Replacing thermostat didn't work--now what?

[The Sanity Inspector]

A lightning strike last night blew out a transformer in our area. We didn't lose our power, but we must have gotten some kind of surge damage. I have a gas furnace and an a/c. The fan will not turn off now, even when I set the fan to "auto" and the thermostat to "off". I replaced the thermostat, thinking it had gotten fried, but I still have the same problem. Plus, the furnace is getting very hot to the touch, even though the chiller is working and cool air is coming out of the vents. What else could be the matter? Thanks.

-- bruce The dignified don't even enter in the game.

-- The Jam

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[Tony Hwang]

First thing I'd do would be to turn off the circuit breaker to reset the control logic board in the furnace. And see if everything comes back to normal. Did you replace thermostat after turning off the breaker? Think logic, first thing first in trouble-shooting.

[CJT]

It sounds like a fried furnace control to me. Probably the furnace and AC are both running at the same time. I'd start by shutting off the gas to the furnace.

[Tony Hwang]

Hi, Actually, by shutting off the circuit breaker and turning it back on will reset(master clear)the logic board. Then most likely it'll flash trouble code if there is something wrong. OP did not mention the model of his system.

[CJT]

I suppose that's true, if it has a "logic board" with trouble codes. In my furnace/AC, the control system is a couple of relays, spaghetti-wired.

[Tony Hwang]

Hmm, Then it's easier to fix compared to digital board. Digital board can be fied as long as micro processor and ASIC are OK.

[The Sanity Inspector]

Thanks for the responses so far. I should mention that resetting the breakers did not fix the problem either. I'm having to turn the fan off with the breaker switch, in fact.

I called Home Depot, and the guy in the electric department tried to walk me through a trouble-shooting checklist involving the

24-volt transformer and relay. But, I don't own the necessary gauges, so I'll probably have to call someone. Know what you're stupid at, I always say.

-- bruce The dignified don't even enter in the game.

-- The Jam

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[mm]

Regardless, I think you are right about shutting off the gas, because if the control sytem is fried, do you think we can count on it to turn off the furnace or the gas when it should?

It's not cold out. He can wait a while to turn the gas back on, if everything else is working.

[<alan_g]

On Mon, 26 Jun 2006 01:44:52 GMT, The Sanity Inspector wrote in part:

A relay probably has one or more pairs of contacts that have become welded. If the relay is socketed it should be able to be unplugged, taken to Home Depot, perhaps tested, and a replacement purchased.

Alan G

[Tony Hwang]

Hi, For ultimate safety, yes. But this logic board is not that dumb. The logic has a fail-safe feature. Micro processor is programmed that way.

[Ken Abrams]

"Tony Hwang" wrote

And I&#39;ll bet you call yourself a technician (or better yet, an engineer)!!

Let&#39;s put you out in the next thunderstorm and see how good your "logic board", "fail safe" and "micro-processor" functions after you are struck by lightning !!!

[mm]

I don&#39;t want to belabor this, but I remember that after I found the keypad/control panel of my burglar alarm smoking, I replaced it with a used one that my friend the alarm installer gave me (that he had removed when he put in his brand of alarm somewhere). It&#39;s a fancy thing with 2 IC&#39;s, and a lot of other parts, and lots of programmable options, It worked fine, except every two weeks or so (counting only times that I was home) it set off the siren even though the alarm wasn&#39;t even armed, and no one opened a door. (Should have disconnected it after the first or second time, but I waited until after the third.)