Heat Pump combi storage cyclinder - the solution to homes with limited space ?

[Moxi]

I have watched this with real interest, it seems to make sense, but on linked in it got a few "sneery" comments and I recall many months ago Adam saying you should never fit a buffer tank in a heat pump circuit and isnt this cylinder acting like a buffer tank or does the coil make the all important difference in that respect.

The other question I have is this - is the water in the cylinder heating coil the same as the water circulating in the radiators or is there another heat exchanger somewhere else that runs the rads ?

It does look a viable solution for my small cottage and maybe after 12 months to let the concept mature I might be ready to ditch the LPG heating and HW system for a HP system.

Moxi

[sharpener]

The other question I have is this - is the water in the cylinder heating coil the same as the water circulating in the radiators or is there another heat exchanger somewhere else that runs the rads ?

Moxi

It is like a standard indirect cylinder but inside out - the body of water is on the HP primary circuit and the (large) coil carries the separate HW flow abstracting heat from the store as you draw it off.

It would properly be defined as a thermal store not a buffer. IIRC it is plumbed via a diverter valve like a standard cyl so not available as a buffer/volumiser to increase the circulating volume when in CH mode.

There is a long discussion here, the main advantage is that it gets round the G3 requirement for a temp/pressure relief valve venting to the outside. Doubtful if it stores more HW than a standard cyl of the same size, and might be less.

[Moxi]

Cheers Sharpener, that link was excellent and answered all my questions.

I’ve been logging my daily gas use and temps for the lpg boiler through this year and done several different heat loss calculations for the cottage as well as varying the boiler DHW and heating temperatures turning them down as low as practicable in cold temperatures and the results are very promising.

On all but the coldest days we can raise the temperature with a flow temperature of 33 degrees C. On the coldest days in Feb and March we sometimes had to raise the flow temperature to 45 degrees C to get 20 degrees C but by my understanding that’s still very acceptable for getting a good SCOP result. Just got to see how the low flow temps hold up in the November and December gales before we bite the bullet and set our stall out for a ASHP system.

Happy to say no WBS since February and no lpg GCH since May.

Got to go back and read some more from your link now as kids are going to bed and I have more time to read and understand stuff.

Thanks again

Moxi

[sharpener]

It does look a viable solution for my small cottage and maybe after 12 months to let the concept mature I might be ready to ditch the LPG heating and HW system for a HP system.

Moxi

What do you have by way of a HW system on yr log burner/gas boiler ATM? Can it be re-used with a HP? Don't be bamboozled by installers saying you need a special HP HW cyl or (even more expensive) the Heat Geek mega coil one, it is mostly BS.

Soon I hope to have the results from adding a destratification pump to improve the heat transfer with an existing cyl at a fraction of the cost of replacing it. And a thermal store to time-shift the heat required by the radiators.

Lots of other good stuff on BuildHub, serious ppl with a scientific/engineering background there too.

[Moxi]

Hi Sharpener,

The DHW and CH is all via an lpg gas fired Worcester Bosh Greenstar 30 Si Mk4 combi boiler, the cottage is 84 or 86 sq meters over two floors, three double finned rads downstairs al 1.2m by 600mm on 8mm micro bore fed from 22mm manifold, upstairs rads are all single (unfinned) 500mm by 600mm, one per bedroom and a towel rail in the bathroom.

Historically we would heat the rooms using the 14kW WBS in the lounge and let warm air circulate around the building and this has always been sufficient with an occasional timed call for the GCH in the morning in winter as we all start to rise from our beds. This year we stopped using the WBS and started using the GCH, This was done by setting the thermostat to 19 degrees C and turning the flow temperature all the way down from 6 to 1 and taking spot temperatures on the out and return pipes (22mm) and the individual rads to see what was what, after a few days of backwards and forwards we settled on a flow temperature of 36 degrees at around mk 1 which had the combi on more or less constantly and on 4 degree or better days this met the room temperature requirements nicely, on windy colder days we had to turn up to mk 2 to mk 2.5 to get to 20 degrees and be comfortable, this temperature appears to coincide with around 40 degrees C flow temp. The DHW is set to mk 3 simply because my wife delights in running a sink of wash water in the morning that coincides with my shower and at mk 2 to 2.5 the high flow demand of the shower and the sink exceeds the boilers ability to keep pace and as I am further away from the boiler its me who gets the reduction in temperature, not masses but enough to notice.

As I noted previously I will be continuing the monitoring of the CH the rest of this year to get a full picture before going further but bit by bit the elements are falling in to place and its looking more and more favourable that a ASHP and some form of small HWC will be ample for our needs.

The previous alternative for me was to consider an outside (lean to building) with large HWC heavily insulated (like Nowty has) but that would entail significant work and cost as it would need to be robustly constructed because we are routinely bashed by 60 to 80mph westerlies in the winter and the odd 80 to 95mph in amongst them to keep my grey hairs in tip top condition!

Theres no boiler on either the 14kW or 5kW WBS and these would only be held in reserve for power outages - which we get often enough in winter to be routine and for the odd winter where we get sub zero temperatures for more than a few days which seems to be every third or fourth year as being so close to the sea we normally stay around 4 degrees in winter.

