Do I need a HP specific cylinder?

[Beelbeebub]

OK, so I've got a boiler coming towards end of life for a small 1 bed flat. 

 

I was toying with replacing with a small HP. 

 

The problem, as ever, is the hot water. Space is limited. I was thinking of a 90l cylinder to be high mounted out of the way. 

 

There are a range of cylinders availible but none seem to be marked as heat pump specific. 

 

However, the coil ratings on meany of them are pretty high anyway, eg 15kw.

 

The rationale for a heatpump specific cylinder is you need a greater coil area to get efficient heat transfer at lower flow temps, and greater flow. 

 

But the HP would only be 6kw nominal, so in DHW mode it would be punting out a bit less say 5kw.

 

Obviously I would derate the coil for the lower HP flow temp, but would using a 15kw coil to transfer 5kw at a lower temp be OK? 

 

Or is there another difference between standard and HP coils, like coil position? 

 

The other option is i save the £500 or so on the UVC and spend it on an instantaneous direct electric heater, the property has a kitchen sink, a basin and a shower. So only the shower is a big draw and 10kw would just about do it. 

 

Slightly less efficient but potentially alot easier. 

[JohnMo]

Trouble I found is being able to manage the dT required to keep the heat pump happy with a small coil. The heat pump will ramp up to max temp very quickly and is unlikely to get the cylinder hot (48 deg) in one hit.

 

90L isn't much usable water at a low temperature, either.

 

Why not just do 90L with an immersion, or heat pump via the indirect coil, see what the max temperature is in one heat cycle, set thermostat 1-2 deg below this, then time the immersion heater to come on and finish heat to 60 deg? At least you will have a decent amount of usable hot water.

[ReedRichards]

My understanding is this.  Hot water cylinders are well-insulated so even an inefficient coil will get your tank to temperature eventually but it will take longer and whilst the heat pump is heating the hot water it isn't heating the house.  So when it's coldest outside the time spent heating hot water might be too long to keep your house/flat warm enough.  Presumably you could get around the issue by up-rating your heat pump output so it can keep your building adequately warm with only 22 hours worth of heat per 24 hours, or something like that.      

[JamesPa]

Lots of discussion on this here from time to time and no definitive answer.

 

Technically the answer is very probably that you don't _need_ a heat pump cylinder, provided you end up with a heat pump capable of a reasonably high temp (eg 65) and modulation ratio (eg3:1). Many modern heat pumps achieve this.  Historically the 'problem', as @JohnMosays, is that at a lowish flow temp the coil doesn't transfer enough energy to the water to keep the heat pump happy, particularly if it doesn't modulate down well.  But with higher flow temps and decent modulation depth, this is not necessarily any longer the case.

 

Furthermore an add on plate heat exchanger and pump can convert more or less any 'standard' cylinder to one with plenty of coil area.  Mixergy sell this but only for their cylinders.  There is at least one installer who is developing such an add on, but the idea does not yet have mainstream traction.  At least 2 heat pump manufacturers sell cylinders with a PHE instead of a coil.  Alternatively @JohnMos suggestion simply to top up with the immersion is sensible, but again not mainstream so many installers won't accept it.

 

In practice however the industry is completely obsessed with swapping out dhw cylinders.  Heat pump manufacturers often specify coil sizes which are larger than makes sense and installers are bound to follow their recommendations. So you may not find an installer that will reuse your cylinder, even through it may well work.

 

So for now it's an unsafe assumption that you will be able to reuse a cylinder without a hp (ie large) coil, despite the fact that it's technically feasible in many cases.

 

Sadly the recently announcement of an increased grant and delay in the boiler ban will reduce the pressure on the industry to get it's act together and become more innovative in how it approaches this problem.

Edited September 23, 2023 by JamesPa

[JoeBano]

I’ve got a megaflow cylinder which I thought was a larger coil but I had to reset the integrated expansion tank inside of it the other day I stumbled the coil size inside. I’m sure it was 0.78 corrugated which is slightly different to your normal coil but I haven’t no issues of maintaining dt or issues of warm up time for my household. It’s 179L which is a bit small for my family size but now I’ve it set up for tank temperature it’s been perfect. 

