Mini Store - Heat Geek / Newark Cylinders for ASHP

[JohnMo]

8 minutes ago, JamesPa said:

so the latter may need up sizing,

Correct, the superflow temporal store required an additional 5 lt expansion if system volume exceeded x lt.

[ReedRichards]

So it's a thermal store but you tell the heat pump it is the DHW cylinder?   

[JamesPa]

30 minutes ago, ReedRichards said:

So it's a thermal store but you tell the heat pump it is the DHW cylinder?   

It's a cylinder containing water from the heating system, through which a (large area) coil runs that is fed with mains water and which feeds the hot water outlets.  The cylinder is connected to the heating system via the usual dhw diverter valve arrangement. 

 

Call that what you will!

[Beelbeebub]

If the regs allow an immersion heater in the buffer/store without bringing the whole weight of G3 into things that would be fantastic.

 

Another 3kw on top of the HP would probably make it near indefinite for showers and taps.

 

My thermal store set up is similar albeit with a very big tank for solar and solid inouts.

 

The gas boiler is maybe 15kw (might be 12, not sure) and that acts more or less as a combi for showers. The trick is having enough stored capacity to provide hot water whilst the system is "ramping up".

 

I've found it I make the storage volume too small (I have a home brewed virtual thermostat on the tank that allow me to set how much volume to store) there is a dip in shower temp before the boiler manages to catch up and hit steady state.

 

If I get the volume right then it effectively works as a combi 

[JamesPa]

In the cold light of day I'm now asking myself, does this arrangement perform better or worse (in terms of water draw off volume available) than a uvc of the same size, also with a large coil and the heat pump controls arranged to reheat more or less immediately.

 

I will try to do the maths unless someone else has a ready answer.

 

If it turns out to have the only a marginally poorer performance, then it's a solution to the G3 problem not the volume problem, still valuable but perhaps a little less valuable than it appears at first sight in a typical combi replacement scenario, where there is almost certain to be an easy route for the vent pipe..  If it's an equal or better performance, then of course it's a definite win (in fact at this point it becomes almost no brainer).

[JamesPa]

25 minutes ago, Beelbeebub said:

If the regs allow an immersion heater in the buffer/store without bringing the whole weight of G3 into things that would be fantastic.

@JohnMois the expert on the regs (and more to the point the accompanying guidance which the industry has decided it must follow slavishly) but my recollection is that G3 applies to stored potable water, which is why buffer tanks don't need the vent arrangement.

Edited June 17 by JamesPa

[JohnMo]

27 minutes ago, JamesPa said:

but my recollection is that G3 applies to stored potable water, which is why buffer tanks don't need the vent arrangement

Correct. The volume of water in the cylinder of the HG arrangement is treated as part of the heating system with expansion and relief provided at the heat pump end. As long as the return leg from this cylinder is always in the flow path to the heat pump all is good. 

 

But if your heating system volume is 100L, and expansion sized for that, you would have to increase the expansion vessel so it's sized for 150L and the appropriate upper temperature limit of the cylinder.

[ReedRichards]

14 hours ago, JamesPa said:

It's a cylinder containing water from the heating system, through which a (large area) coil runs that is fed with mains water and which feeds the hot water outlets.  The cylinder is connected to the heating system via the usual dhw diverter valve arrangement. 

 

Call that what you will!

 

It wasn't the nomenclature I was worried about, it was the control.  But I have now watched the video and it is clear that as far as the heat pump is concerned it is heating a normal DHW cylinder but with the control parameters tweaked to match the needs of this particular abnormal cylinder.

[JamesPa]

5 hours ago, JamesPa said:

In the cold light of day I'm now asking myself, does this arrangement perform better or worse (in terms of water draw off volume available) than a uvc of the same size, also with a large coil and the heat pump controls arranged to reheat more or less immediately.

 

I will try to do the maths unless someone else has a ready answer.

