Furnace won't run past 63-65 degrees

[pburke]

sorry for the long post, but I've been stumped trying to figure out what has been going on with my furnace lately:

I had a service tech at my house about 2 weeks ago after my furnace started acting up and he didn't find anything wrong with it - cleaned the flame sensor, burners, made sure the drain was open, and billed me an hour of mostly head scratching. Since the furnace was cold that day it ran just fine whenever he tried...

The details:

1986 Trane XL90 gas furnace, updated about 10 years ago with new heat exchanger and some other parts, new igniters, etc. Controlled by programmable thermostat. It usually runs just fine for early and mid winter, but over the last few years it has had the tendency to stop heating short of reaching the thermostat setting once we hit the late winter early spring heating season. Not often, but sometimes. It didn't do it at all last year, but this year it started early and as of this week it now does it all the time.

Details: The furnace runs great when the house is cool for the entire day (set to 55 degrees). It kicks in at 5:45pm, runs without a hickup until the house reaches about 62-65 degrees, which is when the flame stops, the exhaust fan keeps going, and it makes no attempt to refire. There's no LED light blinking, and it will sit there spinning the exhaust fan for hours unless I turn down the thermostat and turn it back up, or simply cycle the power on the furnace.

Now, once I do that, and the furnace is still warm, it will light the burners, but they usually will shut off within 10-60 seconds, putting the furnace back into the state it was in before, exhaust fan spinning. I think once in a while I have heard it actually try to refire, many many minutes after it cut the flame, but usually it doesn't.

So then I turn the furnace way down, let the house and everything else cool for an hour or two. After that, the furnace will fire up and start heating, again for quite some time, but once it hits 63-65, it'll shut off again. On a sunny day, when the house is already at about 65, I have been able to push it to 67...

Is the furnace overheating internally? I have checked if the AC coil (on top) is dirty, but I cannot access the business side of it. The top of it is spotless. It has been a while since that has been cleaned, but I don't think that's the problem. Air flow is significant, and removing the regular high efficiency filter to increase air flow didn't change a thing. Also - why will it "overheat" even after idling for an hour, burning for 10 seconds and then shutting off again, when the furnce is pretty much cooled down and doesn't even get a chance to warm up, while it can run up to 67 degrees house temp on other days?

I'd like to hear some suggestions as to what to do next to determine what's possibly broken, so I can determine if it's time to buy a new unit or pay for whatever it takes to make the thing work again. I'd like to avoid getting a new furnace for a year or two since I want to add on to the house and I may need to get a different size furnace at that stage anyway.

Ideas?

Peter

[docholiday]

We cant see for sure why it is turning off from here but it could be a high temperature limit tripping, could be apressure switch opening. My first inclination is a limit tripping issue since you already had to replace the HX once. I would consider a different techncian as it sounds like this one didnt bother to check temperature rise or any other obvious conditions that might explain your problem. Call the same company and explain your problem, ask for a different tech and suggest that you should get your previous hour credited toward the second call or call back.

[pburke]

the old HX was replaced due to a factory safety recall - never had a problem with the furnace before that happened. And guess what, the same people who now can't find the problem did that swap.

could gas pressure be an issue? This town has been growing beyond it's dimensions, as I have barely any water pressure left during peak hours. Just a guess, but maybe similar stuff may be happening in the gas supply lines. On the other hand, it burns fine in the am and pm peak reheating hours. Things only go bad once it hits those 60+ temps upstairs, no matter where I start heating (from 50 or 61 degrees).

Peter

[BaldLoonie]

That oldie wouldn't tell you anything by flashing LED. All it told was control internal fault or not. The limit was an old Honeywell L4064. A tech can take the cover off and see what temp it is reading. They can stick though not near as likely to as todays $2 cheapos.

I'd suspect something in the drain system. By powering down, could be causing the drain to clear and when started back up, it runs. Other possibility is locking out due to flame sense issues, clean the flame rod.

