Mini Store COP and Running Cost

[cjc88]

Long time lurker but have a pretty good knowledge of ASHP now. I am pretty close to confirming final details of my installation, but I have a 600x550x2200 cupboard for the internal gubbins, hence space is TIGHT and installation companies have questioned what is possible!

 

I wondered whether there was somebody in this group who had more details about the HG Mini Store Running Cost and COP (I can probably fit the largest 110L version). Given its design, I guess the following are true:

 

1) The heatpump needs to maintain the higher temperature while drawing water, so whilst you might normally have a cylinder running from 15 up to 45 when refilling an empty cylinder, the mini store can only be fed by 45 degree water for it to work, so reducing potential COP.

2) Rather than being able to draw down the whole tank before refilling you need to re-heat immediately or the water rapidly becomes unusable, even for the larger tank... hence further reducing COP due to cycling. Though perhaps there is enough thermal mass that the temperature doesnt drop that much... (or use a higher store temp which exaggerates 1+2.

3) The heatpump needs to run the when there is demand (rather than offpeak) hence the electricity might also be more expensive than on a normal cylinder.

4) It is smaller so has lower heat losses and doesnt need an expansion vessel (or perhaps just a smaller one....?)

5) Given it doesnt store water very long you dont need to run a high temp legionella cycle weekly....

 

No idea where this nets out at, in terms of money and efficiency, and possibly depends on your usage of it, but was curious if anybody had any other thoughts, or more detailed numbers!?

 

Thanks very much

[JohnMo]

Can you not just fit a slim line cylinder, they are about 450mm diameter?

 

All you should need in addition is a 3 port diverter and expansion vessels X2.

[Beelbeebub]

8 hours ago, cjc88 said:

Long time lurker but have a pretty good knowledge of ASHP now. I am pretty close to confirming final details of my installation, but I have a 600x550x2200 cupboard for the internal gubbins, hence space is TIGHT and installation companies have questioned what is possible!

 

I wondered whether there was somebody in this group who had more details about the HG Mini Store Running Cost and COP (I can probably fit the largest 110L version). Given its design, I guess the following are true:

 

1) The heatpump needs to maintain the higher temperature while drawing water, so whilst you might normally have a cylinder running from 15 up to 45 when refilling an empty cylinder, the mini store can only be fed by 45 degree water for it to work, so reducing potential COP.

2) Rather than being able to draw down the whole tank before refilling you need to re-heat immediately or the water rapidly becomes unusable, even for the larger tank... hence further reducing COP due to cycling. Though perhaps there is enough thermal mass that the temperature doesnt drop that much... (or use a higher store temp which exaggerates 1+2.

3) The heatpump needs to run the when there is demand (rather than offpeak) hence the electricity might also be more expensive than on a normal cylinder.

4) It is smaller so has lower heat losses and doesnt need an expansion vessel (or perhaps just a smaller one....?)

5) Given it doesnt store water very long you dont need to run a high temp legionella cycle weekly....

 

No idea where this nets out at, in terms of money and efficiency, and possibly depends on your usage of it, but was curious if anybody had any other thoughts, or more detailed numbers!?

 

Thanks very much

Pretty sure thermal stores are exempt from needing legionella cycles as they don't store the potable water at high temps, it's instantaneously heated on it's way through the coil

 

[cjc88]

Thank you both, I dont know why I didnt see this response despite being subscribed.

 

I think a slimline cylinder would fit, but definitely harder as it needs the extra expansion vessel, but I was more curious about the actual differences from an efficiency POV. Perhaps I have to try and make some assumptions and build a model myself, but was interested if anybody else had thought about it!

 

Thanks again!

Edited Friday at 15:03 by cjc88

[JamesPa]

26 minutes ago, cjc88 said:

Thank you both, I dont know why I didnt see this response despite being subscribed.

 

I think a slimline cylinder would fit, but definitely harder as it needs the extra expansion vessel, but I was more curious about the actual differences from an efficiency POV. Perhaps I have to try and make some assumptions and build a model myself, but was interested if anybody else had thought about it!

The general rule as Im sure you know is that COP goes up as flow temperature goes down.  So it comes to first order down to the question - whats the flow temperature?  This comparisons will depend on which variables you keep constant.  Both a cylinder and the mini store have one heat exchanger (coil).  In the case of a cylinder heating water circulates through the coil, in the case of a mini store its the DHW (and the mini store itself is filled with heating water).  So in principle the difference between DHW temp and flow temp is about the same, so COP could be about the same. 

 

However for any given cylinder volume and stored water temperature the yield (of DHW) from the mini store is different to the yield from a cylinder.  At low draw off rates it can be more (according to the Newark specs), at higher draw off rates it can be less.  If you are in the latter category and as a result have to increase the stored water temperature to get the yield you require, COP will suffer.  I think the only way to work this out is to understand your demand profile and then plug in the numbers to estimate the temperature of the stored water required and hence the flow temperature required. 

