GSHP heating hot water options

Sustainable Energy
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I think I can see now how I can make this work without having to replace things i already have, so that’s great.

I have a few questions…

Given my 2 well insulated storage tanks, 280L for DHW and 120L for UFH, in summer I am using 5-8kWh per day to heat DHW for showers etc. Zero for UFH. In winter DHW is the same, but the UFH runs all day. I guess on average the UFH energy usage is around 20kWh per day.

if I were to install solar thermal (flat plate or evac tubes)

1) what size would be best for me?

2) How do I make use of solar thermal in the early morning?

My showers are used at 6.30am before the sun comes up! That’s when i use most of my DHW energy.

I (think) my GSHP will monitor the DHW tank, and when it drops below temp, will kick in. Won’t the solar come on later in the morning? Then I will still be using 4-5kWh on my GSHP to heat showers every day.

3) Would boosting the DHW temp in the day solve this?

I‘ve now got a DHW tank at 45°C (thanks for the tip @moojuiceuk nobody in the house has noticed yet!). I’m not sure i understand how the DHW tank temp monitoring works really :confused: What if I got the solar thermal to boost this to, say, 60°C during the day? The tank should only lose about 5°C overnight. As the tank is hotter the showers shouldn’t draw so much as they are mixed with cold to get the right temp. Assume the GSHP will kick in when the tank gets below 45°C.

4) Could I delay the GSHP triggering

I’m not sure I understand how the DHW tank temp works really, I understand heat rises in the tank, gets drawn from the top and cold gets fed in the bottom. I realise the cold water at the bottom will eventually mix to reduce the overall temp, but in the hour we have our showers, won’t we be using the hot water at the top? I think it’s a big enough tank to cover our early morning showers.

So, could I also stop the GSHP heating water in the morning unless I run out of hot water? Say we have guests and have 6 showers instead of 3, I’d need the GSHP to start if the Solar hasn’t kicked in yet. (Although, guests don’t get up at 6.30, so this may not be a problem :wink:

5) Can i do the same for UFH tank?

This currently stores at 35°C. I could just maintain that with solar thermal, or could I boot that to say 60°C then add a mixer to drop it down to 35 for feeding back into my UFH. This could reduce the GSHP operation at night.

Interested in your thoughts, it will help me decide if it is worth doing or not.

Pete

42

I think moojuiceuk has already answered point (2) - sorry. It does have a temperature probe into the hot water tank, which triggers when the temperature goes below a set value. Now I have set the GSHP down to 45Deg I wonder if the answer to this is simply to get the tank up to a high enough temp the day before using solar thermal, say up to 60deg, then we should get showers out of that in the morning without the tank going below 45Deg. Maybe the delay is now a ‘nice to have’.

update…

Looks like i can also use the timer function on my GSHP - happy days!

Setting of the hot water according to clock
The Clock setting DHW accord. to clock (menu 4.3) works in exactly the
same way as Clock setting HP accord. to clock (menu 4.1). This function
can be used to completely disable hot water heating to save energy. This is
particularly effective if peak tariffs are charged.

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  1. based on your tank sizes if you live in a relatively sunny-ish location. 4- 5 kw of evacs would be about the largest i would put on that amount of storage if more cloudy you will have to experiment on the amount abit … I would set the solar collectors at a fairly high angle around 60 - 70 degrees but you might have to play with that depending on your location … so you do not produce so much heat in the summertime, and the heat production increases as the sun drops as the season changes and cools . the flat plate should be rated at least the same size 4-5 kw

  2. the morning temp - it should still be hot enough in most cases still by morning- but like I said I do not use the heat pump to heat my water specifically I use an instant water heater 9kw stieben eltron as a whole house water heater ( set right neat my water storage) that I paid $200 for then built a controller for it that looks at the solar tanks temp the temp of the heating unit and the temperature of the water in the lines then it only tops the water up to what ever my define water temperature is so if the water in the solar tank is 50C the instant water heat stays dormant. if the water in solar water tank is 40c, it waits for the temp in the water line to equalize to 40c ( why use extra power to heat the line) then the instant water heater comes on to raise it 10c to 50c. then after that the sun is allowed to heat the tank for the remaining of the day
    … in the winter time the heatpump heats my hot water tank but the instant water heater still there topping it up as required i could of used a smaller one as it rarely goes over 4kwh versus the 9kw as generally my tank is rarely below 40c. personally from your design I would just set the heat pump to the minimum allowable temp for the DHW tank (say 40c) and let the instant water heater handle the rest. energy wise it probably work out to the same you gain COP on the heat pump , smaller temp diff so the heat pump will not come as often on cloudy days just to maintain and the instant water heater handle the rest. and on sunny days the solar will do the rest.

this works the best in conjunction with the solar water heaters, as the DHW tank can be nearly depleted of hotwater due to water stratification and you only heat the hotwater you actually use… allowing time for the sun to do it work

you will most likely have to put a mixing value on your hot water as a good sunny day your hot water will be hotter then 60C closer to + 70 c is a more likely temp. and in the later seasons somewhere between 55 - 70c except for a one month period where 45c will be your average - since you live at roughly the same longitude as me

all i can suggest is you will have to experiment a little to get the best out of it… and what you want and do not want to install

for me I can not complain it worked well for me and has paid for itself many times over in the last 11 years as my heating requirements dropped to about ~2000 - 3000 kwh per year down from the normal 20000 - +30000kwh per year. and alot of people i know pay more in electricity in a single cold month then I pay in an entire year.

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44

That’s great info Stephen, your setup is really working well. I’m very excited about this now.

I have got in touch with Worcester Bosch and while I could use their Green Skies solar it will work just the same with other solar solutions - as you say, I just need a mixer valve to make sure temp from solar doesn’t go to high.

I will be getting things professionally installed - I do some DIY but for this would rather get the Pro’s in. But I wanted to understand what was possible before going forward.

Thanks again @stephen / @moojuiceuk your advice has been invaluable!

Btw @moojuiceuk my first day of the tank flashing to 60deg was today at 1am this morning - it actually used the additionsl heat element for 1.5 hours which is prob the most efficient way to get to 60deg. Will monitor over the next few weeks and do an energy comparison.

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your welcome no problem, anytime…

you know it working well when the electric company send you a threatening letter demanding to know how you are heating your house, and if you do not full fill their request they are going to bump your rate up from residential to commercial ( about 50% higher rate) as was the letter i got from mine around 5 years ago, i ignored it, because really would they of demanded the same request from someone burning wood or other fuel… nope, i asked other people who did burn wood they never got a letter like that. the only person I can find was me…

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They would get shot for doing that over here in the UK ! They are not our favourite companies at the moment, keep hiking up the prices!

48

Jessica, did you actually read this thread? The OP has a GSHP and was talking about getting solar thermal to assist it. (the GSHP is even mentioned in the thread title!). So your suggestion to get a heat pump seems a little bizarre.

As regards getting gas, the carbon emissions of a heat pump using UK mains electricity compares pretty well against gas nowadays, as does the cost. So there’s no real benefit there. The UK government plans to ban all gas heating from new houses by 2025 as part of the climate change policies, so it’s perhaps not the best suggestion for retro-fitting either. It would be yet another service and maintenance bill as well.