Ground Source Heating Questions for New Build

blobert · 24-10-2019 2:14pm #1

Hello,

I'm planning a large (circa 400m2+) new build passive house bungalow and am keen to have a ground source heating system to help heat it in the winter and potentially to also help keep it cool in summer by running in reverse.

From what I can see you can either vertical bore holes (more expensive and suitable for sites where there is not a lot of space for trenches) or run horizontal loops in trenches seemingly of varying width.

The house is likely to be in the middle of a decent sized site (circa 0.6 acre) so we probably have decent amounts of space to run horizontal loops. We'll also probably be digging up a lot of the existing garden space also, the front garden is largely covered in tarmac at the moment which I want to get rid of and replace with trees and grass) and the lawn in the back garden is really uneven so we'd probably be digging that up and trying to level the ground/lay new grass. So again I think this would make digging trenches for horizontal loops seem a good idea to do at the same time.

My 2 questions with this which I can't seem to find answers to online:

(1) How much space will I need for trenches to have sufficient power for heating/heating water for a 400m2+ house, ie would I need to have 200m2 of trenches, or more, I'm assuming there is a rough formula. I'm trying to get an idea of whether I'd need to run trenches just in the smaller front garden or bigger back one or both.

(2) I'd seen mentions of wider trenches vs narrower ones with the wider ones being more efficient or even running single big loops of 250m spread further around the land. Just wondering what's likely to be the best option, I get the feeling if they are more spread out they are probably more efficient but there's probably more initial expense in digging trenches for them.

Any advice on this would be much appreciated!

Thanks

air · 24-10-2019 4:23pm

You would be best off to consult an experienced installer / specifier at this point and pay them to produce the design.

There are rule of thumb figures for watts per linear metre of pipe that you can use but actual performance will be very dependent on the soil type and moisture content as well as the position of your water table and the proposed burial depth.

The layout should also consider the location of any adjacent trees that you might want to retain.

An experienced individual would be able to guide you in this regard so you can optimally size an lay out the array.

All this is putting the cart before the horse though, the first thing you need is a figure from your architect for the heat demand of the house for the full heating season.

This is the first input towards sizing the heat pump and ground loops.
Underfloor cooling can be tricky as you need to be careful not to create condensation.

Personally I would be more inclined to increase insulation to a level at which ground source isn't worth doing.

Having said that a 400m2 bungalow is always going to be a challenge as the volume to surface area ratio is not in your favour.

Calahonda52 · 24-10-2019 10:48pm

air has summed it up really well here.
The soil type/water content/water table /shading/depth of dig parameters are key.
Once you have all that there is plenty software around to do the math.

Are you having a septic tank.
Re the cooling, first if the passive design is done right, especially with right glazing and site orientation selection, as well as factoring in decrement delay with the roof insulation, should not be much need: rather than chill the floors and ask for condensation trouble, consider a cooling coil in the MHRV, which will have a summer bypass supply, v useful if it's humid

Condenser · 24-10-2019 11:14pm

blobert wrote: »

Hello,

I'm planning a large (circa 400m2+) new build passive house bungalow and am keen to have a ground source heating system to help heat it in the winter and potentially to also help keep it cool in summer by running in reverse.

From what I can see you can either vertical bore holes (more expensive and suitable for sites where there is not a lot of space for trenches) or run horizontal loops in trenches seemingly of varying width.

The house is likely to be in the middle of a decent sized site (circa 0.6 acre) so we probably have decent amounts of space to run horizontal loops. We'll also probably be digging up a lot of the existing garden space also, the front garden is largely covered in tarmac at the moment which I want to get rid of and replace with trees and grass) and the lawn in the back garden is really uneven so we'd probably be digging that up and trying to level the ground/lay new grass. So again I think this would make digging trenches for horizontal loops seem a good idea to do at the same time.

My 2 questions with this which I can't seem to find answers to online:

(1) How much space will I need for trenches to have sufficient power for heating/heating water for a 400m2+ house, ie would I need to have 200m2 of trenches, or more, I'm assuming there is a rough formula. I'm trying to get an idea of whether I'd need to run trenches just in the smaller front garden or bigger back one or both.

(2) I'd seen mentions of wider trenches vs narrower ones with the wider ones being more efficient or even running single big loops of 250m spread further around the land. Just wondering what's likely to be the best option, I get the feeling if they are more spread out they are probably more efficient but there's probably more initial expense in digging trenches for them.

