Radial / Ring conversion.

[JohnW2]

Much, much longer than that, they use a fine spray.

They may - but, as I said, what most people understand by "taking a shower" relates to a flow rate of 10-15 litres/.minute.

 

[flameport]

The anticipated total load will be approx 6000W from the heaters

They need their own separate circuit(s), and that's assuming the supply to the site is suitable for adding such a large load.

but I admit to being s little out of date with the passage of time

It has never been appropriate to connect 3kW+ fixed heating appliances to a ring final circuit, even when such things were actually part of the Wiring Regulations back in the 14th edition during the 1970s.

 

[SUNRAY]

If I've done my sums right, then to heat 10 L of water through 50 degrees (say from 20 to 70 degrees) with a 1,300 W heater should theoretically take about 27 minutes - but that time would obviously reduce if one used gas in addition to the 1,300 W of electrical heating.

I always struggle with these calculations. I've just measured 1L of water into the kettle, it took 53 seconds to raise from cold tap to clicking off, in the absense of a digital sensor I'll make the guess it's a rise of 85-90º at ~3KW.
All things being equal, it should boil 10L in 530 seconds/<9 minutes.

Ergo (3000/1300) x (50/85) x 530 = 719 seconds/12 minutes.

Yes I know the rules of physics... yada yada yada.

 

[Harry Bloomfield]

Thousands upon thousands of touring caravans and motorhomes have / do run with such HW tanks and people shower in them quite happily. It needs users to operate a 'wet' 'soap' and 'rinse' system, so not a continuous stream of water, but we used to cope well with it when we had such a system. (Cold water barrel - external - was no more than 40 litres as was the waste tank).

Ours was 850W/1300W electric and one could run gas at the same time to reheat quicker still if desired. Water was at 70C.

More than adequate as a shower when mixed with cold via the DC pump. We could shower one after the other (first towelled off and dressed before second undressed to shower giving the HW some 'recovery' on electric only).

I've just spotted your post, and I agree completely with your findings, the small amount of water, especially hot water, I find is more than adequate for the purpose. It can seem a little less than adequate, in colder weather, if the 'bathroom' is unheated.

Mine was blown air heated, but the blown air pipe, went under the caravan (terrible idea), so by the time the air arrived in the bathroom it was cold. My fix was to do away with the heat wasting underfloor pipe entirely, and replace it with an outlet blowing into the bathroom door. To supplement that, I later added an electric bathroom heater.

 

[SUNRAY]

The camping shower I had was a poly bag, clear one side & black the other. A fill cap (rather like a hot water bottle) and a short tube/tap/spray head.

Fill it with 8L of cold water and lay it on the ground in the direct sun for a couple of hours then hang it on a tree branch... Boofull.

When there were 4 of us we'd fill plastic milk bottles and leave the on top of the dashboard if the vehicle stood in the sun for long enough to refill. Solar powered hot water system was also used for washing/washing up etc and preheating for tea etc.

 

[Harry Bloomfield]

The camping shower I had was a poly bag, clear one side & black the other. A fill cap (rather like a hot water bottle) and a short tube/tap/spray head.

I once upon a time, had a flat with no bathroom, under refurb, with just a toilet and washbasin. As a work around, I bought a portable shower - a plastic tray, plastic curtain which you hung up, then an 8L pump up, pressurised bottle, pipe, and spray head. You just filled it with water, the right temperature, pumped it up, and away you went.

 

[JohnW2]

I always struggle with these calculations. I've just measured 1L of water into the kettle, it took 53 seconds to raise from cold tap to clicking off, in the absense of a digital sensor I'll make the guess it's a rise of 85-90º at ~3KW. All things being equal, it should boil 10L in 530 seconds/<9 minutes.

I obviously cannot argue with your observation, but the fact is that raising the temp of 1 L of water by 85 °C with a 3,000 W heater requires a bit more than double the amount of time that you observed - specifically about 118.7 seconds (fractionally under 2 minutes)

Ergo (3000/1300) x (50/85) x 530 = 719 seconds/12 minutes.

Again, although I can't disagree with your arithmetic (based on what you observed), to heat 10L of water by 50 °C with a 1,300 W heater requires a bit more than double what you have calculated, namely about 26 min and 51.5 secs (which I previously called "about 27 mins")..

Yes I know the rules of physics... yada yada yada.

Quite so, and I can think of only one possible explanation for your 'misleading' observations, very similar to our recent discussion about the situation in which an immersion heater in a DHW cylinder is controlled by a thermostat near the top of the cylinder ...

