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Discussion Starter #1
I understand that a single speed air handler operates at 100% of its capacity. How about a multi speed? i was told they have different levels like low, medium and high and you set it to one of those.

Does a multi speed unit only operate at the level you set it to or will it change while it is running like a variable speed does?
 

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All air handlers are at least multi-speed these days, but the speed does not vary during a cycle.

They have multiple speeds so that installers can set them up to deliver proper airflow in each mode. Some air handlers can only be wired to use one speed, the better ones may use different speeds for continuous fan, heating and cooling.

The same concept applies to variable, but the motor automatically compensates for duct restriction. The flow rate is set and the motor varies its speed to deliver that.

If there's a 2-stage or variable speed a/c or heatpump connected to the air handler, the airflow will be proportional to capacity the unit is running at but otherwise won't vary once delivering the correct airflow for the mode of operation.
 

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Discussion Starter #3
Thank you. A variable speed unit will more run often, right. Sometimes at a lower or higher speed. A single stage comes on at full blast every time. I'm still trying to understand multi speed. If a multi speed unit is set for medium does it only run at medium each time it comes on? It is therefore working like a single stage unit? I am a little hesitant to go to a variable speed due to my duct work so I am trying to understand if a mutli speed would be ok.
 

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The concept is you have 1 stage or 2 stage furnaces and ACs. A few odd 3 stage units but they are rare.

There are very expensive variable stage furnaces and ACs which are called modulating units.

With a 1 stage unit you use the highest fan speed ( generally for cooling ) and medium for heat. With a 2 stage unit you use the highest fan speed for cooling, medium for 2nd stage heat ( full blast ) and medium-low for low fire.

With modulating units the ECM variable speed fan matches whatever % of full load the thermostat calls for and the furnace or AC follow suit.

Point is you need to know the difference between 1 stage/ 2 stage and modulating furnaces and ACs.

A modulating unit is kinda like your automatic transmission in your car. It shifts speeds depending on the load/how hard you press the gas or whether you are pulling a trailer or going up a hill.

How often a unit runs depends on how well your house is insulated.

Modulating units try to run the furnace or AC at its lowest % so you get more even heat or cooling and more comfort.
 

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Discussion Starter #5
Thanks. The unit I am looking at is classified as multi speed. They say it has 3 speeds, low, medium and high, which are permanently set. Once that speed is set I assume that is the speed it always runs at, correct? If that is the case then the unit does not perform like a variable speed which will shift the speed it runs at. Is that correct? I don't want to go with a variable speed. I want something that runs at the same speed and am trying to determine if a multi speed does that.
 

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What is the brand name and model # of the unit.

Is it a ductless minisplit or a AC with a regular air handler.

There are lots of different combinations of air handlers and furnaces and ACs and heat pumps.

Air handler is a generic term. What type of furnace or AC it is married to it what matters.
 

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Discussion Starter #7
Payne PF4MNB043L00 is the air handler. Not a ductless. There is an outdoor single stage heat pump PF14NB
 

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Your air handler has a ecm motor but doesn't compensate directly to maintain proper airflow.

For a single stage heatpump the blower speed gets set to whatever is required to get proper airflow on the duct system and left alone.

It may have separate taps for continuous fan and straight electric heat.
 
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