Any advice on this would be much appreciated!

Thanks

If you are using a ground source unit then you should be using passive cooling rather than reverse cycle as it's practically free.

When it comes to collector I don't recommend very long runs in trenches as it front loads your cooling load on the beginning of your loop which ends up with large demands and can freeze the ground. Once this happens it can travel further along the pipe and the collector will fail. I've seen this happen multiple times.

Ideally you need to split the load over multiple loops balanced through a manifold system. This evenly distributed the cooling load. The area will depend on the heating load of the house. Heating load minus electrical load gives you your cooling load and this is what must be covered without taking too much energy from too small an area.

air · 25-10-2019 9:11pm

http://www.gshp.org.uk/documents/CE82-DomesticGroundSourceHeatPumps.pdf

It would be worth having a read of this document, it's a good primer on the subject.

murphaph · 26-10-2019 8:41am

Any forums I read here in Germany where someone asks about a gshp for a new build passive house they are usually told the heat load for a passive house is or should be so small that a cheaper small ashp solution will be more economical. In a true passivhaus I suppose you neat no dedicated heat source at all.

Condenser · 26-10-2019 11:12am

murphaph wrote: »

Any forums I read here in Germany where someone asks about a gshp for a new build passive house they are usually told the heat load for a passive house is or should be so small that a cheaper small ashp solution will be more economical. In a true passivhaus I suppose you neat no dedicated heat source at all.

Cheaper to buy but not cheaper to run, not cheaper to own, won't last near as long and has no option to passively cool.

unkel · 26-10-2019 1:56pm

It's probably a stupid question, I have no experience / knowledge of any of this, but in a passive house in Ireland (that doesn't require cooling in summer,) why would you need any expensive heating system at all? Maybe just use a few €15 electric heaters occasionally on a very cold wintery day?

Or do you need a heating system for your domestic hot water more so than for your home heating? And if that is the case, would it not be cheaper to just heat your water with 8c/kWh night rate electricity than have the depreciation and maintenance of a more efficient expensive heating system?

murphaph · 26-10-2019 2:19pm

unkel wrote: »

It's probably a stupid question, I have no experience / knowledge of any of this, but in a passive house in Ireland (that doesn't require cooling in summer,) why would you need any expensive heating system at all? Maybe just use a few €15 electric heaters occasionally on a very cold wintery day?

Or do you need a heating system for your domestic hot water more so than for your home heating? And if that is the case, would it not be cheaper to just heat your water with 8c/kWh night rate electricity than have the depreciation and maintenance of a more efficient expensive heating system?

That's essentially the argument you see presented on German forums. The heat demand should be so small that an expensive heating system should not be required at all.

People forget how much heat is produced by electrical appliances and people.

In our 2016 build here in Germany the heat in the bedrooms in the morning just from us sleeping in them is very noticeable.

With a mechanical ventilation system (especially if it has heat recovery) you should be losing very little energy through the outside envelope and coupled with solar energy the need for an expensive heating system simply shouldn't be there.

We haven't switched on our UFH with ashp yet and it's nearly November. The house is not passive but is energy efficient. We have large solar gains during the day.

We have a towel rad in the bathroom (1:1 electric) which goes on for half an hour morning and evening in the autumn/spring and if there's a really cold couple of days we light a small fire with maybe 4 logs and the place is toasty warm and the fireplace stores the heat in blocks and releases it all night (does still be warm to the touch in the morning).

The amount of energy required for hot water generation is always overestimated.

People here advise big gshp solutions for older properties and cheaper ashp ones for new builds, especially the Panasonic geisha.

For summer cooling the most important thing is keeping the sun out! Our place has exterior shutters that go down and at night the windows are opened wide to let any accumulated energy leave the house. Windows must stay shut during the day of course.

air · 26-10-2019 2:20pm

It's far from a stupid question and I would be of the same view.

I'd insulate to a standard between current regs and fully passive and use a cheap (in capital outlay terms) heat source to make up any shortfall in extreme conditions.

Night rate resistance heating is a definite option.

Another would be a cheap fossil fuel burner.
One can buy a room sealed 5kW diesel/kerosene heater for <€200 nowadays. I have one myself.

