Carrier Moduline Diffuser Retrofit?

[jim_king]

I have a built up VAV system that feeds into Carrier moduline diffusers with Johnson Controls pneumatics in each room on 2 floors of our facility. I am starting to have a lot of trouble with the diffusers, and I'm looking for any suggestions or if anyone else has retrofitted these with a standard vav box? I'm looking to upgrade to digital controls also, to our existing AutoMatrix system. Does Carrier offer a replacement?

[jimgpe]

No reason why you can't replace these with a standard VAV box with ductwork and diffusers. The air coming to the Moduline would have plenty of air pressure for the VAV to operate, on the order of an inch of pressure if they ever did operate correctly.

I'm not aware of a Moduline swap-out solution.

Go for it.

[gyhooyabat]

Jim,
one big problem you will have is the supply fan will slow down and speed up as the existing modulines fill then relieve as the air volume or fan speed changes. All because you have now added VAV's or mixing boxes. What's the difference? VAV's don't care what you supply them with initially. Their controls do the thinking and adjust the damper accordingly. On Modulines, they first have to inflate, which requires a minimum of .75" of "CONTINUOUS" air pressure then they can modulate and maintain a comfortable area.

Understand????

There is nothing wrong with upgrading to a VAV system, you really need to think the system through though. Because Modulines needed a minimum air volume the fans speeds or vortex dampers only moved or changed slightly which usually kept a nice air flow throughout the duct system. That same holds true with a properly designed VAV system, but not when the 2 are mixed.

We usually recommend, and practice, that you attempt to maintain a minimum air flow in the main ducts and adjust the VAV controllers accordingly, just for the time being, until you have successfully replaced all of the Modulines with VAV's.

Good luck and if you have any questions do not hesitate to call or email.

[jimgpe]

The original post said the Modulines were being controlled by pneumatics. I assumed (!) that this meant that they had abandoned the bladder valve and its attendant requirement for a minimum pressure to inflate (they fail open if not enough pressure is supplied).

But now I recall that the pneumatics only modulate the system power airflow to the bladders, so you still need as much if not more system pressure to get them to go.

So....

While the system is a hybrid you will need to maintain about 3/4 or 1 inch of static pressure at the worst case Moduline or it will fail open. This is probably a LOT more pressure than the VAV's will need, so buy the VAV's with velocity resetting controls to limit the flow.

Nice catch.

Jim.

[jim_king]

Thanks for the responses. The system runs at 1.25" static.

I performed a pressure close-off test with a pressure bulb and magnehelic guage on the diffuser, and it took about 1.75" of pressure to close off the box. I think the bladder is ok because it stayed at that pressure for 3 minutes or so.

It just seems that some of the boxes require more pressure to seal off than they did, say, 3 or 5 years ago. I've also noticed the fans are running at a higher speed to maintain duct static, and there are more frequent complaints of too much air "blowing" in the rooms.

Any other ideas? should I start replacing the suspect diffusers?