Is 15 litre unventrd cylinder prv discharge ok going to a soil pipe?

[Gary111]

Hi, I am replacing an old electric unvented water heater with a 15 litre one.

The current pressure relief pipework runs into a soil pipe, the room is second floor so I guess they didn't want too long a pipe run down the wall to a drain.

There was no tundish fitted which I guess I will need to fit.

Will that cause smells from the soil pipe?
Or any other problems?

Thanks

 

[Madrab]

Depends, if it's just expansion then possibly ok as it wouldn't notmally be subject to excessive heat even in a fail condition. The MI will usually specify tho so best to check that, make and model?

Yes, if there is a direct connection from the soil pipe to the bottom of the tundish then there will be a risk of sewer smells if it isn't trapped or a one way valve fitted (Look at Mcalpine's Tun5-CL)

 

[Gary111]

Thanks Mandrab, is there some reason that the tundish needs to drain into 22mm pipe?
its currently 38mm.
also it seems like all it does is create an air gap, which it seems I can do myself without forking out 15 quid for one?

 

[Madrab]

All tundishes are designed where the D2 pipe is always at least a pipe size larger than D1, it's to ensure unrestricted/free flow of any discharge from the tundish. Most cases there are more than one pipe feeding the tundish, not so with your installation but the rules don't change.

The tundish is there to provide a visible indicator that a safety valve is releasing therefore showing there is a problem that needs fixed. The air gap is there to stop contamination if water backs up in the outflow pipe and also acts as a fail safe and ensures that if there is was blockage in the D2 pipe or further down that the discharge can still be released, .

 

[Gary111]

Hi again Mandrab, thanks for your help.
I am wondering if I can use 21.5mm solvent weld overflow pipe for the discharge as it's cheaper than 22mm barrier pipe?

 

[Madrab]

It's all about temperature, if it's the cold side expansion then overflow pipe may be ok. Wouldn't be acceptable if it's to cater for the possible temperatures being released from a T&PRV. So depends on what is attached to the unit.

The rules need to be followed whether that be in the form of the MI or the Unvented standards, what make/model is the new unit?

 

[Gary111]

Thanks, I'll do it in 22mm then, it's an Ariston 15litre one

 

[Madrab]

Again if it's a PRV from the cold side for expansion then it should be ok. if it's for a high temperature T&PRV then 22mm barrier pipe wont cut it either and then it would really need to be 22mm copper continuous or >=300mm copper into HT PP/PB plastic or cast.

Again though it should be quite clear in the MI

 

[Gary111]

Thanks so much, does it normally need to be compression fittings on the copper also?

 

[Madrab]

If it for heat resistance then yes either that or soldered, push fit, like the pipe, isn't deigned for much over 70DegC