Outlet from a boiler PRV

[Bob Minchin]

I'm planning the plumbing side of a new boiler install (Gassafe man will be doing the gas side) It is a standard W-B boiler 24Ri I have an external expansion vessel fitted with a PRV I'm not sure on the requirements for the discharge pipe from the PRV. Ideally I'd like to go straight through the wall in copper and down to a rainwater gully terminating the pipe just above the gully grating. I feel that this will give an air break to stop back siphoning and a visible means of noticing a leak. Any jet of hot water will be directed downwards safely into a trap full of cold water.

PRV has a 1/2bsp outlet, so I assume 15mm copper will be adequate?

The WB installation manual shows a pipe through a wall with a simple downward facing bend as soon as it exits the wall - seems more dangerous than my method

Will my method be OK?

Searching through building regs, G3 talks about discharge from unvented hot water tanks and the need for a tundish etc but I've yet to find chapter and verse on boiler PRV discharge and I really don't want a tundish indoors.

TIA

Bob

[Onetap]

Yes.

You need a tundish on an unvented water heater because it's permanently connected to the mains water supply.

A boiler should lock out on low pressure after discharging the volume of water in the expansion vessel, so no continuous waste of water.

[Gazz]

Think the usual method is to take a bit of copper pipe through the wall, then put 2 x 90 degree bends on the end, so any discharge is directed against the wall,

There may be regs on where this can be situated, mine is about a foot to the left of the front door and about 2 foot higher than the door top, mind, the cowb.... plumbers fitted the condensate drain to go out the wall above the lintel above the front door, then run along the top of the lintel to the end, and that's it,

Never had it freeze up, but the acidic water is turning the brickwork white where it runs down the wall, and the rain ledge part of the lintel fell off about 6 months after the new boiler was fitted.... it was already loose due to water getting in and rusting the rebar, thus expanding and cracking the concrete, so i can only guess condensate water filled the cracks up and froze, and crack, off came the rain ledge bit,

btw, rented house, and the landlord isnt too fussed, otherwise i'd have changed the arangement myself.

[Tim+]

than my method

tundish indoors.

Cowboy installation! I rather suspect that the landlord must have been the installer or else he *would* be fussed. Still ought to be but he is clearly not that smart.

Tim

[Bob Minchin]

connected to the mains water supply.

water in the expansion vessel, so no continuous waste of water.

Thanks Onetap

[John Rumm]

Yup that sounds fine.

The usual idea is to minimise risk to someone close to it. So into a gully as you suggest is perhaps the best solution, however that is frequently not available. So a short drop and turning it toward the wall is a common alternative.

No need for a tundish on a boiler pressure relief valve - its connected to the primary circuit which is not connected to the mains water supply, so there is no back contamination risk (as there would be with an unvented cylinder). The systems usually have other ways of telling you it has operated - typically by not firing next time due to lack of pressure!

[Bob Minchin]

I wonder if I can use a simpler option still. I plan the bring the condensate through the wall to coincide with a rainwater downpipe and drill a hole in the back of the pipe and feed the condensate pipe in through a grommet. I feel this is a good method as there will be minimal exposed pipework to freeze eg about 75mm. The rainwater has its own soakaway which I understand is acceptable for the condensate.

I could use a similar method into the same pipe for the PRV. On the assumption that there will only be a short spurt of return temperature water until the pressure drops below 3bar, this should be perfectly safe as well as being discrete.

Any thoughts?

tia

Bob

[John Rumm]

I would be wary about taking a narrow bore condensate pipe outside, even only for a few inches. It would be ok if you transition to something bigger first though...

It will be safe, but there is some argument that its advantageous to be able to see the end of the pipe for cases where the PRV opens and does not reseal correctly.

[Bob Minchin]

OK Thanks John - points noted.

For the condensate pipe, I could fettle an offcut of the 68mm downpipe to fit tightly between the wall and the curved surface of the pipe and fill it with insulation. Should prevent freezing and look even neater. Shame that they dont make T joints for rainwater pipes.

I think I'm fixated on a discrete solution as next door had cowboys in and their condensate pipe has a long run externally going around window frames etc in 22mm plastic with interior grade grey foam insulation. Looks firkin awful!

[John Rumm]

Yup, that would probably do it.

Yup, I can imagine...

I just took mine into a tee I put into the main 40mm pipe after the basin, and WM traps etc. (the boiler also has a trap that empties syphonically every so often rather than allowing the condensate to "dribble" out - so that also helps make freezing less likely)

[Bob Minchin]

Thanks John, My nearest waste pipe is about 5 metres away and the run would be through a load bearing pier and all behind kitchen units so that ain't going to happen! I think my boiler (WB 24Ri) has some sort of siphon built in the trap. It is being delivered in the next few days so I can have a look in detail. I might add a few turns of insulated wire around the pipe under the insulation so that if I do have a problem I can connect a very low voltage-lots of amps supply to warm the pipe up a little.

Bob

[Gazz]

most PRV's have a red knob on the end, you turn the knob and it lifts the valve off it's seat, but it also rotates the valve too, if you keep turning it, at half way it will snap closed again,

when i had a motorhome with a calorifier in it, the PRV came with instructions to turn the red knob a couple of turns every now and then to ensure any lime scale deposits are broken up and flushed away, but that PRV was operated by pressure every trip, as the calorifier would be full, and as the water in it was heated to about 90 degrees from the engine, it had to expand, despite the 5 litre expansion vessel, which had to be set for the more normal 75 degree temps the eberspacher took the calorifier upto,

So i guess more chances of a bit of limescale getting caught between the valve and it's seat... but i only recall it weeping 2 or 3 times in the 7 years i had it.

Hence if the valve operated normally, and then didn't re-seal after you fixed the fault that caused it to blow, give the red knob a few turns to see if it'll clear what ever is stopping it sealing.

[John Rumm]

Indeed many combi valves are of a similar type.

On combis it recommended that the valve not be manually operated. Since its on the primary side of the system its not in contact with fresh water, and so scale ought not be a problem. However there is the risk that debris in the primary circuit can get on the valve seat and stop it from resealing correctly, causing persistent pressure loss problems.

Fresh water heating systems have different issues. Hence why part of the service procedure on an unvented cylinder is to manually operate the pressure relief valves to test them and ensure they are not scaled.