@transparent We do not connect thermal stoor DHW units to the central heating. If you want a heat pump to work efficiently in heating mode, DO NOT connect it to a multi fuel heating and DHW thermal stoor. Heating is Heating, DHW is DWH, nether the twain shall meet, where heat pumps are concerned.

Well that's pretty clear @heacol ... except that I do not understand why this should be so. 🤔 

What is different in a heat pump such that it lacks efficiency if it receives return water from one 'cylinder' which is being heated towards 50C, but is better able to handle two such bodies of water?

If I were to remove the heat input from my existing gas boiler to my thermal store, and replace it with a GSHP, where does energy then get wasted?

Would your answer be different if the same thermal store also received heat input from a solar-thermal array, thus making it 'multi fuel'? What feature of the thermostatic control would the GSHP find difficult to handle?

To avoid blind alleys in the maths, let me point out that my UFH takes water from the thermal store at 46C. If the temperature of the thermal store is higher than that, then the return water from the UFH manifolds is passed via a mixer-valve. That maintains the 46C maximum feed to the UFH.

Nothing in this suggests to me that switching to a GSHP would be inefficient.

I certainly wouldn't want to feed the UFH directly from the output of the HP because it could be above 46C when entering the manifolds. That, in turn, could over-heat the oak boards on the suspended floor zones, resulting in warping.

@transparent 

Well my understanding of the issue is that the water in the store is hotter than it needs to be for the UFH. Since the COP of the Heat pump decreases as the temperature increases then its inefficient to heat the water to a higher temperature than needed. However since you also have a heat source other than a heat pump I think that muddies the water a bit, but since most of your heating requirement will be in the winter when the solar input is low, it would still seem that the inefficiency would apply

@transparent 

Rereading your post, I think you are suggesting that the heat pump only supplies water at one temperature? Mine has a DHW and an UFH temperature which switches when the demand area is changed.

I've just swapped a Thermal Store for a UVC as part of a new ASHP installation (my TS is for sale if anyone wants it - 277 Gledhill Solar RE).

I initially wanted to keep the TS - I liked the relative simplicity and ease of servicing etc....and given it was already in place it seemed daft not to use it.

I did a lot of research into the pros and cons of keeping it, searching forums, speaking to installers and even contacting John Cantor - the man who wrote the book on heat pumps:

https://amzn.to/379m8UU

Everyone, but everyone said forget it and I'd be better off with a UVC. The final decider was discovering the RHI scheme wouldn't accept Thermal Stores on their paperwork. It meant I wouldn't have got RHI for water heating and would have lost out on around £2k....so on top of everything else it didn't make financial sense. (I actually ended up £1,000 better off having a new UVC).

Here are some of the reasons I was told about why an ASHP and TS won't work efficiently together:

* TS not as well insulated as a UVC so loses heat quicker 

* You need to heat a TS to a higher temperature than a UVC because of losses in transfer

* Increased cost of heating a larger volume of water in a TS compared with UVC

* Hot water from a TS depletes quicker than a UVC (difference between direct and indirect hot water)

One installer, who has contributed to this site, had actually fitted a TS to his ASHP...I emailed him for his thoughts and this is what he had to say:

"It could be difficult using a thermal store. This is because it will have been designed to store water at a fairly high temperature.  The energy-efficiency of a heat pump will drop at high temperatures.    My initial feeling is that it won't be efficient or cheap to run.
Ideally you would operate it as it is and monitor the flow and return temperatures to the store. 
WRT heating.  It seems likely that the radiators should go direct from heat pump to the radiators.

"I would normally not expect a thermal store designed for a boiler to work with a heat pump.  This is because it has a fairly small coil in it and the water flowing through the coil (see page 77) needs to heat up instantly, and to be able to get say 50° at the tap, the store water needs to be well over 60°.  A specially designed heat pump cylinder has a big coil as I was showing in the book.   You are probably best off with a cylinder designed to go with the heat pump"

I'm no expert and only passing this on in case you can make use of it. Everyone's installation is different so it may work for you....but be aware you may end up running the ASHP hotter than you have to - so costing you more.

BTW I've learned a lot from reading @heacol previous posts and glad to see him back on the forum. He seems to know more than most about how heat pumps work. 

Andy

Posted by: @bontwoody

Rereading your post, I think you are suggesting that the heat pump only supplies water at one temperature? Mine has a DHW and an UFH temperature which switches when the demand area is changed.

That's useful to know. In my case I'd be expecting take output from a HP at a maximum 50C for both DHW or UFH use. The ability of a HP to offer two different heating rates hasn't seemed necessary to evaluate.

Posted by: @andygo - quoting from John Cantor

I would normally not expect a thermal store designed for a boiler to work with a heat pump.  This is because it has a fairly small coil in it

Ah... this possibly explains why our views differ.

My present Thermal-store doesn't have a coil to transfer heat from the existing gas boiler. The water circulating to/fro the boiler is exactly the same as that filling the main body of the TS. So there is no heat transfer loss to be taken into account. In effect my 'coil' is infinitely large!

I too am grateful to read @heacol's comments. It's discussions like this which help shape the decision-making process.

Even if I do opt to install a separate DHW cylinder, I would want to check what I'm buying much more closely than I did when acquiring the TS.

When it arrived here, the build-quality was poor.

