Blower motors are rated in HP. Volts x Amps = Watts. I took an amp reading YESTERDAY on a 240v blower motor and it was drawing 1.65 amps. The older the air handler the less efficient the motor. Appliances of all sorts are more efficient. I would expect most blowers 10 years or younger to draw 1.5-3 amps.
If the furnace is in heating mode, and the t-stat is switched to the fan-on position, then I would assume (hope) that the blower is operating at its minimum speed and thus lowest wattage. While the power draw is dependent on resistance to airflow, that particular furnace blower seems capable of running at under 100 watts.
I could see adding a supply run perhaps. But I’d first do the return and turn the fan on. Because that might work.
If it’s a standard PSC motor (and it probably is), switching it to “fan on” will turn the high speed on typically. (You can’t tell the difference frankly berween 100-200 CFM) In the end after taking amp draws on hundreds of motors, they’re usually 3 amps or less. Usually. I haven’t had ONE customer in 25 years say, “man that really jacked up my bill!”
This furnace is a TUH2C100A9V4VAB high efficiency unit with a variable speed 3/4 HP 115v motor (ECM, NOT PSC). As per https://www.bayareaservice.com/wp-content/uploads/2011/12/XV95-Performance-Data.pdf (thanks for the link), the specs show anywhere from 100 to 650 watts of power depending on stage, airflow setting and static pressure – none of which I can assess myself.
I presume Stage 2 means full-on heat mode, so Stage 1 probably applies for continuous operation, and ramp up/down, but that’s a guess. I’m not even going to look at the DIP switches that apparently control the airflow setting, as I’m not an HVAC pro and am not qualified to think about them let alone change them. We run pretty low resistance filters now, after being strongly advised to do so when we had the central air installed.
But all of this still just adds up to guessing.
It would be nice to be able to know, and control, how fast or how often the fan is running when it’s just circulating air. I don’t see any way to do that, and the documentation doesn’t seem all that helpful.
– KW
Typically an ECM motor—-vs a PSC motor—-reverts to a lower speed when the fan is left on. That’s very, very good.
A PSC motor, by contrast, defaults to high speed. (An ECM motor has no defined speeds; think of the dimmer switch on a dining room wall as an analogy)
You have a great furnace. I MIGHT add a supply register downstairs. But you have an IDEAL situation for continuous fan operation. IDEAL.
Just don’t cut in a too large return grille downstairs. Because if that blower can get ALL the air from downstairs it requires—-it will. And you’ll draw very little air from upstairs.
What’s the model number?
Oh I see it
You have a 100k btu furnace. I’d typically associate that with 2600-2900 square feet. (And there’s latitude with that number)
Typically on a high efficiency furnace you’ll get around 115 CFM per 10k btus.
2 1/2 or 3 ton AC? On high speed you’re probably moving 12-1400 CFM. On fan only maybe 40% of that. (W/ no demand for heat or cool)
That’s adjustable via those dip switches. But that’s for an experienced guy.
Thanks raindog.
I think the AC is 2.5. It’s more than adequate. Where we live it gets below -30 occasionally, rarely over 100F.
Not clear why too large a return downstairs is a problem. All the air winds up there anyway, and there are no doors blocking flow between floors.
I figured out why a secondary thermostat is a bad idea. Besides the furnace not supporting it, getting those two thermostats to work together well would be almost impossible. At most, a single stat with remote sensor would be needed.
So I’m agreeing with the continuous operation (although duty cycle control would be nice). Just wish i could predict the cost. Guess I’ll need to call a specialist for that.
KW
It just occurred to me:
A large downstairs return should work fine for slurping up the cold air that settles there in winter, but that’s not really ideal during times of poor sledding, when we want the AC to be sucking in upstairs hot air. Hmm. I’ll need to adjust for that.
One could almost say we want to heat the downstairs in winter and cool the upstairs in summer.
KW
There is a fairly scientific formula for calculating your heating & cooling needs; it’s called a ‘manual J.’
It’s not quite enough to put cool air in an upstairs room; you need to pull hot air back to be ‘recondotioned/ cooled ‘
If you pull ALL your return air from downstairs you’re more likely to ‘get away’ with that in HEATING. If you have problems it will be in cooling.
It would be easy to add a stat for fan only; it wouldn’t be complicated. It’s just unnecessary; the existing stat will do the same thing.
An ECM motor utilized DC electricity and uses 70% less energy. IMO it’s the very best motor for continuous fan—and I’d bet every other tech would agree. Good luck.
Thanks for all the help on this; it’s given me a much better understanding!
In summary:
I now see where the upstairs/downstairs return air balancing can be important depending on the season. I’m going to bear that in mind, definitely.
The difference between continuous fan operation and the theory of having separate thermostats for up and down, or separate sensors, for extended fan operation, is that when the fan isn’t needed for temperature balancing, it stops.
And although I now understand that this furnace has about the most efficient fan available – and where it’s power usage is documented – its actual power draw is still somewhat open to question, depending on settings. The only way I’m likely to get a clearer answer is to get an HVAC person to check it in person.
Thanks for the help!
You could probably get one with a damper that you can shut during the warm months, or fit it with some sort of cover painted to match the room.
Called Therm Guard which will cycle your fan automatically.
http://www.bearmountaindesign.com/
2 wire hookup and 1 button programming - it’s a gem
I used one for years and it worked flawlessly.