 

If it turns out to have the only a marginally poorer performance, then it's a solution to the G3 problem not the volume problem, still valuable but perhaps a little less valuable than it appears at first sight in a typical combi replacement scenario, where there is almost certain to be an easy route for the vent pipe..  If it's an equal or better performance, then of course it's a definite win (in fact at this point it becomes almost no brainer).

OK, I have given this some consideration. At first I thought it will be very easy to prove that a UVC performs significantly better (in terms of DHW yield) than the mini-cylinder of the same volume.

 

However it turns out (I think) that its not as simple as this because of stratification. The UVC does yield more DHW than the mini-cylinder, but the difference is dependent on the degree of stratification and the effectiveness of the coil, particularly at the top of the cylinder. In the limit case of perfect stratification and a coil capable of infinite heat transfer, the performance is, I think, the same!. In other words it becomes an engineering challenge not a physical limit, and its perfectly plausible that a sufficiently well engineered mini cylinder performs nearly as well as a UVC of the same size, but without the G3 complication.

The upshot of this is that the mini cylinder does not solve the size problem (a small UVC would do this just as well, in fact slightly better), but it does solve the G3 problem with potentially a small penalty in terms of DHW yield (or equivalently, cylinder size). So what Heat Geek have really shown in their experiments (if my argument below is correct) is

 

1. that the 200-400l cylinders we ‘need’ are not needed if you make some reasonable compromises

2. that a well engineered mini cylinder can perform sufficiently well to serve a useful purpose (albeit still not quite as well as a UVC of the same size), with the advantage that no discharge pipework/arrangement is needed.

 

The crucial question is, how much is the penalty? If its 10-20%, which is very plausible for a well engineered design, then the trade off is going to be worthwhile for many and what Heat Geek have potentially done is knocked a major hole in the UVC market. If its quite a lot larger then the better compromise for most, if space is at a premium, will be a small UVC

----------------------

Rationale for the conclusion follows, however the implications (if the rationale is correct) appear above so you dont have to read on (but I would be grateful if you do since you are invited to critique in case I have made an error.

 

We are going to consider the DHW yield of a UVC and a mini cylinder of the same size, both of which are heated simultaneously by a heat source. To do this we will work out how much useful energy can be extracted from the cylinder and the heat source.

 

With a perfectly stratified UVC you can extract all of the stored water at the stored temperature and mix it with incoming cold mains water to produce DHW at the desired temperature. The amount of energy which is extracted from the store is VcdT, where dT is the temperature difference between the incoming mains water and the store temperature, and, V the volume of the cylinder and c the volumetric heat capacity of water. In addition, if you are heating the stored water simultaneously, you will be able to extract a further amount of energy equal to the energy transferred to the cylinder in the time taken to drain the cylinder, which, with an adequately sized heating coil, we can take to be PI where P is the power available from the heat pump and I is the time taken to empty the tank (which depends on the draw off rate and desired DHW temperature, neither of which we actually need to know. Actually its a bit more than this because a bit more energy is fed in during the time taken to extract this additional amount of energy. We don’t need to work this out exactly.

 

With a mini cylinder the dynamic is slightly different. The incoming mains water is at the same temperature as the UVC case so will continue to be heated at least a bit until the initial charge of hot water is exhausted, but this is only useful if it is heated to at least the desired DHW temperature (this is the argument generally deployed o explain why heat stores are less effective than UVCs). However, if the mini-cylinder is also perfectly stratified, the bottom of the cylinder will eventually cool to the temperature of the incoming mains (just like the UVC) but (also just like the UVC) the top will remain at the original store temperature. Only when the amount of water left at the top has shrunk to the point where the area of the coli passing through this layer is too low to heat the water passing through it sufficiently, will the emerging DHW fall below the desired temperature.