Edited September 23, 2023 by JoeBano
It’s has an insulation rating of B which better than most of the ones I’ve seen on site.

[JohnMo]

Biggest trouble you will have is convincing anyone that the 90L capacity is acceptable for a heat pump. Typical industry recommendations attached for heat pump cylinder sizing.

 

 

 

 

 

[JamesPa]

Just now, JohnMo said:

Biggest trouble you will have is convincing anyone that the 90L capacity is acceptable for a heat pump. Typical industry recommendations attached for heat pump cylinder sizing.

 

 

 

 

 

Also true.

 

In summary it's not a technical question, it's a question about what the industry will accept.

[TonyT]

1 bed flat, immersion on night time tariff

[Beelbeebub]

If I fitted the UVC, it comes with an immersion so I could use that startetgy as a fall back. 

 

Problem with night tariffs is the day tariff tends to be much higher so the HP gets more expensive to run. 

[sharpener]

2 hours ago, Beelbeebub said:

OK, so I've got a boiler coming towards end of life for a small 1 bed flat. 

 

I was toying with replacing with a small HP. 

 

The problem, as ever, is the hot water. Space is limited. I was thinking of a 90l cylinder to be high mounted out of the way. 

 

I took it that @Beelbeebub has no cylinder ATM i.e. a combi boiler but is planning to squeeze one in somewhere.

 

I have had this experience in spades with three different installers. Am currently awaiting final confirmation from Vaillant that they will accept my existing 210l cyl on a 12kW Arotherm Plus (R290) so long as I fit a secondary circulating pump to stir the contents. (I sent a detailed perfomance model thanks to @JamesPa's spreadsheet). But with a 6 kW HP and the same coil area much less of a problem. You will probably find yr chosen HP mfr has a compatibility table.

 

One installer came up with a "need" to increase my cyl to 300 l (for a 4-bed house) but I think this was only from some mfr's data sheet not any standard. 90l for a 1 bed flat sounds reasonable to me. Edit: see under.

 

IIRC the MCS rules require that the HP provides the HW (though immersion top-up is allowed), so you will not get the BUS grant if it doesn't, someone will be quick to correct me if I am wrong.

 

8 minutes ago, Beelbeebub said:

Problem with night tariffs is the day tariff tends to be much higher so the HP gets more expensive to run. 

 

Octopus Cosy would be good for yr situation as there is also a 3 hr afternoon boost, so long as you can avoid the evening peak 1600 - 1900 i.e. don't cook your supper by electricity.

 

Edited September 23, 2023 by sharpener

[JamesPa]

17 minutes ago, sharpener said:

so you will not get the BUS grant if it doesn't, someone will be quick to correct me if I am wrong

 

Extract from the BUS requirement taken from source legislation.  There is some potential ambiguity but I think that @sharpener  is most probably correct because of clause b

 

(b)it is capable of meeting the full space heating and hot water heating demands of that property, and

(c)it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i)supplementary electric heater, including any immersion heater,

(ii)circulation pump, or

(iii)solar thermal collector.

Edited September 23, 2023 by JamesPa

[Temp]

Presumably the rated power of a coil in a tank only applies at a particular flow rate and temperature delta? Eg 15kW when the flow temperature is 10C hotter than the water in the tank? Something like that anyway. So I would expect the coil would not perform as well at a lower delta temperature. Can well believe tanks for use with heat pumps need bigger coils to achieve the same power throughput.

 

[sharpener]

All that is true as per @JamesPa 's spreadsheet model. But a lot of this thinking is out of date; now that we can get higher flow temps with R290 refrigerant it is much less important than it used to be.

[JohnMo]

12 minutes ago, sharpener said:

All that is true as per @JamesPa 's spreadsheet model. But a lot of this thinking is out of date; now that we can get higher flow temps with R290 refrigerant it is much less important than it used to be.

