SimonD

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