[MikeJ]

Obviously, something is not letting the furnace continue operating all the time. If you had the tstat set at 68 and the furance stopped at 67, well, that would be normal. The residual heat will cause a one or two degree rise so most tstats shut off early too avoid heating above the set point.


"So then I turn the furnace way down, let the house and everything else cool for an hour or two. After that, the furnace will fire up and start heating, again for quite some time, but once it hits 63-65, it'll shut off again. On a sunny day, when the house is already at about 65, I have been able to push it to 67..."


Sunny day? you probably gain a couple of degrees from the sun heating the room.

Let's say you have a cool house, 55-60 degrees. Set the tstat to about 72 (or higher). Now, the unit should start up and continue on until that temp is reached. If it doesn't, there is something that is shutting off the gas to the burners and a good tech should be able to find out what it is. If it is in a lockout, leave it that way until a tech arrives. It is easier to diagnose a non-working locked out unit than one that has been reset.

A temp rise reading can rule out overheating due to airflow problems. Use the search icon on this site for info.

I also would suspect a condensate issue. Have this checked as well as airflow. Short run times don't seem to be a problem, only long runs, is that correct?

[MikeJ]

Obviously, something is not letting the furnace continue operating all the time. If you had the tstat set at 68 and the furance stopped at 67, well, that would be normal. The residual heat will cause a one or two degree rise so most tstats shut off early too avoid heating above the set point.


"So then I turn the furnace way down, let the house and everything else cool for an hour or two. After that, the furnace will fire up and start heating, again for quite some time, but once it hits 63-65, it'll shut off again. On a sunny day, when the house is already at about 65, I have been able to push it to 67..."


Sunny day? you probably gain a couple of degrees from the sun heating the room.

Let's say you have a cool house, 55-60 degrees. Set the tstat to about 72 (or higher). Now, the unit should start up and continue on until that temp is reached. If it doesn't, there is something that is shutting off the gas to the burners and a good tech should be able to find out what it is. If it is in a lockout, leave it that way until a tech arrives. It is easier to diagnose a non-working locked out unit than one that has been reset.

A temp rise reading can rule out overheating due to airflow problems. Use the search icon on this site for info.

I also would suspect a condensate issue. Have this checked as well as airflow. Short run times don't seem to be a problem, only long runs, is that correct?

[pburke]

Originally posted by MikeJ
Obviously, something is not letting the furnace continue operating all the time. If you had the tstat set at 68 and the furance stopped at 67, well, that would be normal. The residual heat will cause a one or two degree rise so most tstats shut off early too avoid heating above the set point.

Sunny day? you probably gain a couple of degrees from the sun heating the room.

Let's say you have a cool house, 55-60 degrees. Set the tstat to about 72 (or higher). Now, the unit should start up and continue on until that temp is reached. If it doesn't, there is something that is shutting off the gas to the burners and a good tech should be able to find out what it is. If it is in a lockout, leave it that way until a tech arrives. It is easier to diagnose a non-working locked out unit than one that has been reset.

A temp rise reading can rule out overheating due to airflow problems. Use the search icon on this site for info.

I also would suspect a condensate issue. Have this checked as well as airflow. Short run times don't seem to be a problem, only long runs, is that correct?

when working normal, the furnace usually shut off at 71 when the tstat was set for 70. Now it will go to anywhere from 61-65, higher on sunny days (and yes, because it's sunny the house starts out warmer when it kicks in - indicating it's probably not "overheating" at a certain temp of intake air).

Leaving it in the "locked state" (that is exhaust fan working, blow fan off once it cools off, and never restarting), is somewhat of a difficult proposition when it's COLD and you know you can restart the heating at least for a few degrees. However, now the furnace has become so "reliably broken" it should be easy to get it into that state.

