Should I Box in My Attic Furnace?


Old 12-12-05, 09:27 AM
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Should I Box in My Attic Furnace?

Hi, I'm new here and a search didn't answer the question I came in with, so I hope someone can help.

I have a new addition (5 years) on my home. It's a second story, and there's a HVAC unit in the attic.

Any time we have snow the it melts off the roof creating ice dams and icing on the sidewalk below.

I suspect that heat is draining up through the HVAC unit and into the attic. I suspect this because the upstairs is always cold, there is cold air coming down into the living space through the vents when the heater is off, and the snowmelt on the exterior clearly shows rafter lines.

The unit itself is less than 5 years old. The ducting is 8" or so, soft/foilwrapped and lightly insulated. The attic itself has R30 on the floor and is well vented

I'm thinking of creating a box around the HVAC unit using 1x2s, chicken wire and insulation batts. I only need access to this unit every few months to change the filter. It's in an attic that I must access with a ladder.

Is there any reason not to do this? Anything I should be careful of if I choose to do it, any precautions? Better ideas?

Any help would be appreciated.
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Old 12-12-05, 04:09 PM
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First and foremost you must find out what is permitter by local building codes.

You cannot enclose the furnace by just using insulation. You will have to have a fire proof material between the insulation and the furnace and the wall of the enclosure, like sheet rock.

I would encase the furnace in an insulated utility room in the attic if I could. Not only would I make the ulitilty room large enough for proper furnace clearances, it would be big enough to make servicing the furnace easy.

You must provide combustion fresh air supply vents for the furnace, unless the system has a fresh air supply pipe. You can tell because there will be two plastic pipes going through the roof. One for exhaust and the other for fresh air. If there is a single metal flue pipe, you have to put vents on the a wall or door of the utility room. Because it takes 15 cubic feet of air to burn 1 cubic foot of gas completely.

The rule for combustion fresh air supply vents is 1 square inch for every 1,000 btu output, high and low. For example, if the furnace is rated at 75,000 btu's output, then you need two vents equal to or more than 75 square inches. One vent installed on the upper portion of the access door and the other on the lower portion of the access door. Or similar separation on one of the walls of the utility room.
Old 12-13-05, 04:52 AM
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Thanks for this very good information.

I hadn't thought to consult the local codes over this, but it's good advice.

My alternative, and I still may do it, is to hire a HVAC man to advise me just on the design and construction. Unfortunately the attic space, while large, is low. A 'room' is probably not as practical as a 'large box'.

I'll check back to see if anyone else adds to what you've said, but I appreciate the reply
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