Carrier 80% furnace consistently running a high heat rise.

[Andr00]

Not quite 1 year old Carrier single-stage 80-percenter, 66k BTU. Can't recall the model and serial. The house is an older 1100 sq. ft. ranch with a single return in the hall and two more in bedrooms.

Unit was consistently going out on primary limit. I found the temperature rise to be well over 80F when I first got there. 40-70 is the correct range.

After verifying the correct blower speed setting and that the capacitor was good, I checked the gas pressure. Was set at 3.7"wc. I pulled it back to 3.2". Data tag says 3.2 to 3.8. A-coil was only three years old, and clean. Venting was fine. And it wasn't the vent limit switch that was tripping.

The filter is a 16x20 pleated that was new. There was also a patch panel over a roughly 10x10 hole on the drop where a humidifier used to be. I pulled both of those just out of curiosity to see what I would get. Temp rise dropped to a bit over 70. Still high, though.

I think the return duct is a contributor to the problem. The bottom of the drop is basically a 16x16x20 box with the filter in it. There's a 16x8 drop on top of that. On one side of the drop, there's 8" round that goes off to a single return (can't tell exactly how far - basement is finished). On the other side of the drop, 16x10 ductwork goes about 8 feet, making two 90-degree turns in the process, before it stops and another several feet of 8" round connects to the panning for the other two returns.

I'm only describing the ductwork from memory. And I'm sure my descriptions of it sound goofy...I'm greener than green on the whole tin side of things.

[rundawg]

Not quite 1 year old Carrier single-stage 80-percenter, 66k BTU.
Unit was consistently going out on primary limit. I found the temperature rise to be well over 80F when I first got there. 40-70 is the correct range.
After verifying the correct blower speed setting and that the capacitor was good

What is the correct blower speed setting? The one set by the manufacturer?

This unit needs to be moving about 860 cfm of air (130 cfm/10k btuh) in the heating mode.

Did you have a chance to do a static pressure test, or use some other means of verifying the airflow? My guess is you are probably pushing .8 - 1.0" wc. in this system.

As an example - the carrier 58STX070 (66k btuh, 40-70ºf temp rise) with a 2 ton blower, will only move 870 cfm on the high speed tap at 0.5" wc. On the med-low speed tap, it is only moving 595 cfm of air at 0.5" wc.

[Mr Bill]

I think the return duct is a contributor to the problem..

Pull the blower door and run it that will answer that question.

[54regcab]

I've found these carrier units sensitive to low airflow, especially when the furnace is oversized. 66k for 1100 sq ft is oversized unless the house is in Alaska, 50% glass, or leaks like a sieve. The solution is to repair the marginal ductwork and returns or install a 44k furnace that has the same size blower.

[Ron]

In my neighborhood with houses as described (like mine) that furnace works just fine after I've done some improvement to the return duct.

[rchristie420]

What is the correct blower speed setting? The one set by the manufacturer?

This unit needs to be moving about 860 cfm of air (130 cfm/10k btuh) in the heating mode.

Did you have a chance to do a static pressure test, or use some other means of verifying the airflow? My guess is you are probably pushing .8 - 1.0" wc. in this system.

As an example - the carrier 58STX070 (66k btuh, 40-70ºf temp rise) with a 2 ton blower, will only move 870 cfm on the high speed tap at 0.5" wc. On the med-low speed tap, it is only moving 595 cfm of air at 0.5" wc.

I totally agree here ive had a few where i hadto bump up to high speed to get more air flow across the heat exchanger.cause it would trip the high limit

[air1]

You need to move more air. Increase the fan speed if possible. If it's not possible to increase fan speed you'll need to increase your duct size to move more air. Most likely the return is undersized. Measure the external static pressure to determine where the most restriction is located.