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Old 07-11-2020, 06:08 PM   #1
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4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Hello,

My furnace use to have a 4 speed motor and that burned up. The HVAC guy put in a new 3 speed motor. This happened in the winter and I did not notice any difference until I turned on the A/C. The blower is running way faster than ever before.

I have been researching this and found the 3 speed motor only has (Blk)-High, (Blue)-Medium, (RED)-Low speeds.

My furnace use to use Medium for A/C and Med-Low for heat. What he did was put the medium speed for heat and High speed for A/C.

After researching this I did the calculations for my furnace and A/C unit and found out the furnace Heat is suppose to be Medium. The outside unit is a 3-Ton unit making it supposed to be set at Med-Low.

Two things are happening now. First the A/C is blowing way to hard in the house and at this setting it is annoying, and also causing the water to evaporate off the coils and that humid air is sent back into the house.

This is what I did so far. Took off the Black wire and parked it. Put the A/C on medium and Heat on Low. Now the air is speed is back to normal and the A/C is dehumidifying as it should.

My question is can I "Y" the (Blue)-Medium speed wire and put it on both Heat and Cool terminals?

Thank you
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Old 07-11-2020, 06:43 PM   #2
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Yes, as long as you "park" the other speeds.
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Old 07-11-2020, 07:41 PM   #3
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Quote:
The outside unit is a 3-Ton unit making it supposed to be set at Med-Low.
How did you determine this?

Unless you have a 4 or 5 ton drive furnace it should be on high.

What's the full model number of the furnace?

You can definitely use the same speed for heating and cooling - requires a wire jumper and:

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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 07-11-2020 at 07:45 PM.
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Old 07-12-2020, 09:16 AM   #4
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Thank you for getting back to me. To answer your question I have a GUD090X35B furnace and A/C unit is ASX160371AB

Here is the chart I used from my furnace.

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Old 07-12-2020, 11:48 AM   #5
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Without measuring static pressure or directly checking airflow using a different method, it's a guess. With a different motor the chart can't be used to determine airflow.

I agree though it probably needs to be on medium for cooling.

I would check the temperature rise with the furnace heating - you may be able to use and actually benefit from using high speed instead of putting both on medium. 90k furnace needs a lot of airflow, more than a 3 ton a/c.

The rated temperature rise range should be printed on the nameplate.

You run the furnace for 10 to 15 minutes and insert and thermometer in supply duct a couple of feet from the furnace and measure return, subtract.

Should be in the middle of the temperature rise range.

50F is a good rise, 60 to 70F is high and stresses the furnace, also reduces efficiency.

The common assumption is that cooling always needs more airflow than heating - often wrong. Blower speed selection needs to be based on measurements, not assumptions.
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Old 07-13-2020, 08:55 AM   #6
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Thank you User_12345a. That is some good advise. I will be testing that out.

I did want to note the pic of the double male tab I can't use. As I was wanting to take a single MED wire (female) from the blower motor and split that wire to two female ends to connect it to both the Heat terminal and Cool terminal of the control board.

Last edited by fliersrr; 07-13-2020 at 09:00 AM.
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Old 07-13-2020, 09:37 AM   #7
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


You can put that splitter on the heating terminal and run a jumper wire from the cooling terminal, the other connection on the splitter gets connected to the medium blower tap.

If you can tolerate the noise and the temperature rise is good with the heating using high speed, i would just do that instead.
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Old 07-13-2020, 09:37 AM   #8
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Quote:
Originally Posted by user_12345a View Post
requires a wire jumper and:

User_12345a has it right. Plug the female end of the double connection above onto the heat or cool terminal on the board. Plug your medium speed on one of the male tabs on the connection. Install a jumper wire (2 female connections, field made) from the double connection to either heat or cool (the one you didn't use for the double connection). Your good to go. There are different ways to do this but the end result should be the same.
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Old 07-13-2020, 10:55 AM   #9
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Thank you User_12345a That makes sense.

I just was looking at a diagram on my furnace and it must have come from the factory with a jumper on it. Note 5 says if the Heating and Cooling speeds are not the same to remove jumper....

I was worried of back feeding the control board, but it must be relay controlled.

Thanks again for your help.

https://1drv.ms/u/s!AnPsn6-NZHWgh9dy...3mOYw?e=ytDelC
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Old 07-13-2020, 10:58 AM   #10
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Re: 4 speed blower motor Now 3 Speed Blower Motor (wiring question)


Thank you 57TinkerMan and User_12345a
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