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I suspect that a major factor in sizing the TS was to make sure there’s room to absorb the heat from a run of the biomass. As I understand those things, you fill it with fuel and fire it up - you then get a huge load of heat out of it over the next few hours, with no way to stop it if you’ve overdone the fuel.

Well, strictly speaking you should have informed them after the install, but since it's such a small demand in practice it's not going to cause any trouble.

No, since that's only a short-duration load (like most other loads), you only need to consider it against the rating of your supply (ie. the main fuse). Of course, if you install enough of them you may need a supply upgrade and be in contact for that reason. It's only for sustained loads which fit within an existing supply but are likely to break their statistical planning that you need to notify - heatpumps, EV charging, PV generation.

There's two separate issues: whether your house's supply is adequate, and whether the supply to the whole neighbourhood is adequate. You may well have a 100A (23kW) supply to the house, but the distribution network will have been planned on the basis of only about 2.5kW per customer sustained capability - on the assumption that you will only use your full 100A for short periods. This forecasting makes assumptions about the kind of load - the number will be a bit higher (but not much) for estates originally built with electric storage heating rather than gas, for example. Your heatpump can be drawing 5kW+ for hours on end, which is much more of concern for the network than your electric shower which might be drawing 10kW but only for minutes at a time - hence the shower is a concern against your 100A fuse but the network operator aren't too bothered, while the heatpump fits easily within your 100A fuse but is a concern for the network operator. So when you install new equipment which changes the nature of the load in your premises, you are required to notify the network operator so that they can plan accordingly; this is almost certainly what you are being asked about. Look here under the heading "connecting electric vehicles and heat pumps": https://www.energynetworks.org/operating-the-networks/connecting-to-the-networks Depending on the circumstances (size of equipment, existing supply), this requires either that you get permission first, or that you go ahead with installation and just have to send in a form to tell them what has happened. I'm guessing from what you said that you fall into that second category - so you (or the installler) just need to fill in the form, send it to the DNO, and keep a copy for your MCS paperwork. But obviously you will need to check the numbers first.

Are there actually 3 separate cells? I don't know anything other than what I've read from others, but I had understood that there was a single volume of the PCM material with 3 sensors at different heights within it (and the heater at the bottom). So it would make sense, starting from cold as in your graph and hence all the PCM in solid state, that the heater initially melts a small volume of liquid that's very close to the heater. Further heat will then rapidly spread through the already-liquid part (by convection) but more slowly through the rest of it since there's a limited surface area of the frozen stuff in contact with the hot liquid and also there's a large amount of energy absorbed as the surface layer melts - think ice floating in a cup of warm-ish water. So you'd have a gradually increasing volume of hot liquid that's much warmer than the average of the whole thing, until everything has melted and you are just putting in the small amount of extra energy stored in the difference between it all being at the melting point vs all being at the max permitted temperature. I didn't think the sunamp controller (at least the original separate unit which has had photos shared) had any means of finely pulsing the elecrical supply to the heating element - it appeared to be a simple relay switching the element on and off. It could potentially pulse slowly (seconds) during the initial warm-up to avoid that small volume of liquid near the heater from exceeding the maximum permitted temperature as it takes time for heat to spread through the mostly-solid content; that should be less necessary later in the charge process as there's a larger volume of liquid and less convection. But it's possible there's also a thermostatic mechanism in the heater itself (independent of the three temperature sensors used by the controller). As above, these comments are 99% speculation, though I'd be interested in any info to confirm or deny...

Here's a few UK suppliers of fan coils that I've bookmarked when I came across them - I'm still not started on my build so haven't contacted any of them yet: https://coolenergyshop.com/collections/radiators-fan-coils https://www.biddle-air.co.uk/en/products/fan-coils/deco-fan-coil-unit http://www.dunham-bush.co.uk/index.php/products/fan-coil-units/ocelot First one has prices (£195+VAT) but no data of any sort; others have data but no prices....

