Unvented 10l undersink heater to fit to gravity system?

[davidsavage]

My kitchen is a very long way from the hot water tank and so approx 12l of water is run off each time the hot tap is used. I would like to fit a small (10l) unvented water heater under the sink and supply it from the low pressure hot water supply to the kitchen. We never use more than 5l hot water at a time and this would avoid wasting so much cold water each time.

Do I still need to fit a pressure relief valve and tundish drain to the inlet to the heater; under normal working conditions expansion will simply take place up the supply pipe which of course is open as eventually it expands into the cold water tank in the loft via the overflow from the hot water cylinder. Even if the thermostat were to fail the undersink heater would discharge into the low pressure 'hot' water supply.

Is it OK to do this? Can anyone think of a reason why not?

Thanks for your help.

 

[gigz]

I would phone the makers of the unit.

 

[bolshy]

So instead of wasting 12 litres of hot water, you will waste 5 litres flowing into the hot water pipe at source and use electricity to heat the 5 litres you've got from the tap, plus reheating the stored 10 litres back up to set temperature?
I understand the principle behind the idea, but apart from not complying with the manufacturers safety instructions for the installation of the heater (as posted, check with them to see what they recommend), I believe you are wasting more money. Particularly if your cylinder is heated by a gas boiler, leccy is 3 times the price.

 

[flameport]

We never use more than 5l hot water at a time

If that 's true, fit a 5l undersink unvented heater, connected to the cold supply in the method stated by the manufacturers.

 

[davidsavage]

Thanks Flameport; If I connect to the cold supply, I will need to fit an expansion vessel as the heater will be very close to the cold water take-off? Is that correct?

 

[davidsavage]

So instead of wasting 12 litres of hot water, you will waste 5 litres flowing into the hot water pipe at source and use electricity to heat the 5 litres you've got from the tap, plus reheating the stored 10 litres back up to set temperature?
I understand the principle behind the idea, but apart from not complying with the manufacturers safety instructions for the installation of the heater (as posted, check with them to see what they recommend), I believe you are wasting more money. Particularly if your cylinder is heated by a gas boiler, leccy is 3 times the price.

That is worth thinking about on the economics; I was concerned on the safety / legal aspect. Thanks for your prompt response.

 

[gigz]

Thanks Flameport; If I connect to the cold supply, I will need to fit an expansion vessel as the heater will be very close to the cold water take-off? Is that correct?

Not always, depends on the length of the cold feed, you will need to fit the prv though. What make and model are you planning to fit ?

 

[DP]

Plenty answers but some are way off the mark

If there is no check valve on the cold feed to the house, and water you are heating is less than 10l (this needs firming though as there is a limit as to how much expansion the service pipe will acomodate), no expansion vessel needed and almost certain no prv needed either. As earlier posted suggested, good advice, call the manufaturers an seek guidance

 

[gigz]

If it comes with a prv then it will need a prv .

 

[DP]

Gigs, that goes without saying

 

[RickyLee53]

Strange, I asked the same question a few days ago. I couldn't find anything wrong with feeding it hot water.

They come with a PRV that I plumbed into an extra port on the trap. (i.e.) dishwasher/washer port.

If you genuinely only need 5 litres of water fit the 10l straight to the cold. I personally think we will need more. Although it only takes 15-20 minutes to get warm again. There will be a time when you want to fill the sink twice or wash your hands.

You can get a 15l unit for not much more money/size etc.

I have this one at work. We have it storing water at the max temp, then you can mix with cold. The only issue is when you have drawn off 10l or so, its pretty cool anyway.

I'd rather have higher pressure hot water, (bare in mind these are 15mm connections).

 

[gigz]

They come with a PRV that I plumbed into an extra port on the trap. (i.e.) dishwasher/washer port.

The prv is supposed to go outside (if it starts passing you wont know and your hot water will go down the drain) i was told the washer on the prv is designed for cooler temps and if you fed it with hot water it would perish quicker (been told that but not 100% certain)

 

[RickyLee53]

They come with a PRV that I plumbed into an extra port on the trap. (i.e.) dishwasher/washer port.

The prv is supposed to go outside (if it starts passing you wont know and your hot water will go down the drain) i was told the washer on the prv is designed for cooler temps and if you fed it with hot water it would perish quicker (been told that but not 100% certain)

I think you would hear it or see steam as its before the trap.

Surely its just a "normal" washer, no doubt it will have a shorter life, but only like any other appliance that has hot water entering it.

 

[gigz]

Thats why i said to OP phone makers to be sure. As for terminating the prv, if it has just a very small drip you wont know but will waste hot water (and leccy). If a jobs worth doing blah de blah de blah