Posted by: @sjc123456789

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Our plan is to install the following (I've marked the bits we already have*).

• Solar, We've currently got 25 panels @440, 11kw*, with another 14 panels, 6kw planned to install now ish, so total will be about 17kw. The solar is on a low pitch roof, we get about 10000kwh* per year at the moment, with the addition I'd be expecting around 15kwh in total for the year. We export in the summer, not so much in the winter.
• We've got 16kw* of batteries, and I'm thinking of adding another 16 to 24kw, as a Yorkshireman I'm trying to keep my purchases to Octopus £0.0667kwh electric, rather than the day rate at £0.2655kwh.

A lot to go through..

While I cannot advise on UFH, etc, I could not stop noticing your plan to install more PV. I would look carefully at the ROI. Unless you have a decent locked export rate, the payback time for it may be very long.

Should you be able to able to invest to shift your exports to peak rates (even if with 8kw you may not have much room), I am guessing the payback may be better, even if less certain in my view. Difficult to say without knowing what type of storage or inverter you have and what is the maximum rate. 

Welcome to the forum and please feel free to ask any question you wish.

Here is my starter for 10 focussing on the ASHP

• 12kW sounds a lot for 200sqm of (now) well insulated property.  I would have guessed 6-8kW is nearer the mark.  Have you had a proper survey done taking into account rhe fabric upgrades and have you checked the assumptions?
• You want to design the upstairs heating to operate at the same flow temperature as the downstairs UFH heating, so you dont get involved with mixing down.  This is important!
• You want a direct connection from ASHP to emitters, no buffer tank, low loss header or plate heat exchanger.  A 2 port volumiser fitted in the flow is OK, but nothing with 3 or 4 ports and nothing fitted between flow and return (other than the emitters of course).  This is also important!
• You will be operating the ASHP principally with its own built in controller, not third party controls.  Ensure that is either wireless or put in a representative room, not in the loft, a cupboard, the garage or anywhere else inaccessible or unrepresentative.
• Dont spend lots of (or indeed any) money on fancy third party controls.  The principal starting point for operating an ASHP is 24x7, using its own weather compensation function, with all TRVs and thermostats turned up to maximum so they have no effect.  Once its working in this open loop mode you may turn one or two TRVs/thermostats down in rooms where there is a lot of solar gain, or occasionally for other reasons, but the majority of the property will be fully open loop on WC only.  If you must have a third party thermostat make sure it is pure no/off, not one of these fancy things that attempts to modulate the heat source as it gets close to the desired temperature, these are basically useless for heat pumps.
• As its a new water tank (Unvented I presume) you want one with a big coil, preferably 3 sq m or thereabouts.  These are sold as 'ASHP tanks'
• Do feel free to post the make and model of ASHP and ask for comments.  Although the principal thing to get right with an ASHP is the system design and commissioning, not the make of heat pump, the controls for some ASHPs are better than others which may matter to you.

The best thing to get the heat from the UFH to the house is concrete screed and tiles. We have 60mm screed and tiles and the floor stays warm for several hours.  It does take a few hours to warm up but once it is warm it is great. It also means the heating can be off for a few hours and the floor is still emitting heat, useful for overnight boosting when the electric is cheap and using less the early morning when it is cold and the sun isn't shining.

The other thing I would want is as much insulation under the floor as is practical. Could you get insulation under the joists running opposite to them as well as 100mm between them if you have a 300+ void under there? A full layer of insulation that covers the timber is required in lofts and walls for new build regs as it supposed to make a significate difference. I'm not sure if it is required under the floor but most new homes don't have joist on the ground floor. I've got 200mm in 2 layers under the screed and it still feels cold if the heating is off for a day.

on the batteries front i would defo get to at least 24kwh. the solar will help to a degree in stretching the capacity as it can add some during the day, but i found recently that my 27kwh was not enough for the Jan 6th cold snap and ran out about 4:30pm, though i was exporting all my solar at the time. i have a 4kwp solar array and a 5kw export limit so not an issue yet. going forward i plan to add another 4kwp and anything above the 5kw limit will automatically go into the house or batteries.

i wouldnt bother with a solar diverter for the hot water as it will be difficult to set it to divert when the solar starts clipping, as i cant see how it would know, the heat pump is far more efficient at hot water heating anyway.

@sjc123456789 I totally agree withJames’s comments on heat pump size and JohnyB’ on floor insulation. Our house is 200m2 not very well insulated (sold concrete floors for example in most of the area) and only needs 8kW ashp which the 7kW Vaillant provides.

On floor covering in our new utility room is LVT and was a decision we made early on and it has consequences (acceptable in our case not necessary in yours). All brands of LVT have an absolute upper maximum on UFH pipe work flow temp of 29C and is due to the upper temperature of the adhesive used. We learned this right at the end of the build. The temperature of the flow is identical to the surface of the LVT (very carefully measured here), no matter what a plumber will tell you. So we need to mix our UFH water down from the standard ashp flow. Fortunately our UFH pipes were spaced at 100mm so this low flow temperature was plenty warm enough, and we’re mostly radiators.