 

In the limit case, where the coil is very large, this mean that you can continue extracting useful heat until just before the initial charge in the mini cylinder is exhausted. Thus the energy you extract in this limit case is VcdT, the performance is the same (in the limit case).  The extra due to the additional heat supplied during the extraction of the water is also the same in this limit case.

 

Now obviously the limit case cant be built, but its quite plausible that its possible to design a coil (and shape the top of the cylinder) so that the heat transfer from water to coil is still sufficient when only 10-20% of the water in the cylinder (at the top) remains at the original store temperature. In this case the penalty is limited to this figure. What we really need to know is what this figure is.

 

So, you are doubtless thinking, if this can be done with a mini cylinder, why not with a conventional thermal store. There are two crucial engineering changes relative to a traditional thermal store namely a) this stores heating water, so the store temperature is the same as thje flow temperature not reduced by the approach temperature of the primary heat exchanger and b) the DHW coil has a very large area, so the approach temperature is small and (presumably) only a fraction of its length is required to heat the incoming mains water to the DHW temperature, allowing much more of the stratified top of the cylinder to be used.

[Beelbeebub]

For what it's worth, my large TS seems to (generally) cool equally from top to bottom ie I lose 5C from top and bottom as I draw off water.

 

Of I only stored water at 45C I wouldn't get much out 

 

*However*, the coil area of the store isn't huge as it was designed for a 65-75C store temp and IIRC it's distributed towards the top of the volume.

 

I can imagine, if you had a very big coil, running from top to bottom, you might get closer to a bottom up draw off.

 

You would still have the issue that, as you got towards the end of the store capacity, your effective coil area would fall and you wouldn't be able to access the last bit of restored energy.

 

I agree with the assesment that this is more of a work around the UVC regs than anything else.

 

Which does beg the question, why couldn't we just have the regs amended.  As has been pointed out before, a UVC hooked up solely to a HP, especially without an electrical booster, has zero chance of overheating.

 

If the regs just carved out an exception for that circumstance then we could fit small "under counter" UVCs that would potentially perform a little bit better.

 

Edit & slightly off topic - in the same way if the regulations on grants for A2A were changed to allow them to collect grants as long as they were firmware locked off from cooling below (say) 30C it would massively help uptake 

Edited June 17 by Beelbeebub

[JamesPa]

9 minutes ago, Beelbeebub said:

Which does beg the question, why couldn't we just have the regs amended.  As has been pointed out before, a UVC hooked up solely to a HP, especially without an electrical booster, has zero chance of overheating.

 

If the regs just carved out an exception for that circumstance then we could fit small "under counter" UVCs that would potentially perform a little bit better.

Regs don't need amending, they have vast flexibility and pretty much specify only that the system must be safe.

 

It's the guidance that accompany the regs, which are followed slavishly by the industry, that is the problem.  The guidance is neither obligatory nor does it guarantee compliance (this is stated in the guidance).  Thus if the industry chose to develop something else it could

[JohnMo]

4 minutes ago, Beelbeebub said:

You would still have the issue that, as you got towards the end of the store capacity, your effective coil area would fall and you wouldn't be able to access the last bit of restored energy

Think that's the advantage of the smaller cylinder, as with the smaller volume and height you are able to to get to decent flow temperature and replenish quite quickly. My 210L slim line, completely destratifies as soon as the reheat cycle starts.

 

8 minutes ago, Beelbeebub said:

agree with the assesment that this is more of a work around the UVC regs than anything else.

I think it's more about making heat pump accessable and installable for combi replacement.

 

12 minutes ago, Beelbeebub said:

Which does beg the question, why couldn't we just have the regs amended.  As has been pointed out before, a UVC hooked up solely to a HP, especially without an electrical booster, has zero chance of overheating

Think the practically and consequences are different. This HG cylinder is in a closed system with a given volume of water spread across the whole house. It's at a pressure of around 1 bar gauge. The cylinder has an open end so any expansion can more outwards to the rest of the heating system which would act as heat sink. An UVC has a blocked discharge i.e. taps are closed, it has check valve in the inlet. It cannot allow expanding water anywhere to go. 

