Changing blower motor

dynamic

I have a customer with an oil fired furnace that the blower only produces 1300 cfm at .2 static. The nozzle size is .75, and the temp rise is 60 degrees. I need 1500 cfm's. The furnace is approx 12 years old.



I am toying with the idea of changing the blower motor to a more powerful motor so that I can get the necessary cfm's. I would only do this provided I can keep the temperature rise within the manufacturers specs. What are your thoughts, has anyone ever done this successfully ?

Paul Fredricks_3

FIrst off

What are the manufacturers specs? 60 degrees is a fairly low TD already. And will a more powerful motor turn any faster? And what does the manufacturer state for required static pressure? If they are looking for .05 you are already way over that. More air will make it worse. Why do you need more air flow?

dynamic

CFM

My customer added a second floor to his home, and the builder set up the ductwork for the second floor. The original trunkline, before the addition, which suppied the basement and the first floor is 20X8. Another contractor seperated the basement from the first floor and supplied the basement with 5 6" round outlets off of 10X8 trunk.  The first floor is still on the 20X8 trunk. The builder supplied the new second floor with 10" round and that floor has 11 outlets with 6" takeoffs. So as you can tell the duct system is in serious trouble.



I balanced the first floor and the basement to their required CFM's with dampers and a meter. I cannot get to the ductwork in the second floor to balance it properly. I am losing CFM's in the ductwork under the floor. I have 700 CFM in the main 10" going to the second floor but I am only getting 400 out of the 11 outlets. So the only thing that I can think of to come up with is to increase the blower speed to try to deliver more air to the second floor. I can always balance out the first and second floor with the dampers.



The only other restriction to overcome in the duct is the 1" filter at the return.



I just reread my original post and I actually have a 78 degree temp rise, the manufacturer post between 60 -90 degrees. So I have some room to work with.  





Paul, where did you come up with .05 ? I got to .20 static by measuring the airflow at the 10" going to the second floor. Their blower charts show between .10 and .50 static pressures.  I can really use any help that you can suggest on this one...

Paul Fredricks_3

Just saying

I know the furnace manufacturers usually spec an SP, I was just saying if they are speccing something much less than .2 that would also be an issue. I have no idea what they would spec for that. 10" is rated for 400 cfm at.1 static. If you have 700 going into the 10" that is going to be very loud. If you only getting 400 out of the 6" take offs, what happened to the other 300? Whatever goes in has to come out, unless the duct is constantly expanding



Also, 6" round is rated for 105 cfm at .1 static. 105 x 11 = 1155 cfm, and there's no way I can see you even getting close to that with a 10" feeding them.



Make sure the unit is sized to handle the load of the house. Find a way to get more ductwork upstairs. Is there a return trunk to the second floor? If so you could use that as addition supply air and let everything return down the stairs to the first floor. Not the best way to go, but it might work. Sounds like the builder did them no favors. And I don't think a bigger motor is going to do anything for you.

dynamic

SP rating

Hi Paul - The manufacturer shows that at a .20 static rating I should have 1250 cfm available. That is where I am seeing the .20. When I measured the 10" and have 700 cfm, that shows to be .20.  And it is pretty quiet, probably because of the 11 outlets with no flow. I measured the 700 in the supply just before it goes into the ceiling. I then measured each outlet on the second floor.



Now, I think that I am losing the cfm through the flex duct. I believe there to be excessive lengths of flex duct between the floors. The other issue is that I cannot reach the dampers to restirct some of the no-flow. For an example I have 300cfms in an outlet in the WIC. I have the floor diffuser closed but that really doesn't move the air to any other outlets. How would getting more ductwork to the second floor help me here ?  My concern would be that all three zones will call at once and I will not have enough air available for the first floor and basement. 

Jean-David Beyer

Have you looked for serious leaks?

If you put 700 cfm in and get 400 cfm out, the other 300cfm must be going somewhere.

In my Quaker meeting we had a problem like that. Turns out the original installer used elephant trunk hose (I do not know what it is really called) to go from the main ducts to the registers. He just slipped the hose over the fitting on the register. THis all in a crawlspace. He did not fasten the hose in any way (e.g,. clamp, screw, duct tape), so some fell off, keeping the ground hogs warm, but keeping the flow into the building cold. We replaced all the duct work for other reasons, and later the two hot air furnaces, but that got the air flow where it mattered.



It does not sound to me as though it is a resistance problem; or at least, not only an air resistance problem.