I'd sooner put one of those in than an elaborate heat pump system or god forbid a stove that would cost thousands up front to deliver what might be a few hundred kWh a year possibly.

murphaph · 26-10-2019 2:47pm

A stove is definitely not economical. We have one because we like to light a fire occasionally.

I've seen people here ditch the boiler/hp completely and use decentralised under sink hot water heaters.

You have no losses due to hot water cooling in the tank/pipes.

In Ireland the electric shower has been a thing for many years of course and people think nothing of it but under sink water heaters make just as much sense I suppose.

I think such solutions can make a lot of sense unless you take baths every day. You also simplify your plumbing, just the cold feed and no real risk of legionnaires.

blobert · 29-10-2019 10:59pm

Thanks for all the replies guys.

With the ground source heating a big part of it is looking for a way to cool the house in summer if needs be without needing/wanting air conditioning.

As I say we lived in an A3 rated house (with no heat exchanger/mechanical ventilation) for 4+ years and it was frequently unbearably hot in summer, even with windows open a lot. This house did not face south.

I'm planning a house that's facing pretty much exactly south and planning to have nearly all of it covered in big windows, it's also going to be far better insulated than the other house.

I might be nuts but I'm assuming there will be issues with overheating in summer even if we have a bit of an overhang above all these windows.

I really really dont want the house to be hot like our previous one was and so I think the ground source might help with this. My understanding is that it should be possible to cool the house this way in summer, not by cooling the floor but by cooling the air. Is this likely to be effective?

If it turns out somehow the house is not too hot and the ground source is just used for hot water and heating in winter that would be fine also.

As we're going to be digging up/wrecking the garden as part of the build process I figured it makes sense to do the ground source system at the same time.

air · 29-10-2019 11:22pm

The South facing glazing shouldn't be a major problem. Once your architect calculates the dimensions of the overhang or brise soleil correctly you won't have any direct sunlight coming in through the windows between April and September or so.
I tried it myself on a small structure with an entirely glass South elevation and it worked perfectly.
I think Condensers suggestion of running the ground source in bypass mode could do enough to cool the slab sufficiently by moving heat to the ground outside.
That combined with running your MVHR in bypass mode should be sufficient if the house is well designed and has plenty of thermal mass.

murphaph · 29-10-2019 11:25pm

I really don't get the need for cooling in an Irish new build. We have much warmer summers (near Berlin) than in Ireland. We get 30 degrees as a rule and a few days it'll hit 35. We have no need for an active cooling solution.

The key is to prevent the sun entering on these hot days and to keep windows shut during the day, opening them at night (or preferably doing this through a ventilation system that waits until the outside air temp has dropped below the inside).

I really can't imagine you should need cooling if we don't. The insulation and air tightness that reduce the heat loss in winter does the very same job in reverse in summer. You just need to keep the sun out.

If you don't like roller shutters (what we have) you could look at external Venetian blinds (Raffstore in German).

blobert · 29-10-2019 11:38pm

Thanks again for the replies.

I agree, we're nothing like as warm as Berlin here, so maybe with an overhang or brise soleil it would be sufficient.

In reality its at best about 2/3 months of the year here where it could get too hot, literally all the rest of the year I'd be more concerned about it being cold.

One of the things with the house it that I don't strictly want to stick to passive principles, ie I'll want a lot of windows on some west and even north facing sides of the house. It's also going to be a very big bungalow which I suspect is not great from a heating perspective vs same size house over 2/3 floors.

So my guess is, even build with serious insulation, it's going to need a decent amount of heating to keep the whole house at 20+ degrees throughout the winter which is what I'd like to be able to do.

Thus I was thinking we'd get use of a ground source system for heating in winter/hot water while also having the possibility to use it for cooling if it turns out the house is too hot in summer.

I think we'll have to have this or solar in order to meet renewable requirements on a new build and my take on solar is I'd sooner wait till the number stack up a bit better for battery storage and then go to town on this also, ie we'll have a very big exactly south facing roof we can cover in panels in a few years and maybe get close to zero energy bills.

All going well we'll be living in this house for rest of lives (am 39 years old now) so I dont' mind putting in stuff that may have a long payback time (like the ground source) now as we'll probably get decent use out of it.

Thanks again

murphaph · 30-10-2019 6:54am

I think the best course of action is to have your heat load correctly determined by an expert and work from there. If you really have the need for a full on heating system you can get quotes as necessary.

Ground Source Heating Questions for New Build

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