ASHP and First World Issues

[Trw144]

I need to start thinking about heating for the forthcoming build in advance of meeting the architect next week and dedicating some plant room space. I'm not 100% decided yet but would like some thoughts on an ASHP's route. I haven't done exact heat loss calcs yet, but house is 600m2 so its going to be a larger heat pump, at a guess something approaching 15kW. Thrown into the mix is an indoor swimming pool - which has a variable demand based on use, but requires a max heat demand of 10kW (much of which is to boost the ambient air temperature when the pool cover is taken off, rather than a constant 10kW). Current quandaries in my head....

 

Looking like two single phase units, or a large three phase unit installed with a large buffer (which will I plan on having anyway as I wish to install an abundance of solar thermal to heat the pool for free for as much of year as possible). Will also have PV (and possibly battery storage) but thinking this is unlikely to see an excess for water heating.

 

Quandaries in my head on this approach...

 

Should I go for a single large unit or two smaller units.

At this size, should I GSHP - there is an acre of field next to the garden that could be used, and I have access to the kit to dig my own trenches. There will be a near identical house next door (without a pool) so again potentiall 15kW and could share the same ground loops.

 

Should I design thinking about using an agile tariff and possible battery storage.

 

 

Edited April 9, 2021 by Trw144

[joe90]

From what I have read here and elsewhere GSHP are not favoured much, lots of brine, noisy indoor units. I have an ASHP and it’s brilliant but only gives low output (5 kW) over long periods but if designed properly works very well in a well insulated airtight house.

 

my brother in law had an indoor pool and it cost him more to heat that than his largish house!!! (Oil).

Edited April 9, 2021 by joe90

[PeterW]

I would go for 3 heat pumps not 2... 

 

The one for the pool doesn’t need to be either an inverter driven one or posh - go for a pool pump and run it hard. Also look at using A2A in the pool room to keep the temperature normalised and also control humidity. 
 

The rest I would go for 2 smaller pumps running in tandem, and even consider using them with a pair of DHW tanks, gives you the option to boost your capacity only when you need it. 
 

GSHP is only worth it if you are going full MCS as the costs outweigh any benefits. How much PV are you installing ..??

[Trw144]

1 hour ago, joe90 said:

 

my brother in law had an indoor pool and it cost him more to heat that than his largish house!!! (Oil).

 

Yes, this is exactly what I would like to do my best to design out, and run as much off solar pv and solar thermal as I can. The pool water and air handling I had been looking to run from a combined unit (Heatstar) that produces both - they can cope with a flow temp of 43 degrees.

 

1 hour ago, PeterW said:

How much PV are you installing ..??

 

I'm thinking as much as I am allowed so if I do go three phase, perhaps circa 12kW. Part of the design has around 100m2 of flat roof (although its not perfect as it will suffer from a bit of shading from the pitched roof next to it). My solar thermal supplier has suggested I could cope with around 16m2 of solar thermal and not cause a summer stagnation issue.

[PeterW]

So if you’re going solar thermal for the pool as well you may want to run some sort of buffer as otherwise you’ll be having multiple heat exchangers. 

[AliG]

Hi @Trw144. I have around 900sq metres, a pool and a Heatstar.

 

The good news is that if the pool room is well insulated and triple glazed it won't cost much to heat. I reckon mine is around £500 a year.

 

I also reckon that the pool pump and the electricity for the dehumidifier cost more. My electricity bill is larger than my gas bill which is pretty unusual.

 

First question - do you have mains gas? I have a standard Bosch 42CDI system boiler. It is exactly the same boiler I had previously in my much smaller house and it easily copes with heating the house and pool.

 

I have considered what I would do at some point in the future if I had to replace it and boilers were banned.

 

On the very coldest days of the year I use 450kWh of gas (sub zero temperatures). So at worst I am using 20kW per hour. I reckon with a few improvements to insulation and air tightness that were missed in the build I might get this figure down to 400ish.

 

Thus my thinking is that I would maybe need 2 heat pumps to replace the boiler. As I am above the normal 16kW max.

 

If I look at my pool it seems to use around 50kWh a day. This heats the pool and pool room which is an area of around 85 square metres. This is a rough estimate from how much gas we use when there is no one at home.