Our new downstairs loo floor has ceramic tiles which have no temperature constraints and despite being an identical temperature feels warmer to toes than the LVT, because the surface has a higher thermal conductivity.

Back to your case. UFH is your main transfer mechanism to heat your house so you need the flow temperature to be the one controlled by your ashp so you need your floor covering to be suitable for the maximum flow temperature and this is the crux of your thermal design. Our system happens to to be designed for 42C flow for -2C outside but in the last cold spell it went up to 46ish. Normal flow design temperatures are 45C or 50C, and these would ruin your LVT, because the adhesive would blow.
Engineered wood flows are good to 45C from memory and ceramic tiles have virtually no limit. We couldn’t change to ceramic tiles late on when we realised the LVT limits because there was not the space under the door, without lifting the lintel.

You are right to ask about everything together on the heating side because it interacts. The solar side is virtually separate and can be treated separately other than the output power of your inverter, which should be enough to drive both house and heatpump. I feel that you don’t need to have enough battery capacity to run the ashp all day. As a good Yorkshireman look at payback time on batteries for the difference in electricity rate. It won’t be exact because rates will change over time but the difference in £&p will be similar. If your solar output has to be clipped you are not getting any export for that extra in summer when you’re not using it. But if you’ve got a hot-tub planned then that’s different.

Posted by: @sjc123456789

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• Our Solar export is capped at 8kw currently (single phase) but we've not really suffered any clipping, we're hoping any clipping of the new system can be diverted to a car charger and or the water tank.

Agree with James' and Judith's comments.

Personally I find that current batteries make it simpler and cheaper to "shift" electricity produced or bought at a good rate to suit our usage.

So, assuming the EVs are not being heavily used, the excess production may be best absorbed by the batteries.  So even if we have the same charger, we are now just controlling the charging via the Octopus app. 

Did you consider the Octopus Agile export tariff, where you can benefit from maximising exports at periods like 4-7pm?  

@batpred Hi Batpred, thanks for the reply.

The current system is made up of a 2 x 5kw inverters in parallel to give 10kw. The 16kw of batteries have a throughput of 5kw each, so i think the limiting factor is 2 * 5kw - I'm no electrician as you've probably guessed. The export is clipped to 8kwh

The equipment is Fox, not sure on the model on the inverters but the batteries are 4300's

I'm paying 7p at night (23:30 - 05:30) for electric and export at 15p.

We were using Gas for heating in 2025 and will be using electric in 2026 with a Heat pump.

We bought 14.5mwh of electric in 2025. Over 2025 at some points we had 3 or 4 electric cars at home.

We exported about 10mwh in 2025, and produced 9.5mwh on solar.

We have 3 electric cars, an I-Pace, a Smart Fortwo EQ and a Mustang EV, I think we used about 7.75mwh in those last year.

Recently we've been dumping surplus electric between 17:00 and 19:00 and filling the batteries from 23:30. Most of the time we didnt buy any peak rate electric, but as said we were using gas last year.

I was under the impression that the panels were the cheaper part of the system, the current system's ROI is about 8 year, I've not fully worked it out for our new system, but I will.

Sounds like you've a similar system to me - but we've only got a 80amp fuse board.

@jamespa 

Hi James, thanks for the reply.

The house is less than 200sqm, upstairs is smaller than down, single storey extention.

We've had 2 surveys done, one by the Heat Geeks which has gone cold - long story, and we've got a second one. I'm not 100% convinced either fully understood what we were doing and put the time in to model it properly, but both came back with a heat loss of between 10 and 11 (forgive me I'm not too technical).

Yes, the upstairs is designed to work at the lower temperatures we're expecting with the UFH downstairs. I didnt put it in the spec, but we might well have aircon fitted upstairs as its usually too hot for us (we have an office upstairs, which needs aircon) but we might move the office downstairs.

I understand where you're going with points 3 onwards, I'll do some research and come back to you.

Thank you. Simon

@johnnyb 

Hi Jonny, thanks for the reply.

Is encapsulating the floor joists a good thing? I didn't know if they needed airflow?? I could put some PIR under them, but it wouldn't be easy to do in big sheets.

I like the idea of screed and tiles, but the wife isn't as keen, the extension is costing me so much I cant afford to divorce her either.

thank you.

@adamk 

Hi Adam, thanks for the reply.

Yes, defo will increase the batteries, and poss more solar.

I'm just thinking of the diverter so I dont "waste" any generation.

thank you.

Posted by: @sjc123456789

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I'm just thinking of the diverter so I dont "waste" any generation

Prior to installing an ashp I had I diverter.  However I disconnected it once the ashp was installed.  Cheaper by far to heat dhw at 8p per kWh and a COP of 2 (ie 4p per kWh delivered) than to forgo payment for export at 15p/ kWh.