Tech checked/cleaned flame sensor, blew air into the drain vent (with his mouth - yuk!), and pretty much didn't do anything else. After that visit things worked for a week, now it's worse. I have a hunch that water drain may be more likely to be the issue here than overheating due to bad air flow, since it will run for 30-60 minutes after it has been sitting for a few hours, but it will then not refire for more than a few seconds unless you wait an hour or more. Slow drain maybe. I'll check that today myself. If that doesn't help, I'll call somebody over and make sure the furnace is in its most stubborn state when he gets there

Thanks for all the pointers

Peter

[tinknocker service tech]

i would recomend calling back and have the tech came back out and look it over again
only this time turn it on before he gets there so that maybe the furnace will act up before he gets there and he can see what it is doing then or after he is there and can see this happen and why.if the problem only happens when it has been running a while and he looks at it cold and waits for it to warm up it still works fine. sometimes it is hard to find a problem when it doesnt show while we are there. called intermitant problem. doesnt mean he doesnt know what he is doing means he doesnt want to guess and and be wrong. call him back turn it on way before he gets there and see if that helps

[MikeJ]

It's getting to sound like a problem with the secondary heat exchanger as BaldLoonie was indicating. Not just a plugged drain line after the furnace but inside the furnace. This could cause the symptoms you have described.

May need to get out the scalpel.

[MechAcc]

The way you describe the problem it sounds like the water is trapping somewhere either in the drain pipe outside the furnace, the drain trap inside the furnace or vent pipe.



Make sure the drain piping/hose is clear all the way to the drain by flushing it with a garden hose. If it is a hose make sure you don't have any heavy objects on top of it that would reduce the flow. If it is a long distance to floor drain you might want to get a condensate pump installed.

Clearing the drain trap inside the furnace is sometimes a bear. By connecting a wet/dry vacuum on the outside and capping the vent openings on the breaking tees sludge and debris can sometimes be pulled through and clear the trap.


Double check the vent pipe ensuring no sags causing water traps. And that it has adequate pitch down back to the furnace.

The inducer on this model is 2 speed. Normal operation is high speed on start up goes through lite off and then times to go to low speed. Normally if the low switch opens the inducer relay will ramp back to high speed then time back to low speed. If the pressure switch opens again it will repeat the procedure. Have you noticed any operation like this?

[Edited by MechAcc on 03-18-2005 at 04:58 PM]

[pburke]

Originally posted by MechAcc
Inside the front cover at the lower left side is the ignition control. Is it grey in color or white with a red LED? If it is grey get the KIT2119 installed which will upgrade it to the white control, new flame sensor, igniter and wiring.

it's white with a red LED on the bottom corner.

Originally posted by MechAcc

Make sure the drain piping/hose is clear all the way to the drain by flushing it. If it is a hose make sure you don't have any heavy objects on top of it that would reduce the flow. If it is a long distance to floor drain you might want to get a condensate pump installed.

I know if it exits the furnace, water will flow freely. I will need to check inside though. There's a new PVC hose on it but I can't tell what's happening inside the last few inches. All that piping was re-done when the HX was replaced.

Originally posted by MechAcc

Make sure that the blower is installed all the way in and secured with a screw. If it isn't installed correctly the limit will trip.

Blower is fine - two screws at the front

Originally posted by MechAcc

The 2 speed inducer relay board maybe going bad causing intermittent problems.

hmm - that little blower seems to go to the lower speed much sooner than in the past. However, it doesn't do that any different in those startups that are successful following a long cool down.

Originally posted by MechAcc


Did someone install or replace an air conditioning coil? Trane requires that at a minimum that there must be a 1-1/2 inch clearance between the bottom of the evaporator coil and the top of the furnace. If not the furnace will trip limit.

The AC coil is what was in the house when we bought it. I looked in there hoping to find access to see if there was too much dirt on it, but I can't see the bottom of it. It seemed to be resting very close to the top of the furnace, but I'll double check. It's always been there, so I don't think this would suddenly pose a problem.

Originally posted by MechAcc

Was the heat exchanger reinspected for possible cracks?

not that I could tell.

Originally posted by MechAcc


The thermal roll out fuse on these will sometimes momentarily open causing the system to recycle.

it won't recycle when it fails - flame shuts off, small exhaust blower continous forever.

