Please be patient - query about use of GSHP in outbuilding (no heated water needed)

[bikerchris]

Hello all,

Hope you don't mind me registering and immediately asking a question!

 

I've been renting an office for a good while but I'm now (finally) in a position where I can build an office in my back garden, so I'm not putting money in someone elses pocket. I've also been told that the cost of building an office is a tax deductible (yey), so that's nice. So far the design is going to be as well insulated as possible (details below if interested or have any suggestions)as I don't want to spend excessive amounts on electric heating (no hot water required), I thought ground source might be a good solution.

 

The building will be around 25-28 square metres (floor space) and the garden, while narrow, gives me around 6 x 15m of clear space for pipe laying at the suggested 1-2m depth.

I had thought about air source, but I like the mostly concealed nature of ground source, even if it means I've got some digging ahead of me but I've got no problems with that.

 

I just wonder if anyone has info about:

 

1) The very approx. cost if I do the trenches
2) How long the trenches should be to get the heat output required (I've guessed around 4KW)
3) If there's even a system that exists for small buildings (so far I've only found info about dwellings)

 

I've contacted a few companies and had nothing back after a week or so, did mention another project as well for a house sized outbuilding/annexe for someone else. I've spoken to a few people who don't use or have direct experience of renewable energy  and they have suggested an ASHP solution, but I'm trying to keep the footprint of plant to a minimum.

 

Anyway, thanks in advance for any responses.

 

Cheers,

Chris

 

Proposed construction:

Floor: Screed / underfloor heating on 100mm Insulation on slab
Walls: Block laid flat with 100mm internal insulation / plasterboard
Roof: Insulation between joists on flat roof with insulation over joists then roof finish (may be EPDM).
Thermal lining will be used throughout

[ProDave]

I considered ground source.  The pipe that you bury and the antifreeze to go in it exceeds the cost of the heat pump.

 

The other consideration is noise. the GSHP unit will be in the building purring away.

 

I chose a monoblock ASHP instead, it is pretty quiet, but the main point is the noise is in the unit outside, not in your building.

[bikerchris]

3 minutes ago, ProDave said:

I considered ground source.  The pipe that you bury and the antifreeze to go in it exceeds the cost of the heat pump.

 

The other consideration is noise. the GSHP unit will be in the building purring away.

 

I chose a monoblock ASHP instead, it is pretty quiet, but the main point is the noise is in the unit outside, not in your building.

 

Cheers Dave, really kind of you to respond so quickly - have a brew on me ?

 

I was thinking about noise as well, I'm planning to have the building divided with a masonry wall, do you think this will give enough acoustic protection if medium block? Don't worry, not looking for a precise  answer, unfair to expect that!

 

I've got a tight area to put the building, which means a ASHP unit would have to be close (just read it should be within 6m to avoid heat loss), it would be nice to open doors...oh hang on, they cool as well, so I'd probably keep them closed...wouldn't it?

 

Really sorry for seeming like a newbie!

[ToughButterCup]

I worked through this problem a while ago when, luckily, @JSHarris was writing about the issue. For 146 sq m (our house area) I drew the conclusion that it wasn't worth it. The discussion is on this board - somewhere - not sure where, though - so long ago that I read it that I can't remember which thread. 

 

But I do know what you mean about liking the idea of GSHP. There's something elemental about it: something deeply connected.

[scottishjohn]

1 minute ago, bikerchris said:

 

Cheers Dave, really kind of you to respond so quickly - have a brew on me ?

 

I was thinking about noise as well, I'm planning to have the building divided with a masonry wall, do you think this will give enough acoustic protection if medium block? Don't worry, not looking for a precise  answer, unfair to expect that!

 

I've got a tight area to put the building, which means a ASHP unit would have to be close (just read it should be within 6m to avoid heat loss), it would be nice to open doors...oh hang on, they cool as well, so I'd probably keep them closed...wouldn't it?

 

Really sorry for seeming like a newbie!

