Expansion for 15L unvented water heater

[Gary111]

Hi, I have an old 80litre water heater that is going rusty inside, I am planning to do a direct replacement for a 15litre one.

Currently I have the stop tap, then kitchen tap nearby, then at least 1.8m of 22mm plastic pipework, then a none return valve, then a short flexible tail to the water heater.

I am planning to add a pressure release valve if the unit doesn't come with one.

Is there anything else I need?

Thanks

 

[Johntheo5]

A expansion vessel if mains fed?.

 

[Madrab]

A expansion vessel if mains fed?.

Can be either or for a 10/15L as long as the supply has an uninterrupted run (no check valves/draw offs) back to the closest draw off and the supply is a certain distance on 15mm pipe for the expansion.

If that criteria (distance from draw off etc) can't be met then a small EV needs to be used but that will normally also need a Pressure Reducing Valve and check valve setup as well as the PRV with a tundish.

 

[Johntheo5]

The first diagram below allways seems a bit strange to me, they seem to say that 2.8M of pipe for 5&10L cylinder and 4.2m of pipe for a 15L cylinder will take up the expansion with a NRV installed, I thought this was the distance to avoid hot water issuing from a cold tap with no NRV installed where the water can expand back through the mains.

 

[Madrab]

As far as I remember - you touch on this when doing the UVHW (G3) - the distance is to allow for expansion where the mains is not over 3bar. Anything higher than that and a Pressure Reducing Valve and EV is required. Most of the installation docs do quote the distance to a cold water outlet, as you suggest, to stop the tap being over pressurised and where no check valve is installed. If a check valve is inline and pressure was over 3 bar then they did require an EV and a PRV to be installed.

Manufacturers always seem to be updating the specs though and I assume this will be to do with more robust materials and design testing proving that other specification will meet the requirements. It will be interesting to see how the new Unvented courses (HWSS) have been updated in line with of the new G3 requirements

 

[Johntheo5]

I'd be amazed if a PRV didn't lift with 2.8M between the heater and the NRV, water is incompressible and 10L of water will expand by 0.16L in heating from 15C to 60C so where does it go?. All combi boiler manufacturers with DHW preheat are now specifying that a (DHW) EV be installed if a flow meter or NRV is installed in the cold water supply because the PHEs were rupturing/leaking during preheat.

 

[Gary111]

Thanks for your help, it's for a first floor flat, it seems like the best option is to use a hotun?
Otherwise I would need to run a pipe down to the grid outside?

The existing water heater discharged to the soil pipe so I guess I can leave that in place still and use the hotun.

 

[Gary111]

Hi again guys, it seems that the tundish needs to discharge into 22mm pipe.
Unfortunately the existing pressure relief pipework which is set into a stud wall then under the floor is 15mm.

The hot water out is 22mm.

Should swop them over to use the existing 15mm prv discharge pipe as hot out instead and use the 22mm got out as the prv discharge?

Thanks

 

[Madrab]

Hi again guys, it seems that the tundish needs to discharge into 22mm pipe.
Unfortunately the existing pressure relief pipework which is set into a stud wall then under the floor is 15mm.

The hot water out is 22mm.

Should swop them over to use the existing 15mm prv discharge pipe as hot out instead and use the 22mm got out as the prv discharge?

Yep, D2 is always at least a pipe size larger.

Could you swap them over, I guess so, a 15mm HW outlet would be fine, length of run dependent but also need to know where the 22mm will terminate.

 

[HERTS P&D]

What is this 15litre water heater supplying?

Andy

 

[Gary111]

A kitchen sink, bathroom sink and possibly a shower, probably just for 1 occupant though.

 

[Madrab]

Sink/basin, no bother - Shower nope, you'd need an electric shower for that. The 15L store won't be enough for a shower and with its 3Kw element, then once the stores used up there won't be much in the way of hot water.

 

[HERTS P&D]

Hey Gary,

Are you a landlord by any chance?

Andy