GSHP heating hot water options


As we have had a particularly hot summer in the UK, I’ve been thinking about thermal solar to help heat my domestic hot water.

Now, I already have a GSHP that covers my underfloor heating and domestic hot water. It’s really efficient to support the underfloor heating in winter, however, in both summer and winter much less efficient to get the domestic hot water up to temperature.

I’ve got deep borehole collectors and get a pretty consistent 12-13degC from the ground all year. But during the summer, the outside temperature is around 21degC and recently we’ve been basking in 30deC in the UK. Surely I would be better during the summer taking advantage of the higher air temperature?

If so, what do you think are my options? Adding solar thermal panel? Adding an small air-source heat pump too?

Interested in your suggestions


Hello Pete, that’s an interesting thought, a small air-source would in theory make sense as you say but then I imagine difficult to justify the expense for summer solar water heating only? Do you have any solar PV, could you automate your GSHP to turn on when the solar PV is generating? It might not be as efficient as it could be but it would still be better than direct electric and would make use of your existing investments, you could soak up the remainder: the PV generation that the ground source cant follow perhaps with a PV diverter…

@stephen has a lot of experience with solar thermal, see his post here: PV vs Evacuated solar if you were curious

Thanks Trystan, I’ll take a look at stephen’s post…

I don’t have Solar PV, looked into it earlier this year and its going to be costly as i have a flat roof and installation is more expensive. Can’t really justify, so i thought I would focus on trying to get the most from my GSHP and large water cylinder that is already installed. John Cantor has suggested perhaps adding a low cost flat plate solar water heating panel to top up the cylinder which i thought is worth investigating. I think i would want to stop the GSHP topping up the cylinder early in the morning when the sun is not out but we are having showers! It’s a large cylinder and i think if i can delay the top up until later in the day that would work. But not sure how to do that :wink:

I personally like the evacuated solar tubs over flat plate and I live a very cold climate… how much hot water do you wish to heat…in my experience 15 - 18 tubes is good enough for 25 gallon tank… you can get them cheap my last quote out of my supplier

the higher priced on are for .8mm wall on the transfer pipe compared to .6mm I suggest getting the .8mm over the .6 they last much much longer then the .6 - reason being when say if your panels stall say a pump outage it get very hot in the collector + 250c the small amount of stream in the the heat transfer pipe stretches the copper then eventually is burst after a few stalls … with the .8 mm it does not suffer the same amount of stress as the .6mm will

Thanks Stephen, I will definitely look into this. How would i make sure that my DHW tank does not start using the GSHP when we have showers in the morning before the sun comes up! I’d like to take advantage of the sunny mornings, but that’s after we have got up! My issue is that I think the GSHP will try and top up too early in the morning, even thought it’s a large tank and we are not using for the rest of the day, so the top-up can wait. I guess I’d like the GSHP to ‘kick-in’ around 4pm if there has not been enough sun to fill the tank with hot water.


Ah, now I’ve seen this picture of yours:

I think I understand a bit more how the Evacuated Solar tubes work together with the heat pump. This looks really neat! I also like the idea of putting the excess solar heated water back into the ground. But not sure how that works. We have deep boreholes (2x250m) so does the pump/plate heat exchanger simply pump the hot water down the borehole pipes for it to be retrieved later by the heat pump? What do you need a plate heat exchanger too?

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Hi - I do like the idea of trying to optimise DHW heating efficiency. There’s a few concerns I would have with the diagram that Stephen has shown you there however.

  1. That diagram shows “potable hot water” coming out of the outlet of the tank, going into the condenser plate heat exchanger (PHX) of the heat pump then back again via the cold inlet. Whilst this will work, it is exposing the PHX of the heat pump to mains water with no inhibitor. The PHX may start to suffer from a buildup of limescale and depending on the manufacturer of the heat pump, could void warranty. Also, if the PHX fails due to limescale and it somehow ruptures, you have water entering the fridge circuit. It is a toss up between a very expensive vac-out over several days, new filter dryer, compressor etc from your fridge engineer, or a new machine!

  2. The diagram does not take into account that the heat pump condensor PHX could also be the same one also serving your space heating. Depending on the manufacturer of the heat pump, you could be trying to pass UFH water, complete with inhibitor, through the PHX one minute, and “potable hot water” for the tank the next.

