SimonD

Usually the cylinder stat, which I'm assuming is what you're referring to as dual stat?, is not carrying anything like the current for an immersion as the immersion has it's own over-temp safety cut out. Usually the cylinder stat just has a safety cut out to cut the supply to the heat source, or to a zone valve to cut flow as @ProDave has done. So safety cut out is only on a 3A or 5A fused spur. I have to say, I'm thoroughly confused as it doesn't need to be this complicated.

Maybe my bad, as I briefly scanned the schematic linked in your previous thread and assumed it was part of the manufacturer's instructions where is has an over temp cut out (the black background schematic) - but perhaps this assumption is incorrect? All unvented installations must have some kind of non-self resetting over temperature device. This doesn't have to be on the cylinder as it can be built into the heat source - i.e. G3 provides for alternatives such as Viessmann's boiler 4-pipe. Now, I know that your heat pump can only go to 75C so there is a reasonable assumption that you don't need this kind of device, but what I would suggest is that you put a call into the heat pump supplier and/or the company doing the commissioning and sign off to directly ask the question because there is nothing we can see that explicitly covers this in the heat pump's documentation. The UVC manufacturer will just supply the whole kit that complies with G3 and tell you that need everything, because that's what they're obliged to do. Technically, the heat pump supplier and company doing the umbrella stuff shouldn't be letting you do the installation on your own and figuring all this out as you go without proper documentation and instructions, and may I suggest, some initial training. You should at the very least have a technical contact to guide you through it all, particularly as at the moment it seems to be overly complicated. Personally, I don't think this is very good at all from the company doing all the regulatory stuff for you.

Ah, okay. On that specific model the lower 22mm thermostat pocket is 700mm from the bottom of the cylinder and the higher one 1000mm. Don't get these confused with the 22mm secondary return which is at 1220mm. Your heat pump sensor needs to be in the lower one. As the wiring diagram shows the wiring in of an over-temp cut out and control, you should follow that diagram. Some manufacturer's like Viessmann on gas boilers for example, don't require it on a 4-pipe system as the safety cutout is built into the boiler itself (yes, I know yours is a heat pump but it's included in the schematic). Data sheet attached: TEMPEST HEAT PUMP DATASHEET -.pdf

Be interesting to see how you get on. I've been keepng an eye on Haier since Graham Hendra went to work there. That's a very good price and surprising the controls come with it as the brochures says controls are sold separate. One question/comment is that the expansion tanks seem quite small at 4.5 & 8l? Is this correct and what does the technical documentation say about system volumes with those?

What UVC have you got? I'd usually expect the pocket for the dhw thermostat to be about 1/3 way up. Can we have photos?

Can you share the data tables you're using please? But I think it's also useful to have a clear understanding of what exactly you mean by oversizing. By how much, proportionally are you proposing oversizing is more efficient? At the beginning you gave an example of almost 100% (6kW - 11kW) now it's more in the order of 40%. This distinction is important I think.

Yes, Vaillant seems to currently be the go to standard bearer of heat pumps at the moment, whether rightly or wrongly. You won't get a 7kW Vaillant for half the price. Being in the trade, I may be able to get £1k off that price for me through a wholesaler only dealing with trade, but as with any business I have to have a margin on this and I have to negotiate for every job. So the price I'm putting here is more likely what you would pay, even if you buy direct off a website as in these case in retrofit, you'd have to pay VAT and all those prices above are ex VAT. You can get cheaper units but often they come with short warranties unless you're one of the manufacturer's 'pro' installers where you can then give up to 10 years in some cases - that process comes with its price too. Given the shopping list above, you can probably imagine the time it takes me to order and co-ordinate delivery of all these things as a one man band - I have to cover my time somehow to maintain a viable business. Yes, go through your costs very carefully before committing - it's amazing what is commonly overlooked by those who are not in the trade and then comes as a nasty surprise.

Yeah, a dream if you can get away with. I'm just hoping those figures can go some way to dispelling the myth that is perpetuated even here on BH that a heat pump installation is just a heat pump, unvented cylinder and 2 pipes and that installers are all ripping their customers off because of the grant - if anything, it's going into the manufacturing/distribution/wholesale/services side, each one taking their slice of the pie. It should also illustrate that the difference between a heat pump and gas boiler is not just the marginal cost of the boiler itself but also the ancillaries like system controller and electrical installation - but on a new build this should not add anything significant to the overall costs of electrical installation. If you are doing a newbuild and you are doing heating, then there is very little difference between the cost of emitter and pipe installation. With an energy efficient build, the unit price difference between a low output heat pump and a premium boiler with long warranty should only be in the order of £1-1.5k and about £3-500 for a standard v heat pump cylinder. If you do end up going down the route of a gas boiler in this instance, then the 11kW Viessmann 100-W or 200-W are ideal candidates as they modulate well, can be installed on 4-pipe priority hot water setups, saving a lot of time during installation.