You've got replies upthread suggesting two possibilities: Take the boost output of the Mitsubishi controller and instead of just wiring it to an immersion, wire it instead to an override input on a solar diversion controller. @PeterW suggests Eddi or iBoost controllers can accept this. Ignore the Mitsubishi's legionella feature and do it all in a separate controller. @Roundtuit suggests that Apollo Gem can do both solar divert and legionella cycle in one controller.

I think the issue is: For efficiency (or just the plain ability of the unit to do it) ASHPs for DHW are normally run with a relatively low tank temperature. The low tank temperature theoretically risks legionella growth. One way to combat this is to, at regular intervals controlled by a timer, raise the tank temperature to 60C in order to kill any legionella. Some systems just run the ASHP harder (less efficiently) to do this, but many ASHPs can't deliver the 60C+ output temperature required. Some systems have an extra resistance heater as part of the ASHP or its plumbing (effectively a willis heater with a fancy label on it!) to boost the output temperature of the ASHP, so the ASHP "system" can handle the legionella cycle. Some systems put a conventional immersion in the tank and control that to run the legionella cycle. So you need either a control system that can do both the legionella cycle and the solar divert all in one system, or else you need a dedicated immersion heater (not part of the rest of the system) to accept the solar divert. If your giant tank has room for two conventional immersion heaters, that's probably the easiest way forward from where you are.

You are right of course.

As I understand it: The "heating load" is what you want. It's expressed in watts per square metre of floor area, so multiply by the floor area 11*241.9 = 2660 = 2.7kW. So a 2.7kW output heatpump, running flat-out 24x7 and with no other inefficiencies (and not running the DHW) would just be enough to maintain their assumed temperature on the coldest days. You obviously need one bigger than that. The difference between "heating demand" and "heating load" is other assumed heat inputs to the building not from the heating system: cooking, electrical appliances, warm bodies etc. So you'd need the larger "heating demand" number if for some strange reason you wanted to be sure you could keep the house fully up to temperature when you weren't living there. There's other adjustments needed to the raw: obviously you've got to add the DHW requirement; a heatpump might not actually produce its full nominal output under the conditions you want (datasheets often quote several figures for combinations of output water temperature and outdoor temperature: since the max input power is typically fixed by the size of the compressor, the output power depends on efficiency, which will be lower on cold days and lower the hotter you need the output water to achieve the heat transfer); and you probably want more than the bare minimum to hold the temperature, just in case you've let the temperature drop for some reason and want to bring it back up within a reasonable time.

You’ve obviously managed to get a good discount, as the list price they quote online is £2641+VAT (or £2511 without the heating element). I could almost certainly get by with 2x 9 units, but 12s give more headroom and don’t cost much more; a single 12 would be barely enough and while much cheaper complicates the plumbing in ny particular case. And the shape of the sunamps vs my house means I have loads of odd voids where a Sunamp will fit but a talk tank won’t.

I don't see anything in the warranty document (PDF linked at the top of this page https://www.sunamp.com/warranty/) that needs an approved installer. It does have a lot of exclusions, and you would be on the hook for any cockup/failure to follow the instructions (where you could otherwise blame the installer - but you would be trying to claim from the installer in that case, not from Sunamp). I'm tempted to have a go myself, though there's a couple of oddities in the instructions I was about to post about: Seems to require copper for the connections. Fair enough - but does a short stub of copper coming out of the case and then convert to Hep2O or similar count? Requirement to have an expansion vessel (unless you are sure that there's no NRV at the meter, and seems in any case dodgy to rely on that as someone might power it up with the stopcock shut). Again, seems to make sense, but there's no requirement for a pressure relief valve, so how do you know when the expansion vessel has lost air pressure?

Reason for not DIYing the last bit?

Are you planning to drink the softened water? I thought normal practice was to provide at least 1 tap of non-softened for drinking use.

Would you mind sharing who from? That's cheaper than I am looking at. (and yes, I'm also probably looking at 2x12s! Reasons in my case include: I want two units for various reasons, and although they are very expensive the delta from the 9 to the 12 isn't that great; household includes teenage daughter who expects 30min+ showers, Japanese wife who expects a bath to be full to the brim with scalding hot water per Japanese practice; I want some headroom to allow opportunistic heating from PV without risk of running out.