  3. Diverting waste heat from the solar panel into the heat pump evaporator PHX for the ground loop sounds like a great idea - but with 2 concerns. Most Heat pumps have a higher and lower operating limit to their ground source collector inlet temperatures. The machines I work with* on a daily basis have a ground loop operating range of -5C to +25C typically. Above that, they’ll refuse to work as there is too much energy for it to work with. Also, if your GSHP is a fixed speed compressor design, the increased ground loop temperature will result in a higher kW output. Sounds great eh? But it also means when servicing hot water soon after via GSHP, you are likely to hit the high pressure limits of the fridge circuit much much sooner. Likewise you could hit high pressure sooner on the heating circuit, especially if flow is marginal.

  4. What sort of control are you thinking about for the solar thermal panel? Ultimately you want to stop the solar energy from going through the coil of the tank above a certain temperature to prevent DHW overheating. In the UK, if the tank is an unvented cylinder, G3 regulations apply. There should be a means of cutting the flow the coil of the cylinder in the event of tank overheat - usually a 2-port valve closing if the tank overheats. If you have an unvented cylinder, only a G3 registered plumber should be doing work on the cylinder, so please read up on G3. Play safe!

So… What would I recommend? Firstly, have a look at dual coil DHW cylinders - one coil for the solar thermal, another coil for the GSHP. Go careful here, check with the GSHP manufacturer regarding coil size as heat pump cylinder coils need to be oversized, especially on fixed speed compressors. If you can’t go that route, it might be possible to use a seperate PHX between the tank and the GSHP, carefully sized. This is essential if your GSHP only has one condenser PHX that is used for UFH and DHW production!

Also, rather than simply powering down your GSHP when solar is at work, it may be possible to send an “inhibit” command to prevent it servicing only DHW during the inhibit signal. It may also be possible to set a “DHW Block” time within the controller of the GSHP to prevent it from attempting to heat DHW during the day when you are likely to get better solar performance.

What make and model of GSHP are you using? Each GSHP manufacturer will likely to have a set of hydraulic diagrams that they recommend for integrating solar thermal with their GSHP so issues like High Pressure, flow issues and operating limit errors are avoided…

  • I work for a UK manufacturer of heat pumps, amongst other heating technologies. All my views are my own however and you should always seek independent advice. I will not be held responsible if it all goes wrong! :wink:
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How is your GSHP wired up?

If there is a “call for heat” for the DHW you should be able to simply interrupt that wire with a standard timeswitch with it “ON” from 4PM until 8~10PM (or whatever time you feel there is no longer any value in boosting the DHW temperature as there is no point going to bed with a full tank of DHW) and otherwise “OFF”, the GSHP would then be unaware of the tank cooling and the solar would it’s thing in the morning.

Yes, that would work if the tank stat triggers demand to the GSHP. If the GSHP uses a temperature probe - ie an NTC10 (a variable resistance probe) or similar, then you’ll need to see if there is an inhibit signal on the controller.

Thanks for the replies, more complex that I thought ;-(.

I have a Worcester Greenstore 11 GSHP, I think it does have a DHW master on/off but it’s from 2011 so only has front panel controls, so not sure how I can switch automatically.

Not familliar with the Bosch Worcester line (I work for a competitor). Is it a combi (DHW tank inside machine) or system with external DHW?

Looking at the manual, what I can tell so far is:

  • It’s got a max ground temp of 20C, so dumping excess solar energy into the ground may cause the machine to fault out.
  • Its a fixed speed compressor (soft starter driven) so you should be careful to ensure any modifications made to the plumbing still ensure good flow rates. Poor flow will likely to trip the machine out on a high pressure fault (the compressor cannot vary its speed to a given delta t of flow & return temperatures to avoid high pressure).
  • Both the System and Combi versions use a variable temperature dependant resistor to sense DHW tank temperature, so interrupting that with a relay going o/c is probably going to give a sensor fault on the controller.
  • There are 2x External inputs B11 & B12. It appears one of them can be configured to “Block DHW production”. This is worth exploring.
  • There is a timer option for DHW. This will be the simple way to halt the GSHP servicing DHW at times when the solar thermal is likely to be doing most work.

If I were you, I would call Bosch Worcester and ask if they have a recommended hydraulic diagram of how to integrate solar thermal into an existing installation. They might just have the right hydraulic for you. It’s only likely if it is a system design with external DHW tank. The combi is much less likely to have an option to add solar thermal.

Good Luck!

thanks, will give them a call and see what advice they have. It has an external DHW tank, so there could also be options there with using a new tank with duel elements, but i’m reluctant to replace it as its quite new.

