Gas furnace short cycling/no flame


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Old 03-03-06, 06:47 PM
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Gas furnace short cycling/no flame

Hi all, Hoping to get some help with my furnace issue.

It's a Weil-McLain HE-4, CP 1554462, natural gas hot water baseboard unit. It has a Hamilton Controls aquastat, a Fenwal 35-655600-003 control box and a RobertShaw gas valve.

Problem: Very frequently the unit short cycles where ignitor will glow, gas valve opens, flame on for about 30 seconds, flame out; ignitor on, gas valve opens, flame on, flame out, and so on. Eventually, the gas valve doesn't open, but the ignitor still glows, and it goes into ignition lockout. The exhaust blower continues to run. I usually find out when the house temperature goes to the mid-60's.

I also think the water isn't heating to 180 degrees. The temperature gauge is hard to read straight-on since it's behind the circulating pump, but it looks like it's getting up to 140.

I've had the igniton control box and gas valve replaced earlier this winter. It seemed to help for about a month, then same problem.

I even took a movie of it (click here - 56k warning, big file!)

Please help! My regular furnace repair guy is out of town all week!
 
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Old 03-03-06, 07:12 PM
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Short cycling

The most common cause for something like this is a restriction of some sort in the exhaust system.
If there is a pressure switch (usually has a small {+/- 1/4"} hose going to it), make sure the hose has no water trapped in it. Also check the venting system for any kind of blockage. If the boiler has a separate flame sensing rod (usually on the opposite end from the ignitor), remove it & clean with scotch brite or fine steel wool. Don't use sand paper.
 
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Old 03-03-06, 08:09 PM
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Does the furnace have a induced fan to 'pull' the combustion gases out and 'blow it' up the flue.? If so I would get it fired up and then jumper the pressure switch and see if it carries on, Other things to check are the flame rod, While it might be getting red it might not be holding the current in, ie it might have some crud on it, try cleaning it, or replacing it, I have a test rig which I connect with a switch and a diode which I connect to the sensor lead and can put the diode in circuit at will which simulates the sensor picking up the flame signal. All the sensor does is recifys the current from ac to a dc current.
Have you checked the gas pressure is constant during the operation?



Hi all, Hoping to get some help with my furnace issue.

It's a Weil-McLain HE-4, CP 1554462, natural gas hot water baseboard unit. It has a Hamilton Controls aquastat, a Fenwal 35-655600-003 control box and a RobertShaw gas valve.

Problem: Very frequently the unit short cycles where ignitor will glow, gas valve opens, flame on for about 30 seconds, flame out; ignitor on, gas valve opens, flame on, flame out, and so on. Eventually, the gas valve doesn't open, but the ignitor still glows, and it goes into ignition lockout. The exhaust blower continues to run. I usually find out when the house temperature goes to the mid-60's.

I also think the water isn't heating to 180 degrees. The temperature gauge is hard to read straight-on since it's behind the circulating pump, but it looks like it's getting up to 140.

I've had the igniton control box and gas valve replaced earlier this winter. It seemed to help for about a month, then same problem.

I even took a movie of it (click here - 56k warning, big file!)

Please help! My regular furnace repair guy is out of town all week![/QUOTE]
 
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Old 03-04-06, 06:07 AM
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Thanks for the tips. Definitlely not an obstructed flue. Checked the ducts inside and looked down the chimney from the top, clean as a whistle. The pressure switch (a small circular thing with one red and one white tube from it) seems OK. I took the tubes off and they looked clean and dry. Is replacing the pressure switch a good idea?

There's no flame sensing rod as far as I can tell. Just the ignitor. I do get flame, it just goes out quickly. Sometimes 10 seconds, sometimes it stays on for a minute or more.
 
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Old 03-04-06, 04:16 PM
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No flame rod

Some equipment used the hot surface ignitor to sense flame & I have seen them glow to light the flame but would not sense the flame was lit. To be sure there is no flame sensor, check the ignition control for a terminal marked "sense", "sen", or something similar. A bad ground can also cause all kinds of ignition or flame sensing problems.
 
 

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