 

I have never quite figured out how the Heatstar sets out to warm up the water. Rather than constantly warm it a little bit it kicks in for maybe a minute every few minutes. Maybe it is dependent on how much energy you supply it and I have it connected to a high output boiler. The temperature of the water is very constant anyway, it just happens that I notice the boiler firing off and on. As you say, when you open the pool it kicks in to heat the pool room. Just in case I have UFH in the pool room, it has never been used and was a waste of money. The room sits at a constant 22C and the Heatstar kick in to warm it up to 30C when you open the pool cover. The reality is I don't usually spend enough time in the pool for it to warm all the way up and don't really notice the air temperature. After a few minutes in the pool you feel quite warm as it is set at 29C.

 

Looking at my original quote there was a 13.8kW ASHP,  I suspect that you need this size to hit the minimum spec of the Heatstar coils, even though the pool doesn't require this much heat. Pool heat pumps seem to be a lot cheaper than house heat pumps, so you are probably looking at £3k ish fitted.

 

Ex your pool room you are going to have 500sq metre of house plus hot water. This is probably going to need a 12kW ASHP depending on your insulation.

 

I looked into solar thermal and everything I read said it was a bad system with a lot of maintenance issues and not to bother. Better to use PV which will power the pool ASHP in the summer.

 

I looked into GSHP and the cost of installing the pipes was laughably high. I think I was looking at something like £26,000 for a system.

 

You will probably want three phase with all the extra elections in the house. The issue with this at the moment i you cannot get a smart meter to be on an Agile type tariff.

 

I hope to be on this eventually. The pool filter pump runs 16 hours a day and the dehumidifier all the time. You might be able to time it so that more of your electricity use is at night, especially if you also have an electric car to charge. I have looked at the Tesla power wall and I don't think it pays for itself, but the Octopus Tesla energy plan might work out well simply because a pool uses a lot of electricity and it cuts the average price paid. You have to have a Tesla though! Actually checking again, this might save about £1000 a year in electricity, but the Powerwall is around £10k installed and lasts around 10 years so it doesn't really work.

 

 

 

 

 

 

 

Edited April 9, 2021 by AliG

[Trw144]

Hi Ali,

 

I read you had a pool so hope you don't mind me picking your first hand knowledge...

 

1 hour ago, AliG said:

First question - do you have mains gas

 

No, or this would have been my first (easiest) choice.

 

1 hour ago, AliG said:

On the very coldest days of the year I use 450kWh of gas (sub zero temperatures). So at worst I am using 20kW per hour. I reckon with a few improvements to insulation and air tightness that were missed in the build I might get this figure down to 400ish.

 

Certainly not near some of the levels on this forum, but our current selfbuild (just under 300m2) has used 16,000 kWh for the last 12 months. If I can achieve a similar level of heat usage per m2 (excluding the pool), then I would be happy. I have never looked what my usage on a cold day is.

 

1 hour ago, AliG said:

If I look at my pool it seems to use around 50kWh a day

 

This tallys with what heatstar have calculated , Do you know how this stacks up between a day you do and don't use the pool? One particular question I am trying to bottom out is how this peaks out and for how long. Heatstar have suggested a 20kW supply for initial heating of the water, and 10kW for the ongoing requirement (air handling and water).

 

1 hour ago, AliG said:

The pool filter pump runs 16 hours a day and the dehumidifier all the time

 

Do you know if it is important when during the day this runs as this would seem an obvious one to time shift (ie. avoid the late afternoon/evening expensive tariffs)?

 

The Tesla tariff I like the look of (even without the car) as it currently equates to 11p/kWh for usage, but also the same for exports. As far as I can see, this is handy with a large PV system as it allows you to export your energy and then get it back when you need it at no cost. This theory doesn't work if they have some small print limiting your PV size/exporting? I also know two Powerwall installers who are customers of mine so hoping I could do some swapsies.

 

 

 

2 hours ago, AliG said:

I looked into solar thermal and everything I read said it was a bad system with a lot of maintenance issues and not to bother. Better to use PV which will power the pool ASHP in the summer.

