DIY Doctor

Advice on Fitting 2nd Under Sink Water Heater

Postby davembk » Fri Jan 25, 2019 1:50 am

I'd really appreciate a bit of advice, I'm a diy plumber who's never flooded the house, not yet anyway. I'm hoping it's been competence more than luck.
About 4 years ago I fitted an unvented under sink water heater in my bathroom. It's got a pressure relief valve on the cold feed side, going into a tundish then out to overflow. All's worked fine.

I've now fitted a second heater exactly the same into another room, same house, same cold supply pipework etc.

I could fit a second tundish etc as well although a bit more difficult due to location etc but no great problem.

The thing is, I was wondering, whether the existing relief valve, could also act for additional heater. I'm just thinking that as the cold feed pipework is common to both heaters, then the existing valve is effectively a shared resource albeit a lot further away from the second heater.

I know I'm ignoring the consequences if say the first heater's cold feed got locally turned off by its close-by in-line isolation valve, but was just wondering if my thinking is basically sound.

I did also buy and expansion tank but
I'm assuming that in normal use any reasonable expansion is taken by the cold feed pipework or back-feed into the mains. I know it's an involved subject as, what about contraction as well, if say the heater cools when the cold feed mains had been turned off, could there be an implosion risk somewhere? Just leave taps open maybe.
The heaters are 5 litres. I'm hoping to put a couple more in, also spaced along the same cold feed pipework.

I could also see a situation where expansion from the heater pushes back up the cold feed which then becomes lukewarm for a short distance but enough for that lukewarm water to be noticed at the cold tap of the sink that the heater is servicing. Obviously all this is taken care of by proper plumbers but I'm hoping I'll get there eventually.

Thanks
davembk
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