S2D2

@zoothorn I answered this back on page 2, and again on page 3 when you typed it out in all caps. Maybe I should have typed the response in all caps? With the above picture it is clear. The hydraulic module contains the backup heater, so your current system requires a smaller system volume. Ask them why they aren't fitting the backup heater with the new system. It will cost more to run than a buffer, which may be the reason. If they are, ask to see their system volume calculations and post them here. If you repeat again that nobody has answered this question I highly doubt you'll get any more replies.

It depends which ASHP and tariff. I think at the new variable October rates it's around a cop of 3.6 needed to be cheaper, easily achieved by a well installed system and modern ASHP. Any time of use reductions to electricity rate likely to make it a no brainer if that works for them.

I don't know if you've stated elsewhere which model you currently have and which they are replacing it with, but the "old" arotherm has a stated minimum volume of 35l for the 11kW model. It's not clear whether this is with or without the "backup heater" I mentioned previously, but a different minimum system volume spec could explain the inclusion of a buffer if you are moving from an arotherm to an arotherm plus.

If you're wanting the over-complication route, temp sensor on the underside of the bath that overrides the timeout when hot (bath in progress)

Assuming they are replacing with an Arotherm Plus, they have a rather odd step in the minimum system volume dependent on size. 10/12kW models: 150l 7kW: 55l 3.5/5kW: 40l I've only just started looking at this but I doubt anyone is fitting a big enough buffer to hit 150l, so I would query this with them as it seems a massive jump from 55l to 150l. Getting down to a 7kW model might solve the headache if you can make up the system volume in the radiators/pipework. You can also drastically reduce the required volume with the 6kW backup heater which is relatively small but will cost more to run, I have no idea how much it would come on for normal usage, I've only just seen that it exists. Including this changes the minimum volumes to: 10/12kW models: 45l 7kW: 20l 3.5/5kW: 15l Which you should be able to hit with radiators alone. Information for the larger units here in appendix E: https://www.vaillant.co.uk/downloads/product-brochures/arotherm-plus-230v-double-2626669.pdf

I think it might have been a typo, they specify minimum system volume, not minimum buffer size. It can be quite large for the bigger units, hence the use of buffers.

It drives me mad when microwaves make this sort of thing difficult. AEG for reference, though not all black: 1000W, 30 seconds: Hit OK button once. Each press increases time by 30s. User experience gold medal. 800W 20 seconds (can't actually do 15): Hit microwave button 3 times (wake at 1000, 900, 800), hit timer button, hit increase button twice (10s, 20s), hit OK I never use the timer button, just set the power then hammer okay for 30s intervals. Anything less than 30s I just open the door. Timer button is just too many extra taps, especially with the major design fail that tapping the timer button starts you off at 0s. This is useful to nobody, and hilariously turns the microwave off if you click okay at this point in the workflow.

Best to ask your installer referencing the exact tiles, we had no issues with lifting and refitting tiles but they'd just come from a job with a difficult type (I can't remember which sorry) where they'd broken and had to replace a dozen or so getting them off.

I find cheap rigger gloves enough for brambles, YMMV and the ones ~15 times the cost will be better.

As a random sample, MCS cost me maybe £1.5k (vs DIY, less vs non-MCS install) and I've had £400 of export payments over the last six months. I filled the roof as the relative cost was minimal so generate more than I need. The company I used are now charging more, so that may be an exception, but certainly worth it in my case. Flux may disappear overnight as well, they've already reduced export rates in line with import prices which track the price cap.

If the holes are too large this will be a problem, they need to be small enough to not cause a significant pressure drop which would affect the flow rate of everything downstream.

As I've said elsewhere, the key battery feature for me is shifting daytime export that would otherwise get 15p to the peak export window at ~30p (new rates). The key feature overall is the higher export rate. The symmetry in rates means you can charge the battery in the cheap window without thinking about it, but nothing says that you have to. Any import is offset by later export, though as I said over in the other thread Octopus just narrowed the inefficiency allowance in their latest price update: Hybrid inverters always prioritise PV for self-use before pulling from the battery, yes. Even if prioritising feed-in or force discharging, the house still uses the energy first before export takes place. My average export unit rate on flux since March has been a little over 25p. As it is a complex tariff, I agree that it needs careful thought before use.

I prefer a platform for above the stairs, like this but without the top steps if you don't need the extra height: Much more comfortable than being up a ladder.

