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Lots of research going into these over the last few years, although they're very old tech - think of them as the Edison battery rather than the Tesla Powerwall! They're best known for copping loads of abuse, happily suffering through deep discharge, being long-lived and cheap to maintain (just top up the water), and having relative benign chemistry. I think one problem is that they don't have great power density, so there's been no interest in them for powering portable appliances and cars, for example. I looked into this a couple of years ago when I was naive enough to think that offgrid could be made to work in a suburban setting with a wife and kids who can't/won't ever turn off a light in the house! Very interesting tech, although perhaps the economics of NiMH now makes it a non-starter.

Before you chuck it, check the brushes. Almost certainly what's wrong and almost certainly replacable.

That's a great looking build and an amazing amount of space! The photo definitely helps btw. I completely hear what you're saying about doing a timber build. I went for the steel originally just because of the span but if it can be done with engineered timber than that's my preference. Guess I won't know 100% if the Glulam will do the job exactly the way I want it until I'm at the point of paying a structural engineer to design everything properly so I'll keep the steel portal version in my back-pocket just in case there's an issue, and make sure my budget covers either option. Just realised I never answered your question about the roof - given that budget is priority number 1, I'd happily use cement particle board or another lightweight board if planning don't raise any objections. I didn't think the weight would be too bad. Neither me or my other half are worried about kerb appeal or having a house to show off on Pinterest so a neat & tidy design that's warm with plenty of space is all we're aiming for. Am re-doing all the basic details now to suit timber only and already it feels ten times easier - no worrying about how do I fix A to B, how do I stop X cold-bridging Y, e.t.c. They only thing I'm not sure about is how to design the load bearing external and internal walls that will carry the roof, floors, e.t.c in timber. Will have to start doing some reading on that later on. Broadly, given the choice would you design the whole envelope using I-Beams and adding Glulam joists for large spans or is there a more cost effective way of doing it by mixing/matching standard timbers or other materials?

JCC is reasonably well known and quite widely used. Its more about the lamp and driver these days TBH, so as they're retrospectively interchangeable I'd not worry too much at this point. look at the hours lamp life and lumens for comparison and see how they compare to others in the same price range. Mini sun have supplied my last few jobs, very happy so far, and Cob are the led unit of choice if I can get the type I want.

I am downsizing from a 5 bedroom house (currently trading as a B&B) to a 3 bedroom house. I had hoped to release some equity tied up in property in this process (not an unreasonable expectation) to help retirement plans. If it gets to the point that the housing market really is so dire (because nobody wants to move to the Highlands at the moment?) I might VERY begrudgingly swallow the bitter pill and accept I am not going to release any equity, but I WILL NOT EVER pay to downsize, so I will not ever sell the old house for less than the cost of building the new one. It is all a mess and a huge worry at the moment, and I know who I blame but I am not going to keep on about it and launch into my normal rant.

Heave: Ground heave is the upward movement of the ground usually associated with the expansion of clay soils which swell when wet. As the soil generally cannot expand downwards or sideways, the result is that the exposed upper surface of the soil rises up. .. The most common cause of ground heave is associated with trees which have died or been removed. As the root network no longer draws water from the subsoil, water accumulates in the ground, resulting in swelling that can move building structures upwards. https://www.designingbuildings.co.uk/wiki/Ground_heave

Are yeah... I've built a few "features" in my time

Make a feature "shelf" on the walls and fit uplighters into it..??

Also could go the the less neat but easier to retrofit way of fitting the joist hangers ready to go and then just boxing around them for now.

Leave a series of slots in the internal block work for the joists and then either fill them with foam blocks or just board straight over. Then just cut them out at a later date and insert the joists - leaving a 100mm gap in one cavity to push the joist into to get them into the walls.

This does not sound to dissimilar to what we are doing. Upstairs is a warm roof with vaulted ceilings. In one (daughters) bedrom it is being kept open as a full height space, then with a mezanine floor above the small bedroom. Our own bedroom has a higher than normal ceiling height of about 2.8 metres so is then closed off to give us some loft space, while the area above the landing and bathroom has a normal 2.4 metre ceiling and a larger loft space you can almost stand up in. So translating this to your scenareo, I think you are meaning a double height space that you can later put a floor in to create an upstairs? Just make sure you build in something substantial to fix joists to when you are ready.

How about exposed "rafters" as part of the build which then become the floor joists later on?

