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Air handler in cold attic

sphaugh's picture

(Opinions wanted)


Have a project that the VE process (you know, 'removing the architecture from a building') eliminated the basement in lieu of a slab floor, and now the air handler must go in the attic.


Well, I'm going to talk to an engineer friend (mechanical) and the subcontractor - but my concern is that if the ceiling insulation is on the attic floor, will the air handler "above" the insulation, it'll be a liability because it's "out in the heat/cold".  Or is this common?  I know in the south, it is really common in spite of the fact that cold ducts + warm air = condensation.


My thought is to build a small room up in the attic truss space with an insulated envelope, but the ducts will still need to be run all over, and I can't see insulating under & over them all throughout.  Can't hang batts between the truss top chords because it's R38 and the top chord is only 2X6 - no way to get a vapor barrier up there.. unless the VB sits between the trusses and the sheetrock ceiling... but that sounds like another detail that is not too common.


Location in question: Southern New England


Thanks for the feedback, I know I can count on you guys.


 

(post #111033, reply #1 of 8)

I've seen attic air handlers done in New England. You can spot them from the outside in the winter. They'll be under the spot on the roof with no snow cover. I'd say an insulated room is the minimum you should do, but those ducts need insulation, too. To my mind, putting a heating or cooling system in an open attic is one of the dumbest things a builder could do, just shy of criminal.


You say you can't insulate the roof because there's not enough depth. How about spray foaming the underside of the roof deck? You could easily get the R-value. It's a code recognized technique now, although your area may not have adapted that provision. I'd talk to the inspector. I think it's the best option, and when you subtract the cost of having to enclose the air handler, it's probably cost competitive.


Andy


Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.

Andy Engel

Never try to teach a pig to sing. It wastes your time and annoys the pig.

(post #111033, reply #3 of 8)

---"To my mind, putting a heating or cooling system in an open attic is one of the dumbest things a builder could do, just shy of criminal."---


If I get this picture right, that "hanging the unit in the attic" is standard for here in all electric houses built on slabs. The units have been designed for that, with an extra drip pan and drain to the outside (they have the normal operations drain into the water heater drain also)


The house they are building for us is done that way and so are all others around here, I asked many people with new houses. Was told in the last 10 12 years, no one does it any other way.


HVAC companies have a computer program that tells them how many runs they need and where they need to be and considers the advantages of the very short runs needed in the attic, well insulated, to be appropriated. No more runs in the slabs, except in very special situations.


I wanted the unit in the house envelope and provided a closet for it in the middle of the house, in the hall by the bathroom, a straight run thru the utility room to the garage, so as to be easy to move in and out, with a plenum etc, but was outvoted by the builder and HVAC people.


They put it in the garage in the houses around here with attached garages in the footprint of the house. In this house, the garage is attached but on one side, so they situated the unit over the tornado safe room, in the middle of the house itself, suspended with eight chains. Code here requires the unit be within 20' of the opening to the attic, that here is over the garage, at 16' from the unit.


The heat pump is outside, on the North side, 43" from the unit in the attic. Even with that distance, they said it was more efficient than on the hot West side at 14' from the unit or the East side, front of the house, at 20', where it would have been an eye sore.


This is supposed to be an "energy star" rated house, so efficiency is looked at closely.


I may have misunderstood something, but if that is wrong, we need to blame it on the computer programs.<G>

(post #111033, reply #2 of 8)

"and now the air handler must go in the attic."

Why

I have a 2 story, on slab, with 2 systems and not a single piece of equipment or duck work is in the attic.

Now the layout of the house does make this easy.

The first floor is mostly open and uses a downdraft system with supply ducts in the slab and a common return.

The 2nd floor system is in the same utility closet and the supply and return ducts are run in a soffit. In all of the areas except the hall the soffit is part of the kitchen or bath soffit.

In the hall it is behind a wood support beam and does not appear out of place.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #111033, reply #4 of 8)

Attic installations can hurt efficiency to be sure. I don't like the hanging system either. Good units can be on the attic decking, usually on concrete blocks that sit in the emergency drain pan. I like to add a layer of foam board to both insulate the bottom of the air handler and help eliminate vibration. Ducts are probably R-6, yet the attic might be R-35 or higher. Doesn't make good sense to blow the most expensive heated or cooled air through ducts located in severe temps. As one poster below noted, the units can be spotted by the lack of snow on the roof. (you can spot the stair set also if it does not have a pull down insulating door over the top of the stairwell.)

