Cold Air Drop Out

Patrick Mosner

We have installed many projects with 100% floor radiant and a/c air handlers in large residences (several over 30,00 sq ft) and have received and witnessed the following complaint. With the DX or chilled water air handling units mounted in the attic we experience a large amount of cold air dropping out of the ceiling mounted air devices. This tends to occur when the attic temperature drops below 30 deg. At times the drop out can be substantial enough where the radiant system cannot heat the home properly. Because many of these air devices are custom wood fabricated and often more than 25 feet high, plus the fact that there can be over 100 of these devices it is not practical to manualy shut dampers in the winter. We have received many suggestions from install heating coils in the units (which defeats the radiant system) to heating the attic (which is not practical) to removing the 2-inch duct wrap and installing heat trace on the sheet metal and then reinsulating.

Any thoughts?

don_42

Bear with me

It sound to me like maybe some more r value on the duct
is in order.Also is the duct seal tight,mastic if you will.

How tight is the building envelope? Is it so tight that everytime they turn on a exhaust fan that the air is being
replace from the leaks in the ducts?

Are all boots coming thru the ceiling sealed? I've seen alot
of crack around ceiling registers where the sheetrock is not tight to the boot.

Airhandler in general have lots of airleaks.Maybe sealing
the doors and where the suction and liquid line come out
of the cabinet would help.

Sysnergistic effects I think they called it.Try looking into
which drivers is adding to the problem.

Dave_38

Attic ductowrk

must be insulated to a minimum R-11 and all ductwork and the air handler must be sealed. If you are getting cold air drop out you may also be getting condensation in the ducts and this will eventually leak onto the ceiling. The amount of cold air infiltration you are seeing also tends to make me think that the house(s) may be in a vacuum so you may have a ventilation issue here also, with full radiant systems we ALWAYS install an HRV set to keep the house in a positive pressure. Also on large houses with very high ceilings we use a variable speed air handler and run the blower at the lowest possible speed for air circulation when the unit is not being used, the air flow is not detectable nor can the blower be heard but the duct work stays warm and the slight air movement prevents air stagnation and temperature seperation from occuring due to the high ceilings. Using this method you can also duct the fresh air from the HRV into the a/c ductwork to distribute it throughout the house. The results are absolutely amazing!! Hope this helped you.

Dave.

jerry scharf_2

conditioned air space

Patrick,

The situation you describe is a poster child case for running ducting in conditioned air space. Would it be possible to insulate to the roof and make the area that contains the ducts conditioned air space? IMO, it's the best solution, though often difficult in a retrofit situation.

I know of no magic bullet that will solve this. You can certainly beef up the duct insulation and create an insulation boot around the registers.

Shutters and dampers can be somewhat effective, but will at best reduce the problem. There is a product out called flexdamper. It's an air bag that is shaped like a flat donut. When you fill it with air, it swells up and shuts off the flow. When you vacuum the air back out it flattens and lets the air past. I thought of these because they are easier to retrofit and would actually give some insulation value by their design (rubber with trapped air.) I see them on sale on ebay, and the company who makes them also builds air/vacuum panels for controlling them.

best of luck,
jerry