In a few weeks we will be taking out our beloved (but decrepit) lpg range cooker and replacing it with a smaller range cooker that is induction hob and electric oven so the only remaining gas use will then be the boiler.

Moxi

[Adokforme]

Thanks for posting @Maxi and all power to your elbow. Sounds like you've done all the background checks necessary to arrive at an all electric solution for your property and needs. Afraid I was nowhere near as thorough, simply understanding that we needed to move away from gas etc. Read up on A2A heat pumps on the MSE board and had one unit installed in the lounge as a trial. Within a month we'd ordered a second to cover space heating requirements for remainder of the property. Having survived two winters without resorting to GCH I stripped all the rads/pipework out and installed a 210 litre thermal store heated by electricity only. Heated via PV in summer, mostly grid in depths of winter and a mix of both through spring/autumn. I do appreciate we are lucky to have Octopus and their cheap overnight tariff but it works well for us, so I'd be very surprised if you have problems going forward. I guess just the microbore pipes raise concerns but you appear to have covered this in the trials already undertaken.

[richbee]

Having watched the latest Heatgeek video with a practical shower demonstration, I'm wondering if this would be good for me.
Our ASHP quote includes fitting a full hot water tank, but there's a lot of pipework and faffing about to fit it under the stairs and get pipe runs to/from.
One of these smaller 'combi' thermal store cylinders would potentially fit in place of the combi boiler and we will be getting a 12kW heatpump, so should be plenty of available power.....

Might have to ask the installer what they think - they are heatgeek registered, but I'm not doing it directly through heatgeek

[Ken]

i liken this to a rad and a thermal store in one

A thermal store because cold water is passing through to be heated by the rad.

The difference here is the small size allows not having to find space for expansion vessel and pipes - that is its unique selling factor.

Secondly the heat transfer in to the tank is not restricted by heat coil sizes and thus heat can be replenished at whatever rate the HP can put out.

Thirdly the huge output coil allows for a higher heat extraction or lower initial tank temp.

In a normal thermal store heated to 60C it is reckoned that one can extract a volume of hot water at 40C shower temp equal to 60% of the thermal store volume and that is after fitting a larger coil than in a normal HW cylinder, ie eg you can heat 108L of hot water at shower temp from a tank 180L.
Now if we keep increasing the size of the coil we can extract a larger % of the heat or start with a lower temp in the thermal store or have a smaller tank. In that calculation no allwance is made for reheating at the same time as withdrawal.

Working on a shower time of 5mins at 10L/min then thats 50L so in a normal thermal store that would require 80L tank at 60C but perhaps 120L tank at 45C with huge coil. I missed what capacity size the tank is in the video although size wise it appears as 50L but smaller after allowing for lagging.

Now the clever bit of this idea is realising that if the tank is is part of the CH system and the HP will come on to replenish the heat being lost in its rad. but what if the CH is already ON then the size of thermal store now becomes the size of the whole working system water and copper pipes,rads and HP ie the whole lot. In this case the rad/thermal store can be very much smaller as it is now behaving as a 100% efficient thermal store with heat energy being instantly replaced.and perhaps all one needs is a 50L tank ie small enough to fit in a kitchen unit.

This leaves me with the question of what happens when the CH is not on.

I would be also feeding the HW through a inline electric water heater that adjusts the electric input and not the flow rate just in case.

[richbee]

Now the clever bit of this idea is realising that if the tank is is part of the CH system and the HP will come on to replenish the heat being lost in its rad. but what if the CH is already ON then the size of thermal store now becomes the size of the whole working system water and copper pipes,rads and HP ie the whole lot. In this case the rad/thermal store can be very much smaller as it is now behaving as a 100% efficient thermal store with heat energy being instantly replaced.and perhaps all one needs is a 50L tank ie small enough to fit in a kitchen unit.

This leaves me with the question of what happens when the CH is not on.

Are you meaning what happens to efficiency when you call for short bursts of heat to top up the small cylinder, eg. for a single shower or a sink of washing up water?
I was wondering that - as the whole point of heat pumps is constant, low and slow heating - which should be fine in the winter, but not in the summer, when the HW is the only thing you are trying to heat.
Wonder what is the trade off between having a large , say 200l, cylinder of hot water heated efficiently and slowly and then sat there all day doing nothing, gradually losing heat - vs a small cylinder heated with infrequent bursts of energy (ignoring the savings in space and extra pipework associated with expansion vessels and vent pipes)

[Ken]

From a ENERGY efficiency point of view it is difficult to beat TOU (time of use) for small amounts due to stop/start losses,pipe losses and dead legs. The greater the amount of heat delivered then these become a much smaller %

For the purpose of the topic being discussed there is no point in comparing to a traditional tank as this could not be used. However the tank losses for a 200L tank would be the order of 2-3kwh/day with much of this due to the conduction of heat through the connecting pipes. This is not severe and therefore it is ECONOMICALLY more efficient to use a HP particularly at cheap leccy rates, or direct at cheap night rates or by solar.

If i did not have room for a large tank i would go with the tank under discussion but i would feed the HW through a leccy in line water heater that controls the leccy input (not the flow rate) to deliver the temp required. I would then have all bases covered.