Help! HVAC Overtemp Cutting Out

Get Off My Lawn

Get Off My Lawn

Artist formerly known as Pink Vapor
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My son’s newer Trane gas pack keeps popping the overheat breaker. It’ll run fine for hours or days. The installer said he has water in the 12” duct restricting air flow. It has new filters. I’ve felt the line from one end to the other, pulled the coil cover to look into the line and the coil. They’re clean and dry. What else should I look into?
Thanks!
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Any straps or anything that are pinching the ducts/causing them to collapse inwards?
 
Are you saying the breaker panel breaker trips or the over temp sensor in the unit trips. Those help here.

2.5 tons should be 15” flex, 14” hard metal duct. 15” flex will never be found but 16” is. Also 10” duct is way too small. Plenum safety will trip unless the speed is hurricane set. Also is this a house or manufactured home.
 
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I am assuming here that it is running in burn gas mode, not as a heat pump. According to the nameplate, the unit has an output of 56,700 BTU per hour and is designed for an output delta-T of 30-60F with a maximum discharge temperature of 160F. The symptom of the overheat breaker tripping suggests that the maximum discharge temperature is being exceeded. Assuming that it isn't a failing sensor / breaker and that it really is getting too hot, the output air temperature is going to be a function of the BTU per hour and the air flow. In turn, the air flow is going to be a function of duct size.

I seem to have misplaced by air flow / versus duct size tool, so I will assume these figures are correct: http://library.coburns.com/hvactechtips/Charts/Coburns Duct Size Chart.pdf According to this, a 12" flex duct will supply about 430 CFM. Next according to this calculator(https://www.coburns.com/hvac-calculators/CFM-with-Gas-Heat), the air flow with a 56,700 BTU heat source to provide a 60F rise is 875 CFM and a 30F rise will require 1750 CFM. For comparison purposes, here's another typical duct size versus CFM chart Of course the air velocity is another aspect to consider, which is CFM (ft^3 )divided by the orifice diameter (ft^2) and you need to make sure that this is within the acceptable range for the material, least you get excessive loss.

Bottom line, it sounds to me like the supply duct is too small. Interestingly, I read a blurb about 12" being standard for mobile home installation and non-mobile home tends to go bigger.
 
BlackGun said:
Are you saying the breaker panel breaker trips or the over temp sensor in the unit trips. Those help here.

2.5 tons should be 15” flex, 14” hard metal duct. 15” flex will never be found but 16” is. Also 10” duct is way too small. Plenum safety will trip unless the speed is hurricane set. Also is this a house or manufactured home.
Click to expand...
The ‘breaker’ is a temp sensor in the outdoor unit by the burners.
The 10” is feeding a box that the larger hose os connected to.
This is a replacement unit in a 10+ year old house.
 
Get Off My Lawn said:
The ‘breaker’ is a temp sensor in the outdoor unit by the burners.
The 10” is feeding a box that the larger hose os connected to.
This is a replacement unit in a 10+ year old house.
Click to expand...
Thats the issue right there. 15" or as Ken said above. You won't find it. 16" and all those issues should go away.
noway2 said:
I am assuming here that it is running in burn gas mode, not as a heat pump. According to the nameplate, the unit has an output of 56,700 BTU per hour and is designed for an output delta-T of 30-60F with a maximum discharge temperature of 160F. The symptom of the overheat breaker tripping suggests that the maximum discharge temperature is being exceeded. Assuming that it isn't a failing sensor / breaker and that it really is getting too hot, the output air temperature is going to be a function of the BTU per hour and the air flow. In turn, the air flow is going to be a function of duct size.

I seem to have misplaced by air flow / versus duct size tool, so I will assume these figures are correct: http://library.coburns.com/hvactechtips/Charts/Coburns Duct Size Chart.pdf According to this, a 12" flex duct will supply about 430 CFM. Next according to this calculator(https://www.coburns.com/hvac-calculators/CFM-with-Gas-Heat), the air flow with a 56,700 BTU heat source to provide a 60F rise is 875 CFM and a 30F rise will require 1750 CFM. For comparison purposes, here's another typical duct size versus CFM chart Of course the air velocity is another aspect to consider, which is CFM (ft^3 )divided by the orifice diameter (ft^2) and you need to make sure that this is within the acceptable range for the material, least you get excessive loss.

Bottom line, it sounds to me like the supply duct is too small. Interestingly, I read a blurb about 12" being standard for mobile home installation and non-mobile home tends to go bigger.
Click to expand...
Mobile home, if using the current ductwork, is a different beast all together. That duct is usually 4x10 or 4x12. That's a high static situation.
 
Get Off My Lawn said:
The ‘breaker’ is a temp sensor in the outdoor unit by the burners.
The 10” is feeding a box that the larger hose os connected to.
This is a replacement unit in a 10+ year old house.
Click to expand...
First you can’t “feed a box with 10” if the other side is larger, say 14. I assume you replaced it per your post above.
Terminology is important. That of course is not a breaker but a high temp safety device for future post here. Breaker is for high amperage disconnection In a panel.
As stated above you are not allowing enough airflow for the delta T, so if it was fine before replacing the damaged duct go back to a 14” larger duct. Also ensure all floor supply registers stay open at all times. Continued low airflow over time will crack the heat exchanger. Plus low airflow on the cooling cycle will freeze the coil. I am assuming the conditions you describe. Any changes in duct connections can be made with fittings to increase or decrease duct connections. PM if I can help.
 
I just came out of the crawl space, it’s 16” inlet ducting. I reset the over temp sensor in the unit this morn, he said it tripped within a few minutes. He reset it and it’s been running all day. We’re reading 130F at the closest register to the unit.
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Get Off My Lawn said:
I just came out of the crawl space, it’s 16” inlet ducting. I reset the over temp sensor in the unit this morn, he said it tripped within a few minutes. He reset it and it’s been running all day. We’re reading 130F at the closest register to the unit.
Click to expand...
Is the 10 or 12 the return?
 
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