Beelbeebub

Honestly, there are too many types to say. Sometimes they have levers on, or "butterfly" handles, a slot for a screwdriver or sometimes thry are removable so there isn't much to see. Without wanting to sound mean, if you have to ask, you probably need to wait until tomorrow morning and get the engineer out. It wouldn't be the first time an engineer has left a Friday job forgetting to turn a critical bit of of equipment back on. Good luck.

Have they left an isolation valve shut somewhere? Eg the ones usually by the HP outside? Pretty poor show though.

Holy crap! Is that wiring center also the controller ie the outdoor unit has no brains at all and it's all inside? That seems like a lot of relays, chips and the like just to connect the various valves, pumps and thermostats.l!

I think the issue is with air escaping. With a drop you essentially get air locks in the pipe. The water is trying to go down and the air is trying to go up. Paradoxically a bigger vertical might make it worse as there is more back pressure from the air lock before any horizontal section where the air can sit above the water. The idea of a tundish always confused me. I get there is a reason for them as an anti syphon device and there is a visual indication argument. Except the visual indication is useless if the thing is in a rarely visited cupboard and the antisyphon argument doesn't seem relevant if the pipe is leading directly outside or even if it's leading to a soil stack. A clear glass section with a properly designed air admittance valve would probably be better albeit more expensive than a tundish.

You can do that too, or you can pump the whole hot water supply - either way you need a pump. On the plus side newer inverter pumps are pretty quiet and can provide nice even pressure without the old on/off/on/of pulsing behavior of the old ones.

If you want decent hot water pressure. The vast majority of the properties I look after are flats so the tank is about 400mm above the shower head so the showers are rubbish. Wasn't a problem 40 years ago when people predominantly had baths but now people expect showers at a decent pressure (coincidentally we have seen a big rise in bathroom damp/mould issues as showers out in vastly more moisture into the air)

Exactly (this is what happened to us). In addition, the approved tundish allows a slight trickle of water to track around the support arms to the outside of thr tundish and onto the floor (again also happened) . I modify the tundishes so the copper pipe can drop 10mm or so below the stop o form a drop edge and stop this. These and various other problems with the venting are what has soured me on UVCs. That said they are neccesary for HPs - the only alternatives are thermal stores, which are inefficient for HPs and vented cylinders, which require pumps. Which it to explain why I keep thinking some sort of relaxation/rewrite on the regs to make it easier to install and actually work on the rare occasions thry are required to work. Now all immersion have the double thermostats/cutouts and boilers also have mutiple systems to prevent them boiling the chances of a boiling event are extremely low (though the consequences are still high)

Fair point

I'm more getting at the routing for the tprv rather than the usual stuff with inlet groups, expansion vessels etc which would all be needed. I'm coming from the perspective of the annual check needed for a vessel that could potentially explode (though most home owners don't get them serviced). If the vessel can't explode then the safety requirements are less critical. Actually, while I think about it, given a significant portion of unvented cylinders probably aren't serviced annually, have we ever heard of one exploding? Or even blowing off and injuring someone? I did see a myth busters where they deliberately sealed one shut and boiled it, and it did go off like a rocket. I'd have thought if that happened we'd hear about it.

Yeah but that's what the usual prv is for. The tprv for unvented has very specific routing and venting requirements to handle near boiling (potentially above boiling) water partially flashing to steam

Good point! The old "nail in the fuse box" fix.

Still not enough to boil a cylinder tho? And if someone fits a co2 HP capable of boiling a cylinder - then it's g3 as per usual. The exemption would only apply to HPs deemed unable to boil cylinders. Could even be a tick box in the spec where the manufacturer declares the HP unable to reach 95C in all fault conditions.

I guess the problem with a prv test is a proper test is to stick the immersion on and boil the tank, but that isn't really safe or practical. The units we had fitted had a knob on the top you turned and that would lift the valve, which isn't really a test of the system only to see if the valve would open. TBH fitting a burst disc would be the safest way but i've never seen one on a domestic system. They do need to get around to sorting the g3 regs so that a system only connected to a HP without any immersion can be treated as a vented cylinder since there is no possibility a HP can boil the tank. That would make installs easier

Years ago spoke to the technical director of reliance controls about "servicing" of their PRVs etc Context was a very awkward tenant who insisted that the valves be serviced every year and wouldn't accept that getting a g3 ticketed engineer to inspect and sign off the cylinder etc was sufficient. Upshot was, there wasn’t anything to "service" - you look at the valve, make sure nothing is blocking the pipework etc and there are no visible signs of leak/degradation. I asked specifically about turning the knob to activate the valve and he said you could but there was always the risk it wouldn't seal back properly and you'd have a persistent drip, which would mean the valve would need replacing. From the safety POV the main thing is that the valve opens when the pressure or temp are too high. They are engineered to be highly reliable and fail safe, ie anything that goes wrong makes them open easier than design. The servicing is more to check they aren't leaking and the venting arrangements are still adequate.

I would suggest that headroom is less important for such a large house. Say we have a very cold snap and your 12kw heatpump isn't able to keep the entire house at 21C (or whatever the design conditions are) Just turn down a few of the unused rooms to 15C for that period. If you have a 2 bed flat and all rooms in use all the time you don't have that ability. But for such a big house the spare rooms can be hibernated for a bit.