I thought that, but having spoken to someone local who has had RWH for some years, he claims that in this area (East Anglia) it's not very seasonal. Met Office data seems to bear this out. Usage on the other hand might be - in my case I'm intending RWH primarily for garden use (so very seasonal), but looking at cost/benefit of adding toilet flushing capability.

A couple of thoughts (not based on any experience, yet): If you are doing a microcontroller project, you could potentially detect the pump running dry by sensing the mains current drawn by the pump (with a simple sense coil). As you say, you wouldn't want to leave the pump running dry, but just for a few seconds as the tank empties and then locking it out until it rains again probably wouldn't hurt. Probably more reliable over the long term than trying to sense the water level in the tank. The roof tank with effectively two float valves set at different heights does seem like a neat option, though I'm not sure about the regulatory issues (need to make sure the overflow is such that runaway pumping of the rainwater can't reach the level of the fresh water inlet?). But as a completely different approach,, how about just running the mains water into your underground tank (perhaps manually) when it gets empty? If you think your use vs roof area vs storage volume means you only need the mains water occasionally, the effort of manually topping up when the pump system has alarmed and shut down is offset by the fact that the pump controls etc are now very simple and need little maintenance.

I was afraid of loss-of-pressure issues, and/or other issues I hadn't thought of that people here would pop up to tell me! If running off mains pressure is that easy, why isn't everybody doing it? Or maybe there was a great revelation in the late '90s and everybody now is? This is really why I posted this thread, I want to understand the issues (and every house I've ever lived in has had a tank). Well, we could knock the house down and start again (and occasionally I think that could have been a better idea, assuming we could have got planning permission). But without doing that, there really isn't anywhere suitable. The house was built to be as low height as possible, I am guessing for planning reasons at the time: being an almost triangular plot with the thin end at the street, if the house was built where it "should be" in line with other houses in the street, then it would have been a tiny house. Instead, we have a gratuitously grand-looking garage built where the house should have been, and the house built further back on the plot pretending it's not there (the original planning application describes it as a "Chalet Bungalow", when it's clearly not a bungalow!). Net effect of this is that all the upstairs rooms are built into the roof shape and any corner you partition off will have sloping ceiling and low height. Anyhow, up to now I wasn't feeling particularly compromised - but maybe I didn't know what I was missing.

