HVAC blower fan testing & diagnosis FAQs set #3

This article series explains how to inspect and test a heating or air conditioning indoor air handler blower fan that is not working. We also discuss fan fan squeaks and noises. The blower fan assembly is the green component in the page top illustration from Carson Dunlop Associates The Illustrated Home.

HVAC Blower Fan Repair FAQs3 - why air flow may slow down

The questions and answers discussed below were posted originally

at BLOWER FAN OPERATION & TESTING - please be sure to read that collection of diagnosis and repair steps.

The blower fan is located inside a horizontal air conditioning unit in many home air conditioning systems, especially when the air handler is located in an attic or crawl area.

Air flow out of our central air conditioner slows in the evening

Forced Air Unit with Central A/C ( with Heat Pump) added about 1 1/2 ago.

So this season the A/C blows cold all day and works fine until the late evening when the out side compressor stays on, but only a trickle of cold air comes through the vents.

Is my blower motor the culprit ?

When I turn A/C on the next day ( after turning it off in the middle of the night) the A/C runs fine again until evening. On 2019-07-17 by Joe

Reply by (mod) -

Sounds odd to me too; One wonders why the fan would know to slow down at night.

Check first for a frost or ice-blocked cooling coil in the indoor air handler.

See FROST BUILD-UP on AIR CONDITIONER COILS where we explain the most-common reasons for cooling coil icing - a condition that certainly would block air flow.

Check the manual for the unit to see if there are control board switches that might vary the fan speed based on temp or other conditions such as

- overhreating equipment

- a failing relay or control board that is responding to ambient temperature

Blower fan failures in American Standard air handlers

American Standard has a wheel fan that is connected to a plastic container that blows the air in duct.

The wheel fan gets disconnected from the core.

When the motor starts, the wheel fan goes berserk inside the plastic container box and will shut down because air won’t release and this patient is no good.

There is a defect from the wheel fan and the motor that the fan continues to spin and fan stays put. Not good On 2019-02-22 by Emilio

Repeated failures of Trane blower motor blamed on humidity?

I had a 3 ton Trane Package unit installed less than 3 years ago from a reputable company in Miami, FL. In less than 3 years, my blower motor had to be replaced twice. To me, that sounds crazy and like something is just not right either with the unit or the install

Company says its due to humidity / environmental conditions etc. I live in Miami my whole life, never remember any other AC units needing a new blower motor yearly. Is this normal behavior for a practically new AC? On 2019-02-19 by Moshe

Reply by (mod) -

I agree with you that the behavior sounds abnormal. There might be something in the local environment such as unusual moisture or even corrosive gases, or low voltage, or some other installation issue that could be a factor.

Certainly plenty of blower motors run in daily use for decades. Let's both do some research on electric motor failure rates as a function of climate such as Florida.

See ELECTRIC MOTOR LIFE

Poor air flow from 1978 GM Gas Furnace

I have a GM gas furnace from around 78. My problem is I don’t have much air flow coming from the registers. Blower isn’t dirty, used a leaf blower on it. Air filter is clean.

The blower does rattle which I have fixed by using a 2 by 4 and old rag to support squirrel cage. Registers are hot to touch, registers closer to furnace gmhave a bit more air flow but not much. The furnace has 1/3 hp blower motor. It’s for 1900 sq ft house.

Is it undersized? A couple of years ago I replaced my swamp cooler it has 1 hp motor. Why would the furnace only use a 1/3? On 2018-11-09 by Josh

Reply by (mod) -

Josh

Look VERY closely at the squirrel cage fan blades - even a modes amount of dirt there can cut air flow significantly.

Then have your service tech check the fan speed (and motor and voltage).

I don't assume the blower or fan is under-sized unless the problem has been present since original new installation of both the blower and the ductwork.

For detailed diagnostic help

see AIR FLOW TOO WEAK

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Continue reading at BLOWER FAN OPERATION & TESTING - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• BLOWER FAN OPERATION & TESTING
• BLOWER FAN SQUEAKY NOISE REPAIR
• ELECTRIC MOTOR DIAGNOSTIC GUIDE
• FAN LIMIT SWITCH
• FURNACE FAN WONT START
• FURNACE FAN WONT STOP

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