1.0 Hp vs. 3/4 Hp


Hi, When I was replacing blower motor on our furnace there were two replacement choices 1.0 Hp or 3/4 Hp 3 speed motor. Between two only difference was Hp rating and full load current draw. As sn after thought I wonder if there'd be any advantage choosing the 1.0 Hp one. BTW, our a/c unit is 3.5ton. TIA for any experienced comments.
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wrote:

You really should be consulting the installation documentation of your system. That is not as simple as just saying bigger is better. The air handler and the condenser have to be matched.
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snipped-for-privacy@aol.com wrote:

Hmmm, Not knowing anyting HVAC. I put in 3/4 Hp one as the manual states. But curious duck wannna learn. Any pointers where to look? Getting old guy has to exercise his brain, LOL!
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Tony Hwang wrote:

As everyone said, replace with what the manufacturer says. It is an engineered system.
You could run a larger HP motor at a higher RPM (which as others have said is not necessarily a good idea). Running at a higher RPM requires changing pulley size if belt driven. Not easy for direct drive.
The motor might run very slightly faster because it is not loaded as near to its rated HP. That would slightly raise the current and electrical power used. It should run cooler. I believe it runs more inefficiently, with increased losses. I believe efficiency is the major reason not to increase the HP. Starting current would be higher, which existing equipment might not like. But it should be up to speed faster.
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No heating air conditioning experience but could it be rather like dropping a V8 motor into a six cylinder vehicle and then finding out that the gear ratio is wrong and due to increased HP the next size universals and/or rear end ratio should be changed! There must presumably be a proper balance between speeds, the load of air to be moved etc. and arbitrarily changing one factor might need some other adjustments? Could also be like trying to use a large pair of kitchen shears to cut ones fingernails; or a using a chainsaw to try and cut plywood panelling? Reminds one of friend who, years ago, put in a 'waaay too large' an oil fired air heating furnace. He got it at a bargain! It ran so infrequently, even in very cold weather, that it was highly inefficient; finally replacing it many years later, with electric heating.
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It's similar, but you're not changing the gear ratios. You have a larger "motor", but are not asking it to deliver more power. The speed of an AC (induction) motor is more or less determined by the line frequency. A motor with a higher HP rating will turn the same blower at the same speed (give or take a little).

Talking about jumping from the frying pan into the fire!
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The HP depends on the size of the fan blades/cfm the blower has to move. If the HP is too much it will only draw more current and cost more to run if the relay and wiring will handle the larger current. If the HP is too small for the blower fan, it will overload the motor, it will slow down and burn out and not move enough air. This could also lead to a burnt out firebox if gas or oil and poor cooling for the airconditioner.
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Ralph Mowery wrote:

Having recently put in a new heat pump, the docs also indicate that too high an airflow will blow condensation off the coils, missing the drip pan and making a big mess.
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On Sun, 28 Mar 2010 21:00:52 -0500, "Ralph Mowery"

The current won't be significantly higher unless the fan is larger. It will draw a little more and is a (small) waste, but it's not likely to bother the wiring.

Yes.
It *should* shut itself down if there is (dangerously) insufficient airflow.
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So long as the motor is not exceeding it's rated current (Amps) depicted on the nameplate, there is no need to worry. There is no point in putting in an oversized motor. So, if you're worried, you need to borrow a meter that can read amps & check it out. But if it was 3/4 HP before, it's gonna be 3/4HP again.
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harry wrote:

Hi, Original motor spec. is 3/4, 2/1, 1/3 Hp, 3 speed of which first two speed are used high speed for cooling and medium for heating. The 1.0 Hp motor had 1.0, 3/4, 1/2, 1/3 Hp 4 speed. My thought was with this motor I could have more options. Also this motor had ball bearings vs sleeve bearings. I understand ball baerings last longer. Any how furnace is back up and running again. Next is time to replace it when breaks again. Price difference was like ~40.00
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Tony Hwang wrote:

If the 3/4 HP motor is large enough you don't need the extra power. Then the comparison is the different motor speeds. I would be cautious changing RPMs from the manufacturer options.

Sounds reasonable.
Some motors are more efficient than others. Electric cost can swamp out value of different lifetime and cost of a higher efficiency motor.

I believe, other things being equal, the 3/4 HP motor run near its rating is more efficient than a 1HP motor run below its rating.
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Ideally, you put in whatever was there before. If you took out 3/4, that's probably because the manufacturer thought that was the right size.
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Did you replace with the same rating, I would not have changed for a different rating without proper testing
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In typed:

What's the old motor? Copy it, unless there are reasons not to.
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