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ac low air flow

1155 Views 6 Replies 4 Participants Last post by  bernie963

I still have some homework to do, but the symptom is low air flow. As of the end of the heating season air flow was good. I cleaned the air filter as I usually do 4 times a year. (allergy pure cleanable) Now the cooling season and the cooling/air flow seems less.

I have a nordyne (tappen oil fired furnace with a/c.) regular pcs motor on the blower. 3 tons (i think) 5 years old. I cleaned the leaves out of the condenser and hosed off the coils. I have two supply grates off, one upstairs and one downstairs for painting.

The house set up is two story with an air manifold in the attic. It supplies all upstairs and 2 downstairs supplies vents, both of the removed vents are off of this manifold.

A nest thermostat was installed during the heating season and works well.

Tomorrow am when it is cooler I will check out the attic to make sure the manifold is not compromised. If memory serves me correctly the blower motor is 2 speed, but not sure if heat or a/c is high speed. If a/c is high speed, it maybe that the nest is calling for low speed. maybe?? I will check out attic and blower speed rating and post back tomorrow.

If anyone has any thoughts, please post.


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thermostat doesn't impact fan speed.

if you have a 2-stage ac the new stat may not be set up right for it, hence low airflow, running on first stage only.

so many things can cause low airflow.

went to the attic this am. no problems found. Therefore, i think it has to be the nest thermostat as the air flow is less on cooling than in heat mode.

The furnace model is Tappan o4ld-140a-16f. The blower motor has several speed taps per the paperwork which came with the unit. (none of which has changed since original installation)

The ac is 4 ton (i misspoke saying 3 ton in the original post). seer 13.

The upstairs has always been a higher temp (heat or cool) since day one 34 years ago. N o different with this new system, the old one lasted 30 years!!! Old magic chef . This furnace/ ac is much better, but certainly not top of the line, but got a steal on it including installation.

Still not sure if the blower speed is higher on heat mode or cool mode. I may just have to fire up the heat and see what happens.

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cooling uses a different tap.

the thermostat doesn't have an impact on airflow unless u have a 2-stage a/c.

possible the y terminal came loose at the furnace control board and it's running on continuous fan mode.
Another possibility is if the refrigerant charge is low the coil can freeze up and block airflow. It would blow normal at first then taper off. If the unit is running a lot it might never totally thaw out and look like a steady blockage. Depending on how bad it gets you can see ice on the line outside the coil case.
If i found the right manual (a nordyne oil furnace has the same m/n) then that furnace can only push the required 1600cfm on the high tap. (Black wire, terminal 2, and default)This is assuming average to better then average ductwork. It's right on the edge, so even restrictive filters will cause you problems. Make sure that your fan of being powered by this tap, or you'll be likely to freeze that coil. Medium high tap, is only about 360cfm, which is really cutting it close. Your refrigerant levels will have to be spot on. (Default tap for heating, terminal 3, blue wire)

Thanks for all the info.

It has been cool here so I have not run ac to see how it is cooling. I am thinking things are ok as it may have had a lot of heat to remove as it was the first real run of the season.

I did check the attic manifold and it seems ok, no air leaks.

I did on a cool morning run the heat, fan only and ac. holding a piece of paper over one supply and one return the paper acted the same in my opinion so I figure things are ok.

The only thing changed since last year was the nest tstat.

Once we get into the cooling season for real, we will see.

on another note, I know the heat and ac are oversized. If I remember correctly, the oil fired furnace is the same btu rating as the original and it had a 3 ton ac. the new one is 4 tons. reason the installer got a much better price on the 4. I do have a lot of glass with direct sun and not the best windows plus 3 skylights also with sun exposure. I have 2400sq ft w 8 ft ceilings and one of the skylight has a 16 ft tunnel to the roof.

Thanks for the responses and we will see what happens.

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