Hi there my specific heat pump is designed of direct water contact( no inhibitor required). and have not had any issues in 11 years now , to reduce build up in my heat pump exchanger the hotwater flows constantly it prevents the exchanger from scaling as it it is the process of cooling where the scales attaches to the exchanger as the mineral precipitate out , also if you are concern and you have extremely high mineral content the company I buy from, also has for has a heat exchanging meant for direct contact with chlorinated and salt water heating that pulls apart for manual cleaning …but really my mineral content is very very high a normal hot water tank in my area would not last 10 years before the bottom element is covered by scale sediment ( probably 40 lbs of scale sediment). it still settles in my tank as that where it precipitates out but not in my heatpump as the fluid is in constant motion keeping the heat exchanger clean

for my specific system the heating cooling and hotwater is all hydronic. no special sidearm which sounds what you are using…
since in my system the heat buffer tank and the potable is basically one in the same. I heat basically ~400 l of water, in the summer all 400 litre is heated by the sun, in the winter the heat pump and the sun. my heat pump only starts to work some times in late November when the sun is no longer strong enough to produce enough heat for heating and domestic… I do not use the heat pump specifically for making domestic hot water it only a happy offshoot… to maintain the desire water level temp I use in conjunction with an electric instant hot water heater. ( you can find mention of the controller I built for it in openenergymonitor … ) it is much better way then using the heat pump when in conjunction with solar water heaters . as generally a buffer tank will loose between 15-25 % of it stored energy per day. simply topping up the temperature with a instant water heater is a far more efficient way. then using the heat pump to keep it at a specific temp. as the heat pump will pulse roughly every hour for 7-10 minutes to compensate for that natural heat loss in a 24 hour period that is 240 minutes at say 2kwh . how much do you actually run your hotwater in a day , if it sunny I have more then enough if it not sunny and it only heats the water to 40C then I only need to top it 10c which means the instant hot water heater might run at 2kw consumption for what 30 - 40 minutes a day at most in the winter time in the summertime it might never run for months . (even if it heavily clouded for days general it always heat it to ~35C which means my instant water heater will run at ~4kw to maintain it at 50c which would make it on par with running the heat pump for constant maintenance of water temp not including you hot water usage ) also when using the heatpump in the winter an it heating my buffer I do not heat it to 50c that i want for domestic i heat it to 45-47C and use the instant water heater to top it (5c) if the sun can not. the reason being those few degree drop significantly improve the performance of the heat pump especially in spring time when the ground loop significantly cools( it all about temperature differential and squeezing that energy out of the unit)

the heat pump controls itself, but I built a secondary control . if my storage buffer tanks are above certain temp ( 55c) the heat pump it turned of completely until the lower limit is reached ( 50C) and for my solar heater they are open vented, ( you can also find mention how to build your own heat exchanger for your domestic hotwater tank if you want too also found on openenergymonitor

my heating system is quite simple but also quite complicated and I squeeze every drop of energy that i can out of it and hence why my normal yearly energy bill for heating probably avg out to $120ish ( cold winter might push $175 a warm winter $90) considering where I live middle of sunny Manitoba Canada that say a lot I use more energy a year running my tv and lights then i do for heating.

it a flat plate heat exchanger your ground loop fluid runs through oneside the solar fluid runs through the other side no exchange of fluid… if you are worried about over temp them put it on the output side . I have mine on the input side as for me in dead of winter when the sun strength is at it weakest and it can barley heat the water to 40c . my system switches the the solar water heaters to direct the solar energy into the ground loop boosting the energy output of my heat pump

the heat pump I have is not that restrictive I believe 35c was the max for the ground loop. but it determines its max by a pressure sensor or temp diff

Stephen, your system does sound well tuned to how you want it to work. If that works, great. Can’t knock it :slight_smile:

My experiences are coloured by the daily support calls I get where systems are not plumbed up correctly, the control strategy is not there etc. All the systems I work with keep the potable DHW completely seperate from heating circuit water via a cylinder coil or PHX. Whilst I will not confess to know the water regulations here in the UK in full, I do know that, generally speaking, we do not allow the two to meet.

edit - Since looking up water regulations in the UK, Wholesome mains cold water is fluid category 1. Stored DHW is fluid category 2. Central heating water for space heating is fluid category 3. Backflow prevention / seperation of them is required via non return valve or air gap depending on category & situation. In other words, in the UK, I would personally not share UFH water with the DHW tank directly.

