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tippet 12-01-2010 06:46 PM

Furnace blower only runs VERY slow...
We noticed recently that our furnace blower barely turns, no matter what
setting the thermostat is in (AC, heat, auto, on..). I tried doing a little
research, and everything I came across suggested the capacitor. I tested that, and it is fine. Whats odd is that everything about the furnace seems to work just fine - the burners light, air conditioning works as it should, its just that the blower barely supplies a light draft of air out of the vents. I have no idea what brand this furnace is, I looked all over but there doesn't seem to be a name plate anywhere. It is probably 10-15 years old.

I know that the blower motor should have 3 speeds (I checked the wiring when testing the capacitor) - the "heat" wire is connected to the "med" speed, and the "cool" wire is connected to the "high" speed. We have 2 of these furnaces, and the functioning one is MUCH louder and blows a lot more air at both settings - I just wanted to clear that up, for anyone who may suspect that it is simply operating on the "low setting." More importantly, it doesn't differentiate between the two settings (heat and AC) as it should - always just a very gentle, VERY low speed.

The final bit of information that may be pertinent is the limit / fan control
- the "manual" button is missing. Otherwise the control seems fine.

Anyone have any idea what would cause this?

kenmac 12-01-2010 06:58 PM

did you check cap with cap tester or just ohm meter ? A cap tester tells what the cap is putting out

tippet 12-01-2010 07:50 PM

I used an ohm meter for that test, with the capacitor disconnected from the motor, the number went from 0 to max over a period of about 30-45 seconds. I don't own a capacitor tester, so is there some other way to measure the output?

tippet 12-02-2010 03:12 PM

An update: Decided to replace the capacitor, just in case that was it (without being able to properly test it, thanks kenmac) - however, no change. I decided to get a better look at the motor, to see what a replacement would cost for that, so I pulled it halfway out. The blower is on top of the furnace (updraft?). Anyway, I wanted to see what would happen when I turned it on, and lo and behold the fan shot up to full speed - sounded great, didn't get hot or anything. Thinking I had somehow fixed it (a loose wire maybe), I pushed it back over the opening for the furnace and it slowed down! Confused now, I pulled it back out and it sped back up. I tilted it up, to expose the bottom to open air, and it slowed down again! So apparently, the fan speeds up when it encounters a lack of airflow (when sitting on the floor, for example), then slows down when it gets plenty of air.

I assume this is a design choice, and not an accident (ie, nothing is actually broken). However, is there some way to bypass this feature and just run the fan on the speeds I want it to run?? It does little to no good operating the way it does!

The motor is an Emerson Rescue 5461, set up to operate at 1/2 HP (10 microfarad capacitor). It appears stock in every way, and is wired into the system in what seems to be the usual manner.

Please help!

beenthere 12-02-2010 04:48 PM

Is the motor on the other furnace also a 1/2 HP? Or is it more?

The speeding up when not moving air is an inherent feature of the design of a PSC motor. It is not bypassable.

Has it always been this way, or just recently?

tippet 12-02-2010 07:46 PM

I haven't taken apart the motor on the other furnace yet (since it works), but I can tell from what little of it that I can see that it is not the same motor - the capacitor is older, and longer, but has no label, and the motor is obviously older (has an OEM look to it, at least on the back). I can't tell what HP it has!

I'm not entirely sure how long the furnace has been this way, we only noticed it once the weather turned cold, as we hardly ever had it pushing AC over the summer and we moved in during the spring. One thing that is interesting is that I can decrease the horsepower rating of the motor by using lower rated capacitors (all the way down to 1/6 HP) - would decreasing the HP help it run faster when there isn't enough pressure in the system?

beenthere 12-03-2010 04:44 AM

No, it wouldn't. Good chance that its not big enough(too low of a horse power) already.

Look at the label of the furnace. It may have a blower horse power rating on it. See what it says.

tippet 12-03-2010 08:02 PM

An update - we solved the motor problem, but exposed a new problem! Turns out whoever installed the motor put in a 208-230 volt model in a furnace that runs 115 volts. Once we put in a new motor, it runs like a dream, if a little wobbly and rattly.

The new problem: all of this started because no air was coming out of the vents. Now that the fan is finally up to speed, no air is coming out of the vents. So essentially I have solved nothing! Can a simple dirty coil block that much air flow? Or is it more likely that the ducts are blocked off somehow?

beenthere 12-04-2010 04:07 AM

A dirty coil can stop all air flow. See it a lot.

Know It ALL 12-04-2010 05:03 AM

Don't know if you have them Tippet but, indoor pets wreak havoc on coils.

yuri 12-04-2010 06:35 AM

Are you sure you got the rotation on the new motor correct. Let it come to a slow stop and look at the blades. Should be scooping the air. If running backwards there are 2 wires on the motor to switch.

tippet 12-04-2010 01:30 PM

Yuri wins! When we installed the new motor, I matched the rotation direction to the fan that we removed (the 230 volt fan) - this was apparently a mistake. I guess the HVAC tech who installed that fan was having an off day. Once I reversed the wires, it worked just fine. Now, the coil is pretty dirty, the filters do need changed, and I am willing to bet the ducts are pretty stuffed up too, but it finally was able to get the room up to 70F without tripping the limit, and it is nice and quiet as well.

Thank you everyone! This probably saved me a lot of money...

beenthere 12-04-2010 04:50 PM

Your return ducts may be pretty dirty, But your supply probably isn't, the A/C coil keeps the supply pretty clean.

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