No, this is a scewed perspective based upon microcosms that highlight poor installations. Last year there were probably around 60,000 heat pumps installed, for arguments sake. Do you have statistics that show that the majority of these installations were not good jobs? That you're more likely than not to get a bad job? Have you run your own figures? Lets take a pretty run of the mill materials list for a retrofit using some publicly available prices at Midsummer Wholesale for the heat pump and other online suppliers for other stuff: Vaillant Aerotherm + 7kW - £4724.24 Vaillant controls - VR70, SensoComfort plus internet gateway - £633.41 Telford UCV 250l- £1225.70 50 liter volumiser - est. £350 (not Misummer who want £455 for a 25l volumiser) Installation kit - anti vibration feet, insulated flexi pipes, isolation valves, mag filter/strainer - £270 External pipe trunking (lets say 5m length) with bends and connectors etc. - £250 External Pipe insulation - £14/m x 10 external + sealant/glue & tape - £190 Internal pipe insulation for uvc and heating primaries etc. - £65 Anti-freeze valves - £200 Type b rcd - £180 Other electrical - cable (power and control), mcb, small enclosures, junction boxes, 2 x kwh meters,32A 4-pole isolator, trunking - £360 3 bog standard radiators - £300 2 nice column radiators for the living room and kitchen - £700 5 x good quality radiator valves - £150 Copper pipe mix of 28/22/15mm - £250 Press fit pipe fittings - £ 250 Pipe clips - £50 3-port diverter valve - £110 Expansion vessel kits x 2 heating and potable - £150 This comes to a grand total of £10408.35 and there are other bits and bobs I can't be bothered to list - like system cleaner/biocide/inhibitor and all those sorts of consumables. No UFH pipe or manifolds here either. No long primaries to house or through it. And this doesn't include materials for constructing a base. This represents a fairly small job for a reasonably efficient heating system. Yes, I can probably get some discount on the uvc and heat pump, but I'm sure this gives you a good illustration of what it actually costs to buy materials for an installation.

That's because the design, installation and commissioning is shite, not because heat pumps don't work. Unfortunately, you only really hear about the bad stuff, and the made up nonsense about how they don't work. I recently did the design and installation at an old house where half of it has 600mm uninsulated stone walls and the other half okay insulation badly detailed. They were having a load of renovation work being done and when the owner was telling the trades including plumbers I was coming to fit a heat pump, they were all negative saying it would never work bla bla. Did the install and the trades came back to do the finishing work and never stopped commenting on how warm the house was..... the owner is delighted saying the house has never been so comfortable. I know I'm blowing my own trumpet but there are plenty of genuine good installers out there that do a proper job and leave their customers very happy. However, I understand your worries. I'm here now as a designer and installer of heat pumps because I couldn't find anyone I could trust to do the design and install in our house. I first went down the route of installing a gas boiler myself and have just swapped this out for a heat pump. I have to say, the heat pump is just so much better by a mile. With your calcuated heat loss you can have something very simple, as @JohnMo says, and it can be very efficient. Now, I know that the BUS Grant is a prickly thing here on BH, but the idea that this just leads to price gouging by installer is IMHO, just nonsense. People need to properly run through the figures to understand why this is nonsense. For example, in the job above, the materials list for the whole job came in at more than £11k ex VAT without any uplift for me. If you're getting someone in to do the whole design and installation, the extra £7500 is something worth having as long as they're a good installer.

This isn't the heat pump size, it's the property heat loss. A proper heat pump design would also look at DHW cylinder and re heat times in the context of property size and occupancy numbers. These assessment have to be considered in the round. Whilst it is sensible to consider heat pump size in relation to defrost, the coldest periods are usually very short so I would suggest the selected heat pump needs to be considered in light of the mean temperature it's going to be running at most of the time as well as its ability to cope with cold snaps. A heat pump twice the size may have poorer modulation. There's also the infrastructure costs where if you double the size of the heat pump, your pipe sizing will inevitably need to be upsized with consequential costs - a double in heat pump size can easily add £2k to the heat pump unit price itself. Input power is also likely to be higher with a bigger heat pump. A properly sized heat pump is always going to provide the best overall efficiencies and balance with infrastructure costs.

I did a boiler service in a very large old mansion - it hadn't been service in donkeys. The boiler was down in the basement with a soil floor and I was a bit perplexed the prv had just been run down the wall to just above the floor. I noticed the system pressure was a bit low so I topped it up. I went to do my gas checks and a gas rate. Got back to the boiler and noticed the pressure guage was up to 3bar but didn't think too much of it at the time. I then started doing my combustion checks and pressure just climbed and climbed until the guage was off the chart. The prv just would not open. I eventually got it to release to a great boom of extremely hot water and a flooding of the ground. Needless to say, I'd always test the safety valves. And it is actually a requirement of G3 servicing.

Although you should listen to the language coming out of the mouth of the electrician I use for heat pump installs when a hp installer put in a 1ph hp cascade onto a 3ph system he'd installed without telling him and without balancing!

No, they don't. You only really put a gauge on the heating side. The multi-bloc should be serviced annually but everyone who does so knows the perils of opening up the various prvs and finding that they don't close back up again. But changing out a faulty multi-bloc is so quick and simple and not very expensive either. You just have to get the right one as cylinder manufacturers spec different prv pressure ratings. Off the top of my head Gledhill slimline heat pump cylinders have a 4.5bar rather than 6bar, for example.

Personally, I'd get rid of the DC connection outside the driver and connect both DC supplies directly to the driver unit. I think there's plenty of space to do this in the Tagras. That way you've got less connections and LED strips can be a bit sensitive sometimes.