 

My view on solar thermal is it works very efficiently for heating water (and is superior to PV and an immersion in this aspect). The issue for most people is they don't have sufficient hot water usage to make the cost worthwhile  (ie. just heating a bit of domestic hot water), and stagnation causes issues. A swimming pool changes this and the ball park figures I have calculated suggest I could produce 60% of the pools water and air heating demand from solar thermal. For instance, I have recently installed a 30m2 system and many a day recently I have seen this churning out 15-25KW (I am planning on a system approximately half this size).

 

 

2 hours ago, PeterW said:

So if you’re going solar thermal for the pool as well you may want to run some sort of buffer as otherwise you’ll be having multiple heat exchangers. 

 

Yes, I would like a buffer to act as a central heat source potentially for everything to run from., it would also work as a battery to store solar energy on a good day, for perhaps a day or two longer. I was initially thinking maybe a 1500-2000 litres. Having this buffer also allows me to consider running the heat pump overnight at higher temperatures on an agile tariff (reduced cop being more than offset by reduced electricity costs?).

 

4 hours ago, PeterW said:

The one for the pool doesn’t need to be either an inverter driven one or posh - go for a pool pump and run it hard

 

What does the inverter bring to the party on a heat pump? Modulation?

 

 

[AliG]

14 minutes ago, Trw144 said:

I have never looked what my usage on a cold day is.

I gave that number purely as an indication of how to size the ASHP as it is defined by the worst days.

 

We use 75,000 total a year. That is around 20,000 for the pool, 15,000 for hot water (we use too much hot water) and 40,000 for heating.

 

15 minutes ago, Trw144 said:

Do you know how this stacks up between a day you do and don't use the pool?

I have never got round checking on days when it is and is not used. It would depend how long it is open for, but I suspect it is 5-10kWh. I will see if I can check this.

32 minutes ago, Trw144 said:

Do you know if it is important when during the day this runs as this would seem an obvious one to time shift

 

It shouldn't matter. You just need to run a certain amount of water a day through the filter. I am sure it could run bit less, but the pool is lovely and clear. The pool guy set it up to run during the day so it did not disturb people at night. But you cannot hear it upstairs, so I would rather it ran at night anyway. It is quite noisy when you are in the pool room (it kicks into a higher speed when the pool is open). He tried to change it once and it didn't work, but I think I could probably change the settings myself if I had a smart meter.

 

40 minutes ago, Trw144 said:

This theory doesn't work if they have some small print limiting your PV size/exporting?

 

It looks like the system is set up to maximise self use and they have total control of the system. I have 5kW of solar. The savings on their calculator seem to go down with more solar. For a simple calculation, say you use £2000 a year in electricity. Reducing your cost from 15p/kWh  to 11p/kWh will save £500 a year. The Powerwall is £10k and only has a useful life of 10 years at best. Most of the cost is the Powerwall itself, so you won't be able to save much.

 

46 minutes ago, Trw144 said:

My view on solar thermal is it works very efficiently for heating water

It does indeed, but I doubt it would produce half of the pool's heating costs as they don't produce as much during winter.

 

The equation is slightly different with gas as although PV produces less energy it displaces electricity which is a lot more expensive than gas. Still with a ASHP water heating costs around 4p/kWh versus pure electricity at around 15p. Thus PV might still produce better or similar savings.

50 minutes ago, Trw144 said:

Yes, I would like a buffer to act as a central heat source potentially for everything to run from., it would also work as a battery to store solar energy on a good day, for perhaps a day or two longer. I was initially thinking maybe a 1500-2000 litres

 

I am not sure this would be worthwhile. When your solar thermal is generating it should all be eaten up by the pool using 50kWh a day. It is an even larger buffer. You will need a smaller one to manage the heat transfer, but I don't think an enormous one. You'd only need a large buffer if there were days the solar thermal generated more than you need for the pool and hot water which seems unlikely.

 

 

 

[Trw144]

1 hour ago, AliG said:

When your solar thermal is generating it should all be eaten up by the pool

Some yes but not all as seemingly most of  the demand is for heating the air, it also allows the pool heating to be kept more constant as opposed to overshooting target temperature, and then dropping back until the next day by simply using the pool as the buffer.

