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Heating and cooling: ASHP choices - cylinder size and zoning opinions needed

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    • CommentAuthorstonecold
    • CommentTimeJun 16th 2025 edited
     
    Opinions wanted on the following ASHP decisions (specs at the bottom). Currently we have EWI and MVHR too recently to get accurate bill data so working from estimates. Planning to get it in within 3 months to avoid refilling oil tank (price has gone up 50% since 2021). Also getting new heating pipes since they are end of life and small microbore
    (leaks keep happening). DHW uses immersion heater which will go.

    1. Heat pump hot water cylinder of under 130L recommended by eco energy retrofit report but quotes all suggest 200-240L. We have 1 mixer shower and second shower may be mixer or electric. Which cylinder size is best - small or large? Is the large cylinder worth trying to use to store extra hot water as an alternative to a second battery for the solar PV we have planned? ASHP shift will move us 100% to electric and main heating need is daytime (I am housebound so always home, get cold fast and very little solar gain).

    2. Cylinder location: no garage, can't use space boiler or current hot water cylinder currently use. Is it really really bad to use a bedroom alcove? Possibility covered by acoustic access panel and nicer boxing in. Another option is far end of kitchen but this crosses a doorway and it can't go under solid uninsulated flow - pipes would need to go in ceiling void above doorway then around corner and down - a long pipe run on coldest side of house.

    3. See photo - could space 850mm x 450mm be used for cylinder? The 450mm wide is the frpnt, basically next to MVHR. Is there enough elbow room and plumbing room? If not would solar PV 2 inverters and 2 batteries fit here? Second inverter should apparently allow solar PV use during our very very frequent power cuts (30+ gets you a cheque from energy network company).

    4. We would be looking at around 8kw solar PV and ASHP between 7.5 - 15 kwh plys obviously some stored hot water. We already use electric for hob and oven, if we have ASHP connected to solar PV with batteries are we going to have enough power to actually run the ASHP (colder temp ok)? This is for daytime since I am home all day, housebound and easily get cold.

    5. Upstairs and downstairs zones for heating - any thoughts? Atypical use means lounge and downstairs office are used all day (I am housebound and get cold easily) and very very little solar gain. It seems daft to heat bedroom when it is empty. MVHR helps kitchen and bathroom temp a lot.

    6. The Gulf Stream looks set to collapse or become dramatically unstable within the next 20 years.. causing Artic temperatures in the UK. We already have -10C several nights a year here. Is it worth getting the larger size of heat pjmp suggested (15kw) rather than 8-11 kw and will the larger size be environmentally much worse? Local experienced plumber always overspecs and suggested 15kw but is not familiar with our degree of insulation or MVHR and Internorm triple glazing. Aira also said the same. Others are in the 8-11kwh range. We have accurate plans (dwg) and accurate U values but energy needs are estimates (only 6 months since EWI and MVHR). Do small or large?

    Basic details are
    1830s stone house, 3 bed 2 bath, 150 sq m
    Full repiping needed, microbore is at the end of its life and like spaghetti
    loft 0.11-0.13 U, walls 0.60 U (EWI), MVHR in use
    no floor insulation and too little head height for underfloor heating
    Oil use should be approx 16,000 to 18,000kwh/year maybe less (from eco report) - because MVHR and triple glazing to near EnerPhit standards
    Water heating 1,800 kwh/year - scrapping immersion heater
      IMG_20250616_234723_(1080_x_2430_pixel).jpg
    • CommentAuthorphiledge
    • CommentTimeJun 17th 2025
     
    By setting short install deadlines you're forcing yourself to commit to things that might not be right for you. My views-

    Pay a Heat Geek engineer to do a heat loss calculation for you which should be the starting point for all heat pump Installs. Once you've got that you'll be able to size the pump correctly and just as importantly the size of all the emitters (likely radiators?) Finger in the air guesstimates from your regular plumber will likely lead to a mis specified system and expensive running costs.

    For best efficiency you want to be designing for a low flow temperature and running the heat pump in weather compensation mode.

    Don't bin the immersion in the new system. Avoid using it but keep it as a back up if the HP is out of service.

    Speak to Newark copper Cylinders about fitting a cylinder into the narrow space you have as they'll likely be able to build you a custom cylinder to suit if standard ones wont
    • CommentAuthorphiledge
    • CommentTimeJun 17th 2025
     
    If you've got concerns over the reliability of your supply then maybe keep the oil boiler and go for a hybrid system with a small generator to keep the lights on and run the boiler indefinitely. Your own battery bank isn't going to last too long running a HP.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 17th 2025
     
    There's a lot to cover in the above, but a few things that jump out at me to resolve, before identifying an answer...

    - the wall EWI value of 0.6 W/m2K seems very high. In the last solid stone house I did, the client only wanted 120mm GEPS, and that gave 0.22 W/m2K.

    - the solid conc(?) floor likely has a Uval of around 0.7 W/m2K, so that's a big heat loss consideration. Might need to think about the FosterTom approach of an insulation skirt around the the outside of the house, under the ground. Not done that myself, but it could be the least disruptive option.