[JamesPa]

3 minutes ago, JohnMo said:

Think that's the advantage of the smaller cylinder, as with the smaller volume and height you are able to to get to decent flow temperature and replenish quite quickly. My 210L slim line, completely destratifies as soon as the reheat cycle starts.

On the other hand in a larger cylinder you can fit more dhw coil in the top (say) 10-20% reducing the percentage lost volume as a result of insufficiency of coil exposed to the top stratum.

 

6 minutes ago, JohnMo said:

I think it's more about making heat pump accessable and installable for combi replacement.

Agree that's how it is presented and very sensibly so imho.  But my conclusion, based on the argument above (which I'm hoping you might critique in case it's wrong!) remains that a uvc of the same size would likely yield more dhw, so the main problem actually avoided is g3 not cylinder size.

 

For the avoidance of doubt I still like it and applaud heat geek for doing something different to solve a real problem).

[JamesPa]

2 hours ago, ReedRichards said:

 

It wasn't the nomenclature I was worried about, it was the control.  But I have now watched the video and it is clear that as far as the heat pump is concerned it is heating a normal DHW cylinder but with the control parameters tweaked to match the needs of this particular abnormal cylinder.

That's my understanding also.

[ReedRichards]

1 hour ago, Beelbeebub said:

As has been pointed out before, a UVC hooked up solely to a HP, especially without an electrical booster, has zero chance of overheating.

But the video suggests there is an option of incorporating an immersion heater in the tank.  That must increase the chance of catastrophic overheating, no matter how small that chance is.

[JamesPa]

17 minutes ago, ReedRichards said:

But the video suggests there is an option of incorporating an immersion heater in the tank.  That must increase the chance of catastrophic overheating, no matter how small that chance is.

I think it relies on the system expansion vessel and PRV to deal with overpressure.  Thats no different to a buffer tank with an inbuilt immersion for backup.

Edited June 17 by JamesPa

[sharpener]

On 15/06/2024 at 21:27, JamesPa said:

Plenty of 6-9kW electric showers around, similar to a more typical 8kW heat pump.

 

I think it is a good technical advance - but there are some optimistic assumptions being made in the video. Adam refers several times to the "11kW" output of the HP. This is attainable by the 7kW Arotherm plus at his test OAT of 7C (but he was lucky to see that at 0730 on 30th Jan!). So with this HP at that OAT you can indeed achieve indefinite shower duration.

 

Less generously specified HPs and/or ones appropriate to smaller houses would not achieve anything like the running times he quotes at winter temps of zero or below, and I think it is notable that he concludes (? to avoid disappointment) by recommending aerator shower heads, which will significantly reduce hw demand with little reduction in the showering experience.

 

Having just stayed for three nights in a hotel with unnecessarily extravagant fittings in the bathroom (twin sinks with no plug at all, massive bath, massive rain head for the shower, optional hand set but no way of diverting all the flow to it etc etc) I think we have a very long way to go in educating ppl to lower their expectations about water usage, hot or cold. It is already a severe constraint on housing development in some parts of the UK.

[JamesPa]

29 minutes ago, sharpener said:

I think we have a very long way to go in educating ppl to lower their expectations about water usage, hot or cold. It is already a severe constraint on housing development in some parts of the UK.

Definitely agree, but it needs to be done.  The unreasonable expectations of the British public (which seems to differ quite markedly from some of our continental European neighbours) is a major challenge, and politicians aren't helping by telling people that they can have what they have been tricked by capitalism into believing they need.

 

But imho the main question (to evaluate the actual innovation) is the comparison with a uvc of similar volume.

Edited June 17 by JamesPa

[Beelbeebub]

1 hour ago, ReedRichards said:

But the video suggests there is an option of incorporating an immersion heater in the tank.  That must increase the chance of catastrophic overheating, no matter how small that chance is.

