ProDave

Or do as I have and fit a dual rate meter that switches rate as the HP switches from heating to DHW (so switched by the feed to your 2 way valve) and then you have heating and hot water energy usage metered individually.

I did think that myself, but having observed what happens in a defrost cycle: The temperature had dropped below 0 having been (just) above 0 in the day. As soon as the HW demand came on and the compressor ramped up to full speed, you could see the ice forming on the air heat exchanger. After about 20 minutes the defrost cycle kicked in and melted the ice in a few minutes. I observed at the end of the HW demand, there was still ice there, but not enough to trigger a defrost, When it ramped back down to a low power for the UFH no more ice formed, but neither did any of the ice there melt. It wouldn't because it was below 0. So I don't think "switch off and wait" would work in any but a very few situations. Mine has a "silent mode" that you can select that limits noise by limiting maximum power. Next time we get a cold snap I will try that and see if it manages to heat the HW more slowly and thus avoid defrosting.

It is not just the missing tundish, but the D2 pipe should be at least 22mm possibly more depending how long it is and how many bends it has. This one looks to be only 15mm. If he can'r be bothered to fit the tundish and the correct size pipe, then how much confidence do you have that he has done the calcs to actually determine what size pipe it should be? And do you even know where it discharges And lets not forget the cage around the discharge point as it could be discharging boiling water.

That is largely why I wanted to mostly ignore the programmer that came with the ashp (and regard it as a curiosity / a means to adjust parameters) and wanted the heating system controlled by an ordinary central heating controller. I must get around to writing the manual and in particular the wiring diagrams so in the future a third party might stand a chance of understanding it.

Makes me laugh that it is frowned upon for someone like me to read the rules and apply them to the letter when installing my own tank, yet someone who claims to have the right ticket just ignores the rules. You see that in all trades.

What about underground cables? Any substations nearby? Know anyone with a CAT scanner?

Would an alternative be to make a new drive parallel to the hedge adjacent to the house, so you enter on the existing drive and turn right. No new entrance, no new dropped kerb, much simpler (but probably more area to pave) The permeable paviers have pips along the sides so when they stack together a gap is left for water to drain down through.

I have never quite understood these types of water heater. They seem a good idea to cool the house in summer and make use of that spare heat, but at any time you are heating the house they don't make that much sense to me as you need to replace the heat they suck out of the house,. I guess if that replacement heat comes from a heat pump, then in effect the water is heated by a 2 stage heat pump?

It depends for a start on what mvhr unit you have and what provision it has for boost and / fan speed control. My own I used immersion heater timer units (which switch a relay next to the mvhr) so all I needed was a 3 core & earth 1mm cable from the relay box to each mvhr boost switch.

What I observed when mine defrosted is the compressor and fan slowed to a stop, the 4 way valve operated then the fan and compressor started up again. It only took 2-3 minutes to melt all the ice then it stopped the compressor, swapped the valve back and re started. During the defrost it energises the inbuilt willis heater I assume to reduce the amount of heat it sucks out of the house.

I wired a house that had a system a bit like that. It had a massive thermal store, I don't know the capacity, but it was huge. It was "odd" in that it was not fed from any top up feed, but you were supposed to check the water level from time to time and top it up. It was heated with a WBS that put only 2KW into the room, and 10KW to water. The theory being you light the stove perhaps once a week and burned it until the tank was hot enough, then that provided HW and heating for the next few days.

Regarding DHW, be careful of over complicating it. I am heating HW to 48 degrees with the heat pump. Most of the time the HP does not need to defrost. However we had a 1 week cold spell recently with an average temperature of 0 and about -6 overnight. For the first time during that cold snap I did see the HP defrost when heating HW (I can confirm even at -6 it did not need to defrost when heating 37 degree water for UFH) the consequence of that is the HW for that week consumed 30KWh compared to a normal week of 22KWh. So it was using roughly 1KWh more per day either to defrost, or through a lower COP (probably a bit of both) I am shortly to be implementing a "boost" system using a modulating instant in line hot water heater. This is being installed mainly to cover those "oops ran out of hot water" moments, but it will allow the HW tank to run at a lower temperature with the electric heater boosting it as it is used. If you are going to use resistance heating then doing so real time heating just what you use will be more economical than heating the whole tank that last bit with the immersion heater. Once this is installed I will experiment with reducing the tank temperature during very cold weather to reduce or eradicate HP defrosting.

I would treat the buffer tank as a thermal store where the "heat in" and "heat out" are usually separate control loops. So the heat out is simple, whenever any room calls for heat, the UFH controller will have a "call for heat" output that will turn on a pump and / or open a valve and draw heat from the buffer. The heat input control will be in it's simplest form: whenever the buffer tank cools down, the tank thermostat will call for heat from the ASHP. That on it's own would be too simple as the buffer would be maintained 24/7/365 which you don't want. So add in a timer so the buffer tank is only being kept heated when the heating is on. In my house I have chosen only to have the UFH working only in the daytime, mainly because I want the house silent at night (the noisiest component of my entire heating system is the UFH circulating pumps) so that is controlled by a conventional central heating controller. That timed output could power both the heat in and heat out control loops so the buffer would only be heated when the heating is on.

I think the lesson here is don't just believe the instructions, actually LOOK at the thing and see if it really behaves as it should.

There are very many ways to tackle a low energy house and you cannot say one is right and the others are wrong. We are in the Highlands a little north of Inverness. We are in a sheltered glen far enough inland that the sea has little influence, and a couple of weeks in winter when it sits at -10 is not uncommon. I very much doubt any house would work well with no heat input at all in that so UFH is a must as far as I am concerned. I have opted for a small 5KW ASHP heating UFH downstairs only (and just bathrooms upstairs) and also doing the bulk of my water heating. Even if you only need say 3KW a little larger is better as it will run at a low level rather than running flat out. I find the heat loss from an UVC to be tolerable it seems they might generally be insulated a bit better than a thermal store but obviously not as good as a Sun Amp for instance. I will be fitting solar PV to try and cut down my electricity bill. So far space heating and water heating is a minor part of my electricity bill. Other "stuff" uses more, thinks like the dishwasher, washing machine, tumble dryer, fridge/ freezer etc. So anything that can help power those will be a big energy saver. The ability to use surplus to also heat hot water makes it even better. Solar thermal would not work for us due to so much shading from trees so it could not go on the roof. You can mount PV remotely from the house to overcome that, but solar thermal would not work mounted remote from the house. If however you are using solar thermal, then turn off any legionairs heating function, let the solar thermal do that. But we had a discussion about that and under certain circumstances there is no need to do that anyway.