 

This should probably be done anyway to be certain that the mini store/small cylinder is big enough, or whether you need to consider additional measures like water saving shower heads, waste heat recovery, or just not showering for 10 mins at 20l/min (if thats what you do - Im not suggesting it is, but apparently some people find this necessary!).

[Nickfromwales]

A heap pump and a thermal store should never meet. For a TS to make DHW you need to be above 75°C, so to ask a HP to do that is just no bueno. 

[JamesPa]

1 hour ago, Nickfromwales said:

A heap pump and a thermal store should never meet. For a TS to make DHW you need to be above 75°C, so to ask a HP to do that is just no bueno. 

That's a bit of a generalisation, and note that the mini store has only one heat exchanger between heat pump and dhw not two like a 'conventional' thermal store plus it's designed for the purpose.  I'm not trying to advocate it just saying I'm not sure you can dismiss it quite so simply.  I think it may actually depend on use profile, bearing in mind that the alternative (in the situations of interest) is a very small uvc

Edited Friday at 17:48 by JamesPa

[dpmiller]

yep, ASHP/TS works for us

[Nickfromwales]

27 minutes ago, JamesPa said:

That's a bit of a generalisation, and note that the mini store has only one heat exchanger between heat pump and dhw not two like a 'conventional' thermal store plus it's designed for the purpose.  I'm not trying to advocate it just saying I'm not sure you can dismiss it quite so simply.  I think it may actually depend on use profile.

Perhaps, but taking heat for DHW production from a HP via any type of instantaneous heat exchanger isn’t great however you cut it (imho). Heat pump to UVC is just way better from many aspects.

 

17 minutes ago, dpmiller said:

yep, ASHP/TS works for us

What temps and COP do you get? And is this an EAHP, or a monoblock > TS with an instantaneous DHW coil in it? You’ve prob said many times but I forget. Good to hear what works when it doesn’t ‘click’ with me. 

[Nickfromwales]

Just looked at the HG more intensively, and it’s just a whopper of a ‘wet’ heat exchanger in essence? So not really a store so much, just water is ‘in it’. 
 

OP says these things in a nutshell. 

[dpmiller]

2 hours ago, Nickfromwales said:

Perhaps, but taking heat for DHW production from a HP via any type of instantaneous heat exchanger isn’t great however you cut it (imho). Heat pump to UVC is just way better from many aspects.

 

What temps and COP do you get? And is this an EAHP, or a monoblock > TS with an instantaneous DHW coil in it? You’ve prob said many times but I forget. Good to hear what works when it doesn’t ‘click’ with me. 

 

Monoblock into 270l store, input and DHW coils both corrugated high-recovery, 3m2 area. Reheats til the thermistor halfway down the tank hits 50 so flow (and the top of the tank) are about 5c higher. That allows a shower before recovery, recovery takes 30 mins or so, but the ASHP ramps up quick enough that flow can be sustained near indefinitely at reasonable flow rates.

 

All heating flow passes through the TS as a buffer volume too, so the whole volume of water rarely drops below 40c during the heating season. PV diverter and woodstove assist in adding energy when possible also

 

Can't be precise about COP as I don't have energy measurement, but the highest daily consumption I've ever seen for the ASHP in the depths of winter is just over 30kWh; our monthly electric bill for the whole house (240m2) all-in is about the same as the oil usage in our previous little bungalow as most of the heating is accomplished during the E7 period 🙂

[JamesPa]

2 hours ago, Nickfromwales said:

Just looked at the HG more intensively, and it’s just a whopper of a ‘wet’ heat exchanger in essence? So not really a store so much, just water is ‘in it’. 

Pretty much.  A few months ago I wondered if its main advantage was the absence of the need for a vent with all that comes with that malarkey.  That said most (but not all) actual applications where a tank this small was the only practical option would probably be in a location where running a vent wasn't too difficult. 

 

Excepting the cases where running a vent is difficult, the question becomes - if you have only enough space for something this small (and not enough money for/dont trust a sunamp), whats the best solution - a 80l (or smaller) UVC or a mini store?  Thats a very specific problem, which likely applies to a fair number of smaller houses which currently have a combi.  In either case you may  be forced to reheat at times of day other than cheap periods (depending on your pattern of use).  I have a feeling that a very well designed small UVC that is well stratified so you can reheat whilst running water off and maintain stratification may well work out better, but Im not absolutely sure, it may depend on run-off rate and is there such a beast?

 

The mini-store is a solution (if its a solution to anything) to the specific problem of insufficient space for a 150l+UVC.  But, in the grand scheme of things, that represents a large sector of the housing stock so the question 'is it a valid solution given these constraints' (and if not whats the alternative given the constraints) is relevant.  Horses for courses maybe?

Edited Friday at 21:03 by JamesPa

[dpmiller]

worth pointing out that this side of the Irish Sea, the Willis heater attached to a vented cylinder achieves superb stratification and allows continuous flow at reasonable rates. Heck, in our last house prior to fitting an electric shower we happily showered using the Willis- turn it on for a couple of minutes to get convection going and cap the top of the tank, then you could draw hot continuously at the rates allowed by the small shower heads of the day

[SteamyTea]

14 hours ago, JamesPa said:

Thats a very specific problem, which likely applies to a fair number of smaller houses which currently have a combi.  In either case you may  be forced to reheat at times of day other than cheap periods (depending on your pattern of use). 

