Pressure relief drain for heat pump heating circuit.

[sharpener]

We need to install a pressure relief drain for a new heat pump heating circuit.

 

Previously there was an oil boiler, we have just realised that the previous owners never had anything connected to the pressure relief valve so for 25 yrs it was potentially venting into the utility room!

 

The boiler does have a condensate drain plumbed to what looks like 19mm overflow pipework that goes into a 50mm waste stack in a narrow void next to a cupboard. This combines two sink wastes and looks like it is done in solvent-welded plastic.

 

So given that the HP will be run at a max of 55C is it acceptable to terminate the pressure relief line to the existing arrangement?

[Kelvin]

I thought it had to go outside? 

[sharpener]

 

24 minutes ago, Kelvin said:

I thought it had to go outside? 

 

Well the G3 requirements for an unvented cylinder which is what you may be thinking of don't actually require that, any discharge must be visible either at the tundish or at an outside discharge point.

 

What we are talking about here is different, it is a discharge from a pressurised heating circuit which cannot exceed 75C, bc that is the absolute max of the heat pump as it has no auxiliary heating, and will in operation be limited to 55C bc that will give us an adequate hw temperature of 50C.

[markocosic]

Practically: sounds fine!

 

Worst case of the branch is clogged it'll only make a minor mess. 

 

Parents place in wales also never had a space heat discharge for 25 years. (empty fitting into airing cupboard) Spotted when draining down for the replacement boiler!

[Hastings]

I believe it is acceptable in Scotland building regs to plumb this to a drain stack rather than directly to outside but I may be wrong. The main issue though is how to prevent gases from the soil pipe entering the house - normally a u-bend trap is fitted but this will dry out and fail without regular topping up - a dry trap, fitted vertically, is better.

[JohnMo]

10 minutes ago, Hastings said:

The main issue though is how to prevent gases from the soil pipe entering the house

If it's not going outside, then use a waterless drain trap.

[sharpener]

Thanks guys.

 

Plumbers have in the past seemed quite happy to fit a Hepworth bladder trap onto my soil stack at first floor level for a boiler P & T relief. But it seems to require quite a head (>2m) to open it up, this caused the D2 drain line to back up to the tundish. So I relocated it to the bottom of the soil stack but it still does not drain freely.

IIRC there are some combined tundish and waterless traps available now, perhaps this is the way to go. Ideally it would have an outlet for 22mm D2 not 32mm waste pipe, is there such a thing?

Edited July 7 by sharpener