 

2 hours ago, AliG said:

but I doubt it would produce half of the pool's heating costs as they don't produce as much during winter.

December and January aren't great but outside of this you'd be suprised how many days solar generation can work in the U.K. (I have a 30m2 system recently installed that has created over 1300KWh in the last three weeks)

[PeterW]

I’d be running a 2,000 litre buffer at max 45°C from the heat pumps and use it as a primary source to a  400 litre UVC. Then put the UVC immersions on an overnight tariff and as the primary dump load to 70°C for the PV. Buffer needs a pair of stats on it that kick one ASHP in at say 42°C then the next at 38°C. Simple switch over will mean that they get equal use and you’ll always have redundancy. Use something like a Finder 72.42 Priority relay. 
 

Solar coils and immersion dump loads can also go into the buffer and just put a differential controller so the buffer heats the main house UVC whenever it’s hotter by 5°C etc. 
 

Pair of 8kW ASHP will give you plenty of grunt but also allow you to have some redundancy if needed. 

[Trw144]

Looking further into two ashps. Anyone aware of any of the manufacturers producing a cascade controller -and potentially manifold as they do on gas boilers?

[SteamyTea]

Right, you have 3 things to do.

Heat a house

Heat DHW

Heat a swimming pool.

 

These are quite different things, done at different times and to different temperatures.

Treat them separately.

 

As for ST and PV, PV will probably be cheaper, more reliable and better looking.

 

Don't discount a GSHP, they are less weather dependant, but don't share the loops, this will be a problem.

They may cost more and have the HP part inside, but it is not as if you are scratching around to save a grand, and you have plenty of room to hide the noisy part away.

 

You also have to think about heat recovery and air conditioning.

You could probably run all your DHW of the extraction from the pool room.

 

Edited April 10, 2021 by SteamyTea

[Trw144]

16 minutes ago, SteamyTea said:

As for ST and PV, PV will probably be cheaper, more reliable and better

Hi. 
 

I will have 100m2 of flat roof below a parapet so it shouldn't be visible, and I have access to a good solar thermal brand so 16m2 of evacuated tubes plus ancillaries for under £4K which I think gives me a good bang for buck.

 

16 minutes ago, SteamyTea said:

Don't discount a GSHP, they are less weather dependant, but don't share the loops, this will be a problem.

They may cost more and have the HP part inside, but it is not as if you are scratching around to save a grand, and you have plenty of room to hide the noisy part away.

I will look into it some more, trying and figure out a bit of a cost and how big a loop I will need in the ground. The land gets quite wet as it's on a slope - I don't know if this is beneficial or not when trying to extract heat from the ground. One thing I do remember after my original post is that GSHP's are nt recommended on swimming pools as the ground isn't given chance to recover during the summer - possibly not an issue if the solar thermal and PV take over in these months but needs consideration.

 

16 minutes ago, SteamyTea said:

it is not as if you are scratching around to save a grand

I probably will be by the end!

Edited April 10, 2021 by Trw144

[PeterW]

28 minutes ago, Trw144 said:

Looking further into two ashps. Anyone aware of any of the manufacturers producing a cascade controller -and potentially manifold as they do on gas boilers?

Can do a cascade controller with the Finder relay and a pair of tank stats. Depending on the heat pump start signal (NV/230VAC/24VDC etc)  then you may need two more relays rather than direct output but it’s all change of £100 stuff. 
 

 

[SteamyTea]

2 minutes ago, Trw144 said:

The land gets quite wet as it's on a slope

It is a good think.  You can extract about 4 times as much energy out of water than dry ground.  GSHP are really water source heat pumps with the water heated by solar energy.

 

[SteamyTea]

13 minutes ago, Trw144 said:

16m2 of evacuated tubes plus ancillaries for under £4K which I think gives me a good bang for buck.

That works out at 250/m².

 

https://edavies.me.uk/2012/01/pv-et-flat/

[dpmiller]

The Carel controller on our ASHP is cascadeable

[PeterW]

 

ST sounds expensive and possibly over what you can easily use.  16sqm of PV is roughly 12 panels or 3.6kW so can be had for about 80p/W 

 

16sqm is approx 120 tubes at 58mm with a peak output of around 6.2kW so whilst it’s giving you a lot more heat it is going to need a lot more buffer / storage as in summer it’s got nowhere to go after that pool is warm. 
 

