Finicky furnace

[charlieb]

I received an email from a past client today. Her home is a bit over two years old. This is an excerpt from her email.

"

The furnace was made by Goodman Manufacturing. Again, the thermostat clicks, the unit cuts on, the burner lights, and the blower begins. At times, it seems sluggish in going through the phases. Intermittently since November 06, the blower will stop with the unit still on in the attic. The blower comes back on again before the unit cuts off. Last winter, the blower speed was increased at an attempt by Climate Masters to correct the problem. The problem returned again this winter. Climate Masters said nothing could be determined and that I shouldn't worry. I had to schedule another appointment due to new noises made by the unit after the first tech person left. The second tech person said the first guy did not put everything back correctly. He reported that he couldn't find a problem either, but advised me to change the "limit". I had it changed, but it did not solve the problem. "

After speaking with her, I believe the blower is shutting down but the burners continue to fire. This sound like an unsafe condition. I plan to connect her with the manufacturer but.......................does anyone have any insight?

FL bound

[homnspector]

The circuit board is the only thing that makes sense to me. Even if the limit switch is bad the burner should shut down if the blower quits unless it is a bad limit switch plus a bad pressure switch which is unlikely.

[energy star]

The goodman thermostat (if one was used) Has no droop. Say if you set the T-stat to 70 and the temp falls to 69.5, the thing will come back on. kinda like it's short cycling. If that's not it, turning up the fan speed to much will move the air to fast through the supply plenum which will lower it's temp. When the temp in the plenum gets to low the blower will shut off, this will give the air in the supply plenum time to come back up to temp allowing the blower only to push hot air out the duct not cold. Dip switches are installed on the board so you can adjust the time before the blower comes on after the plenum heats up if it is a HE furnace.

If you post the Model #, I could post some questions for you to ask the next tech.

Check gas pressure

plenum temp when on

Dip switch positions

Hole behind t-stat (for low voltage wiring) allowing cold air to reach sensor.

Like I said try to get the model # and I could post some questions you could get the tech to answer. He gets 90 bucks for the call, lets get some data!

Goodman equipment is fine, many say it's not. It comes down to how well it was installed. The best equipment installed incorrectly will not function as it should. Also, check the flue size. I don't think that's it because it's installed in the attic.

[Terence McCann]

The first problem is that it's a Goodman - what junk.

Is she sure it's the main blower motor and not the induced draft motor?

I have a Goodman furnace in my condo. When it decides it doesn't like the flame rod any more the main blower stays on but the burners and induced draft fan cycles. It's very audible and to an untrained ear would seem like the "blower" is going off and on.

[inspector57]

First off, modern furnaces include delays, fan cycles, etc. designed to conserve energy and improve comfort.

Seeming "sluggish" is to be expected. Give it time to cycle through so long as it does it's job of heating the house, don't worry.

That said, it sounds like the thermostatic fan switch inside the furnace is cycling the fan due to a drop in air temperature in the furnace or the burners are not firing at their full rating.

This is NOT the limit switch which controls the gas valve, this is the fan switch which only cycles the fan on once the furnace has been on long enough to provide warm air and shuts down the blower once the warm air has been removed when the furnace is turned off.

This is likely more noticeable the colder the house becomes since the air returning to the furnace is cooling the furnace down.

Did the technician run a combustion analysis to make sure the unit is firing at full capacity?

Increasing the blower speed will exacerbate the problem, not help it. If anything a slightly lower blower speed is desireable in the heating mode.

[Terence McCann]

Originally posted by inspector57

That said, it sounds like the thermostatic fan switch inside the furnace is cycling the fan due to a drop in air temperature in the furnace or the burners are not firing at their full rating.

Not true. The indoor blower motor is controlled by a time delay relay. After flame proof the indoor blower starts around 30 seconds or so after.

When the call for heat is terminated it take about 2 minutes for the indoor blower to shut down.

It is not controlled thermostatically.

[inspector57]

Originally posted by Terence McCann

Originally posted by inspector57

That said, it sounds like the thermostatic fan switch inside the furnace is cycling the fan due to a drop in air temperature in the furnace or the burners are not firing at their full rating.

Not true. The indoor blower motor is controlled by a time delay relay. After flame proof the indoor blower starts around 30 seconds or so after.

When the call for heat is terminated it take about 2 minutes for the indoor blower to shut down.

It is not controlled thermostatically.

You could be totally correct since my knowledge is general and old, like me. From the description of the symptoms, it sounds like a thermostatic fan switch common to most older forced air furnaces.

Do we know the age and type of the Goodman furnace?

What is your guess as to the cause?

If as you describe the system, there would be only a couple of logical causes that I can think of, the control board or a blower motor cycling on the inherent protector.

[Terence McCann]

Originally posted by inspector57

Do we know the age and type of the Goodman furnace?

