3-speed Air Handler


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Old 05-07-13, 07:53 PM
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3-speed Air Handler

3-speed Air Handlers are common w/ split Heat Pumps (e.g. Goodman, et al); however, it appears that only 1 speed is selected at installation and that is that. Seems to me it would be better to have controller to select low speed to dehumidify more and a higher speed on really hot days. Is there a reason such controllers are not in 3-speed units? Would it hurt the motor to change speed while running? Do 3rd parties provide such controllers? Thanks.
 
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Old 05-07-13, 10:13 PM
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A 3-speed air handler is really a 1-speed air handler. If you want a variable speed air handler, they have that.
 
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Old 05-07-13, 10:40 PM
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The problem with changing the speed of the air handler is that there would need to be circuitry installed that monitored things like evaporator coil temperature as well as return temperature so that the coil didn't freeze up. If you slowed air flow across the coil then the compressor would have to cycle continuously.

I haven't seen any third party controls that do what you want.
 
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Old 05-08-13, 07:32 AM
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Yes, a 3Speed is in effect a 1Speed, but does it have to be? At initial purchase there is a significant premium for VS and for those w/ existing, working 3S it is very expensive to upgrade. Can 3S be better w/ a simple control?

Please see more in my reply to Pjmax.
 
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Old 05-08-13, 07:36 AM
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Pjmax, I think you ans too quickly. Slowing the air flow across the coil is what VS systems do. Right?

I am an EE & Mathematician mostly developing SW (from embedded micros to PCs) and recently developed energy analysis for an international company which got me involved w/ HVAC. By happenstance I now need to install a new system at my home. Started reading complex ASHRAE eng HVAC theory but it's not immediately practical info.

1. I don't know how tech decides Lo, Med, Hi Speed at installation, but a little reading leads me to believe the lowest speed that works is best. Please correct me.

2. Suppose a system would work at Lo & Med. Wouldn't it be better comfort & better system run Lo under mild weather & low demand and Med during summer heat & high demand? Further, my understanding is that it takes several minutes (maybe as much as 20?) for compressor to cool AH heat exchanger to its min temp. If so, even on hot days, it would be better to run Lo for first few (20?) minutes.

3. If #2 is true, I know there is no problem starting at Lo or Med, but I don't know if switching while running is a problem for the motor. I know they use 3-taps on the same windings.

4. Controls are cheap for me to make for myself & friends.

Thanks.
 
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Old 05-08-13, 08:58 AM
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I'm not an AC tech, but I do know most of these assumptions are incorrect.

> 1. I don't know how tech decides Lo, Med, Hi Speed at installation, but a little reading leads me to believe the lowest speed that works is best. Please correct me.

Nope, not the lowest that will work. A 2-ton AC needs less airflow than a 3-ton, so the adjustment is based on AC capacity which is why it doesn't ever change. The "3-speed" allows the tech to pick the fan speed to match the AC capacity without having to stock 3 units.

> 2. Suppose a system would work at Lo & Med. Wouldn't it be better comfort & better system run Lo under mild weather & low demand and Med during summer heat & high demand? Further, my understanding is that it takes several minutes (maybe as much as 20?) for compressor to cool AH heat exchanger to its min temp. If so, even on hot days, it would be better to run Lo for first few (20?) minutes.

Variable speed air handlers go hand-in-hand with two stage compressors that adjust their capacity based on conditions. If your capacity isn't being changed, than the fan requirements don't change. VS air handlers do sometimes slow a bit for better humidity control. My Trane VS unit does run the fan slower during coil cool-down, for a total of ONE MINUTE. Fan speed has nothing to do with weather conditions with the possible exception of humidity control.

A 3-ton unit puts out 3-tons of cooling no matter how fast or slow the fan run. If the coils don't have enough air flow, the unit efficiency will rapidly drop, and the coils will eventually freeze up. An AC compressor doesn't magically slow down because the fan in the air handler is slowed.

> 3. If #2 is true, I know there is no problem starting at Lo or Med, but I don't know if switching while running is a problem for the motor. I know they use 3-taps on the same windings.

A VS motor uses much less power in low speed. A "3 speed" motor, which is a completely different design, does not use less power, in fact, I've read it can actually use more.

> 4. Controls are cheap for me to make for myself & friends.

They are, but variable speed DC motors are expensive, and that is what you are paying for. Price out how much one of those costs to build also.
 
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Old 05-08-13, 01:40 PM
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Blower speed, or more correctly, airflow is determined on the heating system by the temperature rise across the furnace heat exchanger. Many things affect this such as the firing rate of the furnace and the ductwork installation. Generally the airflow is set to the lowest point that will not allow an excessive temperature rise across the heat exchanger because higher airflow will tend to be noisy.

Air flow on the cooling system is somewhat constrained by the duct installation. Ideally, for cooling the ductwork would be larger than for straight heating allowing for greater airflows with lower velocities. Higher velocity means more noise. It is necessary to have a greater airflow with cooling primarily because the temperature difference between the colder air supplied to the room and the desired room temperature is smaller than when in heating mode. Since it is all about BTU transfer it is easy to see why cooling requires a greater airflow than heating, all other things being more or less equal.

Many (most?) furnaces will have a fan relay on the control board and this relay is wired in a manner to allow a slower blower speed (terms fan and blower used interchangeably here) for heating and a higher speed for cooling. This is accomplished by the control board logic operating the blower through the normally closed contacts of the fan relay and a temperature switch in the furnace. (Some systems use other means but the same principle.) The main room thermostat usually has a fan auto-on switch and this switch is normally left in the auto position. When this switch is moved to the on position it energizes the fan relay and this action will open the contacts to the lower speed and close the contacts to power the blower at a higher speed. Even when left in the auto position when in the cooling cycle the fan relay is energized from the thermostat and running the fan at the higher speed.

Air conditioning systems are essentially a refrigerator and by the refrigeration cycle they move heat from inside the house to the great outdoors. Some home A/C systems use a fixed orifice to meter liquid refrigerant to the evaporator coil (the cooling coil in the air handler or furnace plenum) and these systems do indeed work at one set capacity. The orifice may be called a piston or capillary tube or some other name. Better systems use a Thermal Expansion Valve or TXV to meter the liquid refrigerant. The TXV has the ability to meter more or less refrigerant to maintain a relatively constant temperature on the refrigerant outlet from the evaporator coil. This allows for some variation in the capacity of the entire cooling system. ALL systems with variable speed blowers will use a TXV. Still, even with the TXV the range of airflow is relatively small. Too low an airflow and the coil will start to freeze and too high an airflow and the condensed moisture from the circulation air (humidity removal) will be blown off the coil and re-evaporated in the airflow. The subtle changes in airflow used with a variable speed blower simply cannot be matched with a three or four speed fixed speed motor.
 
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Old 05-08-13, 05:10 PM
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Airflow requirements for furnace have increased significantly over the last 20 years, so as much airflow could be needed for heating as is needed for cooling in cold climates.

A certain amount of air is needed for proper operation -> for cooling, it's 400-450 cfm per ton. Get too far below that and the evaporator coil will freeze.

For heating, it really depends on the type of furnace. An older model may run at a 140+ F supply air temperature while most newer models run at 110-120F under ideal conditions.

Now, air handlers are different from furnaces in that heating and cooling may use the same blower speed depending on the controls.

There are aftermarket variable speed drive controllers for multi-speed air handlers though. One of them is called fan handler.
 
 

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