... I can but presume that your kettle 'clicked off' when the temp near the top of the water in the kettle got to 'target temp' (about 100 °C) but that when that happened a lot of the water lower down in the kettle was at a much lower temp, such that the overall temp of all the water in the kettle (i.e. if thoroughly mixed) would show a bit less than half the temp rise you thought had happened.

Al;though, as I said,I can think of no other explanation for your observation, I'm off to '#do the experiment', and will report back fairly soon

Kind Regards, John

 

[JohnW2]

Al;though, as I said,I can think of no other explanation for your observation, I'm off to '#do the experiment', and will report back fairly soon

Hmmm. Well, my observations are certainly very different from yours, with my kettle taking appreciably longer to heat the water than calculation indicates - presumably due to 'heat losses' (whereas yours seemed to heat the water in under half of the calculated time!)!

I used what is described as a 2,000 W electric kettle, although it only seemed to be drawing about 1,850 W at the time of my experiment. With 1 L of water, initially at 19.3 °C, it 'clicked off after 222 seconds (3m 42s) when the temperature at the top of the water was 96.1 °C - but when I mixed all the water up, it's temp was not all that far behind, at about 89.0 °C.

To heat 1L water through 76.8 °C (96.1 -19.3) with a 1,850 W heater requires about 174 secs (2m 54s), so the 222 secs that it actually took implies an 'efficiency' of only about 78%, the other ~22% of heat energy being 'lost' to the environment.

Why my experiences are so different from yours, both in absolute terms and in comparison with the theoretical amount of heat energy required is very far from clear - at least to me

 

[JohnW2]

Why my experiences are so different from yours, both in absolute terms and in comparison with the theoretical amount of heat energy required is very far from clear - at least to me

Thinking a bit more about this, it would seem that what I experienced with an ancient "2 kW" kettle, with minimal insulation, was probably about right - in that it took around 20% longer to heat up by a certain amount than the Laws of Physics say should have been the case.

What is much less understandable is what you reported - that your kettle seemed to heat up twice as fast as would be dictated by the Laws of Physics (even if it were 'perfectly insulated' with no heat loss). That obviously makes no sense, and I still can't think of any explanation other than, despite what you thought, the full 1 litre of water did not get heated to the temp you were assuming.

Any other thoughts/suggestions?

 

[Rodders53]

Ergo (3000/1300) x (50/85) x 530 = 719 seconds/12 minutes.

Yes I know the rules of physics... yada yada yada.

Well the manufacturers of the unit I used to have reckon 30 minutes-ish on electric or gas only (1500W). 16 minutes if both used (2800 W).

Truma Ultrastore Rapide 10L Gas & Electric Water Heater With JG Fittings - Grasshopper Leisure

Truma Ultrastore Rapide 10 Litre Gas & Electric Water Heater Boiler - The newly updated Ultrastore Rapide GE boiler features a compact modern design and is now even more powerful and cost effective. Water Heaters. Caravan Water Equipment, fittings, hoses, portable showers, tanks & carriers...

www.grasshopperleisure.co.uk

That probably bears out what happens in practice, if my memory serves me.
Note the gas takes slightly longer than the electric due to inefficiency/losses from the flue.

Mine was blown air heated, but the blown air pipe, went under the caravan (terrible idea), so by the time the air arrived in the bathroom it was cold. My fix was to do away with the heat wasting underfloor pipe entirely, and replace it with an outlet blowing into the bathroom door.

The accepted fix for that (the pipe was double-wall stuff that had an air gap for "insulation") was to use large bore Armaflex insulation around the external pipe (12mm thick iirc). Worked quite well. Mine fed from the fire to the front lounge vents. The bathroom stayed inside the bed base. Bathrooms invariably needed supplementary heating I found.

Many high end caravans now have wet heating radiators under the seat backs and towel rails in the bathrooms https://www.alde.co.uk/alde-heating-system/ that also heats the "domestic" water. Never experienced that style myself.

 

[Harry Bloomfield]

That probably bears out what happens in practice, if my memory serves me.
Note the gas takes slightly longer than the electric due to inefficiency/losses from the flue.

That is not what my Truma manual suggests, from memory - gas is quite a bit quicker......

