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Certification of any sort is a quality control process, i.e. a prescriptive means of meeting a particular standard. If you're a fully engaged, hands on self-builder who actually knows what they're doing, the value added is pretty limited - you've already driven the quality into the process and know it without someone else to tell you, and are free to chose your own trade-off between capital cost and energy consumption (15 kWh/m2/year isn't a hard number after all). If you're hands-off and relying on someone to do the work for you, it provides you with a valuable means by which you can ensure that your builder does what you are paying them to - although there are other ways and it may not be the most cost-effective. For volume housebuilders, some sort of audited external standard is probably the only way they're going to improve what they do - without it they'll end up continually salami-slicing the spec to save costs. Given our circumstances I think we're in group (2) and are quite likely to go for certification - we need some sort of external quality assurance, and PH is pretty close to what we want and is a relatively common standard is likely to be more cost-effective than some others.

Isn't that one of the ones that only does the hot water? It looks exactly like what @Thedreamer has fitted:

Most of the maintenance issues seem to be related to the MCS requirement to use Glycol antifreeze. Glycol really doesn't like being hot, and so degrades to sludge over a number of years. If you use plain water instead, and rely on the controller to do freeze protection using low-grade heat from the hot water cylinder then they seem to be pretty robust. Having said that it doesn't make sense if you can use a heat pump and PV - you'll get more heat per unit cost or roof area from PV than from solar thermal. The only way I can make it work is if you're chasing a particular standard which doesn't count solar thermal heat against the energy in the standard (I think Passivhaus still does this), and even then it's only worthwhile if the alternative ways of hitting the same standard are more expensive.

Sprinklers are a planning condition - which doesn't yet appear to have been discharged. I'm curious whether their water supply will be big enough to support it. There seems to be quite a bit of history here - permission was originally granted in 2012, the current structure was built in 2013 to a different plan and enforcement action was taken against it, they eventually got full planning for the as-built structure but it doesn't look like very much has been done since then. Looks like they've upset the neighbours during building work too...

There's an existing foul drain through the tunnel to the edge of the footpath which seems to be part of their site which they're permitted to connect to, so that should be OK. Surface water is an issue though - they don't have permission to discharge to the foul sewer, haven't provided a percolation test or evidence of groundwater levels and it's a pretty tight site so they may not be able to get 5m from any structure to build it either. HS_CD_20_00770-SOUTHERN_WATER_PLAN-838858.pdf

For dishwashers it's pretty simple - they've only got a single inlet, but most (not all) will take water at up to 60°C. I switched from cold to hot fill at a previous house where we had a pretty rubbish Smeg dishwasher picked up off Gumtree and it was a revelation - cleaning was vastly better and it ran through cycles much faster. Typically they use about 10 litres per wash for the more efficient ones - about a bucket of water - and if your plumbing takes anything like that before it runs warm you've got a problem. Any cold slug isn't a big deal - essentially they fill up with the required amount of water, and then the electrical heater kicks in to warm it to the temperature required by the cycle (which is why hot fill runs faster). I just teed it off the kitchen sink and didn't worry about it after that. Hot fill washing machines is rather more controversial - it only works if you supply both hot and cold fill to the machine, and there is only one manufacturer I'm aware of who does them (Ebac). I'm not convinced that they're particularly valuable, especially as washes are getting colder over time, but £100 extra for 150 kWh/year is a better deal than £7k extra for 1000 kWh/year: not that either is good or you could keep a straight face describing them as an investment, but the washing machine is significantly less bad. There's an additional problem - the battery controllers out there are the moment are pretty dumb, and don't really connect to what the grid actually wants. If you've got PV at home batteries will try to charge off it rather than export, even if there is a lot of demand during the day with the gas plants working hard but they're expecting a storm with loads of wind to hit that night and drive spot rates negative. Hot water storage makes sense with a very large cylinder, and I'd also argue for a very well insulated house a thermostat which turns up and down in response to electricity prices makes sense too. Well insulated houses - particularly ones with cellulose insulation - seem to have a time constant in the region of 24 hours, and that effectively allows you to use the house itself for a lot of thermal storage. The problem is that for a bigger system you're going to be MCS registered so can be paid for what you export. That means you're only saving the difference between the export rate (5p or so) and the ~15p you're paying, for the 20% or so of your total consumption that your battery can provide - overnight loads in summer, basically. It really doesn't add up to much money. The only way I can almost make it work is the Octopus/Tesla tariff, if you don't have a Tesla car. No standing charge and net metering with any deficit or surplus valued at 11 p/kWh. If you've got a very big PV system and are exporting most of it, the difference between 5p and 11p/kWh might hit the £500/year or so at which you can just about justify a Powerwall. It's worth noting the batteries in that scheme are managed remotely, so will form the sort of virtual power plant required to make batteries actually useful, as per @SteamyTea's comment above. That's reliant on a particular tariff being available for the next decade or more though...