Yes, it is the "on/off" nature of the heat pump in these things that is a part of the problem (and it's not just the Ecocent, the Genvex and Paul units similarly have a non-modulating heat pump). The basic units are extremely common in China, where there are literally hundreds of companies making them, and they are all very similar in design. The Ecocent was originally just a badged Chinese unit, but when I spoke to the guys from ESP (who are very helpful and technically pretty switched on) they told me that it was all made in the UK now. I have to say I have a few doubts about that, as there are physically absolutely identical units still on sale on Alibaba etc, and ESP has a long history of importing Chinese equipment, getting it approved to EU regs, and then re-badging it. However, where it's made doesn't matter, it's the "all or nothing" nature of the heat pump that's key. The reason for this is two fold. Firstly, these units are designed to replenish the hot water tank as quickly as possible after use, so there is always hot water available. Secondly it is a heck of a lot cheaper to build a non-inverter controlled heat pump, that only runs at a single speed. Depending on the pattern of hot water usage, an inverter-controlled version might well be a good option. For example, our major hot water demand is first thing in the morning for showers, then there is virtually no demand at all through the day, with a small demand in the early evening for washing dishes (just the stuff that doesn't go in the dishwasher) and very rarely an evening bath. A unit that could very slowly recharge during the day, drawing only as much heat from the exhaust of the MVHR as allowed by normal MVHR flow rates, would be fine for us, as our available recharge periods are always going to be 8 to 10 hours or more. The alternative, for those that need the maximum recharge rate, is to have a heating system that is responsive enough to increase its heat output in time with the additional demand that the water heating heat pump needs (I'm trying to be non-brand specific here, as this applies to all exhaust air to water heat pumps). There is loads of available energy in the MVHR exhaust, but to extract it at normal flow rates needs a very small capacity (or variable capacity) heat pump. The alternative is to increase the ventilation rate to get enough mass flow of air through a larger air to water heat pump. However, if the ventilation rate is increased, then the MVHR heat exchanger efficiency will drop, a lot, as heat exchange efficiency is very closely tied to flow rate. This means that incoming cool air won't get warmed anywhere near as much and so will be a lot cooler, and tend to cool the house down more than it would when the ventilation heat recovery system is working at normal flow rates. There is no good way around this. One sort of fix (and one that I think some systems use) is to draw outside air in and mix it with the MVHR exhaust, so that the MVHR doesn't end up delivering cold air. The downside with this method is that although the MVHR efficiency then ends up OK, the efficient of the air to water heat pump drops, as its inlet temperature is reduced by mixing outside air with the MVHR exhaust (which is always a couple of degrees warmer than the outside air). I've seen another fix where there was a valve fitted to the MVHR external inlet to just throttle the incoming fresh air, and allow air leakage to deal with the resulting flow imbalance. Clearly any incoming leakage air will be at outside air temperature, so will cool the house, but because it would probably be distributed over a lot of small, tiny, leakage points the effect may not be as obvious. For me, a small, inverter controlled, variable output, exhaust air to water heat pump could work well, as I could easily accept long recharge periods. I've looked around, and so far I've not found a modulating unit like this. My guess is that this is just down to demand. There is a massive demand in China for these things, do a search on Alibaba and you'll probably be as surprised as I was at how many manufacturers of them there are. Nothing in the Far Eastern market is driving manufacturers to invest in inverter controlled units, and I have to say that it's really only people with low energy homes, plus a pattern of hot water usage that allows for fairly long recharge times, that would really benefit from such a unit.

Exercise bike + dynamo + wife?

They've almost certainly excluded the appliances, I think I saw two side opening Gaggenau ovens and 2 Siemens top of the range microwaves. Thats £8k give or take right there. There was also a very large induction hob. My kitchen designer tells me that they have never sold more Gaggenau, Sub Zero, Wolf etc than in the last year. Originally she specced a Gaggenau fridge and freezer which were over £10,000. I nixed that. Sub Zeros are £12-16k. I don't think they offer any real advantage over other fridges. I don't mind spending extra to get something that works way better or looks much better. I will be having a Miele washer and dryer in the new place, they are demonstrably quieter than other makes and on average much more reliable. However, generally speaking I turn on the oven to 180-200C and that's it. The cheapest Pyrolitic Siemens oven is £759. Now not cleaning your oven, that is something useful. Also after 10 years it is likely that the kitchen equipment will look tatty and I might want to replace it, I don't fancy £800 a year in oven depreciation. In saying all this, it's just a decision for each of us. I drive an expensive car, they are my weakness. Some people like Caribbean holidays. Whatever makes you happy. I still can't figure out a £400K build cost based on a few extravagances.