SimonD

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.

You really need a bending press or folder to get a proper finish in these kinds of things. If you're only doing a few, I'd recommend nipping down to your local metals supply place, ask to buy some off cuts and then bung them a few quid to shear and bend it all to your needs. They're usually happy to do this kind of thing. Once or twice I've also just nipped into a sheet metal fabricators and asked them if they have a few minutes spare. Never been turned away yet and you get quality.

Ah, okay. Yeah, that all means your heat loss 'estimate' is worth less than a pinch of salt. Unfortunately.

Did the company have specified u-values to work with for all building components, including internal walls? And did they have a floor plan to use? Do you know what heat loss tool they used? The most common factors I find regarding heat loss calcs are: 1. air change rates being over-estimated (this is expecially risky when you have a deep retrofit because the designers might put in build date and use default figures for those, rather than use the new figures based on the retrofit upgrades) 2. using default u-value figures for fabric elements instead of clarifying - even on solid walls the default values are know to be pessimistic. Default window u-values are just so far off it's not even a joke 3. don't correctly adapt the floor u-values. In the CIBSE book you need to adjust the floor u-values according to depth and how many exposed edges the floor has and some floors can be calculated using an exposure factor. I've seen a constant u-value inserted for this. 4. don't corretly look at building construction details to adjust the psi values for thermal bridging. In a well detailed, good quality and highly insulated deep retrofit, I'd expect to be able to used SAP reduced thermal bridging values instead of default 5. then there are other minor details like the geographical position used to determine outdoor air temperature, building exposure (which is used to calculate infiltration ventilation losses) and is subjective. etc. All these and it adds up to a lot. On one recent design, the default values applied by the design tool I was using suggested a heat loss of over 11kW on the property and after I adjusted the relevant values it was 6.9kW. On my own house, Heat Geek's AI tool tells me I need a 12kW heat pump, which is obviously total nonsense.

Okay, so I'm self-employed, but have a run of servicing appointments. As real example of a day, I turn up to my first one and find that the boiler was installed with its prv just sticking out the bottom of the boiler and terminating just behind some boxing in the bathroom. Not safe. So this needs to be dealt with. Next customer gets a text message - I'm sorry I'm running late but there's a safety issue. Arrive at the next customers house to find a shite install where the magnaclean has never been cleaned (even the bleed port is filled with plaster from when the kitchen was done years ago). Then, as I close the service isolation valves on the magnaclean, one of them springs a leak and it becomes apparent that when installed the maganclean connector threads had been damaged so as soon as I touch the thing to take the lid of another leak starts. So next customer gets a text. Then I go to a service where I did the installation and I've made up my time and can pick up my boy from school in time 😉 Life in the day of......that's what appointment windows are for ☺️ but every customer was happy to see me, some of whom I'd rescheduled from before Xmas. I find they're pleased to hear if I've found problems and rectify them as that gives them confidence I'll do them same for them. Sorry long ramble.

No, not a bodge. I always have a pack with me when I install a boiler. Sometimes I use it, sometimes not. Just depends on the particular situation. Viessmann actually supply a grey flexible condensate pipe wtih their boilers and these have combined condensate and prv drain. I keep on coming across installers, including for heat pumps that still turn up their noses at anything other than copper, saying plastic/composite pipe is a bodge. But of course it depends on where and how - plastic/mlcp as a single run joint free through a house - yes please. Just like with these condensate packs with 3m of hose which can get you out of a lot of pinches. Just make sure you clip it properly.

For a little comparison, my place is deep retrofit with EWI and new timberframe 1st floor with 3g glazing. We had a heat loss of 3.8kW at -6 with a floor area of 176sqm. We have natural ventilation. If yours is really going to be 12kW, you could also consider 2 smaller units in cascade to provide the modulation you need for the shoulder months, in particular when you have a great deal of solar gain.

That may be too much expectation. Usually assume a few degrees - but usually enough to take the edge off excess heat to provide more comfort.

I'm guessing it's these kinds of straps? Best to go back to the SE and confirm locations as it doesn't seem entirely clear.