Archer

I'm still trying to get my head around trying to size the inverter. Our ASHP (12.5kw) says it's power output is 4kw or 19amps. I'm assuming that would be the max if it's working flat out for a period? And then we'd want to allow for an electric oven/ induction hob (3kw?!) and maybe some spare capacity for adhoc things... But an 8-10kw inverter seems much bigger than almost everyone has so I'm assuming that I'm misunderstanding and most appliances only draw the peak for a very short time, is that right? I'm getting some expert advice now, but it seems to be that 5-8kw inverter is probably the sweet spot for our usage. Does that sound about right?

Good luck with it all however you decide to progress. Our house is also 70's and we ended up keeping the ground floor steel ducts and adding a second zone in with new ducting and vents from the ceiling in the first floor. Obviously with heat pumps the supply temperature is much lower than gas warm air so you need to run the system for longer essentially to achieve the same input of heat. Most of the time this is beneficial because you can get the same level of comfort at a lower set air temp when the system is running. Ours feels a tiny bit underpowered when it's really cold. It still gets up to temperature but it takes hours to do so. Only seem to notice this at freezing and below, I think because of the heat pump defrost cycles and generally greater heat loss ( our house isn't that well insulated) I'm still not entirely sure how we got it wrong. The A2A heat pump itself is oversized so should be fine. My suspicion is that the 2 indoor air handlers are too small because the upstairs duct work is custom made so unlikely to be the problem. Either way, it's a bit annoying but otherwise the retrofit has worked fine. Much better than the 70's original gas system we replaced.

This is all really helpful stuff, thank you. Just to add that I think the restriction on VAT to solar installs has been lifted - at least according to this. All battery installations are now exempt (is this right?) https://contact-solar.co.uk/blog/uk-government-scraps-vat-on-battery-storage/

I'm normally sceptical but Fogstar seem to be well rated for budget batteries. Uk customer service, long trading history, lots of previous customers etc. Not as dodgy as it seems. But also riskier than getting a big brand through a well known installer

Definitely getting warmer I'd say. By the time you've paid for an installer and with real life / non Nirvana usage, it would likely be a 5-6 year payback. But that's decent given it has an 8 year warranty and should last 10+ Feels like a good deal, but still brings me back to trying to work out how to get a decent installer who would fit one and know what they are doing to get me started on the energy management side.

That's the one! Well a chap on Money Saving Expert reckons he's got an "all in" price of £4.5k and that was based on buying the Fogstar for more (£2500). Feels quite daunting to find an installer though unless anyone has suggestions https://forums.moneysavingexpert.com/discussion/6457277/fogstar-home-batteries-domestic/p5

Also, another thing I don't understand - from reading other posts it seems there are efficiency losses between the grid, your battery and your home. Is it easy to quantify what these losses are? I've seen people use 10% to 25% which is quite a big range

No, this is what I needed to understand and is really useful. All of your risk points are totally valid. There is also the question about whether the technology now gets superceded quickly by something better/ cheaper and also whether increasing adoption flattens out the difference between peak & off peak power demand. You can get interest free finance and 10/12 year warranties which reduces some of your risks above but obviously not completely. I'm not sure if I'm allowed to link products in forum rules, but there is a 15.5kw Fogstar home battery with great reviews that is currently retailing for £2000. Changes the payback to 4-5 years but (allowing for the inverter and capable electrician to install). Does anyone have any experience with DIY installation - it's definitely beyond me but I'm not sure how to go about finding someone who will be comfortable installing this sort of kit...??

Putting the environmental arguments to one side for a moment, I could use some advice on how strong the business case is for home batteries in our circumstances. Our house is not well insulated by Buildhub standard - it's very much average for the UK stock with around 70mm mineral wool in walls and floor and a better insulated attic. We have a heat pump running heating and hot water but no solar PV (partly because of shading and partly cost). I estimate on our current tariff we're spending around £1300 a year on electricity at 23p/kwh and perhaps around an average of total 16kwh per day (if my calculations and understanding are sound... always a danger). Installing a 13.5kw home battery seems to cost around £7.3k with an advertised saving of £900 per annum which is maybe a bit optimistic but feels broadly in the right ballpark (if we can get most of use through the battery at a time of use tariff around 8p/kwh) So perhaps a 9 year payback on the investment? Not spectacular but pretty solid and with the added benefit of some protection in the event of a powercut. Advice appreciated - are my logic and sums above ok... what are the gotchas here?! Compatibility with high demand heat pumps, risk of the future Time of Use Tariffs being less generous, sizing the battery for winter peak demand or summer use...? Thoughts appreciated

I've just fairly recently installed a ducted A2A system (mostly retro fit of existing 70's gas warm air). I can confirm that it's virtually silent in the bedrooms, haven't noticed a major difference in comfort from the radiator/ wet systems that we've had in other houses but with the benefit of the option of cooling. The supply air is still "warm" but not as hot as that coming from the gas warm air system it replaced. If anything this is an advantage - you get a much longer, gentler heat and it modulates the fans up and down as it nears the target temperature. Our system is a 2 zone VRF - which is overkill, but was done for noise reasons and the outside unit is very quiet most of the time - barely audible from 5 metres, despite having a Sound Power rating of 70db. Biggest negative is the amount of space the fan units and plenum's take up; also the lack of grant and general wild west nature of AC installers. There are plenty around but trying to find a good one...

I think in principle that should work, the larger VRF systems are hideously complex (any large scale MEP), but we had the same need - to separately control each zone/ indoor unit and to have a single outdoor unit (mainly because of space and noise issues). It was surprisingly hard to find a unit that ticked all of the boxes. Many of the multi-split's seem to work off a single control (ie. you can't adjust each flow separately). The VRF branch box is a neat solution - hopefully it won't turn out to be hideously unreliable or hard to maintain. It looks quite straightforward and the piping for ours at least was very simple. Just seems that there isn't really a market for smaller VRF / individually controlled systems at the moment, hence the lack of choice.

Just to add that we have a 14kw residential scale VRF system (Mitsubishi Electric). It's a City Multi ducted A2A system so probably designed for small retail or office units primarily but works fine - we have 2 indoor units running off a single outdoor unit with the intention to add a 3rd in the future which this gives us the flexibility to do easily. There is an indoor junction box which controls refrigerant flow between the indoor units and the outdoor. It was about £500 more expensive than the equivalent non-VRF multi-split but has a lower operating noise (not sure why).

That's helpful @severnside and also a bit worrying from my perspective. We are still waiting on planning (looooonnnnngggg story) but will be retro-fitting warm air in a similar way to you. How has the cost compared with your old oil fired system out of interest?

This unit does look perfect as a direct replacement. You still have an outdoor and indoor unit by the looks of things but it's very design led. The wall units are some of the nicest I've seen. Having gone through the mill on this with the current gas warm air providers (including J&S etc), I'd be a little wary of an unproven American import. The warranties all seem quite weak and you wouldn't get great aftercare support etc. The commercial A2A products are a bit niche, but Daiken, Mitsubishi etc do sell products with decent warranties and engineers that know what they are doing with them. Government should look again at A2A systems, they are not getting the support that A2W does and it makes no sense. HVAC has additional benefits to air quality etc that could really improve uptake imo

Your post on the other website that you linked further up was incredibly useful. Good luck with it, sounds stressful