Technical data

• Capacity: 10 Litres
• Heating time in mixed mode from 15°C to 70°C: 16 min
• Output of gas burner: 1500 W
• Boiler gas / electric: electric heating element (850 / 1300 W)

The accepted fix for that (the pipe was double-wall stuff that had an air gap for "insulation") was to use large bore Armaflex insulation around the external pipe (12mm thick iirc). Worked quite well. Mine fed from the fire to the front lounge vents. The bathroom stayed inside the bed base. Bathrooms invariably needed supplementary heating I found.

The pipe was double walled, nonetheless, it was impossible to get any warm air coming out of the outlet in the bathroom, in any but warm weather. Emails of complaint to Bailey, produced no solutions. The heater is mounted in a unit of the near side, the same side as the door. The only way to get a duct/pipe, from there, past the door, was under the van, then cross over to the off-side, then up into the wardrobe. A 3m long pipe. No surprise that air travelling such a long way, was cold, before it arrived.

Entirely pointless, so I removed it, and made better much arrangements.

 

[JohnW2]

Well the manufacturers of the unit I used to have reckon 30 minutes-ish on electric or gas only (1500W). 16 minutes if both used (2800 W).

They appear to be saying that it will heat 10 litres of water from15 °C to 70 °C in 29 or 30 minutes using the electricity-only mode (I presume 1,300 W, the higher of the two electrical powers apparently available.

The physics/maths is simple enough. Assuming no heat 'losses' (i.e. 100% 'efficiency') to increase the temp of 10 litres of water by 55 °C with a 1,300 W heater should take 29 minutes and 32.6 seconds - so they're dead right (assuming 100% efficiency) - and that figure is totally consistent with the 27 minute figure I quoted for raising the temp by 50 (rather than 55) degrees.

So, as one would expect, the Laws of Physics are correct, per my initial post about this, but there is something very wrong about the experiment SUNRAY did to arrive at a very different suggested answer.

Kind Regards, John

 

[ericmark]

Theory we had a shower in the caravan, used it once, it used nearly a whole aqua-role of water for one shower, never used it again. The hot air central heating worked A1, so never looked at how it worked, Colvid 19 resulted in not being able to visit the caravan in storage, and by time we could water ingress had written it off, and it was scrapped.

But a static caravan (If there can be such a thing?) when I was involved came in two flavours, standard and winterised, the latter designed for all year around use, with a price tag of a third again over the non winterised. The only difference between a winterised static caravan and a house was it does not have to comply with building regulations, and it can't have a TN-C-S supply.

So with a 40 amp supply would one fit a instant water heater? I have not seen a reference to it being used for a shower. Sorry if I have missed some thing, well I suppose yes must be the answer, I have two 3 kW one cup boilers plugged into a 4 way adaptor and the fuse in the plug of the adaptor in 3 years has ruptured once.

Yes it is not ideal, but it means in the TV adverts I can walk into kitchen and make two cups of coffee and go to loo, and return with the two cups before the adverts have finished. I now use only one as wife can not have real milk, and so we both want the larger of the two one cup boilers, and also using one at a time means we are using solar or battery rather than grid power, but it is all down to time, using 6 kW for enough time to make two cups of coffee will not rupture a 13 amp fuse, but much longer and it would.

 

[JohnW2]

I always struggle with these calculations. I've just measured 1L of water into the kettle, it took 53 seconds to raise from cold tap to clicking off, in the absense of a digital sensor I'll make the guess it's a rise of 85-90º at ~3KW.
All things being equal, it should boil 10L in 530 seconds/<9 minutes.

So, as one would expect, the Laws of Physics are correct, per my initial post about this, but there is something very wrong about the experiment SUNRAY did to arrive at a very different suggested answer.

@SUNRAY ... we never got to the bottom of what went wrong with your experiment.

There is no getting away from the fact that to heat (all of) 1 litre of water by 85 degrees requires 98.93 Wh (356,148 Wsecs) of energy - so, with a 3,000 W source of heat, even if one ignores losses (to environment) it would take about 119 seconds. It is therefore impossible that you achieved what you thought you have achieved in a bit less than half that time. There are really only three possible explanations (or a combination thereof) that I can think of

1... That your kettle was supply a lot more than 3 kW. Given what we know of kettles, and the limitations opf a 13A plug, I think we can discount that possibility.​

​

2... That the temp rise (in the water as a whole) was a lot less than you thought.​

​

3... That the temp rose by the amount you estimated at the top, and thereby caused the kettle to 'click off', but with most of the water in the kettle then being at a much lower temp.​

What do yiou think happened? As you will see as above, when I tried an experiment myself, it took about 20% longer than the theoretical time to heat water by a certain amount, which sounds about right.

Kind Regards, John