Goodman Furnace - High Limit

[BrewerFan]

Hi all, hope you had a great Thanksgiving!

Last winter, I noticed that our furnace (2005 Goodman, GMS90904CXA) was triggering the high limit on fairly regular intervals (every 15 minutes or so), in which it would take about 4 minutes to reset. Once the limit is reached, the blower fan continues to run, but no heat is provided since the burner has stopped in order to allow the heat exchanger to cool.

We’ve lived in the house (built in 1989) since Jan 2008, but I never paid much attention to the furnace before last year, so the issue very well could have been occurring the whole time we’ve lived in the house…

So, in order to gear up for this winter, I ended up having an HVAC guy come out a few days ago to perform a tune-up and troubleshoot the high limit issue, in which he concluded the following:

1. Furnace is oversized for the house (90,000 BTUs for a 2,800 sq ft house including basement), and evaporator coil is too narrow for the furnace.
2. Flue/vent currently uses hard 90 deg elbows instead of sweeping 90’s. This could restrict heat from exiting. He recommended that the hard 90s are replaced with sweeping 90’s.
3. Additional return could be installed right outside our furnace room, which could provide additional cooling air to furnace to help prevent heat exchanger from overheating. We currently have six 16” returns through the house.
4. He recommended opening all registers on the main floor. I had nearly half closed in order to force more air into the second level where all of our bedrooms are. After he left, I opened all registers, but still saw high limit being reached.
5. He changed the fan speed from Med to High.

His statements seemed reasonable, but I always like to hear other's thoughts. Do the HVAC tech's recommended corrective actions (sweeping 90’s, additional return, opening all registers) seem like a good next step in eliminating this problem? I hope to avoid sinking money into "potential" solutions that end up not fixing the problem.

One thing I noticed last winter during my own troubleshooting, is that if I pull the air filter 1/3 of the way out, the high limit is typically not triggered. So, lately when we've been heating, I've typically had the filter partially pulled out, which I'm sure is a big no-no. If we pursue adding another return, do you think it will actually allow more air to pass through the filter?

Thanks in advance for any input you can provide!

[paul52446m]

Goodman Furnace - High Limit

Hi all, hope you had a great Thanksgiving!

Last winter, I noticed that our furnace (2005 Goodman, GMS90904CXA) was triggering the high limit on fairly regular intervals (every 15 minutes or so), in which it would take about 4 minutes to reset. Once the limit is reached, the blower fan continues to run, but no heat is provided since the burner has stopped in order to allow the heat exchanger to cool.

We’ve lived in the house (built in 1989) since Jan 2008, but I never paid much attention to the furnace before last year, so the issue very well could have been occurring the whole time we’ve lived in the house…

So, in order to gear up for this winter, I ended up having an HVAC guy come out a few days ago to perform a tune-up and troubleshoot the high limit issue, in which he concluded the following:


1. Furnace is oversized for the house (90,000 BTUs for a 2,800 sq ft house including basement), and evaporator coil is too narrow for the furnace.
2. Flue/vent currently uses hard 90 deg elbows instead of sweeping 90’s. This could restrict heat from exiting. He recommended that the hard 90s are replaced with sweeping 90’s.
3. Additional return could be installed right outside our furnace room, which could provide additional cooling air to furnace to help prevent heat exchanger from overheating. We currently have six 16” returns through the house.
4. He recommended opening all registers on the main floor. I had nearly half closed in order to force more air into the second level where all of our bedrooms are. After he left, I opened all registers, but still saw high limit being reached.
5. He changed the fan speed from Med to High.

His statements seemed reasonable, but I always like to hear other's thoughts. Do the HVAC tech's recommended corrective actions (sweeping 90’s, additional return, opening all registers) seem like a good next step in eliminating this problem? I hope to avoid sinking money into "potential" solutions that end up not fixing the problem.

One thing I noticed last winter during my own troubleshooting, is that if I pull the air filter 1/3 of the way out, the high limit is typically not triggered. So, lately when we've been heating, I've typically had the filter partially pulled out, which I'm sure is a big no-no. If we pursue adding another return, do you think it will actually allow more air to pass through the filter?

Thanks in advance for any input you can provide!

