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rebuilding my HVAC ventilation plenum

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  1. #1
    1K Club Member DavoSyd's Avatar
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    Feb 2017

    Question rebuilding my HVAC ventilation plenum

    hi, I need some advice on the best material for building a new ventilation plenum for my whole house ventilation system.

    I built the prototype as a 600mm cube with two inlets (external & house) and a door (that opens the plenum to the roof air) on it.

    it sits inside my roof and attached to a 200mm inline fan (that sucks through a "dust shroom" HEPA filter) that attaches to 7 outlets in my house via 150mm R1.5 duct.

    I made the plenum out of 6mm plywood and stapled Earthwool R-1.3 "space blanket" to it.

    some of the components:

    the box:

    the box in the roof:


    fan and damper set up:


    I can control whether the plenum takes air from the roof space or the house space or the external air inlet (that pokes out of my roof tiles).

    in winter it sucks air from the house (heated by my fire) or the roof (heated by the sun) and sends to the living room, in summer it sucks air from external (shady south side of house) and cools all the rooms and uses the roof air that cools overnight to cool the whole house.

    If I am really desperate I can run the A/C that blows into the hallway the house inlet is situated in and then send this conditioned air around the house too... (but prefer passive cooling)

    anyway... that's the backstory.

    it works really well - we're harvesting warm air in winter and cool air in summer (using 0.1kW/h) , but obviously the plenum itself was a prototype, mostly because it is:

    • not properly sealed (lets in cold air in winter & hot air in summer)
    • not properly insulated from the 40'C & 0'C roof temp (stapling 70mm Earthwool blanket to means it is only 70-80% covered)
    • it is permeable - and this is not ideal for a HVAC system

    I am looking at building a new plenum, but not sure what to make it from, and it needs to be built in the roof, which is cramped (I cant build the box down in the garage then take it up to the roof!) so needs to be easy to assemble.

    the options could be :

    1. do better wood cube (maybe 12mm ply) and paint it with primer/sealer paint + traditional insulation
    2. make a new cube out of the 50mm thick walls used for cool rooms
    3. build it out of XPS or foilboard - many brands here!

    I am really thinking the cool room stuff, as it is easy to cut and then use 40x70 ali angles to form & seal the edges.

    I can get a sheet of 2400x1170 (perfect for 6 squares to build a cube) for $100 so not too expensive...

    has anyone built anything similar?

    is there any other material I could be considering?

    is the cool room stuff easy to cut with a jigsaw and circular saw? I need to cut large circle out for the inlets & outlets...
    freedom of expression freedom from consequences...

  2. #2
    4K Club Member OBBob's Avatar
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    Jan 2006


    Cool. The cool room stuff seems a sensible option. Reckon a jigsaw would cut it with the right blade (assuming it's not too thick).

  3. #3
    1K Club Member DavoSyd's Avatar
    Join Date
    Feb 2017


    thanks Bob,

    Quote Originally Posted by OBBob View Post
    Cool. The cool room stuff seems a sensible option. Reckon a jigsaw would cut it with the right blade (assuming it's not too thick).
    the stuff I am looking at is 50mm thick, and the supplier I spoke to said you can cut the panel liner with tin snips, then a blade to cut the polystyrene...

    but needs to be clean cuts to be able to seal properly around the duct take-off for air-tight seals, so I am banking on using a fine tooth jig blade...

    however, with the coolroom stuff - should I add some sort of reflective foil stuff on it too? I am not sure of the reflective qualities of painted steel?
    freedom of expression freedom from consequences...

  4. #4
    3K Club Member
    Join Date
    Oct 2005


    Looking at the plenum on our geothermal AC, the inside is lined with insulation and shiny foil. The outside is just galv sheetmetal. All well sealed.

    The coolroom panels would do the job. Adding a foil would reduce the conduction a little. Foil on the inside would be my bet for the best effect.

    Those who want to ignore the science are increasingly alone. They are on their own shrinking island.

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