Heating water electrically

[sparkybird]

A customer has asked if he can get rid of his immersion heater and fit something else...
There is no gas and space heating is via convector heaters and underfloor heating. He will not be interested in anything that looks remotely naff

What are the alternatives?
The only thing I can think of is an instantaneous hot water heater.

http://www.tlc-direct.co.uk/Products/SOILX009.html

How easy is this to retrofit, say where the old immersion tank was? (I won't be doing it, don't worry, but I'll need to run a new supply to it)

The pressure must be a bit rubbish as there's a pump fitted for the shower at the mo.

What are the benefits/disbenefits to these things?? aside from price...

Many thanks
SB

 

[JohnW2]

AThe only thing I can think of is an instantaneous hot water heater.
http://www.tlc-direct.co.uk/Products/SOILX009.html
What are the benefits/disbenefits to these things?? aside from price...

The main issue with any sort of instantaneous water heater (whatever the fuel) is the rate at which it can produce hot water.

If I've got my scribbled sums right, if one assumes cold water at 15 degrees C and hot water temp 55 degrees C (the hottest that model can provide), 9kw would equate to a max hot water flow of about 3.2 litres per minute - maybe fast enough for many purposes, but perhaps not ideal for filling a bath!

Kind Regards, John.

 

[dingodeano]

Sparkybird have you looked at Rointe. I have installed a number of their elec rads and I have always been impressed. Depends on customers budget though.
http://www.rointe.co.uk/water-heater-rd-series.html

 

[plugwash]

AThe only thing I can think of is an instantaneous hot water heater.
http://www.tlc-direct.co.uk/Products/SOILX009.html
What are the benefits/disbenefits to these things?? aside from price...

The main issue with any sort of instantaneous water heater (whatever the fuel)

but especially with electricity because domestic electricity supplies are simply not big enough to run an instantaneous heater than can fill a bath at reasonable speed.

In my flat I have a worster greenstar 24i junior gas combi boiler. Supposedly it can deliver 24kW of central heating output or 9.8 litres per minute of domestic hot water at 35 degree temperature rise. I can tell you from bitter experience that running a bath with it is annoyingly slow.

Unless the customer never takes baths I would advise him that getting rid of the tank is a very bad idea.

 

[ban-all-sheds]

if one assumes cold water at 15 degrees C

I've just measured mine at 10.6° and last January I saw 6.9°....

 

[ban-all-sheds]

http://www.tlc-direct.co.uk/Products/SOILX009.html

Redring do a 12kW one, but even that is half the power of Plug's boiler (although we don't know how efficient that is).

The specific heat of water is 4.19J/gK.

A 12kW shower will be able to put 720,000 joules per minute into the water.

If we assume a required delta of 35°, the amount of water which can be raised by that by 720,000J is:

720000 ÷ 4.19 ÷ 35 = 4909.6g.

So a the 12kW one would deliver about 4.9l/m.

Dunno how big a bath is, but that's what - about half the flow rate of your average gravity fed shower? I've seen shower pump manufacturers quote 15l/m as the rate at which most people think "Ooh yes - that's a nice powerful shower".

 

[JohnW2]

If we assume a required delta of 35°, ....So a the 12kW one would deliver about 4.9l/m.

I agree with your calculation, and also that plugwash talked of a temperature rise of 35° (and that 4.9 l/m is pretty inadequate) - but, bearing in mind that you said of cold water temperature:

I've just measured mine at 10.6° and last January I saw 6.9°....

... the reality is that a 35° rise would clearly be pitifully inadequate for a DHW supply - last January that would have only got you up to 41.9°, only slightly above body temperature and therefore probably wouldn't feel even warm, let alone hot, to the touch.

Kindest Regards, John

 

[sparkybird]

thanks for the detailed and helpful replies everyone.
I don't think he should get rid of the immersion and this has pretty much confirmed it to me
I'll suggest he seeks more advice from a plumber if he's that keen!

Cheers

SB

 

[ericmark]

I have a 27kW gas instant water heater for normal taps not a problem but the bath takes an age to fill. With under half that power it would be silly.

The advantage is one can go under 55 degs without problem with legionnaires but would one want under 55 degs?

I have also used a lot of under sink heaters and they are good. They have around 7 litres of water stored and use a special hot tap which feeds new cold water in to force hot water out. Very well insulated and because so local to sink there is no delay in hot water at the tap so must save money as there is no hot water left in pipe work to cool down between each use.