ASHP should not be noisy  if you pick one that is plenty big enough so its always running at low speed for your heat requirement.

with garden space you have for GSP --a bore hole would only way +that will end being 3 times the price of ASHP system 

main thing is to get building insulation   as good as poss  then you will need so much less heat to start with 

at size of floor you quote i doubt its worth either --UFH as you say and a simple electric inline boiler and economy 7 .

only if you want to use ashp for cooling of slab in summer might it be worth it 

lot more tecchie people than me on here who will direct you better

 

[newhome]

24 minutes ago, bikerchris said:

Floor: Screed / underfloor heating on 100mm Insulation on slab
Walls: Block laid flat with 100mm internal insulation / plasterboard

 

Welcome! I don’t know anything about GSHPs other than they can be spendy if they go wrong and it seems to be overkill in what sounds like a garden office, but your insulation would seem to be at a fairly minimum level. 

 

You should probably consider more insulation than you have in plan. 

[bikerchris]

8 minutes ago, recoveringacademic said:

I worked through this problem a while ago when, luckily, @JSHarris was writing about the issue. For 146 sq m (our house area) I drew the conclusion that it wasn't worth it. The discussion is on this board - somewhere - not sure where, though - so long ago that I read it that I can't remember which thread. 

 

But I do know what you mean about liking the idea of GSHP. There's something elemental about it: something deeply connected.

 

Hi mate, thanks for sharing your experience and I'll have a nose at @JSHarris past thread thingys and see if I can find it, be really interesting to read. 

 

It's funny isn't it, I like the idea of an (almost) self sustained building...apart from the materials of course, but can't have everything.

[bikerchris]

15 minutes ago, scottishjohn said:

ASHP should not be noisy  if you pick one that is plenty big enough so its always running at low speed for your heat requirement.

with garden space you have for GSP --a bore hole would only way +that will end being 3 times the price of ASHP system 

main thing is to get building insulation   as good as poss  then you will need so much less heat to start with 

at size of floor you quote i doubt its worth either --UFH as you say and a simple electric inline boiler and economy 7 .

only if you want to use ashp for cooling of slab in summer might it be worth it 

lot more tecchie people than me on here who will direct you better

 

That's good to know, although what kind of size VERY roughly, may be 2 x 3 foot is needed for the plant? Not sure how much of the garden the other half with allow me...although part of the building will be for her, so that's a bargaining chip ?

 

I certainly plan to make it as well insulated as possible, the limitation is the width though - 6m wide garden, 300min walls x 2 and I've already lost 600mm, then it'll be around 100mm from the side fences ?

 

That economy 7 could be a good fall back idea, especially with storage heaters...though I don't like the dry air they produce much. I guess I'd be OK during the summer, as there will be (hopefully) 2 nos roof lights with electrical openings if budget can stretch. The office I use at the moment has one 13 Amp socket and I've got a computer, printer and a 800w oil filled radiator on a separate plug-in thermostat (don't trust the inbuilt ones).

 

It would just be nice to have one solution that has me sorted for the coldest and hottest this little island can throw at me ?

 

[bikerchris]

9 minutes ago, newhome said:

 

Welcome! I don’t know anything about GSHPs other than they can be spendy if they go wrong and it seems to be overkill in what sounds like a garden office, but your insulation would seem to be at a fairly minimum level. 

 

You should probably consider more insulation than you have in plan. 

Hey newhome...you guys have got some odd names ?

 

I would go further with insulation, but I don't want to reduce the usable floor area any more, also I don't want it getting too high either - at the moment I'm allowing an FFL 150mm above GL, then 2.3m ceiling height, then upto 400mm (worst case) for roof thickness. Definitely need Planning permission (over 2.5m height within 2m of boundary), but I need it for the extension anyway, so I'll just pay the £226 (via planning portal, including their cheeky £20 "fee") and get them both in at the same time.

[ProDave]

Why bother with masonry?  Make it timber framed with some form of cladding then almost all the wall thickness can be insulation.

