OT, need efficient motor for large attic fan

Hi all,

I just got my A/C bill for the last month and it is over $300., that is way high. My A/C system is new however I have a big (4 or 5 feet dia.)attic fan that runs all the time from April till it cools down. It is 40 to 50 years old. I suspect it is terriably inefficient when it comes to energy use. I think it is 1/2 to 1 hp ,1725 rpm,110 vac and a NEMA 56? Frame motor. It is about 140 up there so I’m not going up just yet to read the exact data off it.

What would be a good replacement and what kind of saving might I expect? I remember years ago thinking I should do this and then it cooled down and so did my inspiration but now at $300 + per month I’m motivated again.

Thanks in advance.

besides switching motor, consider a thermostatic control. it isn't always 140 up there is it? is your venting set up right? seems like it should be cooler with the fan running,

You should also consider checking/replacing the fan bearings. Even a little excess friction will add up over time.

I live out here where it get stupid hot and this fan just takes the heat out of the attic. It lowers the upstairs temputure by at least 10 degrees. This is a 100 year old house with no insulation and original windows. The system all works fine it is just fighting an old house an 100 degree weather that doesn’t cool down that much even at night.

This is also the first August I have been home in 14 years.... normally I’m where it is much cooler.

Good idea on the bearings....

Around 1981, I bought an "energy efficient" 1/2 HP 1725 RPM 110 V 56Y frame motor for belt-driven fan and blower service. I know it is efficient because it says so on the label and it is painted green. I think it was in use for about a year until I bought a whole new Lenox high efficiency HVAC system. I saved the near new motor when they hauled out the old furnace and still have it. If 1/2 HP will do you, it is yours for the shipping cost.

Larry

If your attic has a peak temperature of 140 deg F the fan is not being allowed to do its job. In the ideal attic layout the gable at one end of the house has a filtered inlet opening. The opening has a area of about 5 times the exhaust fan enclosed area. The filter can be a disposable element or a washable metal mesh design if rodents or burning embers from a exterior fire are a concern. The opposite gable has the exhaust fan. For a 2000 to 3000 ft**2 attic plan on 8 ft**2 inlet opening and a 1500 FT**3/min exhaust fan.If the installation is done properly the attic temperature will not exceed the outside air temperature. The objective is to replace the air inside the attic every 10 minutes.

The 24volt DC Comair 10" diameter fans are rated at 500 cfm at .05 inch water pressure with 30 watts power consumption . The full name is Comair Caravel CD24B3. You would need three of them for a 1500 ft**3/min airflow.
The fans will easily fit between the studs at 16" centers. If you do not install a large air inlet you will need to use an impeller type fan capable of working at much higher air pressures. The Comair 24volt DC 10" diameter impellers are designed to run at .85. " water pressure at 500 cfm. The impeller power required is 125 watts. There are better impeller designs available. A single large impeller will be more efficient than three smaller impellers. The DC fans are inexpensive. The impellers are very expensive.

The fans should be controlled with a thermostat and shut off using one of the home fire detector sensors.

Additions:

The attic inlet filter is something that is needed in a rural area. For a house located in a city the inlet filter could be replaced by a wire screen to keep the insects and rodents out. The wire screen will have a low pressure drop and will allow the attic inlet opening to be made much smaller.

There is one company that sells a kit consisting of a small 24 volt solar array, two of the Comair fans and a sheet metal housing. The housing is mounted between rafters at the roof peak midway between the gables. There is a hinged lid that lifts up when the fans are operating. The kit is intended for houses that do not have electrical service in the attic. The only difficulty is that an attic fan will often need to run for a hour or two after sunset.

Robert

I don't think your fan is the problem. a 1hp motor uses about
745 watts if my math is correct 745 watts per hour x24 hours
x 30 days = 536,400 watts used
536 kilowatts x .11 cents per kilowatt = $59 per month.

I could be more or less depending on price per killowatt.

Use clamp on amp meter to be more precise.

If you're using an attic fan with ac you're just sucking all the cool air out of the house.

So the fan is exhausting hot air from the attic?

Unless your house is huge, that should keep the place cooler. We have no attic fan, just vents, and mine only gets to maybe 125 or 130. With venting should do better. Definitely needs to be thermostatically controlled.

So, a 3/4 HP motor, at full load might draw as much as 13A. NEC rating, which is probably high, is 13.8A. That is at perhaps 0.6 power factor, so actual power is about 900W or so. Roughly a kwH per hour.

So, 24/7 operation means about 24 kilowatt-hours per day, times 31 days per month gives a maximum of about 744 kWh If you pay 12 cents per kilowatt-hour, that means the fan could cost up to about 90 bucks a month. Most likely it is less than that, but I don't know by how much. Your electric rate will change the costs, obviously.