• the ports entering/leaving the tank hadn't been assembled with a jig. Consequently they weren't aligned to the radius, but stuck out at odd angles!
• two valves and two thermostats were pre-wired to an electrical box. This had 7 loose connections, including the neutral-in. The wires used were standard PVC rather than high-temp (105C). Four of the earth wires in the connection box had been made using red wire 😡 
• the suppliers (who also designed the product) explained that they never see the TS before it gets shipped because they come direct from a 3rd-party manufacturer. As such they offered no redress, claiming that 'it isn't our fault'!

Caveat emptor!

No wonder this country manufactures so little these days.

Posted by: @transparent

Posted by: @bontwoody

Rereading your post, I think you are suggesting that the heat pump only supplies water at one temperature? Mine has a DHW and an UFH temperature which switches when the demand area is changed.

That's useful to know. In my case I'd be expecting take output from a HP at a maximum 50C for both DHW or UFH use. The ability of a HP to offer two different heating rates hasn't seemed necessary to evaluate.

Posted by: @andygo - quoting from John Cantor

I would normally not expect a thermal store designed for a boiler to work with a heat pump.  This is because it has a fairly small coil in it

Ah... this possibly explains why our views differ.

My present Thermal-store doesn't have a coil to transfer heat from the existing gas boiler. The water circulating to/fro the boiler is exactly the same as that filling the main body of the TS. So there is no heat transfer loss to be taken into account. In effect my 'coil' is infinitely large!

I too am grateful to read @heacol's comments. It's discussions like this which help shape the decision-making process.

Even if I do opt to install a separate DHW cylinder, I would want to check what I'm buying much more closely than I did when acquiring the TS.

When it arrived here, the build-quality was poor.

• the ports entering/leaving the tank hadn't been assembled with a jig. Consequently they weren't aligned to the radius, but stuck out at odd angles!
• two valves and two thermostats were pre-wired to an electrical box. This had 7 loose connections, including the neutral-in. The wires used were standard PVC rather than high-temp (105C). Four of the earth wires in the connection box had been made using red wire 😡 
• the suppliers (who also designed the product) explained that they never see the TS before it gets shipped because they come direct from a 3rd-party manufacturer. As such they offered no redress, claiming that 'it isn't our fault'!

Caveat emptor!

No wonder this country manufactures so little these days.

Was it manufactured in China by any chance? 😌 

Interesting suggestion @derek-m

But no. It was manufactured in Telford. And it won't take you too long to work out the highly-reputable British manufacturer who decided to cut corners once they realised that the product designers had put no QA checks in place.

Posted by: @transparent

Interesting suggestion @derek-m

But no. It was manufactured in Telford. And it won't take you too long to work out the highly-reputable British manufacturer who decided to cut corners once they realised that the product designers had put no QA checks in place.

I'm afraid it is one further example of the world in which we now live. 'Never mind the quality, feel the width', or in this case increase the profit.

@andygo 

Hi Andy

Thanks for this, all sensible stuff. I think there are a couple of caveats though.

* TS not as well insulated as a UVC so loses heat quicker

My thermal store was custom built by Newark and has 125mm of foam insulation

* You need to heat a TS to a higher temperature than a UVC because of losses in transfer

Absolutely true if you dont count legionella purges. How often are you intending to do these with your new UVC?

* Increased cost of heating a larger volume of water in a TS compared with UVC

Again true, but I heat my store to 51C and never to 60C for the purge using an inefficient imersion element, so bigger volume of water but lower temperature and a more efficient heating system, so it depends on how often the legionella purge is done

* Hot water from a TS depletes quicker than a UVC (difference between direct and indirect hot water)

True, but you have a larger reservoir of heat to deplete.

Also with a custom thermal store you can specify any length of heating coil you want to match a heat pump.

I think i am coming down on the side of an UVC but the cost differences dont seem that big, particularly if the purge is done daily and you factor in annual services. One big adavantage of the UVC that we havent mentuioned yet is smaller size and weight 🙂

Mark

Ive just found some temperature data for my thermal store for the past year.

Its average minimum daily temperature was 43.4C, so plugging this figure into my spreadsheet and altering the efficient down from 325% to 300% to mirror the efficiency of the Cylinder 2 figure and changing my size to 400 litres from 450 litres as I was mistaken about its capacity, gives the following:

This shows that my store costs about half as much to heat as the cylinder if a daily legionella purge is specified and is a bit more expensive (£2.19 v £2.48) if a weekly purge is used. I am of course assuming that my store and the cylinder produce the same amount of useable hot water.

To be clear Im not a die hard Thermal Store advocate, but given these figures the choice doesnt look as clear cut as some might suggest. Im more than happy for anyone to pick holes in my analysis as I would like a definitive answer too.

Ive attached a plan of my store below

Mark

@bontwoody I'm not even going to pretend I know what I'm talking about, but everything I've read suggests a weekly not daily legionella purge. Also seen lots of chat about them not really being needed in sealed systems or only needing heating to 50c. Lots of opinions on the internet....some might even be correct.

At the end of the day it's up to you to use what you think is best for you.

Posted by: @bontwoody

One big adavantage of the UVC that we havent mentuioned yet is smaller size and weight 🙂

I'd actually disagree with this - the 200l uvc isn't much smaller than the 277l TS  it replaced - and there's other gubbins which comes with it. We've now got three expansion vessels, an exchange unit and a buffer tank crammed into the room where the store used to sit....