That may be true, but the heat pump to get best efficiency starts flowing at as low temperature possible. Only increasing temp to maintain a circa 5 deg dT. This where a small coil is rubbish, as it is unable to transfer any meaningful heat to the cylinder, until it gets to a high temperature, then you are unlikely to get time required to heat the contents before tripping out on high temperature due to dT margin not being maintained.

[sharpener]

It's all relative. The OP is talking about an unusually small cylinder with a disproportionately large coil (15kW, presumably rated for a boiler though) and a small heat pump which can probably turn down to 3kW or less.

 

9 hours ago, Beelbeebub said:

But the HP would only be 6kw nominal, so in DHW mode it would be punting out a bit less say 5kw.

 

Obviously I would derate the coil for the lower HP flow temp, but would using a 15kw coil to transfer 5kw at a lower temp be OK?

 

[JamesPa]

21 minutes ago, sharpener said:

It's all relative. The OP is talking about an unusually small cylinder with a disproportionately large coil (15kW, presumably rated for a boiler though) and a small heat pump which can probably turn down to 3kW or less.

 

 

Exactly.  And if it is a HT capable pump even a small coil will transfer sufficient heat to avoid excessive cycling.   There is a cop penalty for running at a higher temperature for dhw, but the relatively small extra running cost is very unlikely to be sufficient to make the business case for replacing a newish cylinder stack up.

 

And if it does cycle excessively (which it won't if it's a 6kW unit with a half decent modulation capability), add a plate heat exchanger and pump rather than ripping out (in some cases) lots of pipework in addition to a perfectly good dhw tank.

 

This obsession with replacing perfectly good dhw systems in almost all cases is an old-thinking based myth, which it is convenient for the industry to perpetuate because it de-risks installations and makes the whole thing look more difficult than just replacing a boiler.  In fairness the industry is aided by the modern obsession with 20l per minute showers for the whole family in quick succession.

 

As I said upthread, the question op poses is not a technical one in practice, it's more a question of what the industry will allow you to do in your own home.

Edited September 23, 2023 by JamesPa

[sharpener]

 

12 minutes ago, JamesPa said:

As I said upthread, the question op poses is not a technical one in practice, it's more a question of what the industry will allow you to do in your own home.

 

As it happens I have this week received a revised quote qualified by the following wording

 

Subject to a 2ndTechnical survey visit with Vaillant Technical to confirm existing cylinder is compatible as a replacement has not been quoted

 

so maybe I am beginning to wear them down...

[Beelbeebub]

40 minutes ago, sharpener said:

It's all relative. The OP is talking about an unusually small cylinder with a disproportionately large coil (15kW, presumably rated for a boiler though) and a small heat pump which can probably turn down to 3kW or less.

 

 

Yeah, pretty much this. 

 

I'm thinking a coup that can do 15kw at boiler temps, should be able to transfer 3 or 4kw even at the lower delta T of a HP. The only issue might be the flow rate achievable as boilers tend to flow less at higher temps, but the coils are almost always 22mm so that shouldn't be an issue if the HP and cylinder are relatively close. 

 

The only other thing that might cause issues would be if the coil geometry was different for HPs and boilers. I'm mainly thinking about the vertical distribution of the coil, but wouldn't both be best with the coil as biased towards the bottom as possible?

 

The idea is to replace the boiler with the cylinder (actually put the cylinder above where the boiler was as it's above the worktop so the extra stickout would be a problem but it's a tall room so we could put it on top. The HP will be on the wall just outside and the pipes will come through the existing flue hole (the walls are about 500mm thick hard bricks as it's the lower part of a tall building so avoiding drilling holes is good!)

 

All the HP compatible cylinders I've seen seem to be 125l plus and too tall to fit into the space available. 

 

[JamesPa]

49 minutes ago, sharpener said:

 

 

As it happens I have this week received a revised quote qualified by the following wording

 

Subject to a 2ndTechnical survey visit with Vaillant Technical to confirm existing cylinder is compatible as a replacement has not been quoted

 

so maybe I am beginning to wear them down...

Good and I really hope you succeed.

 

The current position is unsustainable in the long term, of that I am certain. 