Interesting modelling study done here which nicely shows this - Passivhaus with a big PV array generating 11 MWh/year and consuming 4 MWh/year. With no battery they were predicting 2.2 MWh/year would come from PV. With a 5 kWh battery they were predicting 2.9 MWh/year would come from PV (700 kWh/year saving, worth about £70). With a 10 kWh battery they were predicting 3.1 MWh/year would come from PV (900 kWh/year saving, worth about £90). With a 40 kWh battery they were predicting 3.3 MWh/year would come from PV (1100 kWh/year saving, worth about £110). It's worth noting that they could probably have saved ~200 kWh/year by switching the dishwasher from cold fill to hot fill and another 150kWh/year by doing the same with the washing machine, as well as increasing the fraction PV directly used by both as hot water can be time shifted more easily. If you want to have a battery - say you have issues with the reliability of your electricity supply - then it really doesn't matter how much it is, you get nearly all the savings from a very small battery. The PV in that case (well, if it were possible to export it all - they had a severe export limit) would be worth ~£770, and at current prices the system would not have cost much more than a 13 kWh battery.

So the boundary was drawn by the same guy who planted the hedge, and has been there the whole time they've lived there. They've now got a survey which states it's not very accurate to the title plan, and in any case the Land Registry themselves state that the title plans aren't definitive. It's worth trying to be polite to the neighbour - you don't want this to escalate since that will get expensive - but I don't think he's got a leg to stand on unless there is anything in your deeds which defines the boundary much more accurately than the land registry map does. It's worth checking, but I'd be surprised. Edit: apparently what your neighbour is doing is common enough to get it's own FAQ answer.

Correct - there are a handful of "modulating" boilers out there which as I understand it just have two burners which they can use individually or in parallel, but they're rare. Essentially it means you'll be dumping 12kW into a house which needs maybe 1kW on an average day, which will make controlling the temperature for comfort hard. Ideally the boiler would be matched to the design cold day demand, in which case on an average day it would be on maybe a third of the time in which case the temperature stability will be a lot better. Cycling is an issue for reliability, but my main concern would be comfort.

Minimum size you can get an oil boiler is about 12kW - if you're doing a reasonably efficient build then that's going to be massively oversized and give you all sorts of problems.

