SimonD

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.

I don't think that's a good suggestion as all that does is end up increasing the load on the other rooms that are heated (as internal heat losses increase and can do so significantly) and can result in higher flow temps from the heatpump as well as reduction of system volume which can also cause cycling and defrost problems. When you're designing a system for a deep retrofit like this one, there's no excuse not to design and install a system that can supply 100% of heat requirements, which is also required by MCS.

Personally, I would go with what the SE suggests, even if it seems over kill. We have uplift restraint straps throughout the house and they take a few seconds to fit with a metal connector nailer and the traps themselves are cheap as chips by the box full.

+1 for the comments so far. Just a couple of questions for you: - what is your rationale for using fan coils in the locations you're proposing? - how much cooling are you expecting the ASHP system to deliver? A lot of people over estimate this.

I'm not an expert of ground works but my understanding is that new driveway surfaces need to be permeable which means Type 3 not type 1. With our groundworks company for driveway prep, they specified a compacted finished depth of 200mm of hardcore (type 3) and we are on clay soil.

I'm guessing the drips are warm? It definitely appears to be an expansion vessel issue, but it doesn't mean that the expansion vessels need to be replaced. They may just need recharging. From what we've seen we can't be sure that the service involved a proper check of the expansion vessels. Personally, I find it slightly surprising that both have failed a the same time after just 5 years or so. It's actually easier to check the expansion vessels on an unvented cylinder than heating because you just need to isolated the incoming mains (there's usually an isolation valve just before the multi-bloc and then you open a hot tap to take out the pressure, leave the hot tap open while you check and adjust the expansion vessel pressure).

Honestly, I wanted to avoid this like the plague, but it's the building warranty underwriter that is being a **** about this and a few other things. However, we are technically okay as it's still an existing house that used to have a combined system. It looks like there is only one down pipe that will have to do this. I suppose the other option is like the previous owner/builder of the house did - which was to bring the down pipe down into the ground, making it look like it was going to a drain but in fact it was only going into the ground by about 40cm where the pipe just finished! (I'm only kidding but it's amazing to find what some people do)

Expansion vessels do lose pressure over time so it's likely it's been re-pressurised at some point, although probably not in this case. And yeah, nobody carries dry nitrogen, or specifies that in MIs for domestic heating systems.

Yes, as mentioned check pre-charge, but also useful is there is a system pressure rule of thumb that it's 0.5 bar for single storey dwelling, 1 bar for 2 storey dwelling, and 1.5bar for 3 storey dwelling. Obviously not relevant for now is that most system/combi boilers will stop on an error message below 0.8bar. But some fittings like flow meters require a minimum pressure above this to prevent cavitation. Hence why most installers just put it to 1.5....probably

+ 1 The heating one needs to be tested and it's not always possible to do this at one visit because of the test that are done: 1. tank is found at low pressure - you've got to re-pressurise and see how it performs over a few heating cycles 2. If the valve squirts out water when under pressure, this does not immediatley indicate the bladder is shot as it can be condensation build up over the years. So always re-pressurise and see over a few heating cycles. The indication is that because the system has to be de-pressurised and cold to check the expansion vessel pressure properly, it probably wasn't done. But, if the expansion vessel has failed, you will see an increase in system pressure when the system gets hot and this will continue to rise as the system is topped up until the pressure relief valve starts letting by. It's therefore a good idea to check to see if there's any indication of let-by from the heating system prv tundish. The pattern to watch for is - top up pressure, pressure goes down, top up more pressure, pressure goes down until the expansion vessel is completely full and there is no expansion space left, then pressure increases to let by of prv.

. In my instance, it's an existing building that had a combined system before we began works and due to the terrain, being on a steep slope and where the down pipes are located, there is no choice other than to do it because the existing underground drains are in the way so we can't dig any new ones. And yes, will absolutely be into a trapped gulley

I have this question as an issue I've been kicking down the road. Building Regulations says either into a drain (by whatever means) or soakaway. My first BCO was happy with a french drain out onto the sloping ground in front of the house (but this wasn't followed up in writing). Now I'm being asked to either have it into drain or soakaway. A customer recently had an extension done to their house and they used a drainage channel to get to the foul drain. I might do the same if I can.