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Discussion Starter · #1 ·
I have a new (only a couple months old) goodman ghm81005cn 2 stage 100,000 btu furnace with a propane conversion kit (55 orafices). When the propane gas pressure manifold propane gas pressure is at the proper amount (9.7 in w.c. to 10.3 in w.c.) the unit shortcycles after about 30 seconds. If I drop the propane gas pressure down substantially, it will stay on, but doesn't heat the house very well at all.

I've tried bypassing the high limit or "temperature" switch with no luck.

While there is some soot, the piping that removes the carbon monoxide is clear with no obstruction. I've had certified technicians try testing the pressure switch on the combustion motor, they tell me that's not causing the issue.

The flame sensor's been cleaned, no effect on the problem, and as previously mentioned the unit stays on at low gas pressure. So that shouldn't be the problem either.

I've already replaced the circuit board, with no effect on the problem.

There's no blockage in either the air going to (the filter is brand new) or going from the unit (all vents are open and unblocked and the unit is properly sized).

The incoming propane pressure is properly set to 12 in of w.c.

The circulator blower is running at full strength.

There are no error codes that show up on the circuit board.

I've had 6 different technicians out to try to troubleshoot the unit. Despite a lot of testing, none can get resolve the problem.

Any help is greatly appreciated. This is driving me crazy and my house has been cold all winter.

Thank you very much.
 

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Never bypass limits.

Don't change more parts.

Considering there's soot, don't run this unit (except for testing) until verified to be burning properly.

When it fails, is there an error code? Does the inducer stay on?

Exactly what happens?
 

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Discussion Starter · #3 ·
Sorry, I meant to say the certified technicians bypassed the limits and it did not help resolve the problem.

As initally mentioned in the first post, no error codes showing on the circuit board. Since there are no error codes on the circuit board, the unit just cycles indefinitely.

There is a small layer of soot due to initially using the wrong orafices (45) on the unit causing the propane to burn improperly, but that issue was fixed.

Inducer does stay on, I believe.

Never bypass limits.

Don't change more parts.

Considering there's soot, don't run this unit (except for testing) until verified to be burning properly.

When it fails, is there an error code? Does the inducer stay on?

Exactly what happens?
 

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Sorry, I meant to say the certified technicians bypassed the limits and it did not help resolve the problem.
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There is a small layer of soot due to initially using the wrong orafices (45) on the unit causing the propane to burn improperly, but that issue was fixed.
Find another contractor.
 
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Discussion Starter · #5 ·
In regards to exactly what happens... I'm not exactly sure of the order of events, but the circulator blower comes on. The electric pilot light heats yellow, the propane lights in the burners, it goes for about 30 seconds. The gas valve clicks and then the burners shut off. Then the process repeats.

Never bypass limits.

Don't change more parts.

Considering there's soot, don't run this unit (except for testing) until verified to be burning properly.

When it fails, is there an error code? Does the inducer stay on?

Exactly what happens?
 

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Discussion Starter · #6 ·
I'd love to, but do you know how incredibly expensive a technician is? At least $100 a visit and that doesnt include the cost of any actual work, just diagnosing the issue, and they dont gaurentee to even fix the problem. I've already already called 6 certified technicians and been billed 6 times. And the problem still isnt any better. Why would I call a 7th? It's crazy. I need to get the problem fixed, not to single handedly pay the yearly salary of a technician.

Sorry, I meant to say the certified technicians bypassed the limits and it did not help resolve the problem.
----------
There is a small layer of soot due to initially using the wrong orafices (45) on the unit causing the propane to burn improperly, but that issue was fixed.
Find another contractor.
 

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Where's the original contractor in this?

You shouldn't have had to pay to fix a 2 month old furnace, especially if it never worked right since day 1.

You shouldn't have been billed 6 times attempting to solve one problem.

Could be anything - bad connection, marginal ground, pressure switch momentarily opening, call for heat getting interrupted for a fraction of a second, etc.

If the interruption, say a pressure switch opening is very short, the board may not pick it up and flash an error code.

A competent tech doesn't actually need error codes to troubleshoot.



Need a multi-meter that reacts fast, can read dc microamps and potentially a manometer to troubleshoot properly.

I would start by verifying proper line and low voltage to the unit, jumping R and W on the board to bypass the stat. If it continues to shut off after 30 sec, move on to things like checking the draft/how pressure switch is reacting, flame current.
 
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Verify the line voltage is connected with the proper polarity... I have a hard time believing that would make a difference but have heard that it does on certain units... worth checking. Hot to black and neutral to white and green to metal housing.

Verify the flame sensor is actually well into the flame at all times. Also that it's free of soot.

It's odd that no error code is being set on a repeating short cycling unit. Call the manufacturers help desk and give them all the details. If they can't help, they may at least put you in touch with their authorized service company in your area. A qualified tech should be able to fix this on a single trip.

I suspect you may have installed this yourself and the local service techs are miffed that you bypassed them and may not be giving you their best effort.
 

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Discussion Starter · #9 ·
I cant get ahold of the orginal installer any longer.

