GSHP Under A Passive House

[Onoff]

Numpty question I'm sure but here goes:

 

As I understand it a GSHP absorbs heat from the ground into the liquid within the "long" pipe run.

 

The pipe has to be at a certain depth for the ground temperature to be stable.

 

What I wonder is the ground temperature directly under say the 300mm EPS of a passive type house? It should be warmer at a shallower depth? This because of the insulation above trapping the heat.

 

The progression of this thought process is would it be viable to include a ground loop under the footprint of a passive type house.

 

[IanR]

Horizontal ground loops rely on solar energy to warm up the surface and rain to draw that energy down to the loop to recharge the ground the loop is in during the non-heating season. So I expect there won't be sufficient energy in the ground under the slab for the second heating season, and after a few years you'd have a perma-frost.

Edited April 10, 2017 by IanR

[Onoff]

31 minutes ago, IanR said:

Horizontal ground loops rely on solar energy to warm up the surface and rain to draw that energy down to the loop to recharge the ground the loop is in during the non-heating season. So I expect there won't be sufficient energy in the ground under the slab for the second heating season, and after a few years you'd have a perma-frost.

 

Ta. For some reason I thought it was "inherent" heat in the ground or heat rising.

 

You're suggesting I guess that it'd be a dead spot underneath. No chance of that bit of ground being warmed from the sides - and heat kept there by the slab?

 

Be interesting to know though what the ground temperature is under a passive slab.

[IanR]

Yeah, vertical loops that go down circa 75m - 100m rely on geothermal energy (and ground water), but not the horizontal loops.

 

You'd only recharge the ground under the slab by ground water passing through, and for the first meter or so of depth under the slab they tend to be drained so that there isn't any water.

[TheMitchells]

I love it when others ask questions that I want to know the answer to - I'm not the only one; thanks OnOff - keep 'em coming!

[A_L]

46 minutes ago, Onoff said:

You're suggesting I guess that it'd be a dead spot underneath. No chance of that bit of ground being warmed from the sides - and heat kept there by the slab?

 

Be interesting to know though what the ground temperature is under a passive slab.

 

Given the low level of extraction with a passive slab type house I do not think there would be much danger of the ground actually freezing,  with the summer to recharge and the intermittent nature of heating superinsulated houses it should be O.K. Might have to watch it if year round DHW was also provided by this source. Remember the floor heatlosses will be higher because of the lower ground temperature so possibly somewhat counter productive.

 

This may provide some information tonyshouse

[Jeremy Harris]

1 hour ago, Onoff said:

Numpty question I'm sure but here goes:

 

As I understand it a GSHP absorbs heat from the ground into the liquid within the "long" pipe run.

 

The pipe has to be at a certain depth for the ground temperature to be stable.

 

What I wonder is the ground temperature directly under say the 300mm EPS of a passive type house? It should be warmer at a shallower depth? This because of the insulation above trapping the heat.

 

The progression of this thought process is would it be viable to include a ground loop under the footprint of a passive type house.

 

 

In most of the UK the temperature of the ground, below about a few metres or so is pretty constant for around a couple of hundred metres or more, at around 8 deg C.  This heat comes almost entirely from solar gain, with percolating water from the surface carrying heat down to a fair depth.  I used to go caving a lot, and wherever you went in the UK the cave temperature would be a pretty constant 8 deg C or so, even 400ft or so down under Wales................

 

Ground source heat pumps, whether they use surface, water or borehole collectors, are really solar heat collectors, by a round-about route.  If you take heat out from under a house, then the ground will cool down more than if you took heat out of a bit of open ground, as there would be no direct replenishment if heat from above.  Depending on how permeable the soil is, and how well heat can transfer sideways from adjacent land, you could find that you drop the soil temperature under the slab by a lot, it may well even freeze and heave. 

 

Because the heat comes from solar gain and surface water percolation, the collector wouldn't work as well as one on open ground or down a borehole (and our 53m borehole is also a constant 8 deg C, all year around).  An under-house collector would also increase the ground floor heat loss, by increasing the temperature differential between the soil under the house and the room temperature, so more insulation would be needed to maintain the same heat loss as before.

 

It's quite common for ground source heat pumps to be wrongly referred to as geothermal heat pumps, domestic ones aren't at all, unless they have collectors going down many hundreds of metres, and only then when in an area where there is either a high level of natural radioactive decay causing localised heating or the Earth's crust is thin.  I once went down a Cornish tin mine (Wheal Jane) and at about 1500ft down it was fairly warm (around 30 deg C), due almost entirely to the high natural radioactivity of the granite, rather than true geothermal heating.  True geothermal heat (as in from the Earth's core) doesn't really get that noticeable until a lot deeper than this mine, unless you live in an area where the Earth's crust is thin, like Iceland, Yellowstone Park etc.

 

[Onoff]

35 minutes ago, TheMitchells said:

I love it when others ask questions that I want to know the answer to - I'm not the only one; thanks OnOff - keep 'em coming!

 

PM me any idiot questions and I'll ask for you. I have no shame!

[Declan52]

I am only back from Iceland on sat and they are world experts on it. They have plants all over the country that they pump the water from  through a  large network of pipes you see on the surface everywhere. Their hot water, electric and fresh air are the only things that are cheap there!! The guide reckoned they pay around 4-5p per kWh for electric.

