SteamyTea

Welcome. Like any project, make the big decisions first, the rest is just detail. A "box" like @joe90 has is a good start as has a low surface area to volume/footprint ratio. There is also some walk on glazing for sale near you, you could do all of us a favour by designing that into your built.

That will be runc then

Yes. When I had a lodger, and once I had trained her to take a shower that was only a few minutes, my usage was not much higher. One thing about putting extra people in a house is that fixed loads do not increase, so there is only a marginal increase in usage. I use a basic vented, 200 lt cylinder with E7 heating. I had to buy a new cylinder a couple of years back, cost £220, or £7/year, as it lasted 28 years. I don't know how much it would cost to fit in a new build, I suspect it is about the cheapest installation one can do. I have added extra insulation to the cupboard it is fitted to, just some sheets of PU and voids stuffed with mineral wool. This over halved the losses. I also run it at a relatively low 45°C or so (not checked the actual temperature, but have turned it down from the 50°C I used for years). If I have guests, I can easily turn the temp up, or use the upper element, so never had a problem of lack of hot water (since I trained my old lodger). If I am costing out a system, I always compare it to what I have, as I know the costs of what I use as I have been monitoring for a decade.

I use about 4.5 kWh/day for everything. That £2500 would buy me around 23 years of DHW. This is the dilemma I have, energy is just too cheap for me to change anything.

I think the scaling does not optimise too well. Generally, when it comes to any energy production, bigger is better.

There is nothing wrong in directly heating air, it is just the practicalities of it. To heat a litre of air, by 1 °C, takes 1.2 J. So if you have a total airflow of 10 litres/second, and you need to raise it by 10 °C, you need a power of 120 W. Trouble is, you may need a lot more air and that gets noisy.

Right, may be worth modelling to see what could be achieved with my place.

Without looking at the details at all, those limits tend to favour larger houses. 6000 kWh / 45 kWh/m2 = 133m2 Or about 2.7 times the size of my house. And it is kWh, not kwh.

and, like my house, your walls are not a yard thick. I think the problem with SB is that people think that straw is a waste product (we used to burn it in the field) and should be cheap. That, and it is natural, well I don't think there is much natural about a cultivated crop that has been intensively farmed and processed (I don't have a problem with industrial agriculture). It does have a lower embodied energy and CO2 but as it is not the major mass in a building, the benefits are probably not that great.

My house uses less, I suspect yours does to.

Straw Bails have an RSI of around 0.26 m.K/W, aged rigid PU is around 1.1 m.K/W So about 4 times the thermal losses. A quick look on eBay and they range in price between £3 and £19. As these are feed quality, I suspect that building quality is at the upper end. OSB3 is around £20 for 18mm. Lime Render is about £8 for 25 kg, Portland Cement is around half that. I suspect that the foundations have to be the same, all depends on what the local BC and SE says. I seem to remember that the cost of SB was discussed over at the other place and it was decided that it was not cheap.

Yes you can, you can also put PV on it and earn living .

Is Straw Bail construction any cheaper than block, really? Not as if straw is valueless, and it needs processing. May be easier to just cost out some components. So stairs, build a bungalow. Bathroom, just fit shower. Kitchen, small and basic. Windows and Doors, find the cheapest and make the hole to suit. Trickle vents. Heating, storage heaters. Plumbing, E7 and vented (as you can fit that yourself). Electrical, basic, two double sockets per room, pendant lights, one per room. SIPS, stick built or bought in timber frame, integrated roof PV as probably same cost as tiles. Basic cladding/render. Probably make a 50m2 house like mine for £40k. Or just spend that contingency on the kitchen and bathroom, more CAT7 and HA than you can understand, and convince yourself you have a better building as it is flashy.

If this is just a costing exerciser, then I would think it is quite easy to 'build a house' for £100,000. Probably 2/3rds of the world do it for a lot less.

Does it have all the 'services' i.e. water, power, road access, environmental surveys, structural 'stuff' etc etc? There are some on here that have probably spent close to that before they start digging.