That's annoying for anyone hoping to get one in for winter, but I guess demand for air con will be high at the moment. I've had it on a couple of times for this in the recent heatwave and it's great at getting the heat out of downstairs, where I have the unit fitted. Not much gets to upstairs because it's positioned for heating and I haven't been running it for very long. It uses more power to cool, around 600W compared to 400W for heating but again not sure if that's because I haven't run it for long.

Some interesting pricing changes to Flux from July: Import fees are reducing by ~8% for all three time periods Export has different changes for each though: Peak reducing by ~11% Overnight reducing by ~20% (not that you would be exporting during this) Day reducing by ~13% That changes the maths a little to make this tariff not quite symmetric: Old overnight import vs day export allowed ~8.5% efficiency losses to be symmetric, new is ~4% Old day import vs peak export allowed ~5% efficiency losses to be symmetric, new is ~2.5% This makes peak export trickier, there will be some inefficiencies for a full discharge followed by buying from the grid until 2am, but still an incentive to not have charge left in the battery by 2am that you could have exported during the peak window. Altogether not a game changer, still a very good tariff.

Last I heard they're all still variable tariffs so no fees for switching.

The figures I quoted here: Are from billed data from Octopus, i.e. it is the real life performance of the system. That allows you up to 16% losses whilst still being a "symmetrical" tariff - the seasonal shifting I referred to before. I had some comments about use case/costings here, I think I have a different view to many on what the battery should be doing on this tariff: Agree that AC batteries have even higher losses, so the maths would change for those.

Flux seasonal offset couldn't be working better here, I've just hit £0 net bills since joining in mid March - gas and electric including standing orders. I'm now building up a reserve to offset September usage. Average import price 21.73p Average export price 25.16p 8% of the PV/A2A ASHP investment repaid in those three months of Flux

Looks great! Good price on the slabs too, was that a local company or national? The planters are nice and low so won't affect the feeling of space at arm height which is good.

I'd also have Octopus fit the meter, might be worth enquiring with them what the lead times are in your area before switching though?

For those interested, I took another look at figures and clipped the data to where solar gain overtook heating demand which gives two months worth of data: Modelled gas heating demand for that period was 2662kWh Actual gas consumption was 640kWh The ASHP consumption was 372kWh That means every 1kWh the ASHP consumed offset 5.4kWh of gas This is likely to be accurate/an underestimate as I tried to push the ASHP for maximum gas offset meaning the house was on average 1°C warmer than the regression fit data 76% reduction in gas heating usage from a single multisplit unit Assuming a boiler efficiency of 90%, that gives an estimated CoP of 4.9, though this was in the March-May period so will be more favourable than mid-winter performance. Still, it compares well with the stated SCoP of 4.2 Even accounting for "lost export" on Flux's high rates, that means the unit has repaid 20% of its cost in 2 months (~30% of the heating season) - not bad at all No more data until the next heating period, though I'm interested to test the cooling performance over the summer.

The unit you linked is R32 so legally must be installed by an fgas installer. R290 units don't have to be, but the DIY unit I got has 1.5m of pipe on the indoor unit then a 3m extension. If the joint is internal it has to be brazed but outside it's just mechanical. Unsure if you can DIY braze an R290 line, someone else will know the appropriate regs.

To offset what, 31kWh of gas per day? Sounds like you need to limit the shower flow rates first!

One thing to note with this plan is that you would end up with an internal join due to the length of pipe, ruling out a DIY R290 install. If I had paid the heavy premium for a professional install (despite 0% VAT...) I would probably have gone the whole way with a multisplit. I do think you'll get short cycling with a unit that size in that space, perhaps aim for a smaller unit and run for longer. I'm no expert though, just a hunch based on what I've seen from mine. Summer lets will 100% turn on the air con, so that may also offset savings!

Yes compared to direct electric it's very good. You may find a humid climate slightly above zero worse for defrosting cycles, but even still the CoP will stay favourable compared to direct electric. This one is probably personal preference. I wouldn't want the internal unit on when in a bedroom, it's not quiet enough. More expensive units will be better I'm sure. I'm able to overheat the room the unit is in for most of the day when it's empty and leave internal doors open, which is the only way it can heat more of the house. There are still rooms it doesn't reach, hence the boiler still being on. If you can't overheat the central space I'd be very worried about a 7kW system cycling too much, depending on heat loss/size of the space. The other obvious disadvantage is if you choose to shut a door to a room, the room will get cold. Long term I'm likely to fit an A2W ASHP to get stable temps in all rooms. The A2A will do the large room it's in and provide whole house cooling if required.