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Old 11-21-2010, 03:51 PM   #1
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getting the most out of a heated crawlspace

hello, live in Comox BC,...Vancouver Island
Built a house....single floor....1976 sq. ft....ICF blocks for full 3.5' totally insulated crawlspace....floor....5"gravel-vapour barrier-3.5 " foam insulation-2.5 finish coat (cement). 6.5" insulated rim board (SIPS) - so joist ends are inside the heated space....6.5" (SIPS) walls....10.5" SIPS roof panels. 3/4" T&G plywood subfloor...tile and Bamboo throughout..Bamboo was 5/8 solid, vertical grain.
Carrier heat pump 13 Seer....ducting runs for under window venting (I think usual for forced air?)....Unfortunately no heat vents were cut into ducting the year it was built...I found this out on my own inspection...just before the cold weather hit we cut in 'boots' for register vents in the hot air side of the whole crawl space (directing the warm air towards the floor and naturally will flow up under the subfloor) here's the layout.....3 -4x12 vents in the main hot air ducting shooting straight out the sides......6 more 4x12 on the installed ducting all away from any outside walls but in the corners of the crawlspace so the return ducting would pull the warm air through under the whole space of the floor.
Put digital indoor/outdoor thermometer in kitchen...(outside sender on sill plate middle pony wall middle of the crawl space on the wood).....cranked up the definately got warmer...much warmer than before, but to my mind....with such a big heat pump (used for twice the sq footage) it would never be overworked....the heat it produces in the crawlspace needs to up to about 24 C to keep it at 72 F upstairs......Have shut most floor venting to capture heat under the floor......with only 1 4x12 return vent in crawl space more needed?...
..There is absolutely no moisture or condensation in the outside fact this house is perfectly sealed and contained in the crawlspace (everywhere for that matter).. this type of house should the hot air ducting vents be closer to the main hot air duct for maximum 'heat push'? Does the return air venting need to be increased so that it balances?
Lit a small candle, put the flame near the return air vent upstairs....9x32 (close to floor), the flame is definately 'pulling' towards the return side noise, just flow. Because there is very little moving air upstairs (most vents closed) could the return air vents be downsized for the winter ?
My thoughts?....bigger than needed heat pump....3.5' enclosed, sealed, insulated, dry crawlspace... hoping I have enough hot air vents in the crawl space....the flow at the farthest ends do not seem to be much, (or at least as much as I think they need to be) Dont know about the balance of +or- side of pressure but am thinking to get this situation straightened out before too long. What am I missing? I sometimes feel a slight cool air flow across the top of the floor...but I'm certainly 'tuned in' too all of this so I am very aware of almost everything.....would appreciate a 'learned' enlightenment on this whole, anything and everything would be most welcome. I have hear somewhere that in sips houses with crawlspaces the warm air flow should come from as close to the hot air main as it can?...make sense? with more 'flow' in the warm side the crawlspace would heat up more evenly and faster.....staying that way with the prompting of a digital Carrier Infinity the best we could we need the efficiency......thank you for your time.....G


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Old 11-21-2010, 04:57 PM   #2
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The crawlspace isn't suppose to be heated to human comfort temp.

1 supply, and 1 return is all it should need to stay warm.


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Old 06-08-2011, 07:42 PM   #3
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Crawlspace efficiency capitalized

Update - June 8th, 2011
Initial install was completed with minimum heat vents. The house was designed for a heated crawlspace hence additional heat vents were installed and the heat distribution has now eclipsed expected efficiencies.
Average hydro (heat pump) for the past 12 months is $86.00 ! The house does have a gas fireplace, gas cooktop and gas fed on demand hot water tank which averaged at $45 for the past 12 months. Heated crawlspace resulted in a totally warm home. This is a ranger style home so there is basically approx. 4000 sq. ft being heated.

The original posting was to put the information out there as the additional venting was just being installed with the hope that it would create some dialogue with others interested in this type of building.

This build is a complete success with energy efficiency of 85+

Gerry Fossum
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