dogwood said:
Heaterman and ewdudley, I am putting in an oversized w/a heat exchanger over my forced hot air furnace. Might it be workable to avoid impeding the air flow too much and overtaxing the blower motor, to find a way to switch out the w/a hx out seasonally with the a/c cooling coils over the furnace. The blower motor on my Lennox Pulse furnace is only 1/3 horsepower and my w/a hx is just like the one pyrbyr pictured in previous posts. I'd appreciate any thoughts on the matter.
Mike
It makes perfect sense that adding restrictions to the air ducting system would add load to the fan motor, but please consider that it's not true.
Adding restriction to the system will simply move the operating point of the fan further to the left on its fan curve. This means that as restriction is increased, flow decreases, and the pressure drop across the fan increases. Here's a nice short article on the topic:
http://www.jeacoustics.com/library/pdf/72-Evans.pdf
There's two ways you can get in trouble with the fan motor.
If the motor is too small for the fan itself then it will work too hard and overheat. This is a design issue. The motor could only be too small if the unit was badly designed or if somebody swapped-in an unsuitably dinky motor. If the motor is too small then you'd likely be in trouble at any point along the fan curve.
The other would be a situation where air flowing through the duct is required to cool the fan motor. If this is the case then restricting flow
way too much would allow the van motor to overheat for lack of ventilation to the fan motor itself. And again, only if the motor needs airflow from the fan it is driving to keep itself cool.
So having too much restriction -- or too little restriction, for that matter -- is an efficiency problem, not a motor overloading problem. If your air handler is designed for a certain flow at a certain pressure drop at a certain RPM, then adding restriction would cause the fan to operate more into its stall region, resulting in somewhat more noise, somewhat less air flow, and -- counter intuitively -- somewhat less power input from the motor.
You can prove this to yourself by putting an ampere meter on your fan motor and record current draw as you increase restrictions in the system by closing dampers or stacking stuff on top of the registers. Or place your hand over the outlet on your shop vac and listen to the motor speed increasing towards synchronous speed, which shows that motor load is decreasing. Or note that the engine RPM decreases at constant throttle as a light airplane takes off down the runway.
--ewd