What is your guess as to the cause?

If as you describe the system, there would be only a couple of logical causes that I can think of, the control board or a blower motor cycling on the inherent protector.

From the original post Charlie said the home is 2 years old.

If it's truly the indoor blower motor then it's either the motor, the wiring going to the motor or a bad circuit board.

I'm leaning toward the induced draft motor/flame proof/differential pressure switch for draft motor proof.

[energy star]

I would check the installation of the stat. It's not the blower motor, it either works or not. Is it a Goodman stat? Something is telling the burner to shut off. Is the stat set to auto?

[charlieb]

As always, thank you for the responses.

I'm waiting to hear from the owner and will post the model info. I'm at IW so there's no dropping by.

I'll post the model info when she emails. I'll most likely take my AC guy out when I get back.

[charlieb]

Ok guys. The latest info.

Mr. Sessums,

I was finally able to pull panels off today and find the model number on the furnace. Goodman model number GMS81155CNA

There a light inside this model that remains solid if the unit is operating normally. Once inside the unit, I found a typed list of what it means if the light does other things. If it is off, there is control failure. Two flashes... pressure switch stuck closed. Three flashes...pressure switch stuck open. Four flashes... open high limit. Five flashes... flame sensor without gas value. Six flashes... open rollout. Seven flashes... low flame signal. Currently, the light is burning solid.

The flue size is 4 inches.

I don't know if the condo owner understood my description of what is happening. I don't know all the technical words. What I mean by the "blower" stops while the unit is still on upstairs... is that air is no longer blowing out of the vents in the house... but the unit is still running. It is almost like the unit is trying to cut off by going in reverse order. However once the air stops coming out of the vents and the unit runs for 15 to 20 seconds... the air blows out of the vents again before everything cuts off as it is supposed to.

Hope this makes sense.

Thanks again for any guidance.

[Terence McCann]

Well Charlie, from that description it sounds like an older style furnace with the bi-metal style fan control - much like what Jim was describing.

I was hoping find an installation manual, to look at the electrical schematic and see what the fan control was, no luck.

This web site had the manual but when I clicked on it the link came up broken.

I did see this picture

Click to View

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and there is some kind of limit mounted to the blower housing - not sure what it is - over temp limit?

I also ran across this part which is an integrated fan control but unfortunately no picture.

Without an electrical schematic I'm out of bullets.

[charlieb]

Terry, Jim and Bill,

This unit is in the home of a single mom who has faithfully called when she has any questions. I try to go the extra mile for folks like her. Since I'm at IW, I fell unable to give the level of service I like to give. You guys have been a help and allowed me to show her the commitment I have to clients. Thank you for the input. I've forwarded this info to my A/C man and he is going out to solve the problem. He is a good fellow and doesn't take offense to input.

have a great weekend.

[Terence McCann]

Originally posted by charlieb

Terry, Jim and Bill,

This unit is in the home of a single mom who has faithfully called when she has any questions. I try to go the extra mile for folks like her. Since I'm at IW, I fell unable to give the level of service I like to give. You guys have been a help and allowed me to show her the commitment I have to clients. Thank you for the input. I've forwarded this info to my A/C man and he is going out to solve the problem. He is a good fellow and doesn't take offense to input.

have a great weekend.

How's IW going Charlie? Would have liked to gone but it wasn't in the cards for this year ($$$).

Is Douglas Hansen there?

[charlieb]

Well it's all over.

Interesting IW this time. In response to your query, no Hansen. For that matter, no Cramer. It's his home turf too. A lot of the usuals were absent.

ASHI made a real effort to have some variety in the offerings this year. Not the same old stuff. Unfortunately some were real duds. Some classes also carried additional fees. A lot of hit and miss, if you know what I mean. I took the family so they could play in the parks. From what they saw, a LOT of guys were inspecting the rides instead of attending the classes.

The vendor turn out was way down. Not much free stuff this year. I did get my traditional blue screwdriver from Dearborn. Let's see how long I make it this year before I lose it.

Another change was changing the Gala to Friday night. Perhaps the thought was to try and capture attendees before they left town. Like N.O. the attendance was off. To meet the financial commitment, the continental breakfast became a full breakfast. For those that need a bit of lubricant before they dance......it was also an open bar. I can't tell anything about the night life.

To continue on the business of such an event, the economy is making it difficult to have event's like this. With decreased attendance and less vendor support, changes must be made. The society is working to be fiscally responsible. No lavish spending but lots of value for the attendees.

I would like to say that I got to meet a few folks I'd wanted to meet. And spending time talking to old friends is always a great fringe benefit. Sharing ideas and contacts will help keep 09 on track. 2010 will find IW in Vegas. I will not attend. It's a great location for lots of reasons but I'm not a Vegas kind of guy.