furnace blowing all the heat up the chimney

[Stormin Mormon]

That's not good.

[Stormin Mormon]

Of course, that's totally possible. Bad fan limit switch. I've seen that before.

[dpb]

Isn't that what "One would presume those are .. input/output ratings" says??? :(

I gave an alternate outside chance possibility but clearly stated it as such...

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[dpb]

Well, certainly isn't here and it gets pretty cold as well...0F and below pretty common...

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[cshenk]

Yup. Deliberate too. It's to protect the pipes from freezing.

[dpb]

Well, here it isn't--they'll duct it normally w/ vents in used space. Seems a terrible waste otherwise to heat w/o any control all that volume.

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[dpb]

dpb wrote: ...

Anything built in the last 20 years or so anyway, that is. Back in the

60s and earlier one saw lots of things done that make no sense whatsoever under today's operating costs. That sorta' changed in the 70s and by 80s pretty much gone around here.

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[Stormin Mormon]

I reply to messages as I scroll through. I do not read all the replies before commenting.

[Tony Hwang]

Hi, To me the furnace is a bit undersized to begin with. First thing first, If blower motor has a speed tap, it should run slower than when in cooling mode.(warm air is light vs. cold air) Does cooling mode work OK in summer? Had a look at evaporator coil? Is it clean?

[Ed Pawlowski]

My first thought is that the unit is a bit on the small size for that house but I don't know the construction. That said, if it has been there for 10 years it must have done the job at some point. You should be getting more than 70 degree air While the stack will be hot, it may be much more than needed. Sounds like there could be a problem with the heat exchanger if heated air is getting by. I think what you really need as a more competent service tech. They should be taking the temperature of both the stack and the plenum.

[clare]

That furnace is NOT undersized if the house is reasonably well insulated. The only reason the furnace in my house is as big as it is, is I could not buy one smaller. Perfect size would have the furnace running full time on the coldest day to maintain temperature. My furnace has NEVER run over 8 hours in a day. - which means at 50,000 BTU it is TOO BIG. You loose a LOT of efficiency if the unit is too big.

[Tony Hwang]

Hi, If it is limit control problem, flame will be off with blower running. Easy to see it.

[dpb]

you should damn well at least read the message to which you _do_ reply.

[trader4]

What temp the fan cuts on and off at doesn't explain how he has only

70F air coming out of the vents with the furnace fired and running steady and why most of the house is cold.

[ransley]

It can be explained by a clogged AC coil, I saw that once at a friends house, we just removed it for the winter and he got heat until he had time to powerwash it. Has he or anyone even looked at his AC coil yet, nope I bet not.

[norminn]

Just for future reference, how would a clogged AC coil cause the issue? I barely understand basics of HVAC :o)

[Van Chocstraw]

Because the burner cuts off too soon.

[Stormin Mormon]

Both the heat air and the AC air go through the AC coil. If the AC coil is clogged, you won't get much air flow through the furnace.

[hrhofmann]

If the air flow is low, the over-temperature sensor may cut off the heat whether it is gas or oil-fired, causing not enough heat. It could also be a bad sensor, or a sensor that is set too low that is the problem.

[mkirsch1]

Nope, it's actually COLDER.