JohnMo

One issue with combi preheat is the cold slug of water between TS and combi, the combi always fire up every time a tap is opened. So you need a solar diverter valve close to the TS outlet. Take a look here also. https://originaltwist.com/2016/06/23/eco-heating-system-for-heat-pumps/

Generally I think you have to much time contemplating and ended up with a complex solution. Way to complex. Read some of my threads I have been there done most of it. Sounds like you have most of it already. With what you have I would do the following. Sell the thermal store and get an UVC. Combi, blank off the DHW side. Use the boiler as an UVC heater on its own or in combination with the ASHP, in hybrid mode. Use the ASHP to drive your heating. Don't bother with the back boiler.

I'm in the same boat as you. Equivalent really means nothing. Don't see why me following all the MCS guidelines isn't the equivalent of a MCS person doing it. My noise is easy, main road 150m away nearest house 400m away. The heat pump can do all heating and hot water without issue, even if I used MCS calculation and ignored MVHR.

Nothing to do with your air test results. It's how much your ventilation is moving in and out of the house, the figure is multiplied by the MVHR efficiency. For a leaky house you would add like 5 or 10% of your leak test figure to the air changes per hour. Your lowest mean is for the coldest day. The monthly average in this case is meaningless. You really need the historical average not this year's data also

If that's the case there looks to be something not right with the heat loss calculation - rubbish in - rubbish out

Air changes per hour will be ventilation rate, so if MVHR with 0.3 or 0.5 MVHR hourly volume, did you add that figure, not sure where it came from, as it doesn't appear on my copy? Not sure you your mean temperature make much sense, your coldest day is 5.3 degs, we had -9 for 4 days straight this year and last year.

That is what I use, but the model with a timer. Set it to come on morning and evening and set the return temperature to about 35. Only runs for a couple of minutes time. There are two sizes, you just have to watch the head required, the small one is a very low head output.

My heat demand is around 3.5kW at -7. The heat output is 6kW at -7. On that temperature day the heat runs for around 12 -14 hours. On an 8 degree day it runs for a few hours. If you installed an 8.5kW heat pump with a demand of 8kW, your heat pump would have to run at nearly full load 24/7 to cope. If you want to batch charge the floor oversized is ok, in fact it has to be oversized.

In simple terms To get the grant the heat pump has to be capable of heating the house and got water at the lowest design temperature. So using easy number 3 kW heat demand, AND requiring 2 hours for DHW heating, that leaves 22 hours for heating. 3 x 24, is 72kWh. You now need to put 72kWh into the house in 22 hours. 72 divided by 22 is 3.3 kW. Now add your numbers to those and see what you come out with.

A quick look and the next size down is 8.5kW, so by the time you have allowed for DHW heating, it would be too small.

No - but is the output of the heat pump a your design outdoor temperature?

No way I would do a borehole without it. Never know what could come out the ground. Not sure I would be comfortable drinking the water without boiling it, without the UV.

I wouldn't put the condensate out the roof it really needs to be down a drain, it also needs a drin trap. Otherwise it will suck outside air in also, adding to the issues. I can spiral wound duct without insulation, not good. If the supply and extract flows are poorly balanced the flow temperature gets messed up, these figures should be within a degree.

My boiler in 2020 cost a £1000 in a pre COVID sale, around £400 off. In 2023 I swopped over to ASHP all self funded. The asap cost £1300, some other bits and pieces added to that cost, but not much. Running costs are cheaper than gas overall for me and I get summer cooling. Win win

I am currently running about 30% cheaper on ASHP, compared to gas boiler. When comparing all electric usage, to gas and electric. Battery and E7 helps, but neither would help with a gas boiler.

Me also, assume you UV treat and element changed annually, that wil kill anything anyway.

A copper tail - then convert

Get one with 3m2 Yes Not needed 2x 2 port one normal open one normally closed or 3 port diverter - not mid point Two ways balancing or amount of pipe in the floor, or a combination I use a Computherm Q20RF at 0.1 hysterisis and charge floor at a set temperature, not WC, once 0.5 degrees below target it switches the heat pump off. Run on E7 tariff. Download the Chofu manual and install as per their instructions not Grants, use their controller to control temperature - no need for any external controls.

If electric boiler is being proposed a well designed heat pump would cost a 1/3 to run. Storage heaters would be a better alternative to an electric boiler.

I assume you are talking gas, oil or LPG not electric?

Or these, cheaper than making them, just add screw as many sections together as you need. No need to reinvent the wheel. https://www.screwfix.com/p/reliance-valves-4-port-potable-water-manifold-15mm-x-3-4-/248TY?tc=TA5&gad_source=1&gclid=Cj0KCQjwwMqvBhCtARIsAIXsZpbeISzhvFO7hmEFMvAOeigrG2p8bINv9aaj4gLrCuFMPsFQidtZfR8aAhEmEALw_wcB&gclsrc=aw.ds

Been there got a t shirt. High temperature required, rubbish and avoid (mine had a further 150mm of insulation under it). Even with fantastic levels of floor insulation, your downwards heat loss is at least 20% more than radiators. With minimal insulation it becomes a money pit. Not really an issue, you really don't want to be zoning things to death. Boilers don't like it. Couple of things to consider Run a boiler the same as you would a heat pump, assuming you go that route. Have a boiler suitable for priority hot water or X plan. Your heating run at low temps with dT25 radiators and hot water production is at two different temperatures to get best efficiency out of the boiler. Use a heat pump UVC store water at 50 degs. Fast recovery through a big coil. Ventilation needs to be considered early. Especially if changing windows. Best in my head is dMEV with humidity activated trickle vents. A low wattage fan in each wet room running all the time with automatic boost based on humidity. Cheap to install only ventilates as required. Greenwood CV2GIP are good, silent and generally cheap of eBay.

Not sure in a domestic setting there is a legal requirement, but get the facts and do a risk assessment. We reheat our cylinder generally twice a day because we use the contents, not because we loose the heat. If you are consuming the cylinder contents daily there is zero risk. If you go away for a few weeks at a time, you may want to use a sterilisation program to keep you safe. If you have anyone with a weak immune system, avoid the risk use a sterilisation program. Not every case is the same, do the research make your own mind up. Commercial property is different.

Not really sure that is actually true. It's the stopping and starting that breaks machinery, steady running doesn't and never has. It's basically the changes in temperature of the moving of parts, breakdown of boundary lubrication as bearing parts start to lift and be supported by lubricant, that what kills mechanical equipment.

The heat loss test basically heats cylinder to 65 degs in a 20 deg room and maintains that temperature for 24 hrs and measures how much energy is required to keep it at 65 degs. But storing water at 45 to 50 your losses become really low. Reducing by around 50% for both cylinders. I would think the biggest difference to cylinder heat loss, is the piping install and insulation which is never mentioned or included in the heat loss figure. Store at a lower temp and don't worry about it. But some perspective the oso delta geo coil is £1800, my slim line cylinder was £1000. It would take 40 years of savings to make up the cost difference.