how to increase a furnace fan speed

tobybul · 07-04-2012, 01:28 PM

Quote:

Originally Posted by beenthere

I take that to mean you have never measured the static.

No I have not. But its fair to say that I have much less static thru these filter system than if it was just the standard flat filter perpendicular to the air flow. Plus, I've designed plenty of commercial systems to know.

**beenthere** · 07-04-2012, 01:33 PM

V shape filter set ups are very turbulent. You should know that.

Commercial usually has a T&B done, and the pulley is sized/adjusted to get the required air flow. Seldom are they low static duct systems.

tobybul · 07-04-2012, 01:50 PM

Quote:

Originally Posted by beenthere

V shape filter set ups are very turbulent. You should know that.

Commercial usually has a T&B done, and the pulley is sized/adjusted to get the required air flow. Seldom are they low static duct systems.

I have to disagree. The velocity thru a filter with more surface area is slower than if it were flat thus reducing static or "turbulence". Of course commercial systems are designed with higher static pressures for obvious reasons. They push more cfm because they serve more square footage. Residential systems are proportionally smaller thus can be simulated accordingly.

**beenthere** · 07-04-2012, 02:07 PM

There are videos showing the increased turbulence of the V shape filter set up.

If, IF, your current blower is moving 1200, and its a 1/3 HP motor spinning at 1075 RPM. Then you would need a motor RPM of 1290 to move 1440CFM(the 20% you said you want). And the new motor would need to be a 1/2HP with a service factor of 1.15. If your current motor is a 1/2 HP, then the new 1290 RPM one would need to be a 1 HP motor.

Doc Holliday · 07-04-2012, 02:40 PM

Quote:

Originally Posted by tobybul

I have to disagree. The velocity thru a filter with more surface area is slower than if it were flat thus reducing static or "turbulence". Of course commercial systems are designed with higher static pressures for obvious reasons. They push more cfm because they serve more square footage. Residential systems are proportionally smaller thus can be simulated accordingly.

Always amusing to see a newb argue with BT.

Batter up!!!

tobybul · 07-05-2012, 04:22 AM

Read your fan curves. Static pressure is not a bad thing. You just need to overcome it.

**beenthere** · 07-05-2012, 05:14 AM

When you have to increase RPM and HP above normal, which increases operating cost, then the static is too much. Which is a bad thing.

tobybul · 07-05-2012, 08:51 AM

Quote:

Originally Posted by beenthere

When you have to increase RPM and HP above normal, which increases operating cost, then the static is too much. Which is a bad thing.

Am aware that there may be a need to jump from 1/2 hp to 3/4 hp motor resulting in to an increase in utility cost. But I am willing to accept that if it will improve my heating an cooling. A small price to pay for the convenience.

Better than what the mechanical contractor was proposing (take measurements $600, change to variable speed motor $1000) when in the end I know what results will be. It'll be a while before I recoup the cost. Motors are cheap and I can do it myself. This is not a commercial system with 40 hp-100 hp motors where I'm spending $10000-$20000 a year in utility cost.

Doc Holliday · 07-05-2012, 09:23 AM

Young Skywalker < YODA

kilosos2 · 07-05-2012, 10:16 AM

Toby

Why don't you just adjust your supply runs? Isn't that better to do and most likely cheaper? You just need some material from the supply shop... I mean why mess with your system if nothing is broken. You may end up with a bigger headache. I would change my ducts if I can access them.

Doc Holliday · 07-05-2012, 10:36 AM

It's indefinite that his system will mess up, what BT just said. Higher static pressure will REDUCE efficiency as it extracts less heat due to air flow restriction equaling warmer and more humid supply air, eventual catastrophic floodback, dead compressor.

Good luck OP.

tobybul · 07-05-2012, 02:54 PM

Thanks all for your concern. I know I can make it work by increasing the cfm. Think of it this way: If this were a belt driven blower with adjustable pulleys, I would not even have this discussion. Unfortunately, its a direct drive. Finding a higher rpm motor to make it work will be my biggest challenge. The unit may be limited to just that size motor but I can't imagine that to be the case. I just need to get the specs of the existing motor so I know what to upgrade it to.

As to the ductwork, my system is properly ducted. It just needs a bit more airflow. This is not rocket science for me. Nothing catastrophic will happen. This is merely a modification on the motor. I expect that the worse that will happen is a higher supply air temp in cooling mode and a lower supply air temp in heating mode. Also, there is a slight possibility of air noise due to the increase air flow. But as long as I stay within the velocity range of the duct and the coil, it should be good. Consult your ASHRAE manuals and fan curves.

And the system will be easy to balance after.

Doc Holliday · 07-05-2012, 04:38 PM

Quote:

Originally Posted by tobybul

Thanks all for your concern. I know I can make it work by increasing the cfm....

As to the ductwork, my system is properly ducted. It just needs a bit more airflow. This is not rocket science for me.

Good luck, bud.

**beenthere** · 07-05-2012, 06:55 PM

Quote:

Originally Posted by tobybul

Thanks all for your concern. I know I can make it work by increasing the cfm. Think of it this way: If this were a belt driven blower with adjustable pulleys, I would not even have this discussion. Unfortunately, its a direct drive. Finding a higher rpm motor to make it work will be my biggest challenge. The unit may be limited to just that size motor but I can't imagine that to be the case. I just need to get the specs of the existing motor so I know what to upgrade it to.

As to the ductwork, my system is properly ducted. It just needs a bit more airflow. This is not rocket science for me. Nothing catastrophic will happen. This is merely a modification on the motor. I expect that the worse that will happen is a higher supply air temp in cooling mode and a lower supply air temp in heating mode. Also, there is a slight possibility of air noise due to the increase air flow. But as long as I stay within the velocity range of the duct and the coil, it should be good. Consult your ASHRAE manuals and fan curves.

And the system will be easy to balance after.

If your system was properly ducted. The those 2 rooms wouldn't need more air.

The motor you want will be a special order. And if your current motor is 1/2HP, you'll need a 1 HP, not a 3/4HP.

This is what you want