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Drainback to protect against overheating | 25 comments (25 topical, editorial)

Re: Drainback to protect against overheating (3.00 / 0) (#10)
by DamonHD (d@hd.org) on Mon Dec 10th, 2007 at 10:24:02 AM MST
(User Info) http://www.earth.org.uk/

Hi,

Yes, at this house DHW (and cooking) gas use is ~10kWh/day, and CH (central/space heating) load for half the year is approx an additional 20kWh/day (and zero the rest of the time).

So total year-round DHW/CH demand is approx 7.3MWh/year.

But under the scheme that I've outlined here:

http://www.earth.org.uk/towards-a-LZC-home.html

the heat-pump would only need to be working flat-out to supply all the CH needs in mid-winter, as solar thermal would be contributing some or all DHW/CH needs at other times.

We can of course work on reducing the CH demand by increasing insulation, etc, and that will make GSHP more plausible.

I am assuming that if we move to a new place we can keep DHW/CH demand similar or even lower.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#11)
by fcfcfc on Mon Dec 10th, 2007 at 12:10:44 PM MST
(User Info)

Hi: Ok, so at 20KWH that's ~70,000 Btu's per day average which means there are times in the Winter you will use 140,000 Btu's or higher in a day and less than 70,000 Btu's other times. Add in the DHW etc.. and you are around 100,000 Btus per day average. Last questions, do you have any idea what the sunshine percents are in the heating season for your location..??.. Second, you mention roof area <50M2 but what are the roof dimensions, ridge length and length down the slope of the east west roofs and roof pitch..??..

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#12)
by DamonHD (d@hd.org) on Mon Dec 10th, 2007 at 01:27:09 PM MST
(User Info) http://www.earth.org.uk/

Hi,

For the current house it's probably hopeless, but I can tell you that PVGIS (http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php) suggests:
886Wh/m^2/day at about our roof pitch (36*) and 944Wh/m^2/day on a vertical surface in December south-facing, with year round averages of 3100Wh/m^2/day and 2180Wh/m^2/day.

I think that those numbers are roughly halved on east- and west- facing pitches, though tubular collectors may do better than that suggests because of their profile.

I don't know our exact roof pitch, but by eye 36* isn't far off.

The north-south ridge of the roof is roughly 5m long and the house front to back (ie east to west) is about 8m.

Rgds

Damon

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#13)
by fcfcfc on Mon Dec 10th, 2007 at 03:05:27 PM MST
(User Info)

Hi: Boy, that is tough. You could run two down the East slope and two down the West slope facing true South but your roof is about 10 degs to steep, to much shading. You could only use that approach if you mounted all the way at the South end of the house so the shading problem goes away except the whole rest of the roof will then be in the shadows of the thermal units, not good for PV. That would give you 120 tubes which would yield about 100,000 to 120,000 Btu's on a perfect solar day, which would get you part way there. I suppose you could put some PV on the North end facing south but you would have to raise them up a bit for shading issues.. Not an easy install either way....

[ Parent ]

Re: Drainback to protect against overheating (3.00 / 0) (#14)
by DamonHD (d@hd.org) on Mon Dec 10th, 2007 at 03:54:32 PM MST
(User Info) http://www.earth.org.uk/

Hi again,

Interesting. Thanks for that.

I think the situation is not at all good for this house for real RE.

Really time to look for a better place I think... B^>

Rgds

Damon

[ Parent ]

Drainback to protect against overheating | 25 comments (25 topical, 0 editorial)

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