 

 

 

[Trw144]

1 hour ago, PeterW said:

 

16sqm is approx 120 tubes at 58mm with a peak output of around 6.2kW so whilst it’s giving you a lot more heat it is going to need a lot more buffer / storage as in summer it’s got nowhere to go after that pool is warm. 

 

These panels (4 of them at 4m2 each), have peak power of 2700w each so four gives me 10800 watts, and puts me at 37p/W.  I do need to model it further but the technical guy at my solar supplier suggests I should be ok with this (he will model it fully once I have all the data to hand) with a 2,000 litre accumulator.

[SteamyTea]

1 hour ago, Trw144 said:

and puts me at 37p/W

No so different from PV then.

PV has the advantage that it can deliver some power at lower light levels as it does not rely on temperature difference as the main switching mechanism.

And it don't need servicing, pipes, mechanical valves, antifreeze, expansion vessels, over pressure and temperature safety systems.

Wires are easy to fit.

And when the pool us hot (which you may be able to do with just resistance heating to save on installation costs), you can divert it to charge an EV.

 

But get those thermal models fine and checked first, you may find that better pool insulation changed a price point somewhere else.

And don't forget the evaporation losses.

[AliG]

In terms of heat loss from a pool, when I calculated it, the loss is really the heat loss from the pool room. The losses to the ground are much smaller. Although I do have EPS under and around the pool. The reason pools used to cost a fortune to heat is that they need quite large rooms and people would often put them in rooms not much better than sheds.

 

 

[Trw144]

1 hour ago, AliG said:

In terms of heat loss from a pool, when I calculated it, the loss is really the heat loss from the pool room. The losses to the ground are much smaller. Although I do have EPS under and around the pool. The reason pools used to cost a fortune to heat is that they need quite large rooms and people would often put them in rooms not much better than sheds.

 

 

These are the figures I am working from....

Edited April 10, 2021 by Trw144

[AliG]

I have the same spreadsheet still from my original order.

 

They filled it in with some standard numbers and never redid it for the actual numbers for the house.

 

I calculated it myself using the JSHArris heat loss calculator.

 

Looking at the calculation though, it is indeed based the structural heat losses of the room. The way the pool makes a difference is that it requires more ventilation than a normal room which loses energy from the evaporated water. Otherwise it is the same calculation.

 

It looks like yours is in a basement? No windows in the calculation.

 

Looking back at my calculation there was an allowance for the electricity for the dehumidification and filter pump. They would be around £450 (3000kWh each) a year each which seems about right. Actually i tried to work out my electricity consumption a while ago and I had the dehumidifier at just under 3000 and the pump at just over which is almost exactly what is in the original calculation. I think I calculated the pump use from its specs.

Edited April 10, 2021 by AliG

[Trw144]

1 hour ago, AliG said:

It looks like yours is in a basement? No windows in the calculation.

Yes it is in the basement (on three sides as it is into a bank) although has since moved position in the basement so will have one, possibly two patio sliding doors.

 

Glad to know their figures are somewhere near accurate. 

[Trw144]

Looking at heat pumps, and a pair of of Valliant Aerotherm 12kW look like they would be up to the job (and appears to have one of the higher SCOPS)... 12.5kW output at -5 (@45 degree flow), rising to 14.9kW @ 2 degrees, should cope with house and pool heating even in the coldest months. Is there any downsize to oversizing a heat pump if its being installed with a decent size buffer?

 

Will take a look at GSHP next and see if the improved SCOP justifies the additional expense.

 

EDIT...

 

The quick answer appears to be ASHP. Valliant ground source heat pump at 10 degree ground temp/35 degree flow temp is 5.5 vs SCOP of 4.5 for Valliant Aerotherm @ 35 degree flow. Assuming 50,000 kWh needed a year in total (I'm hoping it will be less), then this equates to 9000kWh electric needed on the GSHP vs 11,000 on the ASHP. Assuming 15p/kWh then a saving of £300 a year. Have I missed anything?

Edited April 13, 2021 by Trw144