ASHP low pressure help pls

[Beelbeebub]

1 hour ago, Dillsue said:

OK, but that's the shower that may need a pump, not a vented cylinder. We're open vented for CH and DHW and have a pump only on an indulgent en suite shower. Rest of house incl a second first floor shower isn't pumped.

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.

[Beelbeebub]

20 minutes ago, sharpener said:

 

As installed mine had a compliant 22mm D2 setup taken into a first floor soil stack via a Hep2O bladderless trap.

 

Thinking this was where the restriction was I modified it so there was first a 2.5m drop to the ground floor and the trap went into the stack at that level instead.

 

But it is not much better and will still not take the full flow from the tprv. 

 

So I think that the whole concept that the pipe below the tundish needs to be only 22mm - one size up - is flawed, the regulations do not achieve their aim and are pointless. And a tundish hidden away in an airing cupboard slowly filling with water is NFG either.

 

Personally I think it would be better to have the relief valve piped in 15mm directly to waste with an electrical or mechanical alarm to indicate there is something wrong and possibly a small air admittance valve to vent the pipe but prevent smells. 

 

As a further twist the MCS surveillance visit on another house meant the HP installer was called back to move the tundish to somewhere slightly more visible (even though it was pre-existing and not part of his work). The D2 pipe is 28mm bc of the run length but has a 28mm tundish that will overflow too. So the modification is purely cosmetic, the tundish is still in an alcove off a loft above our bathroom, access is still via a ladder and a door which is kept shut so we will neither see or hear any overflow condition. Which is why I test the tprv whenever I am up there, at least I know it is not stuck and the valve seating is washed clean.

 

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.