Actually, I think that is a pretty good way to model it. Certainly a lot easier than working out the forcings as the sun moves around the house during the day. If the rain stops, and the sun comes out, I may get my IR thermometer out and see what sort of temperatures the walls get to.

Only the wall area that is exposed to the sun at any one time. May not have made that clear, in fact I didn't at all.

MVHR will only contribute to cooling when the external temperature is below the internal temperature. But you still need to change the air. So this may change the rate at which you change the internal air when you need cooling i.e. a lot at night, none during the day. 72W x 500m3 = 36 kW (Ah, that was at 3 ACH, a third of that would be 12 kW) This seems very high to me, so I may have made an error somewhere. Thing is that this is the worse case, and it only happens very infrequently, and then for a relatively short time. Consider you would start cooling well before the temperature reaches your desired maximum, the peak power needed would be smaller. I would have to knock up a spreadsheet to work it out properly. Wall area, but minus window aperture. This is affected by the insulation levels as they slow the passage of heat into the dwelling. Windows can be considered the same as floor area as the light passes though and hits something, and starts to heat that. That implies that it is 8.8 times oversized, which sounds odd. I think you are on the right track to sorting it out, just needs the details filling in.

https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

To a certain extent it is. And how many people think that spending 5K extra on a kitchen will save them that on not eating out. What is the payback on a £800 bath compared to a £200 bath? Or a £1000 kitchen tap that boils water compared to a £50 one and a tenner for a kettle.

Air loss can be anywhere between 0.3 ACH to 10 ACH. As it is over heating you are interested in, and windows may well be open, work with between 3 to 5 ACH. Air 'hold' around 0.00278 kWh of energy for every cubic metre, for every degree. So if you are changing that air 3 times an hour, that is around 0.009 kW, or 9 W, so relatively small. But at 8°C delta, that is 72W for every cubic metre. Quite a lot. Is SG single glazing or solar gain. If single glazing work on 2 W/m2.K If solar gain, then it is a bit more tricky as this changes during the day, but with noon probably having a lesser affect that 9AM and 3PM. This is because at noon, the sun is hitting the roof more, and the hot air in the roof void is trapped (to a certain extent) and is acting as insulation. When the sun is lower in the sky, it is less powerful, say 800 to 900 W/m2, but is hitting the walls and windows more directly. as a rough estimate, work on 400 to 500 W/m2 as a worse case. Unless you like working with compound angles an want to model the whole day.

You really do need to do some basic thermal modelling. It is not hard, just a case of multiplying the thermal insulation number, the W/m2.K, but the wall, windows, doors, ceiling and floor areas, the m2, by a temperature difference, the K or °C. Then add all those numbers together. Add in some air loss figures, and some solar gain figures (these can be estimated from the external wall and window areas. Air is not a good method to shift energy around, I blowing air into a corridor and hoping it will go into rooms is a bit hopeful. And remember that if the A/C unit is all indoors (like a portable one), switching it on will actually increase the overall temperature.

It may be that the heat pump is not set up to well and the inbuilt resistance heater is on a lot (I am assuming it has one for the legionella cycle). This may be able to be improved with a bit of tinkering. How hot is your hot water? Many on here with heat pumps only run up to 42° to 45°C. Though with only one of you in the house it should not be the major energy usage.

Decent insulation and a timed re-circulation pump would have done it. All hot pipes need to be well insulated. That should have been a warning.

kw, is really kW (1000 watts) and is power, or an instantaneous reading. kWh (1000 watts, for 1 hour), is the energy. 32 kWh a day is still a lot. Do you have a hot water cylinder, with an immersion heater, that is constantly on? Even then, you would have to be using a lot of hot water every day. Is there any chance that there is a wiring problem and you are supplying a neighbour. The easy way to check this is to isolate your house at your consumer unit/'fuse box', and see if there is any movement on your meter. If there is, then you are supplying something else. What do these heat? Where does the power for your sauna come from?