Lightning strike to furnace

[kylemeadows]

Hi all,

Lightning struck very near our home this past summer frying a lot of stuff. The furnace was not on (summer), but shortly after the strike my wife realized that the furnace had actually started running all by itself. Adjusting the thermostat did nothing, and since it was already warm enough she turned off the power to the furnace and we had a great summer!

Now that fall is here I suppose I should try to find the problem.

The two wire thermostat appears to be working (clicks on and off with appropriate continuity) but the furnace runs even with the thermostat disconnected.

The two wires (red and white) from the thermostat to the furnace appear unharmed and have no continuity between them.

I followed the wires to the furnace circuit board, disconnected them and found that the two spades they attached to test positive for continuity. My guess is that's not right, since the thermostat is supposed to control continuity, if I'm thinking correctly.

Here's a photo:

There are five black boxes attached to the circuit board, one or more of which may be fried and in need of replacement. Trouble is, I don't know what they are or what they are supposed to do. Am I even on the right track? Any advice?

Thanks a bunch.

p.s. It's a Goodman, model # GMNT080-4

[RegUS_PatOff]

Furnace model number ?

pictures are best when they're about 800 pixels wide MAX.

[kylemeadows]

Hi RegUs,

Thanks for the reply. I've edited my original post to reduce the image and include the furnace model #. It's a Goodman, model # GMNT080-4.

Thanks for any help!

[RegUS_PatOff]

GMNT080-4 Service Manual

I'll take a look ..

[kylemeadows]

I found a replacement control module. Would it be best/easiest to replace the whole thing for $135 or is it possible to isolate problem to smaller and cheaper part?

[RegUS_PatOff]

I think replace it for now,

lightning can do so much damage ..

The (5) black "boxes" are Relays and may be OK,

but one of the Integrated Circuits may have been damaged..

(28-pin device)

Keep the old one in case you need other parts parts .... Relays, etc ..

[kylemeadows]

Thanks RegUS. That's what I'll do. I appreciate your help!

[AmTec Services]

Agree that the best fix is to replace the board. If the furnace runs with no t/stat connected than the board is fried internaly. We replace a lot of them in Fla due to lightning.:shock:

..............................AmTec

[kylemeadows]

Thanks, AmTec. The board is on its way!

[kylemeadows]

I replaced the board and solved the problem of the furnace running constantly. Now I have found that the burners won't ignite. The ignitor gets bright orange, but never any flame. It tries to light 3 times then gives up. In reviewing the manual, I see there are at least 7 items to check. Any advice where to start? Is there a reset button?

Thanks again for all help.

[JumpSteady]

Make sure the gas is turned on.  Also some gas furnaces may not operate without the cover on.

[kylemeadows]

The gas was turned on, thanks Jumpsteady.

The burners wouldn't light because of a faulty Flame Rollout Switch ($4 at a local supply house), which I diagnosed with the help of the manual I downloaded from the link supplied by RegUS_PatOff in his previous post.

Thanks to everyone for all the suggestions. Furnace is now running nicely, house is warm and wife is happy! Plus, I saved probably $100 or more I'm guessing for a service call.

Cheers!