I'm currently starting on a major refurb/minor extension of our 80's house (upgrade insulation from really poor to Passivhaus Enerfit levels, etc, etc). Current plumbing is traditional for the era - gas boiler feeding vented cylinder, large water tank in attic, only kitchen tap is direct from mains, all other hot/cold are from the tank. Plan is UFH+ASHP replaces heating, 2x sunamps replace hot cylinder. Main reason for particularly favouring sunamps here (over and above the usual trade-offs) is space to put them: the house is an odd shape with low roof height and while there's plenty of odd corners to put stuff they are all low height - existing cylinder cupboard is a triangular space in the eaves with a fairly small cylinder and no height for a bigger one, never mind a meaningful thermal store. Anyhow, the exact detalis of the heating system is a discussion for another day; I'm fairly convinced I want the sunamps and the question is how to plumb them. Obviously the easy thing is to just put them in place of the existing hot cylinder - cold from the attic tank runs through the sunamp(s) and out through the existing pipes to all the taps, cold supplies unchanged, feed in and out of the Sunamp just adapts the existing pipes that go to the cylinder. Nice, quick, easy job..... However, I'm tempted to convert the system at least partly to use mains pressure water to the taps, for the following reasons: As mentioned, the house is an odd shape, and in particular is split into two halves (half single-storey, half 2-storey), with only one possible route from one half to the other for MVHR ducts (low ceiling heights and a structural beam in an awkward place means there's literally no other option). That space is currently filled with a total of 8 pipes running from the tanks in the attic to the cylinder cupboard. Two will disappear (boiler circulation is also vented with a header tank in the attic), but the others - 6 independent cold feeds to various taps/showers etc - will need to be relocated somehow. At minimum, this makes me rip everything out and put it back so keeping the existing arrangement is no longer the "easy option"; also, cold mains comes from the other direction so probably makes it easier to find new routes. Due to the low ceiling heights, there's not enough head for a shower on the 1st floor - existing 1st floor shower room has an electric pump to make it work, which it would be nice to eliminate. Also, given 2x sunamps for total capacity, one option is to split them up - place one of them in that 1st floor shower room right next to the shower and also serving the kitchen immediately below, with the other one in the existing cylinder cupboard over the other side of the house feeding the bathroom and gnd floor shower room there: this would considerably shorten the hot runs and so time/water/energy wasted running the shower/tap waiting for it to warm up. I've never liked having "not really drinking water" at bathroom taps, having been stored in the attic tank. Though maybe this point is moot given I'd probably feed everything from the output of the water softener so that's not ideal for drinking either. Item 1 has suddenly made this urgent, where I'd previously been leaving this stuff until the end of the build . So I could now: Leave as-is, just find alternate routes for all/most of those pipes. Put the 2nd sunamp in the 1st floor shower room, and feed that with mains cold water (easy as it enters the house over that side), mains cold also used for the cold feed to shower and other taps nearby, but leave the other side of the house running off the attic tank. This (I think) eliminates one of my problem pipes, and also frees up what was a hot-out pipe through the (also crowded) floor void that could be used in the reverse direction to replace another one. Scrap the attic tank altogether and run everything off mains pressure. What are the pros and cons here? Current system has dedicated cold feeds to all the main users (though not so much for hot), hence no issue with one tap turned on reducing pressure to others. Is this going to become a big deal if I start using mains cold and at least some of the pipework is shared? Water softener probably makes this worse (it is currently located right next to where the cold enters the building, so no problem for routing, but bound to add to pressure loss). Are there regulatory concerns? Sunamp install guide suggests I need an expansion vessel if the water meter contains a check valve (haven't yet determined if this applies to me). Presumably only a really tiny one given the volume inside the Sunamp - do extra-small ones exist? Note that I've written the above as if I'm doing the work myself, and I might have no option but to get do the first bit DIY since I think plumbers aren't currently working non-emergency jobs, but I'm open to being told that it needs specialist design if that is the case.

You might be in luck - it appears they haven't completely stopped. My planning permission was submitted on Feb13 (so before CV restrictions) but never got validated or appeared on the website - allegedly IT problems. Then suddenly yesterday it appeared - with the "validated date" set as Feb13 and deadline for decision next week! I'm not holding my breath for a decision as they haven't advertised it yet (and maybe can't?), but if you are just looking for discharge of conditions you might find the pipeline is now unblocked.

I was assuming you'd do something like: When PV generation is predicted to be good (summer) and/or your store is fairly full, run the HP only if there's enough excess to cover the whole expected power draw. When generation is predicted to be poor, run the HP as soon as there's any reasonable amount of PV and top up with grid power - on the basis that you are going to have to use grid power when the sun's gone down if there isn't enough hot water in your tank at that point. If you've got something like the Daikin unit (which has 4 configurable power levels controlled by external inputs, corresponding consumption configured in 1A steps), then you can track rather closer, but obviously not as close as something with proportional control. Still, if for example you've set it up for 4 equal steps and you have a COP of at least 2, as soon as you are over the first step to let you turn it on you are then you are automatically winning compared to the resistance heating - the proportional resistance heater could use up the balance between actual generation and the next step of your power control, but the HP is putting out more heat even with that bit "thrown away". As ever with these things, it does depend what you are trying to optimise - overall energy consumption, running cost, initial cost, independence etc. And it also depends what would happen to the excess generation if you can't follow it exactly - the FIT-with-deemed-export, the paid-a-small-amount-for-export, or the store-in-a-battery scenarios give a different level of imperative to closely follow the generation.

