[meredithcaudill]

I have an old Honeywell gas furnace with the old mercury limit switches..the fact that I've learned that much about it and can keep the belts from squeaking and change filters is the extent of my hvac knowledge. Recently the blower suddenly decided to run constantly. The furnace heats just fine, but it will blow cool air when the heat stops. The thermostat does not appear to be the issue as a) I do know its not in the "on" position but rather the auto, and my husband has replaced a thermostat before so he said it doesn't appear to be any kind of short there. http://www.heatinghelp.com/files/articles/768/216.pdf is a copy of old manual I found online, but the limit switches seem a lot different than new stuff out there, so I don't want to be messing with them. The LA412 seems to be what I have...

I am desperate, we can't afford to replace a furnace

 

[gregzoll]

If your unit is that old, it is time to place it out of its misery. It is not that it can be fixed, the problems lie in that the exchanger can crack once they get really old, along eith the fact that parts are non-existant 99% of the time.

 

[joecaption]

Can you afford not to replace the whole thing should be the question.

 

[how]

It could be a part change or just an adjustment.

Inside your furnace will be a fan/limit control. It controls the temp at which the fan comes on & off. It will usually have three (sometimes four)wires connected to it and often has a white manual summer fan button. It is usually located inside the furnace above the burners.
The controls are adjustable and it sounds like the lowest temp nib might need to be adjusted higher to shut off the furnace fan at a higher temp.

Locate it and take a picture to confirm. Turn off the furnace power and pull off it's outer cover to expose the inside settings which should also be photograghed.

 

[hvac5646]

If furnace is old as you say the fan limit control could be jammed in the run position. If you can check the resistance across the fan terminals and they show zero (or very close to zero) than the control is in need of replacement.

 

[yuri]

They stopped making that one 35-40 yrs ago. The replacement is quite different and I doubt you will find anyone old enough or experienced enough or wanting to work on that old dinosaur. Mainly because it has old brittle wiring which will fall apart once moving it and the new control is a combo fan and limit control. I did hordes of them yrs ago and if I had to do one today it would cost at least $500 to replace and that is w/o any major rewiring. Time for a new furnace as the gas valve and other parts are probably obsolete and there are serious safety concerns witrh cracks in the heat exchanger etc and that old bird producing CO Carbon Monoxide. Some gas companies have reasonable financing rates and you may be able to finance one and get it installed and pay it on your gas bill.

 

[how]

Yuri
A check of the heat exchanger, a $100.00 fan/limit and an hours labour if your replacing the wiring as well means your labour rates are $350.00 plus an hour???

 

[yuri]

Huh,

Do you sell your parts at cost and give away your services?

We double ours so the control is $200-$250, plus the service call etc

 

[how]

@Yuri

They say they are money desperate. I think I'd like to wait on a photo to see if a simple fan temp adjustment might solve their problem.
or pulling out the summer fan switch
or replacing the fan/limit control

but a heatexchanger check is really the important point.

Still think someone is charging you too much for your parts..

 

[hvac5646]

@Yuri

They say they are money desperate. I think I'd like to wait on a photo to see if a simple fan temp adjustment might solve their problem.
or pulling out the summer fan switch
or replacing the fan/limit control

but a heatexchanger check is really the important point.

Still think someone is charging you too much for your parts..

Yuri is a true businessman. That's why he has a top tier Lennox dealership.
He's not going out of business anytime soon cause he knows what it costs to stay afloat and make a profit.

Uncle used to charge ridiculously low prices when he first started out 'cause he thought that was what made a business. For reasons that those-in-the-know are aware of, he was running long hours with little to show for it.

He soon changed his ways and started making a profit with a smaller client base.

End of life lesson.

 

[how]

@hvac5646

Wasn't criticizing Yuri, just questioning the math of his argument. Don't know if he's lennox's man anymore but I am often impressed with his recall of specific furnace problems.
Most op's come here with questions and not all these answers should fall to the prerogative of a business mentality.
I think it's the diversity of all our hvac experiences that is the real depth of the gift here, not that we all sound the same.

 

[meredithcaudill]

We found a great local guy with knowledge of older furnaces, problem we've found is need to replace old Mercoid Switch, this particular item is no longer available. Are there good resources for finding what parts have replaced these old mercury switches? We are tentatively planning to call Dwyer, the folks that now own Mercoid.

 

[jagans]

I think Yuri already said this, but you could probably take a loan, have a high efficiency furnace installed and pay for the loan with the savings in fuel. Someone on this forum said that mercury switches are very dear for obvious reasons, and purchasing them would be spending money that might go down the drain rapidly.

Just something to consider.

 

[yuri]

He cannot be that great if he does not know that Honeywell STILL makes the L4064 combination fan and limit control easily obtainable. Need to measure the length of it and depending on whether the duct is flat may need an adaptor which i believe is obsolete to mount in on a sloped duct. Used to come with the control but there is VERY little demand now so they stopped giving it away.

http://www.nextag.com/honeywell-l4064/products-html