Has anybody used the design & build route for their project, or does anybody know of anybody who has? We had a builder come over yesterday to discuss our project and get a second opinion on a few things, mostly relating to costing and feasibility. One of the interesting things to come out of the discussion was that he prefers to go down the design & build route if given the choice - i.e. they tender to deliver the whole project once we've obtained planning permission and provided a reasonably detailed spec. They then take responsibility for the detailed design and deliver something close to a turn-key build. This builder seems reasonably clued up, is fairly local and has delivered a number of similar buildings before including at least one certified Passivhaus, and several of his points resonate pretty well with my own experience at work (I'm a mechanical engineer). Doing this is obviously in their interests - it locks in the project management for them, for instance - but at the same time may suit us quite well. We've taken a while to get to the starting point here so my wife is a bit fed up with the whole process, and project managing it ourselves isn't on the cards due to other commitments (jobs and young kids). While we're happy that our architect will do a good job and deliver the design we want, their initial cost estimate included not far short of £100k in double-counted costs (they took a QS report from another one of their other projects then added in a load of costs that were already included but they'd assumed weren't without checking - so e.g. 10kW of PV got counted twice) so I would not feel very confident relying on them to supervise the build. Technically I'm happy that it would be fine, but I suspect the cost control would have issues. It should also be noted that we aren't very fussed about finishes, etc. - a few things we want are a step up from a standard developer level of finish, but we have young kids and are assuming that anything really nice will get destroyed by them so are working on the assumption that kitchen, bathrooms, etc. will be replaced in 10-15 years time. The thinking is that we'll take it to getting planning permission with the current architect, who will also do PHPP, etc. for us. At that stage we'll have a full QS report as well as the planning drawings and so have something to compare a design & build tender to and see if it really is cost-effective. If we did go this route we'd probably also insist on it being a certified Passivhaus - that's up for grabs at the moment, and I think with this type of contract the additional quality control aspects would be particularly valuable. Comments?

Looks like you can get off-the-peg concrete lintels up to 4800mm as a special order, and 3600mm is a stock item. How wide are your bi-folds? Catnics look to be easy for the builders because they only have to put in a single lintel. If you put in two, one on each leaf, then the width of the cavity doesn't come into it. It's also worth noting that there is no requirement for a structural engineer to calculate every aperture even if your bi-folds are too big for anything but a steel - you use off-the-peg concrete lintels for all the other windows, and get the calculation done for the bi-folds. For very big bi-folds you probably need to get a structural engineer's calculation anyway. Something that simple should only be £50-100 at most. So far as "pain in the arse client" goes, the critical word there is "client". If you wanted a 200mm cavity, the people you are paying for a service should be figuring out how to get it for you within your budget, and if they can't do so explaining what the impact of doing it would be and suggesting alternative ways of achieving the same insulation value. There **may** be good reasons for using a 150mm cavity, but "a particular type of lintel which is easy to draw the details for isn't available in this width" isn't one of them.

A Sunamp and a hot water cylinder with an immersion heater are essentially the same - the difference is that Sunamp use a specialist chemical mix instead of water, which allows them to pack more heat energy into a given volume. That costs more, and in addition of course they're the only supplier for this sort of device so will be pricing at a premium. You can get a box for a few hundred (e.g. the Eddi) which plugs into the immersion heater in a normal cylinder and uses PV in the same way as a Sunamp. The whole shebang shouldn't cost more than half the cost of an equivalent Sunamp. If you're going for MVHR, look at fitting a post heater - it's effectively a hidden fan heater, basically all of the supply air from the MVHR flows over it and gets warmed up in the process. Covers the whole house at 25% of the price of the heaters he's trying to sell you, but there is a limit to how much heat it can provide. Unless you're insulating much better than currently planned, it's only going to be good as supplemental heat rather than your only heating source. @joe90 can answer this one - he used 200mm Dritherm 32 in a brick and block build, so can tell you exactly how to solve that one cost-effectively.