Pretty much any motor is going to run about the same, max savings might be $20 or so. Efficient motors are probably only 15% better. You can probably do better than that by thermostatically controlling the fan at some reasonable temp. Then it will run only when you need it to, which could be half the time it runs now, considering nighttime, etc.

And maybe do better on cooling by improving the insulation between the attic and the top floor.

Also, make sure the fan has ample inlet area in the attic.... you do not want it to end up sucking cooled air out of the house......

2 options.

One is simple thermostat for on-off operation.

Second is locate a suitable 3 phase motor and VFD.

now a couple thermostats and some pots will allow multi speed depending on need.

As attic gets warm fan speed increases.

Can use a 3 speed swamp cooler motor but where is the fun in that?

Sent from my SAMSUNG-SM-G930A using Tapatalk

Solar Power could be an option.

<1 hp single phase and half hp 3phase motors usually dont break 70% efficiency and are run saturated with high no load losses, 300watts or so for a single phase 1/3rd hp motor.

One solution is to drop the applied voltage. For example i have a 1/4 hp 230v furnace blower operating on 120vac. Watts dropped from 300 to 86, after changing the pulleys to drop the fan rpm by <50%. So i move half the air for 1/4th the watts.

If you have a variac and a watt meter you can find out the optimal voltage, then buy a new larger motor (how large needs to be calculated) and run it at half nameplate volts.

Why not a couple solar panels and exhaust fans, no batteries, fans just run during day.

I live in the central valley of California and that fan sounds huge. Way too big for my climate if it has any rpm's. You say it is 140 in the attic with the fan running. I do not think my attic gets that warm with no fan just turbine ventilators. I assume your climate seldom goes above 100 or so?
I suspect you have way too little intake venting. You could make a simple tube mamometer to check attic pressure. It should not be more then a few inches if the venting is correct.
Intake vents are calculated by taking fan cfm and dividing by 300 to get square feet of venting intake needed. Note that you need to increase this figure by about 25% to allow for vent restrictions caused by bug screens, ember guards, spider webs, etc.
A 1/2Hp fan I saw vents 10,000 cfm so it needs 333 square feet of intake vent.
Logic should tell you that you need at least the same area as the fan so at least 12.5 square feet +25%. just to be anywhere near balanced.
Bill D.

With a $300 a month incentive, I'm sure insulation could be made to pay for itself in no time.

Blown in cellulose in the walls (old houses normally get the holes drilled outside,
and dense packed), and the attic.

I suspect that the OP's idea of an attic fan and that of most the respondents here is different. I grew up with an attic fan. It was about the size of the OP's. It pulls air through window and door screens and out through the attic. I never lived in an air conditioned house until I was 13 and in Alabama, Louisiana, and Georgia, when it finally came about it was amazing. The old attic fans were mounted vertically in the attic joists with a louvered shutter below them. When they were on the louvers moved slightly with the blades producing a rhythmic noise that lulled one to sleep. As clearly as I remember it, I do not want to go back.

I know about those.... they are common around here. Neighbors have one, wish I did, because we do not use A/C except in bedroom, and those fans really do ventilate well. Better than my big window fans.

But I assume the OP does not have that type because he mentions A/C, and the fan of the "house ventilating" type would completely nullify the A/C, by sucking outside air through the house.

Not sure about your utility provider, but added charges (delivery, meter read, meter and equipment maintenance, taxes, etc.) can account for 30% of the total electric bill. All those costs can increase incrementally and yearly.

johansen said:

<1 hp single phase and half hp 3phase motors usually dont break 70% efficiency and are run saturated with high no load losses, 300watts or so for a single phase 1/3rd hp motor.

One solution is to drop the applied voltage. For example i have a 1/4 hp 230v furnace blower operating on 120vac. Watts dropped from 300 to 86, after changing the pulleys to drop the fan rpm by <50%. So i move half the air for 1/4th the watts.

If you have a variac and a watt meter you can find out the optimal voltage, then buy a new larger motor (how large needs to be calculated) and run it at half nameplate volts.

Click to expand...

What about installing a 3 phase motor and VFD?

What do you feel would be a reasonable efficiency gain in a fan duty electric motor? 10 %? 15% would be unrealistic.

So instead of a $300 /mo electric bill, you would have a $270 bill.

Would that make you happy?

It would not make me happy. I would look for other ways to "keep things cool".

You know GRAVITY will cause warm air to rise.
hmmm.. there are possibilities there.

Now if you could help me with the electricity cost to run these d*** dehumidifiers.... It's been a wet summer!

Thanks everyone, to clarify the fan is not sucking the A/c out of the house. The attic is sealed off from the rest of the house.

A number of you brought up a good point, the intake vents. I had a new roof put on in Nov. and haven’t been in the attic since, I wonder if the vents are put in correctly? And not blocked... i’ll Have to check.

Also I have been keeping the house at 72 which is a bit cool.