Optimizing Furnace Fan for Air Conditioning

Dealmaker1945 wrote: ↑ I got a new furnace last fall and got some great advice here for optimizing the fan settings for heating.

I am wondering if I can also tweak the fan setting for cooling. I am holding my cooling setpoint OK, but I have the sense that my air conditioning is not as strong as before, i.e. it seems to run longer than it used to. Last fall I pulled off the aircon unit housing and dusted and washed the cooling fins, so I think my owner maintenance is up to date.

With a thermostat setpoint of 76F, and measured as such at the return plenum, the output is around 62F, a difference of 14F. I am reading that the difference should be between 16F and 22F as a general rule of thumb.

I have the option of tweaking the cooling airflow up or down. Can/should this be used to adjust the output temperature?

Dealmaker1945 wrote: ↑ I got a new furnace last fall and got some great advice here for optimizing the fan settings for heating.

I am wondering if I can also tweak the fan setting for cooling. I am holding my cooling setpoint OK, but I have the sense that my air conditioning is not as strong as before, i.e. it seems to run longer than it used to. Last fall I pulled off the aircon unit housing and dusted and washed the cooling fins, so I think my owner maintenance is up to date.

With a thermostat setpoint of 76F, and measured as such at the return plenum, the output is around 62F, a difference of 14F. I am reading that the difference should be between 16F and 22F as a general rule of thumb.

I have the option of tweaking the cooling airflow up or down. Can/should this be used to adjust the output temperature?

engineered wrote: ↑ It's probably fine. You might be pulling a lot of humidity out of the air. What is your humidity level at?

insertname2020 wrote: ↑ If I remember from your PMs, you have a carrier performance 96/60k btu. Is that correct?
Available in two different blower capacities so would need to know the full model number.

A. The 59TP6B 060V17-14

What size is your a/c? This will dictate if the blower speed settings need to be changed.
For 1.5 and 2 ton, the factory speed is likely too high.
The installer is supposed to adjust it on startup, but probably did not.

A. I think 2 Tons. It is a York AL6B024F3CA

At 50% humidity in the return air, supply air should be 18 to 22F colder than the return.
The more humidity there is, the lower the expected drop.

Having a higher fan speed than normal, provided it is not through the roof, does not reduce cooling capacity; what it does is reduce dehumidification so it feels warmer at the same temperature as well as increase the supply temperature.

Have you noticed a reduction in cooling ->ie not keeping up?
Are you using an accurate thermometer and measuring near the furnace?

A. I have a couple of them and they are consistent with my thermostat. I am in the same place I measure the heat rise - at the main return drop into the furnace and the supply ducts very close to the plenum. The aircon is keeping up today with 33C outside.

Since your furnace blower is not the constant airflow type, it can take a duct pressure measurement and comparison with the manufacturer's charts to get the cooling airflow right.
You probably don't have the tools for that, can try to get it pretty close without.

What was your temperature rise (delta-t) on high heat mode? That can be a good indicator of how restrictive the ductwork is and how to set the dip switches.

A. well that was in spec but on the high side in your opinion and you suggested that was due to the weak blower in this furnace.

insertname2020 wrote: ↑ Yes - the blower is weak for heating mode, not in cooling mode.
What was the actual temperature rise on high heat mode? Like 60F? I can go back and check the PMs if you told me before.

I can check the chart and get an idea of what the duct pressure is relative to airflow.

Based on the info I have at this point:

See attached chart. Which switches to change are labelled on there.

The factory setting is good for a 2.5 to 3 ton a/c so it should be lowered a tad if it hasn't.

The normal target for 2 tons is 800 cfm at 400 cfm per ton.

Factory for setting is OFF OFF OFF.
My safe pic for 2 ton is ON OFF OFF, and this may still be a little high if your ducts are not undersized for what's installed.
I won't recommend going lower than that without.

Ignore the recommended settings/cfms in the manual and or label on the furnace access panel for the tonnage as the actual cfm will be lower due to the motor than what's stated from not being able to compensate for ductwork at higher pressures like the infinity version can.


After making changes and unit has been running for at least 15 minutes, check temp drop again and larger refrigerant line for frost.

Dealmaker1945 wrote: ↑ I think I got the heat rise down to 63F when running on stage 2, by increasing the fan speed. (It has rarely if ever run on stage 2 since I transferred the staging to my thermostat.) I will look at the dip switches tomorrow. In the meantime I am pondering the benefits of achieving a higher delta-t when cooling, since I am getting good dehumidification as it is? Would the aircon unit run a shorter cycle?

insertname2020 wrote: ↑ With a 63F rise on high, I definitely would not reduce the fan speed setting for cooling beyond what I recommended.

A greater temperature drop with less airflow will not result in shorter cycles.
Fewer cubic feet per minute of cold air being supplied, more cooling per cubic foot of air cooled.

Reducing the fan speed shifts the capacity from cooling to dehumidification - end up running a little longer due to less actual cooling which would further enhances dehumidification.
With lower humidity, the t-stat can be set a little higher.

Keep in mind that we haven't had super high humidity yet, once the outdoor dewpoints get to 68f/20c+, it may not dehumidify adequately.
Today was not that humid.


Now, when the return air is already quite a bit colder than what the thermostat reads due to poor return air vent locations, lets say its 70F and the thermostat read 75 or 76f, a higher fan speed than normal is beneficial, as it can help offset the negative impact of that. Higher than normal airflow is also helpful if you have lousy ductwork/trouble getting cold air to second floor, if applicable.

Otherwise, I recommend cutting the airflow down to where it normally should be.

If you have enough thermostat wires, you can actually get the best of both worlds and stage the fan in cooling mode.
Staging fan in cooling mode in your case would mean having airflow closer to normal and ramp up to a higher than normal only when the a/c isn't keeping up or temp is above the setting to maximize cooling over dehumidification.
Need Y2 connected, another wiring change and low cooling dip switches set correctly.

Dealmaker1945 wrote: ↑ Thanks for the suggestion. I flipped the dip switch today to reduce the fan speed as suggested. So far I don't see much difference in the temperature. I will need to monitor and see how it works when we get back over 90 again. But I am starting to think my cooling is not as effective as it was in years past.

insertname2020 wrote: ↑ Does it feel warmer or running more? Feeling warmer but maintaining as before would be due to worse dehumidification.
What's the temp drop now? (at what indoor humidity, roughly?)

Do you hear gurgling or gas bubbles in the small uninsulated refrigerant line?

May need to have gauges and thermometers put on the system. (pressures alone can not be used to definitively diagnose unless it's really low on gas, superheat/subcool test required which involves measuring pipe temperatures)

insertname2020 wrote: ↑ It's probably moving roughly the same amount of air in that case; as you try to shove more air through than the ductwork was designed for, the back pressure just shoots through the roof and the blower motor has its limits.

It is very possible you changed the wrong dip switch settings - there's a separate bank that's similar for low cooling/continuous fan.

Temperature drop is just a starting point and not a great metric itself, need to know airflow and wetbulb return air temp which takes humidity into account to get a better handle on what's going on.

Check the temp drop at the same humidity level and temperatures every few weeks this cooling season and if it declines, get it checked.

62F supply is too high under normal conditions though, it should be in the 50s.