Upgrading blower motor

[dbooksta]

What do I need to consider if I want to upgrade to a faster blower motor?

I have a forced-air, oil-fired system that has had its duct, filter, and living area volume expanded by about 50% over the original install. Airflow to the system ends is pretty weak (my subjective assessment).

Obviously the heater core and blower unit are staying the same size. (Blower wheel is 10"x10" and rated for 1750RPM.)

Current motor is 1/2HP 3-speed 1075RPM. (9amp, 115V)

Do I need to boost my HP-rating or change any of the other motor specs if I'm looking for a 1625RPM replacement?

Do I need to worry about anything in the system's functionality or reliability if I put in a 1625RPM motor?

[2hot2coolme]

Why in the world was the ductwork increased by 50%???

Sounds like everything was increased but the heating system.

[markwolf]

"feeling"air is a bad thing it is uncomfortable.If the rooms are within 2 degrees of eachother & the furnace is keeping up don't worry about how much air you"feel"Moving to much air across a heat exchanger will cause you problems.Furnaces are designed to maintain a minimum & maximum temperature rise across thier heat exchangers.If your rooms are not within accepable temperatures then contact a contractor to look over your system.

[dbooksta]

I added an addition and increased the plenum and filter while I was at it. So yes, everything was increased in size except for the blower-heater-A/C stack.

In general the system can "keep up" in terms of keeping the rooms within a few degrees of each other. But during extreme weather it does start to lag.

So if I raise the RPM from 1025 to 1625, how much will that boost the airflow over the heater core? And what problems can that cause?

[Gunnery Sergeant Hartman]

More air moving over the exchanger faster will result in cooler supply temps when heating, longer run times, and higher energy bills.

Why not call a pro and get the correct system for your home?

[dbooksta]

I believe that supply temp during heating isn't a factor because I run my blower continuously anyway (both to filter the air and to keep my basement humidity and wing temperatures balanced since I only have one zone).

Guys, I'm not looking to put professionals out of work -- I pay to have my oil heater serviced every year. But neither am I interested in buying a new heating system just because I want to increase airflow, nor would I drag a contractor out here just to ask him what specs would be appropriate on a higher-speed motor.

[just_opinion]

dbooksta,

increasing the blower speed (hoping to increase the airflow)
is not an easy task as you think. It is not straight proportion.

you must know the
a) current CFM of you blower
b) current static pressure
c) current TRUE RPM of you blower
d) current fan performance of your wheel (rated 1725 rpm on you current wheel means it can withstand the RPM. But at what static pressure??). So you can see how fast you can run your wheel. If the wheel runs faster than its performance curve, it will free wheel and not deliver the CFM.

So it is complicated.

But if you want to trial and error - the motor you need is 1HP. Once you buy it you keep it.

[mark beiser]

To get a 50% increase in airflow with the current blower would require a huge increase in the motor size, and is likely not practical, if it is even possible.

Originally posted by dbooksta
Guys, I'm not looking to put professionals out of work -- I pay to have my oil heater serviced every year. But neither am I interested in buying a new heating system just because I want to increase airflow, nor would I drag a contractor out here just to ask him what specs would be appropriate on a higher-speed motor.

From what you have described was done with the ductwork and addition, you most likely NEED a new system, or a 2nd system, to keep up with the increased heating and cooling loads you have now. Just moving more air through the system won't do much to fix your problem, and will likely decrease your comfort in both the heating and cooling modes.

[dbooksta]

Yeah, I know I'm not going to get 50% more heating/cooling out of the existing system.

I can live with the system the way it is; I just thought that if I can drop $100 into a new motor to boost my airflow, and I'd be a little more comfortable, then why not. I know I won't get any more heat or cooling, but if I raise the blower RPM 50% and only get, say, 20% more airflow for 25% more electric consumption, that's fine with me. (Given that the blower's running constantly anyway, higher airflow just means it's blowing conditioned air that's less hot or cold per unit volume than before, which to me is actually MORE comfortable. I don't like walking by a vent and getting a huge temperature differential.)

So my question is really, "How can I avoid doing this wrong?" I don't want to try something that could burn out a motor, waste gobs of electricity, damage my heater core, or something like that.

