hot air blower

Mellow_2

I think we found the problem with the noise. The old system was installed in 1952 and I did not think about the amount of dust that could have been collecting in the returns. The pic is looking in the end of a return duct (we cleaned the duct going up the wall that is why you can see the metal). I removed one of the end caps to find this dust in the duct. this proves to me the unit is only getting about 1/2 the return air it needs. I can see how 1/2 the return air can make things louder and make the fan run longer. the ducts close to the unit were cleaner so I didn't see the problem at the time of install but we wanted to return to find the problem so we did.We have a duct cleaning service on the way and can not wait to see the improvement. thanks for all the helpand ideas.

Mellow_2

fan noise

I have a new furnance that makes alot of air volocity noise. The old furnance was a belt drive from fifty years ago and I know the ducts are too small for the new furnance. The new unit is the smallest direct drive unit and set on the lowest speed but I still have some noise from the speed that the fan runs. I can't change the ducts but what could I do to slow things down. I think by changing some of the regesters it would let more air out but what about a bypass between the supply and return plenums? Would that slow the air though the ducts? I know by adding more ducts it would move more air but I do not have that option here. The old unit heated though the ducts with very little noise. Thanks for any help. jake

Brad White_51

Noise

Being a subjective thing sometimes, are you sure the noise is velocity noise? That tends to be a higher pitch (versus a rumble as would be the fan).

If it is a rumble or drumming noise it could be impact noise where the fan discharge hit the opposite wall of the plenum before changing direction.

Yes, try opening the dampers as much as possible but weigh that against room comfort. Ideally the furthest rooms need no damper at all (ie: Fully Open if they are there). As you get closer to the furnace and rooms get too much air, close them down. But that too can create noise as you absorb excess pressure.

Think of the duct system as a balloon with controlled leaks (your registers). Squeeze one, the balance goes out the others.


Some judicious duct lining may help here, even a "target area" opposite the fan discharge. Short of changing the ductwork I do not see a tremendous amount of viable options besides what I note here.

Before I would change the fan speed, I would want a full understanding of the BTU output and air flow at your lowest setting. You may wind up increasing the discharge air temperature unacceptably. Is this a condensing furnace? They often have higher airflows to enhance the condensing process (lower air temperatures too which is why you need more airflow).


You could do a bypass between supply and return, but you would lose efficiency and save no money. Don't forget your pressure differential is greatest there so a little crack in a damper means more air bypass than further away from the furnace. Hard to control.

CHARLES_4

Noise

Brad touched on a subject that's going to dictate just how much you can reduce air volume/noise. Check specs for temperature rise through furnace. You can only reduce your airflow to that maximum allowed value. Two ways I've been able to do that is by 1) Installing an air shutter @ the blower housing opposite the motor. Adjust as required, not exceeding maximum temp rise. 2) Install a speed control @ the motor. Again, adjust to temp rise but also stay within spec on motor amp draw. Before you start either project, check temp rise through furnace. If temp rise exceeds spec, call the installer back and have him explain why an oversized unit was such a good idea in the first place...

Techman

duct noise.

Hi Jake .what size supply and return duct's do you have ? What size furnace/fan do you have ? Checking your temp rise across the heat exchanger will tell you alot about your furnace sizing and CFM flow.

Brad White_52

That's one for the books-

How gross is that?

You know, one time I had a "no heat" call, lots of burned fuel and no air delivery to speak of but not much noise. HUGE fuel bills. I lifted the cast iron floor return grille. It became apparent that it had been used by the housekeeper as a place to sweep to. The duct was all but full to within a couple of feet below the floor. Bottom collar was sucked in and that is all the return air she got.

But never was a noise issue- Another angle. Learned something once again- thanks.


Your sleuthing paid off; let us know how it works out, would you?

BEW