[bikerchris]

Just in case anyone's interested, here's a rough plan so far. I've not included the fences on this view, but there's one to the back that's sort of mine, but the sides have normal 6 foot close boarded in a rickety condition.

 

Oh, top left room is the tool store, bottom left room is her room if she wants it...otherwise more tool / workshop area for me, yey! ?

 And yes, not put doors in yet - minor detail on paper ? 

 

Edited February 4, 2019 by bikerchris

[A_L]

Look at it from a heatloss point of view. Assuming an outside/inside differential of 20°C then with 150mm of PIR or equivalent on all six surfaces the heat loss is about 340watts, plus say 180watts for 6m² glazing,   ventilation of  0.5ach with no heat recovery is about 200watts. So about 720watts, then subtract input from solar/bodyheat/appliances. Do you really need a GSHP?

[bikerchris]

23 minutes ago, A_L said:

Look at it from a heatloss point of view. Assuming an outside/inside differential of 20°C then with 150mm of PIR or equivalent on all six surfaces the heat loss is about 340watts, plus say 180watts for 6m² glazing,   ventilation of  0.5ach with no heat recovery is about 200watts. So about 720watts, then subtract input from solar/bodyheat/appliances. Do you really need a GSHP?

Thank you AL, that's a great perspective - thank you very much for doing the rough calcs, really kind of you. Has to be said that around 500-800 watts would be generated from the computers/monitors. The only thing that concerns me is when the temps go way down (or up), I know that Scotland can be extreme, but I can't work at 0C sitting at a desk. What I'm trying to create is a building that maintains an even temp all the time, just to preserve tools and electrical equipment as well as my own comfort. I want to be able to go in there any time of the year and be comfortable - sometimes I'd have to be in there for 10+ hours be it snowing or the hottest day of the year. The office I rent at the moment is horrible in the peak summer and only tolerable in winter. For the first time ever, I want to be able to control the ambient temperature and have it be the same all year round.

 

The other thing I don't want, is to have to do GSHP after it's built, it would just be hassle and regret. Of course, if someone said that a GSHP config for what I want is around £5-10k, I'd walk away from the notion of having GSHP. Having an ASHP seems to be cheaper, but then you've got external noise, plus it'll look fuggly.

 

Anymore comments really are welcome. ?

[jack]

10 hours ago, bikerchris said:

Having an ASHP seems to be cheaper, but then you've got external noise, plus it'll look fuggly.

 

An ASHP is much, much cheaper than ground source. For what you're trying to achieve, you'd be buying the smallest possible unit. You can probably get something for £1700-2000, depending on brand (you might get it cheaper if you're willing to go the ebay route).

 

Installation isn't too hard, and with an installation this size you can probably avoid a manifold + pump (etc) and have the ASHP drive the UFH directly. I do worry that even the smallest ASHP will be so oversized that it will be cycling on and off a lot.

Edited to add: ASHPs are fugly, for sure, but usually you can tuck them out of sight. The issue you'll have with your situation is that you can't put it at the back or side of the building, because there's no space. You could put it at the front, but you'll need some form of screen to camouflage it. I wouldn't worry too much about noise: if you're using it, the doors/windows will be closed, and decent inverter-driven ASHPs really don't make very much noise, especially when they aren't being driven very hard (as would be the case with your setup). 

 

Given how low your heating requirement will be, I'd really think hard about whether it makes financial sense to go for such an expensive heating option for a space like this.

 

Alternative idea:  @TerryE plans to use an ASHP with UFH, but is presently experimenting with a Willis heater to get a feel for how much heating he actually needs. A Willis heater is basically a standard immersion heater in a sleeve, which heats water as it's pumped past. It's incredibly cheap and simple - eg, here's one for less than £50. You'd need to add an UFH pump to this, but it's still a lot cheaper than an ASHP. They're very reliable, and if it breaks you can buy a replacement immersion heater off the shelf for peanuts. In comparison, we had a pump go on our ASHP after less than three years and that cost several hundred quid to replace (had to replace a circuit board and pump together).