A relatively small PV array will offset ToU costs.

 

I am starting to think that any thermal storage needs to in conjunction with PV if possible.

 

 

[JamesPa]

4 hours ago, dpmiller said:

worth pointing out that this side of the Irish Sea, the Willis heater attached to a vented cylinder achieves superb stratification and allows continuous flow at reasonable rates. Heck, in our last house prior to fitting an electric shower we happily showered using the Willis- turn it on for a couple of minutes to get convection going and cap the top of the tank, then you could draw hot continuously at the rates allowed by the small shower heads of the day

Indeed.  Thats sort of the idea of the mini-store.  Once you start drawing off water the heat pump switches to reheating the stored water thus maintaining flow for longer.  In principle you could do this with a UVC as well (perhaps some do), provided the design was optimised for stratification during reheat (perhaps they are!).

 

The 'problem' is the (apparently) common 'requirement' for 5 off 10 minute showers at 20l/s in quick succession.  Seems to me like a very first world problem!  Perhaps we should just get over it!

[Beelbeebub]

On 05/04/2025 at 08:18, dpmiller said:

worth pointing out that this side of the Irish Sea, the Willis heater attached to a vented cylinder achieves superb stratification and allows continuous flow at reasonable rates. Heck, in our last house prior to fitting an electric shower we happily showered using the Willis- turn it on for a couple of minutes to get convection going and cap the top of the tank, then you could draw hot continuously at the rates allowed by the small shower heads of the day

Any idea why the Willis heater seems to be very common over there but almost never on this side of the sea?

 

In GB an immersion in tank seems to be the norm for unvented electric setups.

 

Is there some reason beyond "that's just how we do it"? Like some sort of odd regulation that banned electric heaters in tanks or something?

[dpmiller]

Willis is a Belfast company...

 

https://willis-heating.com/about/

[Gone West]

I had never seen a Willis heater until around sixteen years ago at a building exhibition. Then when I moved into my current house I found that the solar thermal panel was connected to the vented hot water tank via a Willis Solasyphon. It seems a clever external heat exchanger and I'm surprised it doesn't seem more popular for retrofits.

[JamesPa]

2 hours ago, Beelbeebub said:

Any idea why the Willis heater seems to be very common over there but almost never on this side of the sea?

 

In GB an immersion in tank seems to be the norm for unvented electric setups.

 

Is there some reason beyond "that's just how we do it"? Like some sort of odd regulation that banned electric heaters in tanks or something?

I have for some while thought that the combination of a Wiillis heater and a PHE could be a very powerful combo in heat pump retrofits.  A 3kW Willis heater and 6kW heat pump together will do 9kW which is much as top end electric showers if used for instantaneous DHW heating (no storage).  Alternatively simply plumb the two to any normal existing cylinder, add a circulator pump, and you have enough heat exchange capability for the heat pump, but without having to fit a new cylinder.  Better still, given that the willis heater is not physically in the tank, organise a couple of pressure relief valves as part of the package, and you could make a good argument to do away with the vent pipe arrangement which adds half a day to a typical retrofit.  Better still the PHE/willis heater can be sited wherever its convenient to do so, it doesnt have to be near the cylinder.  There is a packaged solution in here somewhere which has the potential to make a significant saving in time, money and disruption for heat pump retrofits.

 

However all this requires thinking out of the box and therefore is something that's unlikely to happen.

[SteamyTea]

5 hours ago, JamesPa said:

3kW Willis heater and 6kW heat pump together will do 9kW which is much as top end electric showers if used for instantaneous DHW heating

Do you mean in series.

Cold feed -- HP -- Heat Exchanger -- Willis -- Shower.

 

 

[JamesPa]

5 hours ago, SteamyTea said:

Do you mean in series.

Cold feed -- HP -- Heat Exchanger -- Willis -- Shower.

 

 

Cold feed feeds heat exchanger the other side of which is fed by the heat pump (thus heating the cold feed - 6kW).  Now heated cold feed passes through willis heater (another 3kW) and leads to shower and other outlets.

 

Or substitute for cold feed the potable water circulated through DHW tank/PHE/Willis during reheat, thus loading the DHW tank with both an external plate heat exchanger and an external electric element - which can then be locally prevented from sending water hotter than say 90 degrees to the the tank (circumventing the need for D1/D2 vents provided that the industry is provided to think outside the box and follow the building regulations rather than avoiding responsibility by sticking slavishly to the 'guidance').  Cheaper and in many cases more flexible in terms of configuration options than buying a 'heat pump cylinder'.  Im not saying this replaces 'heat pump UVCs' altogether, but surely its a sensible additional option particularly for retrofits, particularly if suitably packaged.

 

Edited 1 hour ago by JamesPa