Normally 3kW but I have changed the element to 1kW for use in a caravan and it worked very well.

Clearly they can't replace the cistern but can mean you only turn on immersion for when one wants a bath.

There are now pressurised tanks which work on mains pressure rather than need a header tank draw back need testing every year but will allow standard thermostatic over bath mixer shower outlet at mains pressure. Also the new tanks are better insulated.

 

[big-all]

Dunno how big a bath is, but that's what - about half the flow rate of your average gravity fed shower? I've seen shower pump manufacturers quote 15l/m as the rate at which most people think "Ooh yes - that's a nice powerful shower".

4 inches [tap end]in a bath is around 50L 8" is around 120L 12" is around 210L
this is a 66" cast iron bath

 

[ban-all-sheds]

... the reality is that a 35° rise would clearly be pitifully inadequate for a DHW supply - last January that would have only got you up to 41.9°, only slightly above body temperature and therefore probably wouldn't feel even warm, let alone hot, to the touch.

Kindest Regards, John

Take a thermometer into the shower.

Either you are going to be surprised or I'm a total wuss, cos I measured mine this morning, and 40° at the shower head was fine. Above that and I'm into the range where my initial reaction is "Ooh that's hot". I don't know how much above that is the point where I don't stop feeling uncomfortable no matter how long I stay under the water.

At no point in a 40° shower do I ever think "I wish this was hotter".

At what temperature is the safety detent in thermostatic mixers that have them?

But there is a valid case for having a higher delta, as adding cold will increase the flow rate.

 

[ban-all-sheds]

I have a 27kW gas instant water heater

What make?

for normal taps not a problem but the bath takes an age to fill.

Same with my 28kW combi.

The advantage is one can go under 55 degs without problem with legionnaires but would one want under 55 degs?

Depends on flow rate. I guess if you want to fill a bath the hotter the better, as you can add cold at the same time and fill it faster, but for a shower if the flow rate is OK with just the hot supply then you don't need (probably could not tolerate) 55°.

I have also used a lot of under sink heaters and they are good. They have around 7 litres of water stored and use a special hot tap which feeds new cold water in to force hot water out. Very well insulated and because so local to sink there is no delay in hot water at the tap so must save money as there is no hot water left in pipe work to cool down between each use.

Dragon's Den Idea:

For people who like the function of stored hot water, but don't want to give up space for a conventional cylinder, I reckon that by making the bath a little bigger on the outside, mostly by adding height, you could store the water around it, and lose virtually no bathroom space - imagine a large rectangular vessel lying on a long side with a bath-shaped depression in the top surface.

 

[timtheenchanter]

At what temperature is the safety detent in thermostatic mixers that have them?

44 deg C IIRC. this is also the max temp allowed for showers in residential/nursing homes, 41deg C for hand wash basins.

 

[JohnW2]

Take a thermometer into the shower. Either you are going to be surprised or I'm a total wuss, cos I measured mine this morning, and 40° at the shower head was fine. Above that and I'm into the range where my initial reaction is "Ooh that's hot". I don't know how much above that is the point where I don't stop feeling uncomfortable no matter how long I stay under the water. At no point in a 40° shower do I ever think "I wish this was hotter".

Yes, I am a little surprised - perhaps I'm wrong, but I don't think that most people would describe 40 as particularly 'hot'. However, I think you may be confusing two issues - I was talking about desirable DHW temperature, not the output temperature of a shower. DHW is used for lots of things other than showers and for some of those purposes, 40° would be very low for many people.

One of the most obvious examples of such 'other uses' is when filling a bath and, as you went on to imply, it is then highly desirable to have a DHW temperature way above the desired bath temperature, so that one can use a lot of cold water as well; apart from anything else, even a whole 'average size' DHW cylinder full of water at 'bath temperature' would probably not be able to fill a large bath very deeply.

Kind Regards, John.

 

[SimonH2]

Also, when thinking about shower temperature, if you are heating the water generally rather than running an electric shower, the DHW supply needs to be considerably hotter than the shower setpoint for the thermostatic valve to work. Some of the better valves will work with only 5˚C, others need as much as 10˚C - so for a 40˚ shower the DHW would need to be at least 50˚. If the DHW temperature isn't high enough, then the shower won't shut off on loss of cold supply.