    - I'm based in Scotland, with a different grant system, but I assume you'll be using MCS registered installers. I do all the heat loss calcs and design for my clients, as I want control over the insulation methodology and some certainty around the outcome, BUT the MCS installers HAVE to work to the MCS calcs system, so you might as well get the likely supplier to price up the system and do the calcs, cause that's what any MCS based supply will be sized on. (incidentally, my calcs and their MCS cals come out resonably similar for older property renovation projects, so that gives me some confidence in their calcs, for these renovation scenarios).

    - the DHW cyl size is only part of the problem. Even monoblock ASHP's have additional kit, like an over pressure blow-off catch-pot, diverter valves, control panels, buffer tank, etc, which would be fine for the volume of space you've mentioned, never mind the DHW cyl. Speak to the potential installers of the ASHP first, as they will advise what needs to be accommodated, and how much space for access and servicing. Also the best location for the outdoor unit, and pipe entry into and through the house.

    - taking a DHW cyl with even 75mm insul, leaves you with less than a 300mm diam cyl, to fit in the 45 space, and that's a tight squeeze. You'll be looking at something like 100 litres volume, maybe a little more. That certainly sounds low, though with a 12kW ASHP, you have a reasonably good heat up rate, but will give rise to a lot of cycling on the ASHP, something the ASHP will prevent itself doing, and so cool DHW is a definite possibility.

    - rads with TRV's will effectively give you heating zoning.

    - consider investing in more batteries and less solar PV (lots of both is great, but I imagine your budget is not unlimited). Use the batteries to import low tariff elect. 20kWh of batteries would give you say 60kWh of ASHP heating on top of also running the ASHP on grid feed during the low tariff periods.

    The above are not positive solutions, rather pointers to hopefully help narrow down some of the uncertainties.
    • CommentAuthorstonecold
    • CommentTimeJun 19th 2025
     
    thanks all for the replues - I am still digesting them but have more clarity on some

    Location - we can maybe fit the cylinder under the stairs (depends on heighy)/are actually (surprisingly) at the far end of the area set aside for possible future ensuite (or far end of kitchen which would be less flexible), still unsure of noise but the super large (1.5m tall) acoustic access panel we have elsewhere is good. This leaves MVHR room for solar (conveniently next to electrics).

    Chat with engineer today told me MCS standards stays minimum cylinder size, and since we are thinking of Aira that means probably 250L all-in-one unit so 600mm x 670mm.

    Heat pump quote from a few years ago was clearly calculated as 15kwh based on MCS calcs before EWI etc, checking recently its 7-10 kwh almost certaintly. Since the original quote was from full calculations (which we didnt keep a copy of hopefully this tine things will be lots clearer. We have exact plans from a survey including all ceiling heights so btu calculations can be done (I'm jumping ahead with guess work on my estimate above).

    Looking at costings, looks like a dead cert to get solar PV at the same time due to its really good payback time - house wil be 100% electric by then. Just checking our consumption pattern from bills, it looks like a second battery for solarPV won't be worth it since we will be using all we generate for 6 months of the year. It wont be enough surplus to be using a second battery.

    Yes walls are highish at U of 0.60 - 55mm of Diathonite Evolution - its vapour open and a big plus for the damp, could feel the difference before it was dry. One wall has less since it enchroaches on neighbour's.

    Are heat geeks really that good? Any ballpark figure on what they charge? I saw that they were against zoning which concerned me with our atypical daytime usage.

    Rads with TRVs are a poor substitute and the chance of a signal to all of them is pretty much zilch. A timer on rad may be some help but that takes away central control. And fancy TRVs are another thing to break and maintain Hmm..

    Cheaper electric at off peak means needing a regular smart metre signal which is highly unlikely given location of meter, thickness of walls, terrible radio and 3G/4G signals. Everyone here is on same mobile provider but only some parts of house have a signal so we can't bank on that.

    Floor insulation will be limited but we are going to try in 2 maybe 3 rooms, some floors are below ground level already and we have beams.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 19th 2025
     
    not sure all of the assumptions above are quite accurate eg,

    - needing a smart meter to get low tariffs
    - 6 months of solar PV covering demand, only being during the warmer months, not when there is the highest power demand
    - solar PV payback calcs are often dubious (I've reviewed a lot for my clients), and do they consider the grid feed costs they offset on low tariff or only the high rate. 30p/kWh versus say 7p/kWh pushes the payback out considerably. I've a number of clients who have realised this with their battery-only systems

    but no doubt dicussion might clarify some of those.
    •  
      CommentAuthordjh
    • CommentTimeJun 19th 2025 edited
     
    Posted By: stonecoldCheaper electric at off peak means needing a regular smart metre signal which is highly unlikely given location of meter, thickness of walls, terrible radio and 3G/4G signals. Everyone here is on same mobile provider but only some parts of house have a signal so we can't bank on that.
    It's the supplier's problem to make suitable arrangements that ensure a good connection to the DCC. You don't need to worry about it.

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