Pulsating air out of vents

[hvac7]

Went to a call today that the homeowners were complaining about the air pulsating out of the vent. Which is noticiable but not that bad. Real picky homeowners. The fisrt thing i did was place the fan setting on low and left the door of to see if it was starving for air but the unit still pulsated. I did notice a slight drop in power going to unit. I mean real slight drop, which would be .5 volts at most. This particular system is a 2 ton york. The duct system is fairly simply with only 7 supplies coming of 10 inch round duct off of each side of supply plenum. 4 supplies off of one side and 3 off of other. 6 inch flex for each supply. I know, I know do a manual d for duct design. But should the air still pulsate on low speed which according to papers with airhandler says at a .1 static pressure cfm is 650. Each 6" round is around 100 cfm but 6 inch flex duct only producing 80 cfm. If you take the 7 supplies and mutliply time 80 you have 560 cfm which still seems short. but enough to cause the pulsating?

[coolwhip]

Could be a weak motor winding.

[hvac7]

I should have mentioned that I did replace motor and capacitor because I to thought that something was wrong with the motor.

[rimek]

any chance the flex is sagging or too long and moving with the positive pressure?

[james mo]

I can't imagine a psc blower motor changing speeds in a way that would cause an airflow pulsation. I would be looking for things like water in the flex ducts where they sag. Could you have an electric bypass damper that is driving or a barometric damper that is bouncing?

[hvac7]

you can hear the pulsation in in just about all the supplies. Not terrible but it is noticable

[Wild Leg]

Is the return flex?

[hvac7]

the returns are flex. Two returns which are a 12 inch and the other a 10 inch. This unit is in the attic and there seems to be no sags in flex.

[trane]

You answered my question while I was typing.

[Edited by trane on 12-14-2005 at 06:10 PM]

[coolwhip]

If you had the blower door off and it still did the same thing, then I doubt its a return problem. You say you replaced the motor and capacitor and still have the same problem,,hmmmm..(scratching my head). It has to be a voltage problem then. Are you sure you did not replace it with a 230vac motor? Check the rating plate. Maybe the distributor gave you the wrong motor.

Then again a 230 motor would be moving alot slower.

[trane]

If my memory is right, we had a York air handler doing this and had to add more static to the return.

[beenthere]

Remove one of the 10" flex trunks and see if it still does it.

Could there be water laying in the flex from this summer.

[hvac7]

the two 10 inch round supply ducts are metal duct with 6 inch flex tap offs to each vent. a total of probably 30- 35 feet of main trunk line 10 inch all the way with 2 45's on one side to offset because of tray cieling. 15 feet on one side and maybe 15-20 feet on other side of main trunk line. on both sides of trunk line the last takeoff seems to be a good foot from each end

[contactor]

Ill bet on the coils plugged up