Under sink heater

[ajh]

Following on from the "heat loss from hot pipe" thread; our vented hot water cylinder is about 12 metres of 1/2" pipework away from the kitchen sink and takes about 30seconds before the hot tap runs hot enough to do washing up.

I was wondering if any 2kW 10litre under sink unit could be fed from the hot supply pipe to avoid changing the taps?

My thought being when the main hot tank was up to temperature then the water could be run from that through the under sink unit (wasting 10 litres first??) but in summer when the water tank was not being used during the day the under sink unit could be switched on.

[Theo]

If I did my numbers right, a 12.5mm internal diameter pipe (presume 15mm OD) that's 12m long holds 1.47 litres of water. (1.23ml per cm). Even if your flow rate is terrible, I'm surprised it takes 30 seconds - which would be 3 litres/min. Is there perhaps more pipework at the end of the run?

(I suppose some of the first heat from the tank goes into heating the pipe, so it isn't simply a case of replacing cold with hot)

Have you considered insulating the pipe run?

I can't answer the question about running hot into the sink heater, but it would seem unfortunate to now have to run 11.5 litres of water to get hot rather than 1.5.

Theo

[Chris Green]

That's what I have done, twice now, works very well. The advantage of running from the hot water supply is that there aren't any major issues with pressure etc.

[Tim Lamb]

Me too. You need to make sure there is enough pipe length for expansion (no non return valves). Consider if there is a water softener in the hot circuit. Who knows which tap lady chefs use for cooking water:-) (I discovered recently mine uses the Quooker!)

>

[newshound]

I did that too, when I had an underpowered combi with a long-ish pipe run to the kitchen. No longer a problem with an unpressurised tank maintained at a decent temperature from a system boiler, and a pumped DHW feed.

[Fredxx]

If the HW feed is by a tank then no need for expansion pipework or vessel.

[Fredxx]

I've done this because the only way of heating the HW tank was electric. I then only used the immersion if I wanted a bath.

No need for any expansion vessel where you have a vented HW system.

[ajh]

Just for you I timed it, it took 100 seconds to run hot but only filled a bowl with 6litres.

Now I know that is a lousy flow rate and I hope to replace the Cold water cistern this summer and I'll see about replacing the pipe then and insulating it.

[ajh]

That's a positive result then, any recommendations for make?

[Chris J Dixon]

What type of tap do you have? If it is a mixer with anti-backflow valves, that can really slow the flow.

That is the only thing I am not happy about in my "new" kitchen, but in 14 years I still haven't been sufficiently motivated to sort it.

Chris

[Chris Green]

They are Redring from Screwfix but no longer available I'm afraid.