Disjointed thoughts since it's late and I spent 4 hours on the phone today: I'm really not convinced a Sunamp is any better than an unvented hot water cylinder for 90% of people - it's a lot more money for a small space saving. Unless you're building on a very compact site you're going to be much better off with a cylinder. For the capacity they're quoting 4-6x the price of a comparable cylinder. I'd suggest a fan heater or something would be a far cheaper way to be safe - you're very unlikely to need the safety if you do your sums right and it'll cost you a lot of money up front to build in that safety margin with a heat pump. If you're fitting MVHR you can get post heaters which go in them for a few hundred pounds, capable of providing quite a bit of extra heat (the Passivhaus standard is essentially written around one of these heating the whole house). I can't remember the last time I saw a wet heating system leak - I've got 1970s pipework at the moment which has all sorts of problems but leaking isn't one of them. Sounds like an excuse to sell you a dry heating system. I would question the value of being able to control the heating system from your phone - it sounds like a lot more hassle than a heating system that just sits there and does it's thing without your intervention. With a well insulated house, there really isn't much benefit to turning the thermostat down during the day (particularly if PV is most available then), so you may be as comfortable and have less to do with a very simple thermostat. The bit about "German style electric storage heaters" is pure snake oil. I wouldn't worry about two circuits - oversize the radiators upstairs as much as you can (good for heat pump efficiency), and you'll presumably have a mixing valve for the underfloor heating anyway to bring the flow temperature down from that in the upstairs circuit to what you need. Remember that heat will rise within the house so you shouldn't need as much heat from upstairs radiators as you would from ground floor ones. Several people on here use Willis heaters for the underfloor heating, with Economy 7. The only reservation I have is about your insulation - 150mm of mineral wool isn't all that much (~0.23 U-value) and using direct electric heating might be expensive, those who do it are at or close to Passivhaus. Check the heat loss calculation before you decide.

I think of it like pushing a weight up-hill. Heat naturally wants to go from hot to cold ("downhill") - but the bigger the hill (the bigger the temperature difference between the outside air and the water flowing around the heating system), the more work you need to do to push it up the hill. Flow rates make a small difference, basically there's a sweet spot for efficiency where the work you do pumping the water or air is more than the improved heat transfer. Best just to leave the heat pump to do it's own thing there - that sort of thing is programmed in at the design stage.

Sunamp is really awkward for heating hot water with an ASHP - the PCM58 they use in most of their units needs water at ~65°C to recharge, and the PCM34 is really too cold for DHW use.

If it's only a few seconds I wouldn't worry about it. When electric motors (such as those in ASHP compressors) start up or change speed they need a brief spike of additional power to do so, meaning you should see the spikes corresponding with the unit starting to provide heat or changing power levels. The energy consumed by these spikes is trivial - you're billed on (power consumption) x (time), so the energy bill you're getting will be driven by the steady state ~500W (12 kWh/day) rather than the odd spike of a few seconds.

That's still going to be heating the working fluid though and eventually the house, so the only one that should contribute towards any reduction of COP below 1 would be the fan given that the working fluid for that is external to the system. Given that the fan power is <1% of the total, a COP of >0.99 is to all intents and purposes 1. Yes. So how do you get a COP below 1 unless you're significantly heating the outside world? Essentially that requires your working fluid to be warmer than the outside world. That's fundamentally not possible with a split heat pump, and realistically coming only from the water loop losses for a monobloc. Not totally sure about that - when you get into conditions that cold you start seeing a significant risk of permafrost. See http://cchrc.org/ground-source-heat-pumps-cold-climates/

If it has a COP of less than 1, where is the rest of the heat going?

What's the benefit of copper in these areas - is it just aesthetics, or is there some functional benefit?

I'm on totally the other side of things - I hate lawns with only one species of grass in them, no wildflowers, etc. Mowing will kill off anything that can't survive being in a lawn (grasses grow from the base, most other plants grow from the tip - so if they're not low-growing mowing will take them out), and then over-seed with a lawn seed mix to deal with gaps. You end up with a slightly patchy effect, but it's a much more diverse and wildlife-friendly zone.

Point is that it's excluded from their report, which is a bit shady - it's likely to be half of their total energy consumption.

The other key point is hot water - that's provided by flat plate solar and natural gas, but appears to be excluded from the figures. For a well-insulated house that has the potential to be a very significant omission.

Are you telling me James Watt has been de-platformed for being a dead white man?

All comes down to what you expect to pay for your tiles/slates. It's about break even for bog standard interlocking tiles, if you want slate or something a bit nicer it starts to get a little bit cheaper. It's certainly not a major cost saving, but I think it makes sense if you want PV and IMHO it certainly looks a lot better.