Dillsue

I don't know the vailant controller but it likely you have the following options- Permantly ON with a fixed but adjustable flow temp. This allows you to set what ever temperature you want for the water leaving the heat pump. The heat pump will run continuously producing water at the temp you set. As an example you could set the heat pump to heat the water to 35 degrees and it would produce water at this temp 24/7. Timed heating blocks with adjustable room temps for each timed block. This sounds like how you've been using it with a heating block in the morning and a second block in the evening. In between these blocks your heating is off during the day and overnighf. Add a timed heating block between the morning and evening blocks and between the evening and morning blocks but set the temperature 2 degrees lower than that set in your morning and evening blocks. This will leave your system heating 24/7 but with a 2 degree set back during the day and overnight. This leaving the system ON with set back You don't need a thermostatic valve to keep your bedroom cool as you can likely use one of the valves currently on the radiator. If your system was set up correctly there should be a valve either end of the radiator. One should have a cap that stops you easily adjusting it and this was used to balance the flow through each radiator.....you shouldn't alter this valve. The other valve should have a knob on it that allows you to open and close it. Close this fully then open it a quarter of a turn and see how warm the room gets at night. Open the valve to get the rad warmer or close it to cool the rad but only change it by 1/8 turn at a time. Most people would want the room heated but cool enough to sleep so tweak it to suit what you want.

And what if the "pro chap" is wrong? Thats perfectly plausible. A "professional" company told me I needed a 12kw heat pump when my calculations said 8kw was my heat loss. I put in a 7kw pump and the house has sat at 21 degrees since mid September. Not all professionals are as professional as they should be so maybe have a rethink on what people are saying in this thread, some of whom I believe are professionals. Don't forget the professionals that have managed to heat to old stone building linked to previously. Google historic England or the energy saving trust for lots more examples.....they're all professionals

It's not the uninsulated nature of the rooms that's the problem, it's the way it's being heated. By that I mean the way you are trying to heat it and/or the way the system has been designed, installed and set up. That's not a snipe at you. You've said the bathroom is heated with a towel rail which typically have a very low heat output so they'll only work in a well insulated bathroom or where the bathroom is surrounded by other warm rooms. If you post the dimensions of the bathroom walls, door, window, ceiling height and how each wall is constructed I'll estimate the heat loss and compare that to the likely heat output from your towel rail.

I did a quick heat loss calc on a ground floor stone building 12m x 6m x 2.5m without any insulation and rooms above which gives a 5kw heat loss @-2. If the rooms above are of a similar size and insulation level then a 9kw heat pump is going to heat the place. That's physics. If you know someone you trust who has studied physics then speak to them and see if they can explain it to you in a way that works for you. One thing that you've said several times is that you have run the heating continuously then in the same breath you say the radiators were alternating hot and cold. That's not running continuously. If the heating is turning on and off then you've either got a fault, got the system set incorrectly or the thermostat has been switching the HP which suggests the area where the stat is installed got up to temperature.

Did you read the article on the stone church heated sucessfully with a heat pump? If you accept the basic physics principle that a correctly designed and operated HP system will heat any space then you might be able to work through the issues you've got and get your place warmed up

My bill says my 15p outgoing rate is guaranteed til June. Maybe the March drop is for new customers??

As to whether you can ultimately heat your stone cottage, have a read of this posted in another thread https://interactive.carbonbrief.org/factcheck/heatpumps/index.html Note the stone church heated with a heat pump

If I've understood your upstairs build you got stud wall rooms built within the original stone walls? If that's correct, then that's your "tent".....the stud wall rooms upstairs. If that's what you've got and the gap between the stud walls and stone walls is open and ventilated to the loft/roof then you'll be loosing heat up that gap....25mm PIR between the studs isn't alot of insulation particularly if it's loose fitted and not sealed into position tight against the plaster board. If you're thinking of sealing this gap to the loft then I would get a professional condensation risk analysis done before you seal it up. At the moment most of the water vapour permeating through the stud walls is vented so has little opportunity to condense on the inside of the stone wall. If you block the gap at the top then you'll loose this ventilation and most of the water vapour will remain in the gap between the stud wall and the stone wall. If this condenses then you could have water running down the inside of the stone wall and running into the joists/ceiling/room below. If you want to insulate the stud wall rooms upstairs then you'll likely need to strip the plasterboard, insulate fully between the studs(70-100mm?), insulate over the studs with another 25/50mm PIR, tape the joints and then plasterboard. Cross posted with @ProDave

For a like for like swap it's no different than a boiler swap and maybe easier as there's no flue?? I don't know what the current installer skills are but when the current install base of HPs start to get to the end of life I'd like to think(hope!) there'd be alot more HP savvy plumbers around. Getting back to the "should you be worried about replacing a failed HP" question, I think the answer is no but be prepared to pay a bit of a premium to get it done by someone who knows what they are doing.

That's a very valid view but it would likely be very easy money for any plumber/spark that had done a bit of skills updating ie understood heat pumps

If you got an expensive installer supplied HP now and it fails there's nothing to stop you changing to different manufacturer if and when it fails. For most situations you likely need to change the controller but there's a fair chance the existing controller cabling would work. A replacement for our 7kw LG Therma V is £2700, so not the end of the world if it throws the towel in.

It likely will, but needs time. As others have suggested you'll need to leave the heating on continuously for probably weeks to warm up what's likely to be 10s or 100s of tons of stone. Only then will you start to feel warm without the walls soaking up all the heat. A quick Google search suggests your walls U values are unlikely to be significantly different to an uninsulated brick cavity wall but the advantage the cavity wall has is that it doesn't have the thermal capacitance of your walls so will heat up quicker. When we first switched our heat pump on last September it took a few days of continuous heating before the temperature stabilised. That's in a reasonably well insulated house but we have alot of masonry inside the the house including an old brick chimney breast. It all takes time to warm up.

Just check the max power available on the back up output will power the "heavier" loads. The Solis unit I'm thinking of using is a max of 5kw on the back up output so would need a bit of manual load management if we wanted to cook in the winter with the HP running

I think the grid disconnect is done within the inverter as it is with G98/G99 so no external changeover/disconnect needed?? In the second schematic the "on grid home load" will be dead in a power cut and the "load" will be live, fed from the batteries or generator. If the inverter can't sense a stable grid connection it keeps itself disconnected with internal contactors?? In terms of installers knowing the requirements for backup power supplies/generators, are they actually covered in the wiring regs to the level that the regs spell it out in black and white and every spark would know exactly how to wire things?? I've always understood it to be a bit of a grey area??

But if there's a neutral to earth bond, as in the schematic, anything the inverter might apply to the neutral line will run to earth?? I appreciate UK regulation/best practice may want the inverter neutral disconnecting from the grid as well as line, but if the neutral is earthed surely there's no hazard??