The world famous blue lagoon uses the waste water from the power plant next door for tourists to bathe in, me included.

[Jeremy Harris]

Iceland is a place I've wanted to visit properly for years. 

 

I did spend a night there once, a long time ago, but it was by accident, a result of bad weather.  I never got to leave the base at Keflavik, though, as none of us had our passports..............

[Declan52]

It's beautiful, very expensive but me and the wife loved it. Never seen the northern lights as It was very foggy so gives us an excuse to go back. 

 

[Onoff]

Iceland is expensive. I can get four cans of Guinness in LIDL for a pound a can but they want £4.59 in Iceland!

 

 

(Mum often goes there though).

Edited April 10, 2017 by Onoff

[PeterW]

Been there and seen them ... truly amazing ..!!

 

Tapping the heat and use as District heating out in the highlands is inspiring - tapped at 160c at 70m down, feed 5 houses over a 5000sqm area and the last house is getting water at 90c before it discharges it at 60 litres per minute with no pump ..!

 

Keflavik runway is also heated for de-icing by using hot water from the neighbouring power plant - one way to be able to land in -20c..!

[Onoff]

Hmmm.....there's loads of diagrams on Google showing "earth tubes" under passive houses and "ground heat exchanger". I Googled "the ground under a passive house" and went to "Images".

 

 

[Jeremy Harris]

19 minutes ago, Onoff said:

Hmmm.....there's loads of diagrams on Google showing "earth tubes" under passive houses and "ground heat exchanger". I Googled "the ground under a passive house" and went to "Images".

 

 

 

Earth tubes are a way of being able to feed an MVHR with pre-warmed air in winter, or cool air in summer, but they are not easy to get to work well.  The pipe has a special silver anti-bacterial, anti-fungal coating and is very expensive.  I looked at it briefly, as I'd thought of fitting the pipes behind our big retaining wall, as it holds a fair bit of heat.  Once I saw the price, and read about the horrors of not using the special pipe (see here: http://www.greenbuildingadvisor.com/blogs/dept/musings/belgian-passivhaus-rendered-uninhabitable-bad-indoor-air ) I decided not to bother.

 

They may work well in a dry climate, but it seems that they aren't that easy to make safe in the sort of climate we have, as there will often be condensation in the tubes.  Stopping stuff growing in them seems to be a significant problem, and I'm not 100% convinced that even the special silver coating will have a long enough working life.

[Onoff]

5 hours ago, IanR said:

Horizontal ground loops rely on solar energy to warm up the surface and rain to draw that energy down to the loop to recharge the ground the loop is in during the non-heating season. So I expect there won't be sufficient energy in the ground under the slab for the second heating season, and after a few years you'd have a perma-frost.

 

How about if the site was on a slope? Wouldn't the earth under the house get continually "replenished"? 

[SteamyTea]

1 hour ago, Onoff said:

How about if the site was on a slope? Wouldn't the earth under the house get continually "replenished"? 

Would that not 'wash' the house downstream as well?

 

I think that the Earth emits 62.5 mW/m² on average, not a great deal.

Lord Kelvin (of should that be Lord K°, real name was William Thomson), used this figure to calculate the age of the Earth, but he was unaware of radioactive decay when he said it.  Ernest Rutherford put him right 1904.  Rutherford showed that 1 gram of pure radium releases enough energy to raise 1.3g of water by 100 K (though probably called °C then) in 1 hour.

 

[Ian]

17 hours ago, Declan52 said:

I am only back from Iceland on sat and they are world experts on it. They have plants all over the country that they pump the water from  through a  large network of pipes you see on the surface everywhere. Their hot water, electric and fresh air are the only things that are cheap there!! The guide reckoned they pay around 4-5p per kWh for electric.

The world famous blue lagoon uses the waste water from the power plant next door for tourists to bathe in, me included.

@Declan52@JSHarris

Iceland is such an amazing place. I went there a few years ago to do some photography and took some shots of the blue lagoon and other geo-thermal power plants:

 

 

[Jeremy Harris]

Brilliant photos, Ian, convinced me even more that I must go there.

 

The last trip wasn't much fun, as it was a bad weather diversion, that stopped us getting back to St Mawgan.  The USN made us welcome, even to the tune of lending us money (none of us had our wallets with us.........).  It was memorable mainly for the phone call home, to say that I wouldn't be back that night.  On being asked where I was I said NAS Keflavik, which prompted the question "Where's that?".  At that point the USN chopped the phone call!

[Declan52]

After 4 hrs in the blue lagoon i had baby soft skin. The wife has Psoriasis and it helped get rid of it and 5 days on it's still clear. Apparently if you have Psoriasis in Iceland you get sent here instead of filling yourself full of drugs. You get a lovely silica mud pack for your face and as you can see in the pic you look like an idiot but everyone is in the same boat!! The cranes in the background are the hotel they are building on site.

Make sure you bring your wallet the next time , everywhere takes cards with contactless widespread. 

Loads of airlines do cheap flights there now as it's one of the most popular city breaks. Both of our flights in and out where full to capacity and from talking to other visitors it seems it's a used as a 2/3 day stopover destination for most Americans then on to Europe.

Edited April 11, 2017 by Declan52