Expansion vessel on unvented hot water tank question.

[Collectors]

Hi, we are looking at getting a 2 emersion pressurised hot water tank fitted & it will need an expansion vessel. Will it harm or help the hot water if we fitted a larger one.
Q&A 2 if i can. Our boiler is in the loft next to where this new hot water tank will go & has an 1"&1/2 waste pipe going from the boiler to the bathroom below & joins on to the Bath waste & into the drain on our bungalow. This is from a pressure valve on the boiler. This new tank says in the instructions to run a 22mm coper pipe direct to the drain. Could we not get away with using the same waste as the boiler. ??? As it will be a real pain to get a copper pipe to the drain. Thanks

 

[oldbuffer]

1. I assume you mean "will it harm or help the hot water if we had a larger one fitted". You are not allowed fit an unvented hot water cylinder yourself, they may only be fitted by someone with a current "G3" registration. To answer your question, it will do no harm to fit a larger expansion vessel, but neither will it do any good. The expansion vessels are normally white, sometimes blue, but are not the same as the red ones you get with a boiler.

2. The pipe which is meant to go direct to the drain from the unvented cylinder is known as the "D2" pipe. It is a vital part of the safety system for the pressurised cylinder. In fault circumstances it may have water at a higher temperature than 100 degrees in it, as pressurised water boils at a higher temperature than normal. It is normal to have this pipe in copper for this reason. It is permitted to have the pipe in plastic, but it must be special plastic which can withstand high temperatures. It is very unlikely that an existing waste pipe will meet this requirement. Depending on the length of the pipe run it may need to be of a bigger diameter than 22mm.

3. On installation an unvented cylinder must be registered with the local council, a task normally handled by an approved installer. It should also be given an annual service by a G3 registered person to ensure all safety systems are operating satisfactorily.

 

[OvertonPlumber]

I'm bored so I'll dispense with the bullshit.

Boiler discharge shouldn't be put into waste should be run outside in copper. Or on a dedicated connection ie not shared with anything direct into a soilpipe.

All unvented cylinders come with the expansion vessel when new. Although if you get a megaflow, santon or rm internal or gledhill internal you don't need one. As they rely on an internal air bubble.

Unvented discharge the d2, from the tprv and multiblock should be in copper to outside but could be onto a flat roof for example. If it goes into a waste system it has to be on a dedicated leg and using mupvc pioe ie pushfit/mccalpine. It cannot be teed into a bathwaste.
You could connect it all into a high temp condense pump.

 

[Collectors]

Ah! well done & thanks. The push/glue fit pipe going from the boiler in the loft & connected to the bath waste is dedicated waste pipe all the way to where it connects to the bath which is about 1 meter from the outside drain. So if i separated it from the bath waste & made it direct to the drain, would this be ok. Oh! The waste pipe to the boiler i now see is just from the ultracom condensation trap. Number 10 in photo & is just a flexible plastic pipe from the boiler.
The tank we was looking at is a Telford 200l slimline & comes with a seperate expansion. Mainly as it will fit through the loft trapdoor at only 470mm wide.

Telford Tempest Slimline Direct Unvented Stainless Steel Cylinder 200 Litre​

 

[muggles]

It needs to be high temperature rated plastic, so either MuPVC / PVC-U solvent weld (not the standard ABS sold in most stores) or push-fit polypropylene. Standard ABS solvent weld waste pipe will soften, buckle, and potentially split in the event of a high temperature discharge.

 

[Gasguru]

PVCu/uPVC is standard waste pipe...needs to be muPVC or cPVC ...ie. modified or chlorinated PVC.

 

[Collectors]

PVCu/uPVC is standard waste pipe...needs to be muPVC or cPVC ...ie. modified or chlorinated PVC.

Must admit i can only find 32mm MuPVC pipe & fittings. Which is ok as its a 9m run with 2x45 & 2x90 sweeping bends, so boarder line for 22mm.
Hopping i can find a 22 to 32mm reducer in plastic. MUPVC. Thanks for the tip, & looks like we are running a 32mm MUPVC pipe to the drain.

 

[Madrab]

TBH I think your qualified Unvented engineer may be the best person to describe all that's needed. An unvented cylinder is a regulated appliance and as such all of this should be planned, set out and installed by them. Don't forget this is also notifiable work and as such will require an LABC warrant/sign off or self certified by the installer if they are part of a competent installer scheme.

 

[Collectors]

TBH I think your qualified Unvented engineer may be the best person to describe all that's needed. An unvented cylinder is a regulated appliance and as such all of this should be planned, set out and installed by them. Don't forget this is also notifiable work and as such will require an LABC warrant/sign off or self certified by the installer if they are part of a competent installer scheme.

The young fellow fitting most of it is fully qualified, but i will be giving him a hand. He is also not the easyest peron to contact. Plus he is not VAT registerd & i am, so i can claim it back.

 

[Gasguru]

If he's not easy to contact (within normal working hours) surely it's time to bin him off.

You should also note that a dedicated branch fitting must be utilised on the stack...a strap boss (and equivalents) are not permitted. This is from the BPF that represent most of the main pipe manufacturers.

Have a look at Hotun waterless tundishes..they have become the standard for these applications. Either 22mm copper or 32mm (1 1/4) push fit outlets are available. Their website has many technical articles that will prove useful.

Unfortunately the G3 regulations are woefully inadequate and are considerably out of date. The same can be said of the G3 course and assessment. Get yourself fully aquainted with the regs and work with your installer to ensure compliance.