Is it compulsory in the UK to have a blending valve fitted on the outlet from a Mini-Store?
Trying to understand the potential performance when running a bath.
From a 110 litre vented cylinder heated to say 60 C, the combined hot and cold fill rate from the bath taps could be say 20 litre/minute for 6 or 7 minutes, with the temperature mixing taking place in the bath itself.
Compared with the full volume having to pass through a blending valve and all coming out just the hot tap at 40 C. Does the bath fill then get restricted, take an age, and end up too cold before complete?
Iâve just been specced a MiniStore 110L Tall to go in our low loft space, but there does not seem to be much info about winter performance beyond this thread. And it seems far more fiddly than a tank to get working well. I suppose one worry is the heat pump deciding to defrost when youâre having a shower. Does anyone have real world experience of the recent sub-zero weather?
The only good option is a âstore of heatâ that you fully âchargeâ before starting your shower and the does not result in the heatpump running while showering. But you clearly have limited space. (A few costly heatpumps have a seeprate buffer they only use for degrosts.)
Also have the issue that the regulations for the heatpump grant requires a DHW tank with a large coil area, and small DHW tanks as standard donât have large coils. The grant also excludes the sensible option of having a 10kw electric water heater connected to the output of a ministoreâŚâŚ (Many electric showers can work in country that have âcoldâ water at 35c.)
A normal tall DHW tank can have enough hot water for the shower above the coil, so a heatpump doing a defrost have no effect on a reasonable length shower. A small, heatpump DHW tank will not have much space above the coil.
It would be ideal to have a button you passed to tell the system to do a defrost before a shower (if required) but I never come across anyone doing it.
Iâm sure thereâs some maths that might prove this wrong, but it seems like a simple option would be to have a 3kw immersion in the Mini Store too that would provide a bit of a boost when the temp starts to drop. A bit like your 10kw heater idea above but lower power and inside the heat store. In fact the immersion could kick in as soon as demand starts, before the heat pump has âspun upâ that might prevent short cycling in the summer if the water only needs a small boost, and give a bit more headroom in the winter.
Oh thatâs cool, do you mind me asking where you got the info from that itâs being released later â26? Iâm wondering if I wait for V2 and hope my boiler holds out long enough.
I was reminded by David that Zip do an inline heater. If I ever go the HP route, I think Iâd probably fit one of these for the security of never running out of DHW. I suspect a smaller tank and one of these is probably a good trade-off.
I do run a TMV at the pump/tank end of my system because I have a heat battery in conjunction with the DHW cylinder and I donât want a scald risk. The TMV is set close to 50C IIRC, good for a nice shower or bath, and aiming never to mix down the heat-pump DHW output! 16WW Energy Systems Diagrams - Earth Notes
I believe that what may be compulsory in some UK settings, such as care homes, is TMVs at the hot taps. I doubt that many home owners would bother with that, and thus will have no TMVs anywhere, and so a scald risk after pasteurisation cycles and/or with PV diversion.
I talked to our installer and heâs suggested that we could try the mini store, and if itâs not quite holding up in winter then he could add an inline heater too, which would help bring things up to a set temp once the mini store starts to drop. To my ânot a plumber or heating engineerâ brain this seems like a pretty good compromise; I assume the inline heater would be off most of the time, just kicking in if we shower for too long and the flow temp out of the mini store starts to drop. I doubt it would keep up if the temp drops too much because itâd only be 3 or 7kw or so, but it could probably extend the runtime significantly.
Thatâs right. Inline heater models like the ZIP Inline CEXU which Iâd referenced in the other thread Brian linked have pretty smart electronic controls and bring in their heating elements gradually in order to maintain whatever target water temperature theyâve been set to. So most of the time theyâll stay in âstandbyâ mode and even when theyâre âonâ they wonât be running at full power unless theyâre being fed with properly-cold water. During the Summer they could be switched off at the electrical isolator so theyâre not even drawing any âstandbyâ power.
There are a few practical considerations to be aware of:
The CEXU is rated at 8.8kW (though it can be set to a maximum of 6.6kW instead) so it needs to be on a dedicated wiring circuit using suitable cable
There are some restrictions on the types of âappliancesâ which can be connected - e.g. I expect a âpower showerâ with a pump would not be suitable
The connections are only for 15mm copper pipework and bear in mind the âhotâ water would be running through the unit all the time so the flow is bound to be restricted to some extent - which might be problematic if the water pressure isnât great
As a consequence, I donât think it would be reasonable to expect these to perform as âwhole houseâ top-up water heaters - or for filling a bath
But definitely an option to consider in the event the mini store isnât quite coping in Winter.
[See also this later post which has more detail on experience with the ZIP InLine CEX and its flow rate limitations.]
With a Willis type heater in the hot water outlet from the MiniStore, you are adding to total volume of heated water in the cold feed to hot water line, so need to consider whether that remains below the level permitted before G3 pressure regs apply.
Also the Willis heater would really only be providing instant heat as water is run off, so not able to boost the water temp inside the MiniStore over a longer period of time, say to 65 or 70 C before running a bath rather than a shower.
I contacted Newark Cylinders about an immersion in the Mini Store and they said that while it is being planned for v2, they can build them with an immersion connection now on request. That might be quite interesting as an option - potentially switching on the immersion to boost the store when running a bath or if itâs a particularly cold day and defrosts might be a problem.
So did I, and found they add 100 mm to the height of the cylinder, inserted just below the coil. This adds 11 litres of volume to the 375 mm diameter cylinders. The tapping point for the thermostat pocket ends up just above the immersion element though, so will likely switch off the heat pump before the cylinder is fully charged with heated water when the immersion is active. Prototypes of the v2 are being made for testing, and have a larger surface area of coil. I just hope they release v2 sooner rather than later for a Summer project this year.
Fwiw I have never seen a defrost while the heat pump is doing hot water. Whether thatâs just dumb luck I donât know; weâve only had a few days when the heat pump has done defrosts, and even then those only happened at night when weâre asleep.
Fwiw, Iâve noticed that the 5 kW Arotherm â which produces more like 7 kW at full bore â can keep maintain a store temperature enough for my 38° showers, at 8 L/min flow, more or less indefinitely. Different story for my partnerâs ~44° showers (the blending valve is set to max, 48°, but itâs only 44° when the water reaches the shower head). But neither of us are types who just stand in the shower contemplating life, the universe etc.
You can see from the graph that the store temperature is declining slightly. The store is supplying 16 kW total, 7 kW from the heat pump and 9 kW from the heat energy already in it. It wonât supply 38°C forever but it will for a long time.
What about a Willis Immersion connected between the heatpump and mini store, set to say 55c controlled with a push button timer that enables it for say 20 minutes when pushed?
(Could even use two in parallel if wired safety.)
Willis on DHW output of ministore (before mixer) would need expension vessel and G3 bits.