If you want to stay with me for a while we can see about fixing your problem.
First we need to find out what all the problems are. It does not hurt to over size a furnace. Some of these guy will tell you anything because they don't have the nohow to find the real problem.
The furnace you have has a blower in it that can move about 1,600 CFM of air if ducts are done right. Here are some things i need to know to help you check out the engineering.
How many hot air opening do you have?
Are most of these opening 6" round pipe?
What size are your hot air ducts leaving the furnace?
What size is the return air ducts?
What size is the filter?
How many 3" ells are on your stack.
How long is your stack?
It would really help if you could take a bunch of pic. so i can ask more question.
Answering these questions and taking the pic. is the best way if you want me to tell you what all you have wrong.
There is nothing worse than having a heating system that don't work right.
What size is you air con? Take the model no. of the outside unit.
There might be a few little things we can do to help your problem
Just like you were talking about you filter being part way out to keep it off
limit. No more than 300 CFM of air should go through 1 SQ. foot of filter.
Paul paulm989@hotmail.com

[BrewerFan]

Thanks for the reply.

To answer some of your questions:

1) Number of hot air openings (registers?): 18 [3 in basement, 8 on main floor, 7 on 2nd floor]
2) Size of hot air opening: All visible ones are 6" round
3) Size of hot air ducts leaving furnace: 20x25 (main), 24x8 (minor branch), 8x8 (major branch)
4) Size of return air ducts: 10x24 (main to furnace). Return grilles: four 16x7, one 18x3, one 25x8
5) Size of filter: 16x25x1
6) Ells on stack/flue: One 2" 90deg, three 2 1/2" 45's, and one 2 1/2" 90deg.
7) Length of stack/flue: 15 feet total. Approx 3 ft is 2" PVC (coming out of the furnace); the remaining 12 ft is 2 1/2" PVC leading outside the house.

I'll try and get the A/C model number tomorrow when it's light out and hopefully snow has melted.

I'm only able to upload 5 images, so I focused mainly on the layout of the supply and return branches. Please let me know if there are other angles that would be helpful.

I also plan to measure temperature before & after the heat exchanger in the next day or so in order to determine the deltaT.

Thanks again!

[paul52446m]

Thanks for the reply.

To answer some of your questions:

1) Number of hot air openings (registers?): 18 [3 in basement, 8 on main floor, 7 on 2nd floor]
2) Size of hot air opening: All visible ones are 6" round
3) Size of hot air ducts leaving furnace: 20x25 (main), 24x8 (minor branch), 8x8 (major branch)
4) Size of return air ducts: 10x24 (main to furnace). Return grilles: four 16x7, one 18x3, one 25x8
5) Size of filter: 16x25x1
6) Ells on stack/flue: One 2" 90deg, three 2 1/2" 45's, and one 2 1/2" 90deg.
7) Length of stack/flue: 15 feet total. Approx 3 ft is 2" PVC (coming out of the furnace); the remaining 12 ft is 2 1/2" PVC leading outside the house.

I'll try and get the A/C model number tomorrow when it's light out and hopefully snow has melted.

I'm only able to upload 5 images, so I focused mainly on the layout of the supply and return branches. Please let me know if there are other angles that would be helpful.

I also plan to measure temperature before & after the heat exchanger in the next day or so in order to determine the deltaT.

Thanks again!

I do not agree at all that your furnace is too large. To have a good system to treat the air in your home, for heating, cooling, and cleaning the air, you need to move all the air in the home through the furnace 5 times per. hr.
So if you check out the cu. ft. of air in your home, and you times that by 5, you would have to move 1866 CFM of air through the furnace.
Your furnace depending on the restriction can move max 1600 CFM.
They should have the 5 ton blower in that furnace instead of the 4 ton you have. A 5 ton blower goes up to 2000 CFM.
They make that same size furnace with a 5 ton blower.
Your filter is real bad. It has 2.7 sq. ft of surface. 300 CFM per sq. ft of filter,
means you filter is large enough to move less than 900 CFM. It should be double the size that it is..
Also when you move that much air, the return air should ether come in the bottom of the furnace or both sides.
The main things that restricts air flow is blower not sized right, return air ducts too small and air con coil to small.
With the small filter you have, the finer partials of dust are sucked through the filter and start plugging up the air con. coil.
I usually put twin air filters in that drop when i am moving that much air.
One thing you can do that will help, is use a pleated air filter. The way they are curved they give you more filtering surface.
I hope you have at least a 4 ton air coil on top of that furnace.
You said your man said that your coil was too small, and if it is than that can be a problem. Well its getting late so we can talk some more tomorrow
night,
That temp check from return to hot side should be only about 70 degrees rise. Paul

[BrewerFan]

Ok, so I measured the temperature rise across the heat exchanger in a few different scenarios:

1. Filter fully engaged (high limit reached):
Return side temp = 75.6F
Supply side temp = 152.4F
Rise = 76.8F

2. Filter 1/4 removed (high limit not reached):
Return side temp = 74.7F
Supply side temp = 147.7F
Rise = 73.0F

3. Filter completely removed (high limit not reached):
Return side temp = 73.2F
Supply side temp = 134.8F
Rise = 61.6F

I assume that if I had the filter 1/2 removed, I probably would have seen a rise fairly close to 70F.