Originally posted by MechAcc


If you have the Trane BAYSTAT thermostat and it is the mechanical type with the magnetic reed switch sometimes the contacts on these go bad as well.

Double check the vent pipe ensuring no sags causing water traps.

Thermostat is a 3 year-old programmable aftermarket unit that seems to do exactly what I tell it to do.

I'll check the water drain situation thoroughly tonight.

thanks

Peter

[pburke]

Originally posted by MechAcc

The inducer on this model is 2 speed. Normal operation is high speed on start up goes through lite off and then times to go to low speed. Normally if the low switch opens the inducer relay will ramp back to high speed then time back to low speed. If the pressure switch opens again it will repeat the procedure. Have you noticed any operation like this?

it is 2 speed - starts up fast, drops in speed before the flame ignites, and generally stays at that speed until I cut back on the thermostat setting below actual room temp. If I don't turn the thermostat back, it will continue spinning at low speed.

maybe related - a few months ago I heard some squeaking noises come from either the motor or the assembly driven by it after a cold startup. Usually just for a moment until the thing was up to speed, but it never did that in the past (nor is it doing it now).

Peter

[tinknocker service tech]

pburke

think i missed something here are you doing the repair because we can not possibly tell you what to do to fix this problem. we can give ideas and thoughts about it. thying to repare this unit and using us for tech advice is like swimming arround in the middle of the ocean with a staw for a life preserver and thinking you wont drown. if i am wrong and asuming this please i am sorry and dont want to upset you. this unit can be tuff to find something like this problem and if someone makes a mistake with any one of the controls it can fire without any reason or will not shut down the gas valve when it is sopose to
please for your own saftey do what is in my first post
if it was up graded to the wr control from the fenwal and the rewiring was wrong the same probem could happen
let the service tech look at it again

[MikeJ]

From Burkes posts:

I had a service tech at my house about 2 weeks ago after my furnace started acting up and he didn't find anything wrong with it - cleaned the flame sensor, burners, made sure the drain was open, and billed me an hour of mostly head scratching. Since the furnace was cold that day it ran just fine whenever he tried...

I'd like to hear some suggestions as to what to do next to determine what's possibly broken, so I can determine if it's time to buy a new unit or pay for whatever it takes to make the thing work again. I'd like to avoid getting a new furnace for a year or two since I want to add on to the house and I may need to get a different size furnace at that stage anyway.

****
Tinknocker:
I don't know how you can say;

"think i missed something here are you doing the repair because we can not possibly tell you what to do to fix this problem. we can give ideas and thoughts about it."


this whole business of clammin up when a homeowner dares to ask a question about doing things to their furnace that the very manual from the manufacturer says they should do amazes me. Yes, it is not supposed to be a step by step instruction site. Let Bob Villa do that. I for one, more often than not, end up referring the customer to a qualified service company rather than do it themselves.

This guy is operating and monitoring his unit, which is not working as it should but is not old enough for the scrap heap. And he is posting questions here to understand the operation and be prepared for the technician when he returns.

That's the way I read it. Unfortunately, a solution has not yet been found. I think a tech is going to have to dismantle the unit a bit and do some exploring. (Scapel)

[htinglky]

I dont suppose the tech did a volt check to see where you lost the 24volts exactly did he?
found a lot of 90% over the years lose a little piece of silicon or sludge or whatever, especially after hx replacement, act like a little flapper inside the drain
and guess what happens. Let a tech deal w/ it

[pburke]

I took off the hoses, plugged the top drain from the AC with my thumb and attached a dust spray air can to the main drain with duct tape, a few pushes on the trigger, removed the ir can, and the water started flowing... for quite a while.

House now at 71, furnace running just fine. I wouldn't have done anything fancy myself, but I figured I can try that level of a fix. If it plugs up again, I at least know what is wrong.

Thanks again - almost too warm here now after a week below 65 degrees

Peter