Pressure relief pipework

[Daljit]

Everything i've read tells me pressure relief pipework should be routed continually downwards, ending in a safe position outdoors. Thing is, i have a top floor flat and it would be much simpler routing copper pipework vertically for 8 ft and then horizontally in the loft and out through the eaves.

Is this a huge no-no?

Would an alternative be to upgrade the condensate pipe (which runs continuously downwards into the kitchen waste) to high pressure stuff and simply connect the pressure relief pipe to this beneath the boiler?

Daljit

[John Stumbles]

You mean share the condensate waste with the PRD? That would be a massive no-no! I don't see any problem with having the PRD pipe rise from the boiler provided it's not liable to freeze. The main thing is that it must be unobstructed at all times.

[nightjar]

Pressure relief pipework must lead to a tundish, not more than 300mm from the pressure vessel. That is to ensure that the pressure quickly drops to atmospheric and is not kept at dangerous levels by flow restrictions in the outlet pipes. There will, therefore, be no pressure to drive the escaping water / steam mix up the vertical pipe. They lead down because you need gravity to remove the fluid.

BTW, the exit from a tundish is always one pipe size larger than the inlet, so if the outlet from the vessel is 15mm, the tundish will have a 22mm outlet. If it is 22mm, the outlet will be 28mm. If it is 2½ inches, the outlet will be 3 inches etc.

A domestic plastic waste pipe will not have sufficient resistance to the high temperature of escaping steam. I think the regulations specify that the vent pipe must be metal, but I don't have my copy to hand to check that.

Colin Bignell

[Jim Alexander]

"nightjar .uk.com>"

I stand to be corrected but I have never fitted/seen fitted/had a boiler failed on inspection a tundush to a system boiler. Neither have I ever seen

22mm relief pipework outside a boiler location.

Yes I have a tundish on my unvented cylinder.

However your comment about no pressure to drive the water up the pipe is right which makes the OP's request and support comment wrong.

Jim A

[adder1969]

Yes I think the above relate to unvented cylinders. Sealed heating systems are unlikely to go bang in such a dramatic way I would guess. Can't the PRV exit through a wall? Does the flue go through a wall?

[Daljit]

I don't know if this makes a difference, but it's a combi boiler with a built -in expansion vessell.

Yes I think the above relate to unvented cylinders.

Flue goes vertically through the roof.

[nightjar]

The last size I gave is a clue to the area most of my customers work in, if a bit on the small side for some of them, but I certainly know of domestic boilers that have a tundish. However, the wording of the Water Supply (Water Fittings) Regulations 1999 is a lot less specific than the Water Byelaws and I don't have a copy of the latter to hand to check what was required when the systems I am think of were installed.

Tundish design does require the outlet to be one size larger than the inlet, as can be seen from the sizes of the two about 1/3 down this page, so a system with a tundish would have outlet pipework at least 22mm diameter.

are plastic, so I may well be wrong about the need for metal pipes, but I doubt they are made of the same plastic as domestic waste pipes.

Colin Bignell

[Simon Stroud]

Interestingly, especially in the light of what others have said, my Man MICROMAT (Eco Hometec-badged) condensing combi boiler has only a single outlet for condensate drain AND pressure relief - the output of the relief valve (inside the boiler) goes down a little clear plastic bendy hose and joins onto the main (also plastic) outlet. They do say that the combined outlet (kind of 22mm, but with a "push-on" connector on the output) must go via a tundish, which I have done, but this then just goes into a 40mm plastic waste pipe to the drain. No special metal pipe out through the wall for me!

Regards, Simon.

[John Stumbles]

Depends if it's a PRD from a sealed heating system (usually a system or combi boiler) or if it's from an unvended DHW system. My bad that I just assumed the first and didn't check it out with the OP. (Your bad that you assumed the second ... ;-))

[Daljit]

Depends if it's a PRD from a sealed heating system (usually a system or

I'm getting a little confused... i have a direct, sealed combi system, with a built-in expansion vessel Is it critical that the PRD pipe slopes downwards?

thanks

[Jim Alexander]

YES

though you might have a different interpretation of critical. However I don't think you've mentioned which model of boiler you are getting. The installation manual may offer some guidance/requiremements for correct installation. It's your risk if you fail to comply.

You got some guidance, IMHO wrong, that it was OK to go vertical. The guidance was sensibly qualified that freezing must be avoided. I very much doubt that you would be able to avoid the prospect of freezing PRD discharge in the loft section. Its a seriously bad idea.

Maybe a pipe through the external wall is not as difficult/expensive as you imagine. Another poster has mentioned a particular boiler option that may not need an external PRD discharge.

Think again.

Jim A

[The Natural Philosopher]

There is a world if difference between the output of a hoiw water tank, which under fauklt conditions may go to well over 100C and 250 liters of it too..before the valve blows...and a piddling combi, which is barely capable of generating a lukewarm stream of piss even going flat out.

The Combi, if the primary blows, will only have a few liters of CH water to vent. And if the how water circuit blows, there is no large volume of stored water to vent off - again just what is in the pipes. It wouldn't be enough to heat up the vents.

MY BCO insisted I put guards round the 28mm copper vent pipe from my tundish..

[Ed Sirett]

I need to look up the BS for this. It might be in BS 5449. At the back of my mind is a small allowance for 'impossible' locations for the discharge pipe to rise. However at least one manufacturer requires a continuous fall.

The requirements for a PRD pipe are much less onerous than for an unvented which must be able to take a very substantial flow rate of very hot water.

[Daljit]

The boiler is housed in a cupboard in the middle of a flat with windows to the front and rear. The external walls are two foot sandstone. It's not impossible to route piping under floorbds for about 25 ft and bore thru the rear wall, but a lot of work, particular as the bathroom floor has been laid.

I'll rig it up thru the loft, as described, and see if it passes a CORGI's landlord safety certificate.

thanks for all advice

Daljit

ps FWIW, it's a Potterton Gold HE

[Owain]

"High Explosive"?

Owain

[Jim Alexander]

OK, your decision.

You have chosen to ignore one requirement of the boiler installation instructions. There a few other requirements as well...

"2. The pressure relief discharge pipe should be not less than 15mm dia, run continuously downward, and discharge outside the building, preferably over a drain. It should be routed in such a manner that no hazard occurs to occupants or causes damage to wiring or electrical components. The end of the pipe should terminate facing down and towards the wall.

1. The discharge must not be above a window, entrance or other public access. Consideration must be given to the possibility that boiling water/steam could discharge from the pipe."

Maybe you need uk.legal to debate *should*.

Jim A