I tried 6 different technicians because each technician kept telling me different solutions. And they were all SURE they were right, so sure. So I'd try their suggestion, ie... new circuit board, change the propane regulator and so on... and when it didnt work I'd they'd either give up and just tell me to pay them $1500 to install a new unit or I'd try to find someone more competent (with no luck)

Why would bypassing the thermostat help? If there was a problem with a bad connection, marginal ground, or call for heat interruption, why would the unit work properly when simply adjusting the gas valve pressure lower? Wouldn't these issues cause a problem regardless
Of the gas valve pressure setting?

While I suppose it could be a pressure switch issue, multiple techs said they checked that possiblity. In addition is the pressure switch somehow connected to the manifold gas pressure? Because the Inducer and pressure switch work fine as long as the manifold propane gas pressure in low.

When you say flame current, I assume you mean whether the flame and gas pressure is steady when the burners are on. I suppose there could be very brief momentary changes in the incoming propane pressure causing the issue, but all other appliances seem to be working fine. And I'm not entirely sure what would cause an issue like that. The some of the propane gas pipes were already replaced (because a tech thought they were too small) and the propane regulator was replaced (because a tech thought that was the problem). And the incoming propane pressure is sitting at about 12 in of water column. And manifold pressure is at 10 in of water column.

Where's the original contractor in this?

You shouldn't have had to pay to fix a 2 month old furnace, especially if it never worked right since day 1.

You shouldn't have been billed 6 times attempting to solve one problem.

Could be anything - bad connection, marginal ground, pressure switch momentarily opening, call for heat getting interrupted for a fraction of a second, etc.

If the interruption, say a pressure switch opening is very short, the board may not pick it up and flash an error code.

A competent tech doesn't actually need error codes to troubleshoot.



Need a multi-meter that reacts fast, can read dc microamps and potentially a manometer to troubleshoot properly.

I would start by verifying proper line and low voltage to the unit, jumping R and W on the board to bypass the stat. If it continues to shut off after 30 sec, move on to things like checking the draft/how pressure switch is reacting, flame current.
 

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Why would bypassing the thermostat help? If there was a problem with a bad connection, marginal ground, or call for heat interruption, why would the unit work properly when simply adjusting the gas valve pressure lower? Wouldn't these issues cause a problem regardless
Of the gas valve pressure setting?
It's unlikely to be the thermostat or wiring but it's best to start with the basics and rule out anything external that could cause problem.

The gas pressure can impact pressure in the heat exchanger as well as flame signal.

When you say flame current, I assume you mean whether the flame and gas pressure is steady when the burners are on
No - there's a flame sensing circuit and the current should be checked.

Meter is set to DC microamps and wired in series with the flame sensor.

While I suppose it could be a pressure switch issue, multiple techs said they checked that possiblity.
How was it checked?

Voltage drop across it needs to be monitored with a meter that responds fast.

If it's opening for a fraction of a second, pressure it's seeing needs to be checked.

If the pressure is normal and steady, the pressure switch is bad.
 

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Contact Goodman and demand a new furnace under their warranty. Your original contractor should have done it if he couldn’t fix it.
 
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manufacturers do not deal with end users and the problem may be related to the installation, not a defective furnace itself.
 
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What is the model # again?

I Googled it and nothing matches.

If you ran it with incorrect orifices and it produced soot in the heat exchanger and secondary heat exchanger then they may be partly plugged.

If the manifold pressure is where it should be and you have a partly plugged heat exchanger then the sheer amount/volume of fumes could trip the pressure switch quickly.

A fluttering pressure switch can be hard to diagnose.

If you continue to run it with a lower manifold pressure you are seriously fouling it up/sooting and plugging it up. The damage may be done>> you have a partly plugged heat exchanger.

Reversed polarity will prevent it from running at all. Some newer units have a error code for that.
 

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Discussion Starter · #15 ·
Ghm81005cn

What is the model # again?

I Googled it and nothing matches.

If you ran it with incorrect orifices and it produced soot in the heat exchanger and secondary heat exchanger then they may be partly plugged.

If the manifold pressure is where it should be and you have a partly plugged heat exchanger then the sheer amount/volume of fumes could trip the pressure switch quickly.

A fluttering pressure switch can be hard to diagnose.

If you continue to run it with a lower manifold pressure you are seriously fouling it up/sooting and plugging it up. The damage may be done>> you have a partly plugged heat exchanger.

Reversed polarity will prevent it from running at all. Some newer units have a error code for that.
 

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Consider going to the HVAC Talk forum. There are professionals there who may have run across this issue. And they will NOT tell you how to fix it but may come up with some suggestions.
 

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I found a GMH81005 furnace which is a mid efficiency.

Do you have a chimney with it or plastic vent pipes to the outside.

Either way if it is a mid efficiency unit and the heat exchanger is partly plugged ( you should never see any soot in a furnace ) then that may be your problem.

Goodman likely won't help you and with AC units they insist on a licensed HVAC tech start it up and balance the charge and you need proof of that.

Point is they have no control over hackers who oversize furnaces and have undersized ductwork and improper venting which causes problems.

With dealers they have a basic baseline of knowledge and can assume it is installed properly. Not so with DIYers. Not saying a DIYer cannot do a proper job but how can Goodman know it is not a install problem?

Just that way it is.
 
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