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Old 11-30-14, 10:04 PM   #31
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Quote:
Originally Posted by Servicetech View Post
Blower is single speed, one of my previous mods was replacing the OEM 3 speed 1075PRM motor with a single speed 825RPM. PSC motors have awful efficiency on low speed, old motor used 355W, new uses 150W. Temp rise did go up about 7 degrees when I changed motors. Furnace is rated 25-55f, 45f is on the high side, but still 10f from max.

I did do the mod you are talkind about on my old furnace. It had to be on HIGH speed to stay off limit, and the giant blower produced a cold draft when it 1st kicked on. A sequencer was my "time delay" of choice, had a fixed 30 second delay. Long enough to let the blower clear the cold air out of the ductwork while still on low speed.

Dumping the oversized beast and installign a furance 1/2 the size fixed a lot of comfort issues. No need for a low speed anymore since the blower isn't overpowering to begin with.
Is the 44k an 80% unit?
44000*.8=35200
35200/(45*1.08)= 724 CFM
You are getting 724CFM from 150 watts? Is this 825RPM motor a PSC motor?

The reason I ask is I replaced my old shaded pole furnace motor with a PSC motor and I thought I was impressed by going from 804 watts down to 320 watts on the highest speed tap. I'm a little under 800CFM based on my own output capacity and temp rise.

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Old 12-01-14, 05:16 AM   #32
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Unit is a 90%, 39.6k output, 814CFM. Works out perfect for the 2 ton AC also. The 150W is a SINGLE SPEED 825RPM PSC condenser motor. 3 speed PSC motors have a lot of "winding waste" for the lower speeds that makes a big impact on efficiency.

The fun part is the motor is the 1/3HP 230V motor running on 120v. Couldn't find a 825 RPM 115v motor anyhwere, thought I'd try a 230v and modify the cap. The motor specs a 7.5UF cap, had to use a 10UF to get to to run properly on 115v. Tried a 12.5uf, but it increased amp draw without any change in speed. Motor is rated for 1.8A @ 230v, draws 1.2A @ 120v. Power tests were done with a Kill-a-watt.

Blower wheel is 10x8, original motor 1/3HP 115v 4 speed. "3 ton drive" if you will. Ductwork is all flex, (2)12" returns with 20x20x4 filters. (2)10" plus (2)8" supply ducts off of the main 17"x20" plenum. Ductwork is a bit oversized for 800CFM, old system was 3 ton/88k @ 1200CFM.
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Old 12-02-14, 08:44 AM   #33
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I've considered trying a cap swap to a lower rating but never really had any experience doing this even though it isn't a difficult test. The spec cap is 7.5mfd but I think I have a 5mfd cap sitting around. I suppose I make the swap and watch the amp draw and make sure it can spin up properly and the temp rise is not adversely affected?
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Old 12-02-14, 09:38 AM   #34
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Changing cap value will not always give good results. If you are running rated HP/amps/voltage the rated cap will normally give best results. I only changed the cap value because I'm running a 230v motor on a 120v circuit.
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Old 12-12-14, 08:20 AM   #35
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If you change your capacitor value check before and after motor amps to make sure you're not increasing your power usage.

So if longer furnace run times are more efficient then one easy improvement would be to set your thermostat program settings with more differential. This makes sense if you leave for work or if you have 2 hvac. We sleep upstairs so downstairs can be shutdown all night. After everyone leaves both units can set back 10 degrees.
I also stagger my schedules so downstairs runs before upstairs to anticipate the heat rise and prevent overshooting the upstairs set point. "Losing" the heat to the upstairs also increases my run times.
If your house is already tight and insulated there might not be much of a temperature drop before you return but you should get the extra temperature swing to take advantage of optimized heating temperature.
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Old 12-25-14, 04:17 PM   #36
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+1, always test and verify any modifications you do.

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