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Old 02-03-09, 07:25 PM
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Good data collecting job, and a plausible analysis.


How do the times you report compare with your other furnace that continues to work OK?
 
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Old 02-03-09, 10:54 PM
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Originally Posted by BikerBill View Post
Time from ignitor voltage to gas valve click = 7 secs

In reading through the specifications for the operation of the ignition module again, the time specified for the igniter to heat up ranges between 17 and 21 seconds.

In your post above you say you measured this time as seven seconds, much shorter than it should be.

You might want to review the description in the "Heat Mode" section of the instruction manual and decide whether your module is leving the igniter on too short a time before turning on the gas and giving the ignition test.

If the igniter is truly on that short a time, it might be a good explanation of why it may not be lighting the gas.
 
 

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