If the unit has to go in the attic, I would suggest insulating all sides with foam board and aluminum tape all seams. Leave an inch or two untaped along the bottom so that an internal overflow will drip into the safety pan. Keep ductwork on the attic floor, and blow cellulose over it, a la snowdrift.

If you can, a hall closet can be used with distribution a number of ways. Such as, through the floor below the closet with supply grilles to all floor mounted in the upstairs rooms; or over head into the attic, with drops into each room, or if using 9' ceilings, the hall can be 8' to allow hidden ductwork.

Had one client put the up unit in a hall closet, put in acoustic panels on the closet walls, put a louvered door with filters on the inside of the door. He gets both high and low air return, extremely well filtered. He ran one main trunk line in the attic above the insulation, but kept the room runs between the floor joists and covered with insulation. He triple walled the main trunk with insulation.

Also suggest return air filter grilles and sealing off the built in filter door on the air handler. No sense in lining return air ducts with dust. Return air grille filters seem to get changed more often than those hidden away in the attic... Paul

Energy Consultant and author of Practical Energy Cost Reduction for the Home

Energy Consultant and author of Practical Energy Cost Reduction for the Home

(post #111033, reply #6 of 8)

One followup question Paul:


re: If the unit has to go in the attic, I would suggest insulating all sides with foam board and aluminum tape all seams.


I am under the impression that the fan motor in the air handler needs some fresh air flow to dissipate heat.  Are you suggesting literally cladding the sucker in rigid board?  I'm definitely going to look into this.  Draping the ducts with additional insulation is cheap insurance.


Thanks for weighing in


 

(post #111033, reply #7 of 8)

The only external air input needed on an air handler would be for a furnace input to the burners. The internal blower motor of an A/C air handler or heat pump AH is in the internal air stream between the return and the supply air to rooms.That is all the air it needs.

My own air handlers have all seams al taped, then foil backed foam board (the tape holds well on foil board) cut to fit all sides, and those seams taped. Blanket insulation shrouds the ducts until they are between the floor joists.

You can test the effectiveness of your system by turning to "fan on" and measuring the wall return air grille air temp and the furthest supply air flow into a room on a hot mid day or cold night. The difference is the heat pick up or heat loss. I have helped many clients who have had 15 deg heat gain when air conditioning, out of the 20 degree total cooling at the coil output. 75% loss before they felt the first Btu!

Insulate the system, radiant barrier/ventilate the attic, seal the system, and keep the conditioned air in the house.

Where you live, heat loss in the winter to the duct system can be costly. A ceiling return air grille will allow the warm indoor air to rise into the return duct system and cool. A thermosyphon sets up with cold air falling into the rooms, and more heat pulled into the return. I like low on the wall returns to prevent the winter loss, and to pull cold air off the floor for our forced air heat system. Never found hot air at the ceiling to warm my feet well! Paul

Energy Consultant and author of Practical Energy Cost Reduction for the Home

Energy Consultant and author of Practical Energy Cost Reduction for the Home

(post #111033, reply #8 of 8)

good info. thanks for your continued supply of considerations Paul

(post #111033, reply #5 of 8)

I have an attic air handler, it was added 8 years after construction.  I was also concerned about the extreme enviroment when it was installed, so after the contractors left, I went back up with some insulation. 


First, the unit is sitting on the joists, buffered by foam board for vibration (good call, Paul) and in a pan that is plumbed out a side wall.  (BTW, after installation, I wired the air handler through a high level switch in the pan so if the unit overflows and drain is clogged, it will shut off before damage occurs.) 


I took a whole bunch of FG unfaced rolls and buried the air handler and all duct work to conserve energy.  When it snows, there is no exterior indications that the unit is in the attic, so I guess the insulation is doing its job. 


One possible fallout to the attic installation/insulation is that the electronic air cleaner (electrostatic precipitator) has burned through 4 power supplies in 11 years.  They cost $250 a pop, not including the labor charge to change it out (and the fact that I can't oder it myself just ticks me off.)  They just replaced it several months ago, and claim the insulation is causing premature failure.  Don't see how, but I guess it could be.  The unit in the basement has only burned one, so maybe it's true.


 


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