Surely you'd still want to divert the PV into the ASHP rather than diverting it to reisistance heating, in order to get more bang for your buck? I suppose there's edge cases like high summer when the ASHP is too busy cooling, or wanting to trickle it in when the PV is generating less than the minimum needed to run the ASHP compressor,. In the latter case using resistance heating is only a win if you have low PV output for hours on end - if it's winter time and you aren't going to have enough anyhow, might as well run the HP on half PV half grid power (if you are getting COP of 2, that ends up using grid power for 1/4 of the total heat input,), and in summer time there are probably enough hours where the PV is generating enough to run the HP to get the job done. Daikin at least has modes for limiting the ASHP consumption for this sort of purpose (only a couple of bands, not fine-grain control). Not sure if Valliant does similar.

I happened to be reading the manuals for this thing recently (it's a candidate for my install when I get to that point later in the year). I concluded that it DOES have a seperate hot water mode; there's supposed to be a motorised valve to divert the flow water to either the space heating or the water tank coil. There's separate configuration of the tank target temp(s). There's several documents online, this one "installer reference" seems more detailed than the "installation maual". https://www.daikin.co.uk/content/dam/document-library/Installer-reference-guide/heat/EBLQ05-07CV3-EDLQ04-07CV3_4PEN405544-1D_Installer Reference Guide_English.pdf The exact capabilities of the unit are a bit unclear as it is documenting the whole system including the "optional" backup (resistance) heater, but as I read it the maximum capability of the heatpump alone is 50C (it says you can set it for 55C but won't reach that in low temperature conditions). You can set the tank target temp up to 80C but that will invoke the resistance heater. The bit you quoted earlier about reheat/scheduled setting is to avoid the DHW demand taking away from the space heating since it can't supply both at once: you can have the DHW heated only on a timer (so deliberately chosen times when you don't anticipate using space heating, eg. overnight), or just on demand (so it will take away from space heating), or a compromise that tries to do it on the timer but will top up at other times if it falls below a threshold. There's a huge lot of settings on this thing and it could easily be set up wrongly such that it's only able to deliver less than its full output. Rated maximum input is 15.7A (=3.6kW) - interestingly the 5kW and 7kW versions are almost the same, suggesting that they share the same compressor and the 7kW unit just has a bigger fan/heat exchanger so there's more opportunity for it to run closer to flat out. Stated typical input powers are 1.55kW (at 35C water temperature) to 2.45kW (at 45C water temperature) given 7C ambient. So if it's taking an average of 1.16kW across the day and it's running continuously day and night, then there's quite a lot more capacity there to be turned up (though will of course cost you more money!). If it's only running 12 hours a day (and the resistance heater is disabled), then it's effectively flat out and the only way to get more heat out of it is to run it longer - use the scheduling to get the hot water tank heated in those other 12 hours, leaving the daytime hours to keep the lounge warm. If the 28kWh/day = 1.16kW average included significant amounts of resistance heater usage then there's scope to push the heat pump harder and get more heat for the same money. I think if it was me, I'd start by disabling the resistance heater and see what it's doing by itself. (NB. all the above just based on reading the manuals and analysing the numbers - I don't actually have one of these)

I am doing a major refurb/insulation of my house, and when complete will switch from large hungry gas boiler to a small ASHP, so will have no further need for gas. Currently I have a normal gas meter, in the standard type of white cupboard though mounted on the inside wall of what is now an open front porch, but the porch is about to be enclosed so it will become internal. It's also a prime piece of wall for things like the solar PV inverter. I have read that getting the gas properly disconnected has significant cost (they dig up the street). It appears that having the meter removed but the supply pipe left in place is also an option. Some sources even suggest that it's cheaper to keep the meter due to dual-fuel tariffs being cheaper (though this sounds unlikely). Any views/experience?

I haven't bought any yet, but these people list them (with prices) in their online catalogue (and happen to be local to me): https://midsummerwholesale.co.uk/buy/sunamp-heat-batteries