Yeah, the inclusion of an immersion heater would come with a small chance of overheating (but very small now the units all have built in non resetting cutouts as well as the usual thermostat and presumably the actual control relay as well).

 

But if there was no immersion in the system then zero chance of overheat.

[Beelbeebub]

The other option, which we've discussed here before, would be a small HG cylinder this no G3 with a 9kw instantaneous heater after it.  Ideally the 9kw unit would be modulating so you can bring whatever the HP delivers (say 3kw on a cold night) plus another 9kw. As it's an instantaneous heater no G3 either.

 

And the instantaneous heater is only needed occasionally so most of the time you get the high cop of a HP.

[JohnMo]

12 minutes ago, Beelbeebub said:

would be a small HG cylinder this no G3 with a 9kw instantaneous heater after it.

Or if you're considering a combi boiler, fit a system boiler instead and a HG small cylinder where you could, way better performance than a combi, hot water to the tap as quick as an UVC cylinder, indefinite hot water, just set heating system as PDHW.

[sharpener]

1 hour ago, JamesPa said:

But imho the main question (to evaluate the actual innovation) is the comparison with a uvc of similar volume.

 

I agree with yr analysis upthread @JamesPa. Because heat transfer is reciprocal I don't think there is anything to choose conceptually between doing the heat transfer on the way in and storing it as potable HW, or storing it as primary flow and doing it on the way out. The latter requires a bigger coil, much bigger if they are to be believed, so uses more copper. The concept has been quite well packaged with the reversible design and unversal wall bracket/cradle. I was surprised at the number of size variants but they are all part of Newark's existing range so they will have all the tooling apart from a stock of the lurid paint colour.

 

As you and others have said, the main advantage to what they have done is dodge the G3 requirements, which I agree is worthwhile in itself though doesn't affect me personally.

[JamesPa]

17 minutes ago, sharpener said:

. I was surprised at the number of size variants but they are all part of Newark's existing range so they will have all the tooling apart from a stock of the lurid paint colour.

This was one of the factors which led me to question whether the real advance is size, or whether it's avoiding G3.  If the yield penalty relative to a uvc is small, then why bother with UVCs at any scale?

 

Cynically the other benefit is that the slight obfuscation might enable installers to justify departing from the MCS guidance, no bad thing as it might allow an installer to meet the customer requirements rather than the MCS assumptions about what the customer requires!

Edited June 17 by JamesPa

[sharpener]

8 minutes ago, JamesPa said:

This was one of the factors which led me to question whether the real advance is size

 

No, I don't think it makes any difference which "kind" of hot water you are storing, 1 kWh is still 860 litre-degrees.

 

We have had many debates on here in the past about optimising various combinations of storage, heat input and auxiliary electric heating whether in the same vessel or as @Beelbeebub suggests upthread in cascade, and have yet to unearth the silver bullet.

 

I am sceptical that this will address the 80% of the market they claim, bc with smaller properties that lack the space for a cyl you also (should) get smaller HPs being fitted, and Adam is already stretching the claimed performance by using figures for a quite large HP at an unseasonably warm OAT.

 

Also having a largish store of hot water is a good way of time-shifting your usage. If E7 elec only costs 1/3 as much the savings in CoP are overshadowed by having to heat it at the time of use, not many ppl shower between midnight and 0700.

[JamesPa]

6 minutes ago, sharpener said:

I am sceptical that this will address the 80% of the market they claim,

 

7 minutes ago, sharpener said:

I am sceptical that this will address the 80% of the market they claim, bc with smaller properties that lack the space for a cyl you also (should) get smaller HPs being fitted, and Adam is already stretching the claimed performance by using figures for a quite large HP at an unseasonably warm OAT

Agreed.  The real question for me is, does it make UVCs a niche product?  This depends on the yield penalty (if any) which we don't know