 

However, whilst demand exceeds supply and the government continues to subsidise the indefensible whilst failing to scale up the supply, there is a risk that nothing will change because the industry doesn't need it to change. 

 

Hopefully someone, manufacturer or large installer (eg octopus) will break ranks sooner rather than later and deliver the kick up the rear that is urgently needed.

 

The one which does break ranks will, for a while, enjoy a significant competitive advantage. Obviously they need to be able to achieve the volume to exploit this.

 

In my dreams the other dhw problem, namely the uvc vent, will also be solved by someone breaking ranks.  A dhw cylinder heated by a hp cant boil.  If you don't put an immersion in the system then boiling is impossible.  If you need immersion back up, then locate it outside the cylinder with alternative means to ensure boiling water never reaches the cylinder.  Then no need for the troublesome vent, or at worst you open up the possibility of locating it somewhere more convenient than where the cylinder is.  £500 or thereabouts and major disruption saved.

Edited September 23, 2023 by JamesPa

[JamesPa]

5 minutes ago, Beelbeebub said:

All the HP compatible cylinders I've seen seem to be 125l plus and too tall to fit into the space available. 

See comment by @JohnMoupthread about minimum 'recommended' size.  Also fitting a 3sq m coil (the 'safe' size accepted by almost all hp manufacturers, although some specify smaller) into a small tank means that the tank is disproportionately coil not stored water so reducing too far is a case of diminishing returns.  A plate heat exchanger loaded tank solves this problem.

Edited September 23, 2023 by JamesPa

[sharpener]

1 hour ago, Beelbeebub said:

All the HP compatible cylinders I've seen seem to be 125l plus and too tall to fit into the space available. 

 

Both Gledhill and Newark have a vast range and will also do custom cylinders with your own choice of aspect ratio, coil, tappings and immersion bosses.

[TonyT]

Likewise Macdonalds in Scotland too

[sharpener]

 

On 23/09/2023 at 18:11, JamesPa said:

Good and I really hope you succeed.

 

The current position is unsustainable in the long term, of that I am certain. 

 

However, whilst demand exceeds supply and the government continues to subsidise the indefensible whilst failing to scale up the supply, there is a risk that nothing will change because the industry doesn't need it to change. 

 

Hopefully someone, manufacturer or large installer (eg octopus) will break ranks sooner rather than later and deliver the kick up the rear that is urgently needed.

 

The one which does break ranks will, for a while, enjoy a significant competitive advantage. Obviously they need to be able to achieve the volume to exploit this.

 

Regarding fittiing a 12kW Arotherm plus to my existing 210l OSO cylinder, a fortnight ago two representatives from Vaillant came with the installer for a further site visit, and confirmed they will stand by their official schematic (which shows my proposed bronze secondary circulation pump to improve the heat transfer) on their usual warranty terms for the Vaillant-supplied parts. They are understandably not going to guarantee the performance of the existing HW cyl but informally it seems they believe it will work and are quite keen to have an interesting case study.

 

I ran some further modelling with @JamesPa's spreadsheet and it would seem that even without the pump I can heat the HW to 51C with a flow temp of 70C before the HP cuts out. This gives me 5 deg margin for error as its abs max is 75C.

 

Assuming the secondary circulating pump will then improve the heat transfer by 1.5x (borne out by a simple water bath experiment) then I can get HW at 52C from a flow temp of 65C which will give me more margin and a worthwhile improvement in CoP.

 

The installer after some initial teeth-sucking now seems happy with this and is going to issue a revised quote with the extra parts.

 

Separately the approved design includes a 210 l thermal store (to be charged at night rate and then used to heat the bedroom radiators after it has ended). All parties seem happy with that too and the quote from Newark for a customised vessel (which will also serve as a volumiser) is very reasonable. In the mean time I have managed to get DNO approval and PP so it looks like most of the hurdles have been overcome at long last.

 

Thanks to all who have contributed along the way!

 

 

[JamesPa]

Whatever happens next, even if you need to play some more tricks to be satisfied with the dhw, this is definitely a result.  Well done and good luck with the install.  May there be many more who are not forced to replace their dhw cylinder!