 

In your situation, I'd consider a Willis heater driving a radiator or underfloor heating, and wiring up (but not yet fitting) provision for an air conditioning unit that can be retrofitted if you find it regularly getting too hot in summer. 

 

You could spend the money you were going to spend on an ASHP on better insulation (as mentioned above, timber frame is definitely your friend given the space constraints), which will further lower your energy requirements.

 

Pay attention to shading so that you maximise gain in winter and reduce it in summer. 

[scottishjohn]

12 hours ago, scottishjohn said:

ASHP should not be noisy  if you pick one that is plenty big enough so its always running at low speed for your heat requirement.

with garden space you have for GSP --a bore hole would only way +that will end being 3 times the price of ASHP system 

main thing is to get building insulation   as good as poss  then you will need so much less heat to start with 

at size of floor you quote i doubt its worth either --UFH as you say and a simple electric inline boiler and economy 7 .

only if you want to use ashp for cooling of slab in summer might it be worth it 

lot more tecchie people than me on here who will direct you better

 

this is what i would be using for heat supply  in your small build

https://electricaldealsdirect.co.uk/heatrae-sadia-amptec-c600-6kw-electric-boiler.html?msclkid=bc823df1a32818b32ba061a8ea6c5a8c&utm_source=bing&utm_medium=cpc&utm_campaign=Heatrae Sadia Shopping AD&utm_term=4583245503759036&utm_content=AD - heatrae sadia %26 Heatrae Sadia Amptec C600 6kW Electric Boiler (EDDSADIA108)

presuming you still going UFH  

only advantage i can really see for UFH in that building is to get more space  and flexibility of layout

as radiators or heaters will take up wall space

Edited February 5, 2019 by scottishjohn

[bikerchris]

12 minutes ago, jack said:

 

An ASHP is much, much cheaper than ground source. For what you're trying to achieve, you'd be buying the smallest possible unit. You can probably get something for £1700-2000, depending on brand (you might get it cheaper if you're willing to go the ebay route).

 

Installation isn't too hard, and with an installation this size you can probably avoid a manifold + pump (etc) and have the ASHP drive the UFH directly. I do worry that even the smallest ASHP will be so oversized that it will be cycling on and off a lot.

 

Given how low your heating requirement will be, I'd really think hard about whether it makes financial sense to go for such an expensive heating option for a space like this.

 

Alternative idea:  @TerryE plans to use an ASHP with UFH, but is presently experimenting with a Willis heater to get a feel for how much heating he actually needs. A Willis heater is basically a standard immersion heater in a sleeve, which heats water as it's pumped past. It's incredibly cheap and simple - eg, here's one for less than £50. You'd need to add an UFH pump to this, but it's still a lot cheaper than an ASHP. They're very reliable, and if it breaks you can buy a replacement immersion heater off the shelf for peanuts. In comparison, we had a pump go on our ASHP after less than three years and that cost several hundred quid to replace (had to replace a circuit board and pump together).

 

In your situation, I'd consider a Willis heater driving a radiator or underfloor heating, and wiring up (but not yet fitting) provision for an air conditioning unit that can be retrofitted if you find it regularly getting too hot in summer.

 

You could spend the money you were going to spend on an ASHP on better insulation (as mentioned above, timber frame is definitely your friend given the space constraints), which will further lower your energy requirements.

 

Pay attention to shading so that you maximise gain in winter and reduce it in summer. 

Thank you very much Jack, you've certainly given me more to think about - like you say, so far the only ASHP/GSHP's I've seen are well oversized which would be a waste of money. I really do like the idea of one solution that deals with the whole year round...though having a pump go after only a few years would be disappointing.

 

So the immersion heater route, is there a miniature hot water tank you can buy to put it in, or is there a more inline system available? 

 

It would be a shame to need an Air Con unit put somewhere, the roof would be a no-no as the structure will be tall enough as it is. 

 

When it comes to building orientation, I'm kinda stuffed, as it can only be set to the back of the garden for practicality, the glazed elevation faces South South West roughly, though I have incorporated a 3-4 foot overhang to the front which should help with solar gain and if budget permits I was going to include a Brisolei. 