I took the supply measurement about 3 feet above the evap coil in the main trunk, and the return measurement just upstream of the filter (ends up being around 2 feet from the blower).

The return duct drops down to the bottom of the furnace where it goes into the blower. I currently use a cheapo pleated filter, which it sounds like is the way to go.

Based on what both you and the HVAC tech have said, it sounds like the evap coil could be a potential culprit. I've had the coil cleaned, so I think it is more of a design issue than a clogged coil. The furnace was replaced in 2005/2006 (by the previous home owners), in which I'm guessing the coil was not replaced and matched to the size of the new furnace.

Do you think that replacing the hard 90 ells with sweeping 90s could provide some relief on the exchanger? At this point, I'm trying to evaluate the cheapest (but still effective) solutions.

Thanks!

[paul52446m]

Ok, so I measured the temperature rise across the heat exchanger in a few different scenarios:

1. Filter fully engaged (high limit reached):
Return side temp = 75.6F
Supply side temp = 152.4F
Rise = 76.8F

2. Filter 1/4 removed (high limit not reached):
Return side temp = 74.7F
Supply side temp = 147.7F
Rise = 73.0F

3. Filter completely removed (high limit not reached):
Return side temp = 73.2F
Supply side temp = 134.8F
Rise = 61.6F

I assume that if I had the filter 1/2 removed, I probably would have seen a rise fairly close to 70F.

I took the supply measurement about 3 feet above the evap coil in the main trunk, and the return measurement just upstream of the filter (ends up being around 2 feet from the blower).

The return duct drops down to the bottom of the furnace where it goes into the blower. I currently use a cheapo pleated filter, which it sounds like is the way to go.

Based on what both you and the HVAC tech have said, it sounds like the evap coil could be a potential culprit. I've had the coil cleaned, so I think it is more of a design issue than a clogged coil. The furnace was replaced in 2005/2006 (by the previous home owners), in which I'm guessing the coil was not replaced and matched to the size of the new furnace.

Do you think that replacing the hard 90 ells with sweeping 90s could provide some relief on the exchanger? At this point, I'm trying to evaluate the cheapest (but still effective) solutions.

Thanks!

The ell in the stack is not what is making the furnace over heat. The fact remains that you are not moving enough air through the furnace. You have already proved that you can get the temp down by taking the air filter out.
Like i said before your filter is 1/2 the size it should be. If you could have a tin banger cut a hole in the return air drop and install on a slant two 20x 25 air filters. Then she might work and not go off on limit. Did the tech check the gas pressure to make sure the furnace is being fired at the right rate?
Paul

[Hube]

What does the spec tag on the side of the furnace say that the HEAT RISE" should be? Usually it will suggest range of 40-50 degrees . If your hi-limit is shutting the burner down every so often this means that the "heat rise" is too high. In this case you are not getting enough"return" into the furnace{which really seems to be the actual problem} To properly check the "heat rise" have the furnace going thru a heating cycle,and insert a probe type thermometer into the return air at/ near the blower but out of sight of the heat exchanger.lets say it reads 68*....Now insert the probe into the main heating duct as close to the supply plenum but out of sight of the heat exchanger...lets say it reads 158* That means the heat rise is 90* !! way too high for the spec reading on the side of the furnace. That is why the Hi-limit is tripping thr burner off........... Solution would be to ADD more return, make sure all supplys are OPEN.Also havea pro come in and check the firing rate of the burner etc.

[paul52446m]

Goodman Furnace - High Limit

What does the spec tag on the side of the furnace say that the HEAT RISE" should be? Usually it will suggest range of 40-50 degrees . If your hi-limit is shutting the burner down every so often this means that the "heat rise" is too high. In this case you are not getting enough"return" into the furnace{which really seems to be the actual problem} To properly check the "heat rise" have the furnace going thru a heating cycle,and insert a probe type thermometer into the return air at/ near the blower but out of sight of the heat exchanger.lets say it reads 68*....Now insert the probe into the main heating duct as close to the supply plenum but out of sight of the heat exchanger...lets say it reads 158* That means the heat rise is 90* !! way too high for the spec reading on the side of the furnace. That is why the Hi-limit is tripping thr burner off........... Solution would be to ADD more return, make sure all supplys are OPEN.Also havea pro come in and check the firing rate of the burner etc.

I think if you was to read all the post you would see that i have covered all these bases. Paul