There are also legionella prevention concerns sharing heating water running on UFH with the DHW tank. Your typical UFH circuit will be operating at a nice low temperature (30C?) which is an ideal breeding ground for legionella. The heat pump could be heating the DHW tank to 45C daily and once a week 60C to kill off the bugs. If solar is in play, it may be heated to 60C daily, but not necesarrily in winter months in the UK. The water in the UFH would not be heated to 60C however, and legionella could continue to grow there, then get pumped around to the DHW cylinder.

I know a service engineer who has caught Legionaires and thankfully lived to tell the tale. It’s not pretty.

Going back to a solution for Pete. It might be possible to share the same DHW cylinder coil between heat pump and solar via a 3-port valve (and 2-port shut off for G3 if applicable) if the heat pump is told not to service DHW when the solar is running. My concerns is if the heat pump is flooded with excess heat on its return pipe due to the heat from the solar, that it will either shut off prematurely whilst servicing UFH or hit high pressure fault. We still want to be able to service UFH via heat pump regardless of what the solar is doing.

As Bosch Worcester also make solar thermal panels, its likely they’ll have a hydraulic they’ve tested as good to integrate the two, to save Pete pulling his heating system apart as an experiment just before winter arrives here.

no worries my heating operates at the same temp as the domestic water (your not suppose to consume hotwater in general anyways as in contains alot of leached chemicals that really not healthy for you and even showering can be quite unhealthy due to the fine mineral particulate ), if it was to operated at a lower them would place that on a closed loop,… as the temp diff between ~50C and ~30C is relative great for easy fast heat transfer.

here a break down --generally in the good sun for 8 months the water will heat to 60 - 68 c in the winter it might generally make it to 50 - 60 ( there is a 1 month period where 45 is about the top temp 2 week before and after winter solstice

I have two tanks in series ( tank1 - domestic ~100l - tank2- heat buffer ~300l), my solar controller handle both tank directing the energy to the cooler tank first then once thermal fluid passes the temp of the hotter tank it runs the transfer fluid through both tanks. my heat pump specifically heat the tank 2 as when you use domestic hotwater or the heating system is on. it cools that tank not the domestic tank. when the heat pump comes on it pull perhaps 10% from the domestic to maintain it temp. when the heating system is requires it follows this path buffer tank ( mid tank) to heat pump > to top of buffer tank 60/30 split to heating which returns to the cold inlet on the buffer tank ( bottom of tank) when no heating required it moves mid tank to top of tank - with about 10% moving to the domestic so even though I only heat the buffer tank to 45-47C when it runs through the heating cycle it generally heats the domestic tank to +50c. as the hotwater leaving the heat pump is generally around 55 c when it getting close to finishing it cycle which pretty much sterilizes the system for the most part . then generally every couple days the sun heats it up enough to be completely sterilized of any bacteria. and if it not sunny enough and if my hotwater is cool, it sterilized by the instant water heater. like I said 11 years in operation 2007 is when it was installed


I do have 2 water tanks. One services the DHW and one for the underfloor heating. The DHW tank is also Worcester Bosch, it’s a 280L tank, but only has one feed in from the GSHP. My underfloor stores at around 30degC and DHW at around 53degC. All delivered from the GSHP with a 3 way switching valve inside the unit. It does have a booster element, but that only came on once last year when we had a really cold night.

I see from the Worcester website that they now make a tank specifically to combine solar with a 2nd heating source. Greenstore Unvented Solar cylinders.

I’m reluctant to spend again o a new cylinder, so that option is not for me at the moment.

I will give them a call on Monday though to see what else they may have that works with my setup. I know they do flat panel solar but i don’t think they do evac tubes.

Just for interest, I did a chart of my energy usage for a year…

The high values in Nov-Apr are due to the GSHP generating DHW and underfloor. In the other months, although they are low, a lot of it still comes from the DHW needs. We have quite a lot of tech in the house so realise i’m a high user anyway outside of the GSHP. I’m using 10-15kwh/day to power non heat related stuff. In the summer months, the GSHP uses around 5-6kwh per day on water heating.

I clearly need to focus on the winter :wink: But i was interested in making at least a small difference on sunny days to reduce my energy usage for the GSHP. Stephen’s approach looked very interesting to me, and I do like the idea of the evac tubes. But maybe not compatable with my system.

I’ve looked at PV panels before, but my calculations make that not worth while for me - hence my looking at solar water heating.


my energy usage is similar to yours but the difference here is that this energy usage is for 2 1200sqf home ( and a greenhouse) on one meter and my average temp is probably a lot colder then yours ( pretty much split the usage in 1/2 for one home )-- April missing as they over estimated my usage so the next couple months the energy company average it out… I shut down the second house last april and do not plan to heat this winter as no one lives there any more

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