 

You've given me a superb amount of info and I'm really grateful for you taking the time to write.

 

[bikerchris]

9 minutes ago, scottishjohn said:

this is what i would be using for heat supply  in your small build

https://electricaldealsdirect.co.uk/heatrae-sadia-amptec-c600-6kw-electric-boiler.html?msclkid=bc823df1a32818b32ba061a8ea6c5a8c&utm_source=bing&utm_medium=cpc&utm_campaign=Heatrae Sadia Shopping AD&utm_term=4583245503759036&utm_content=AD - heatrae sadia %26 Heatrae Sadia Amptec C600 6kW Electric Boiler (EDDSADIA108)

presuming you still going UFH  

only advantage i can really see for UFH in that building is to get more space  and flexibility of layout

as radiators or heaters will take up wall space

Wow, that's a slim unit, thank you very much John, very much appreciated.

 

Yes to UFH, partly to provide layout options, but also for the even heat.

 

At the moment, the office I rent costs me around £5k a year, so even if I spend that in the first year (on a credit card of course), I can recover that in the second year. Personally I'd like to move to a more appropriate house, but it was difficult getting a mortgage in the first place as I'm self-employed ? 

[scottishjohn]

If you now thinking elec boiler /immersion heater

then small dhw water tank  as buffer and expansion vessel -job done

.nick the plumber will  probably confirm or say i,m talking rubbish , and give you better solution but i think you can get small tanks with expansion vessel built in  , so then its just plumb to mains pressure + add a circulation pump

my reason for using inline boiler is a more accurate constant  temp control  than a simple immersion element .

[jack]

42 minutes ago, bikerchris said:

So the immersion heater route, is there a miniature hot water tank you can buy to put it in, or is there a more inline system available? 

 

A Willis heater is an inline system, no tank required. You could use a small tank as a buffer if you wanted to.

 

You'd need to check with a plumber as to whether an expansion tank is required given the volume of the system.

[scottishjohn]

2 minutes ago, jack said:

 

A Willis heater is an inline system, no tank required. You could use a small tank as a buffer if you wanted to.

 

You'd need to check with a plumber as to whether an expansion tank is required given the volume of the system.

the answer is  adefinate --yes --but it might be very small --like in a combi boiler -if its going to run be a non vented system 

amptec type you get temp control etc all in one package--its what they were made for 

simple heating  solution for a flat 

[dpmiller]

A willis uses a standard immersion element and has a built in stat...

[jack]

8 minutes ago, scottishjohn said:

simple heating  solution for a flat 

 

I'm sure it could be done more cheaply with a Willis heater, but for not that much more money I agree that having everything in one integrated and warrantied unit is attractive.

[scottishjohn]

5 minutes ago, jack said:

 

I'm sure it could be done more cheaply with a Willis heater, but for not that much more money I agree that having everything in one integrated and warrantied unit is attractive.

last one i bought i got on ebay --unsed -bnib --at half price --so if going that way start goggling

[Jeremy Harris]

A Willis is a better option than an electric boiler for this application.  Electric boilers have controls and interlocks that will make them a PITA to run inline with a UFH system, as they are not designed to do this.  Electric boilers are fine for what they were designed to do, which is provide high grade heat in a similar way to a gas or oil system boiler.  That's not what's needed for UFH though, all that's needed is a simple heating element together with a pump and thermostat, as the flow temp probably doesn't need to be higher than about 30°C to 35°C.

[scottishjohn]

yes, you are correct 

just checked manual for the one i have and  temp range  65c --80c ,fine it was feeding a cylinder, then to UFh with thermostat ,but not direct coupled

hard calculation ,but maybe if you had small thermal store --heated to 70c with economy 7 at night

and then UFh uses it as required  with thermostat through the day ,

but then you might as well go for simple immersion element on tank -- 

life is never simple 

Edited February 5, 2019 by scottishjohn