If I'm hearing this right, my risks are:

1. Free-wheeling the blower. Which just means that I've hit the limit of what it can move given the current enclosure and pressures. (BTW, my static pressure across my return filter is .25" WC.) Shouldn't cause any damage or burn much more energy, right?

2. Incorrectly sizing the motor. If I undersize it I'll burn it out, but if I oversize it then there's no damage or energy waste, right?

[just_opinion]

Here go to this website and good luck.

www.edasolutions.com/Groups/Tech/HVACCalcs.htm

[dbooksta]

Wow ... according to that it'll take 1.7HP to get me to 1625 RPM, and my static pressure will rise to .6"! So I guess this isn't a possibility after all.

Thanks for the pointer.

[markwolf]

Originally posted by dbooksta


If I'm hearing this right, my risks are:

1. Free-wheeling the blower. Which just means that I've hit the limit of what it can move given the current enclosure and pressures. (BTW, my static pressure across my return filter is .25" WC.) Shouldn't cause any damage or burn much more energy, right?

2. Incorrectly sizing the motor. If I undersize it I'll burn it out, but if I oversize it then there's no damage or energy waste, right?

3.moving to much air across the heat exchanger/improper temperature rise causing flue gas condensation,resulting in possible life & property damage.

[mark beiser]

Originally posted by dbooksta
Wow ... according to that it'll take 1.7HP to get me to 1625 RPM, and my static pressure will rise to .6"! So I guess this isn't a possibility after all.

Thanks for the pointer.

Yeah, thats why I said it wouldn't be practical, if it is even possible.
The fan laws kick you in the arse in a hurry when you try to increase airflow with an existing blower setup.

If you were able to pull it off, the noise would be simply amazing too.

[aircooled53]

Hey, mark I thought that I saw you the other day off Haltom road, I was just finishing maintenance Estes Park and either you or one of your vans turned on the same street east.

Any how, wave if you see an AirWise van it's me. Only one van the rest are pick-up trucks.

[mark beiser]

Originally posted by aircooled53
Hey, mark I thought that I saw you the other day off Haltom road, I was just finishing maintenance Estes Park and either you or one of your vans turned on the same street east.

Yeah, I was over there on Rushing Creek Ct wednesday, I saw your van too.
We are only a 2 man shop, and mine is the only van.

[dash]

Want more air flow to the additioal space,why not just run a 10 or 12" flex from the plenum,to where the trunk duct starts for the additional space,if space is available to run it.


This will lower the static greatly and increase air flow to the addtion.Can use a larger or smaller duct for this,may want to put a damper in it to balance air flow.

[dbooksta]

Dash,

Agreed -- opening up the ducts is a lot more straightforward. (Anyone else looking to expand your airflow, do this first!)

I expanded the trunks of the original system from a 216sq.in. cross-section to 288sq.in. I also put a booster fan in the run to the addition.

I guess I should check the pressures to see how much could be gained from bigger ducts (assuming I can find room).

What's a reasonable goal for static pressure level between supply duct and room?

[Lorne]

If your pressure drop across your filter is.25 your TSP is going to be to high. .1 is a good drop across a filter. Your duct work is the issue. If the temperature getting there is considerably lower maybe insulating the duct is the answer if you don't want to change the duct. Try using a less restrictive filter, just a cheap throw away. Just for a test remove the door to you furnace(unsafe to leave off and see how much air you get.

[climakers]

If you look at your furnace nameplate, it will state that the maximum external static pressure is .5". If you have air conditioning, the static pressure across a dry coil is usually around .2". Your filter is .25, that only leaves you .05" for both the supply and return ducts. Chances are, your system is probably operating off of the fan curve ( static pressure too high).

If you get the static pressure below the maximum .5", then you could start to close dampers close to the furnace to get more air to the end of the line.

To accomplish this, you need to get a professional certified in residential airflow balance to measure the total external static pressure and plot that out on the fan performance table for your furnace. That way he can tell you if you are delivering the proper amount of air for the furnace. He can then measure the static pressure of the return air ducts and supply ducts to determine if they are properly sized. He will also have the capability of measuring the air flow supplied to each room. After performing a room by room load calc, he can tell you if you have enough air to each room.

After measuring supply and return duct static pressures, he should be able to recommend appropriate corrections.

Changing the motor is not going to deliver any more air on a fan that is off the fan curve.