furnace not getting house up to temp: suggestions?

[Kyle]

When the weather here in the mid-Atlantic turned cold in the last month, we began experiencing a problem in our house we didn't have last year. The programmable thermostat will call for a temperature increase, the furnace will run for a few-to-several minutes, the temp in the house will increase by 1-2=B0 and then the heater will stop without getting close to the target temp.

In the past the furnace has run for ~5 minutes and then shut down for a cooling period while the exhaust fan still ran=97I'm assuming that's to keep the burners from overheating=97and then fire back up to continue to heat. But now the system just completely shuts down and doesn't fire back up again for long periods of time, sometimes a couple hours.

Another symptom is that we have the thermostat set to 65=B0 at night and to come up to 70=B0 at 6:45AM, but when we get up at 7AM the temperature in the house has been 62=B0. Why would it be 3=B0 colder than the minimum overnight temp and 8=B0 colder than what's called for?

What I need is advice on what the HVAC people should be looking for when they come out.

System info:

15 year old Trane gas furnace (don't have the model # at hand) Rite-Temp 6036 flush-mount programmable thermostat We had an AprilAire whole-house humidifier installed by a friend (HVAC guy) last year, and he blew the motherboard on the furnace installing it. We've since had two HVAC people out to service the furnace and fix the motherboard problems. We didn't have this heating problem last year=97the system worked just fine. Nothing has changed on the system since last year. I hard-reset the thermostat and reprogrammed all the cycles, and it still has the problem.

[Red]

Disconnect the humidifier to see if it is causing the problem.

[trader4]

Does the furnace have any LEDs that light up on the control board to indicate error conditions? If so, I'd start there. Also, I'd determine which two wires the thermostat closes to call for heat, remove the thermostat, and temporarily test it by connecting the wires directly together. If the furnace starts and continues to run normally, then you know it's the thermostat. If not, you've eliminated the thermostat as the problem.

[dadiOH]

Just tell them the same thing you wrote here.

[Bill who putters]

I had the "Gas valve" fail last year. Unit would call but fail to deliver except for time but would run with cold air forever.

Good luck

[Vic Smith]

Check out the expected life of the heat exchanger. Before you toss a few hundred here and a few hundred there at it, see what it cost to have it replaced. It's pretty old.

--Vic

[ransley]

I see you never had it checked out, it shouldnt have been shutting off with the exhaust fan running last year. Time you called a pro to clean and go over it.

[jamesgangnc]

That's the behavior of an overtemp safety device tripping somewhere. They are not designed to be tripped continually so after a while they trip easier and sometimes won't self reset.

Whenthe furnace is operating properly it will fire and run until the thermostat stops calling for heat. You can confirm it is shutting down prematurely by setting the thermostat at a very high point and then watching the unit. If it starts up but then after a short period turns off the burners and leaves the fan running then it's probably the safety device.

There should be a schematic in the unit and you can identify the safety devices. There may be more than one in series with the power to the gas control. Best test is to check things with a meter.

[Tom C]

This is a silly question. Don't you trust your HVAC tech?

[J Burns]

I don't want that to happen to me, so I've been keeping an eye on my heat exchanger. First I listen to the furnace, to hear if the flames make an even roar. Then I start it with the cover off to see if the flames look even and blue in the tubes.

Corrosion in a heat exchanger results in holes. They can make the flames jump around. I believe that's called rollout. A temperature sensor is supposed to shut off the gas before the house catches fire from flames going where they don't belong. As holes in the heat exchanger get bigger from year to year, that sensor could shut the furnace off sooner.

Another sensor shuts off the gas in case of overheating. If that's the one causing the shutdown, maybe for some reason your blower isn't carrying the heat away fast enough.

My control board has a red light. If I see it flashing, I count the flashes and look at the decal on the door to see what's wrong.

[mike]

I sure DON'T. I watched the idiot contractor install my furnace. Had to get them back several times to fix things. Reroute the gas connection so it didn't pull the ceiling down cause they were too lazy to use the right length pipe the first time. Several trips to fix leaks in the vent piping dripping water in the attic. Dripped water inside the unit and killed the igniter. Electrical work failed inspection. Still got drips I need to fix myself. Ditto for the insulation contractors. Guy was first day on the job and ripped out the insulation that they had put in two days earlier. They taped it up. Had to make 'em redo it so it wouldn't let moisture in. There may be a competent guy in the organization, but he didn't work on my systems.

Had windows installed. They ordered windows too bug and BROKE the house frame trying to make 'em fit.

All in all, I'm extremely lucky I had some leverage thru an intermediary. If it had been just me, I'd have been SOL.

Don't EVER hire anybody to do anything you can possibly do yourself. If you must hire someone, learn how to do it before hand and WATCH them every step.

[mike]

Has anybody mentioned the obvious: Check the filter??? Verify that the air volume and temperature outa the vents seems reasonable?

[clare]

Just out of curiosity, where do you live?? I know there are pockets around the country where you'd be better off in a REAL third world country when it comes to getting things done!!

[clare]

Do a temp rise check across the heat exchanger too.

[Home Guy]

Of the 3 standing-pilot furnaces that I have first-hand experience with in one way or another (with about 100 years of total service between them) none has suffered a thermocouple problem.

What's that?

For me - not yet.

A cracked heat exchanger is no big deal. Most people don't realize that because of the air-pressure differential between the combustion-side and the air-handling side of the heat exchanger, that a crack means that air will leak *into* (not from) the combustion side. Which means CO can't really leak into the conditioned side of the system.

Now if only these modern furnaces should live long enough to suffer exchanger corrosion...

[ransley]

If it is the overheat sensor realise what can cause overheating and ruin your unit,reuuced airflow from a clogged AC coil, poor or blocked ducts, Closing off registers, a bad gas assembly allowing in to much gas. The heat exchanger has a design temp range it can safely operate at. Go higher than that temp and most exchangers fail fast. You start by checking the AC coil to see if its clogged and air temp just above the heat exchanger. If you never had it cleaned and checked out by a pro now is a good time.

[trader4]

What do you think the efficiency is of that 34 year old furnace that you're so proud of? Yes new furnaces are more complicated and from most reports don't last as long. But saving $200 a year in fuel costs could make for a reasonable paybackm even at today's energy costs, which are almost certainly going higher in the future. Plus, with the current tax credits for high efficiency furnaces, if it were me, I'd be replacing that furnace right now. Of course a lot depends on where you're located climate wise, in terms of what the payback would be.

[Home Guy]

[e-mail] unnecessarily full-quoted:

Because all you sheep are spending thousands on delicate new computer-controlled furnaces with pathetically short life-spans, you're using less natural gas, which is keeping the price of natural gas low, so people like me with old furnaces can enjoy the benefit of low natural gas prices because of all you suckers with expensive new furnaces.

So you'll save $200 a year for 15 years, which is just enough coin to pay for the next furnace which you'll need to do by then.

No thanks.

I sleep just fine knowing that some sensor or controller or electronic motor isin't going to die on me at 1:30 AM on the coldest sunday morning of the year.

[ransley]

Gas consumption hasnt gone down. My neighbor said he saved about 200 in a few months with a new unit, your statements are untrue. But your unit may be very efficient, depending on its type if its maintained.

[clare]

To save $200 a year on heat you would need to increase the efficiency of my furnace by about 25% - and I don't mean incremental - like it's

80% now so you need to increase it by 25% (25% of 80) to 100% - I mean it's 80% now, you need to go to (80+25+)105%

Ain't gonna happen in this world. And mine is better than 80%.

Last year heat (including water heater) was about $700.