Efficiency of in sink water heaters

ISTR some one cleverer than me, not hard, recently laying out the maths of energy required to heat water. Could someone point me in the direction of that post please.

I'm curious to debunk some astounding claims being made about them.

I'm referring to the type that heat the water as it flows. Not the type with a water tank.

Many thanks

Reply to
fred
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The physics is invariant - it costs exactly the same to heat 1kg of water through (say) 90K.

The only argument for instantaneous heaters is they only heat what is needed (plus a little in the block). OTOH they are rather more expensive than a kettle, which can also heat small amounts.

Reply to
Tim Watts

Basically they are just about as efficient as you can ever hope to make it with just a small volume of water flash heated to the required working temperature as and when needed. Entropy says this is *the* way to do it. Pushing heat into already hot bulk water is harder.

The clever ones even wrap the incoming cold water around the heater so that essentially no heat is wasted by escaping to the outside apart from the thermal interia of the hardware itself.

The only thing is they need an insane current supply to do it and they are more complex and perhaps inclined to go wrong if you are unlucky.

Even the ones with a small water tank aren't all that bad since the tanks are typically very well insulated. Snag is when someone comes along and opens the tap full bore waiting for it to run hot.

Reply to
Martin Brown

Although that is true it also costs money to keep bulk hotwater at a high temperature on the off chance that some of it might be used.

Reply to
Martin Brown

Provided that you have soft water. The life is very short otherwise.

Reply to
Capitol

The problem with using them for drinks is that you have to run off the cold water, then fill the cup to heat it, then throw the water away and refill the cup. It's very inefficient compared to a kettle with the right amount of water in it. Must be soft water.

Reply to
Capitol

Going off, or maybe veering off at a tangent, has anyone had any experience of insulated kettles like (random googling) vektra.co.uk ???

These would seem to be a good compromise between the complexity and expense of the sink heater and the frightening inefficiency of the kettle (especially when the latter is used to heat 1.5 l of water for two mugs of tea...

David

Reply to
David

"We guarantee that the water in your Vektra kettle will stay hot for up to 4 hours after it is first boiled."

You have to laugh, don't you.

Reply to
Mike Barnes

I am thinking about getting one, not for drinks but for washing up. Our kitchen sink is so far from the HW tank that it takes several minutes to get hot water. One of these devices installed under the sink might stop my wife complaining. We have very soft water, and anyway it would only need to be hand hot, say 45 degC.

Reply to
Big Les Wade

You eliminate the heat losses /standing losses from the storage tank. Depending on how much hot water you use and if you use the water immediately or not. If you leave this type o fheater permenently on, you will waste a lot of energy. Especialy if your hot water needs are low.

I have this type but I use the water immediately it is hot and (main reason for having them) I use them from my solar PVpanels so water is available in the evening.

Also the hot water pipe run is short, this savesenergy too.

Instant heaters need a big electricity supply or they only heat the water fast enough to provide a miserable dribble. So, commonly used for spray taps for hand washing. But very little heat losses.

Reply to
harryagain

Filling the cup and throwing away would be the same with a kettle

Reply to
Tim Watts

ce of insulated kettles like (random googling)

se of the sink heater and the frightening inefficiency of the kettle (espec ially when the latter is used to heat 1.5 l of water for two mugs of tea...

Not sure I see the point of them. Why not just boil the exact amount of wat er you want? Simply transfer water from tap to kettle via your mug.

A mugful takes 2 minutes at 800w to boil = 1.6kWmins, cost = 13p x 1.6/

60 = a third of a penny. To pay back the £65 excess cost would take 3x6 500 = 20,000 mugs of water not boiled due to the insulation. So just anot her greenscam product.

NT

Reply to
meow2222

ence of insulated kettles like (random googling)

ense of the sink heater and the frightening inefficiency of the kettle (esp ecially when the latter is used to heat 1.5 l of water for two mugs of tea. ..

ater you want? Simply transfer water from tap to kettle via your mug.

6/60 = a third of a penny. To pay back the £65 excess cost would take 3 x6500 = 20,000 mugs of water not boiled due to the insulation. So just an other greenscam product.

Or perhaps just another greenscam price. A vacuum flask only costs £3 aft er all. Adding £3 to kettle cost would only need to avoid 900 cuppas boil ed, with a long lived product that may make sense.

NT

Reply to
meow2222

The water in the kettle is hotter.

Reply to
Capitol

I thought the in sink ones dispensed near boiling water?

Bit useless for tea otherwise...

Reply to
Tim Watts

I fear you've mis-read my post. What I want know is how much energy (electricity) is required to bring cold water, say @ 10 degrees up to 90 degrees as it flows.

Or to put it another what electricity supply would be needed to do this and what flow rate would be achievable with a standard house-hold supply. (Single phase)

My experience of 'Instant' water heaters is that they provide a miserable dribble if you require hot hot water though I think that was from a 13 amp socket.

Reply to
fred

What flow rate do you have in mind?

This is the equation - so you can work it out:

Assuming S, specific heat capacity of water is 4200J/kgK, T is your delta-T (90-10 in your case) Mass of 1 litre is 1kg - near enough

E is the energy to heat mass m of water through T F is flow in kg/s which is helpfully more or less l/s too

So

E=mTS

Given a time t

P=E/t F=m/t

So P=mTS/t m=Ft

=> P=FtTS/t => P=FTS

P is in J/s = Watts

and

S=4200J/kgK So

P=4200x90xF P=378000F or, more usefully, converting to kW

P=378F kW

or

F=/378

So a 10kW heater will give you a flow of 0.026kg/s or 1.59kg/min

A 3kW heater will give you: 0.008kg/s or 0.48kg/min = about a pint/min

Does that help?

Reply to
Tim Watts

3 kW / 13 amp will give you a gentle spray of warmish water for hand-washing, not for washing up. 7 kW / 30 amp will give you a weedy-but-adequate shower except in the coldest weather (lowest incoming water temp) 10 kW / 45 amp will give you an adequate shower in almost all conditions.

for comparison the smallest gas combi boiler is usually 24 kW. That will give you a good shower or hot tap flow for washing up (at one tap at a time).

Therefore if you want *hot* water for washing up, you really either need gas, or stored hot water even if it's only a 15 litre unvented heater.

Owain

Reply to
spuorgelgoog

They are not hot enough for a tea bag in a cup. Hence the need to preheat the cup. The reliability is a bit of a problem also, at least for the some of the US ones.

Reply to
Capitol

Ok - thanks for explaining that. I assumed they came up to around 100C.

As I said - "bit useless".

In fact worse than useless. Last thing I want is visitors trying to wash their hands under it - so I rate it as a really dangerous and stupid, pointless and bollocky invention - a solution without a problem...

SWMBO mentioned it last year prior to us fitting the new kitchen.

I saw that off and explained why. Luckily my SWMBO knows I'm right about stuff like that and as I do as much domestic stuff as her, I have an interest in making stuff work too :)

Reply to
Tim Watts

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