When to switch off the immersion

[Roger Tonkin]

When going away for a few days, we are always in a quandry about switching off the immersion heater. We are an all electric house, with ecconomy 7 electric supply. The immersion is timed to switch on for a couple of hours towards the end of the overnight "low" period.

When we go away for just a couple of nights, we leave it on, and when we are away over a week, it goes off. It is the in between period we are never sure about. Obviously there must be a break even point between reheating a cold/cool tank and keeping it hot.

Anyone any ideas as to how (if at all!) I should go about working out a solution.

There is no obvious reliable way of measuring as there are fridge and freezers also running, and I do not have the expertise or will to try and insert some cleave ciruitry into the immersion system to measure usage.

Prpbably the answer is the same as the answer to "How long is a piece of string?"

[Tim Watts]

On Wednesday 01 January 2014 20:50 Roger Tonkin wrote in uk.d-i-y:

As long as you plan to reheat the tank from Economy 7, then turn it off.

It's quite simple: If the tank is host, it is losing energy to the house which must be replenished.

If it is allowed to go cold, it is losing no energy once cold. It is also losing less and less energy over time as it cools, so the integrated sum of losses over the week (or any time) is clearly smaller if it is allowed to go cold, compared to be kept fully heated. The losses are balanced exactly by the electrical input that you must pay for.

It's a similar argument to "leave the heating on or let the house go cold".

[Vir Campestris]

Nope. If the tank is cooler it is losing less heat to the environment. Ultimately it cools to room temperature, and ceases to lose any.

Of course, with economy 7 you have to think - will I be reheating it with peak-rate power when I get back? That makes it a non-obvious problem, you have to balance the saved power through not keeping it warm against the extra cost of re-heating it with expensive electrickery.

Andy

[pcb1962]

Here's one way of looking at it: Let's say for example that the water will cool to 20 degrees if left unheated, and your immersion heater heats it to 60 degrees. Each 10 degree increase in temperature requires the same energy input, let's call that 1 unit of energy. Therefore the maximum energy you will require if the water is left to go cold will be 4 units. Let's say that in a 24 hour period the water temp will drop 20 degrees. So if you leave the immersion heater operational, then each day you will use 2 units of energy returning the water to 60 degrees. So if these figures were accurate, you break even at 2 days, and any longer you are better turning it off. So your first step is to find out the temperature of the water at the end of the heating period in the morning, and the temperature it has dropped to just before the heater goes on the next morning. A simple min max thermometer will tell you that. (Obviously it needs to be measured when you are away).

[spuorgelgoog]

If you are reheating the cylinder on off-peak rate then switch it off, as said.

If you are going to reheat the cylinder on return on peak rate, then,

P to heat the cylinder at peak rate C to heat the cylinder at cheap rate L is the cost in keeping the cylinder warm (heat losses, per day)

then (P-C) is the extra cost of reheating on peak compared to cheap rate.

L x days is the extra cost of keeping the cylinder warm

if (P-C) is greater than (L*d) leave the immersion on. if (P-C) is less than (L*d) turn the immersion off.

the heat loss per day can usually be obtained from cylinder manufacturers' websites for new cylinders. Older clyinders will probably be less well insulated.

I am not a mathematician so all the forgoing may be complete bollox, of course

Owain

[Andrew Gabriel]

If you are not going to shower/bath until the morning anyway, heating it up the instant you come back is probably unnecessary - can probably wait until it kicks in its normal E7 cycle. If you do want some hot water just for hand washing, just 5 or 10 minutes on at peak rate will give you that.

I don't know what peak rate electricity costs, but I'll guess 20p/kWhr. Even if you did run it for 2 hours on peak (assuming 3kW), you're only talking about £1.20 anyway, as a once-off expense.

When I bought my last house, took me a few weeks before I moved in. When I did, I found the immersion heater had been left on 24x7 (there was no timeswitch) by the people moving out. Mind you, electricity was probably only 5p/kWhr back then.

Something else I would consider is that immersion heater circuits can suffer from connections going bad, over-heating, and burning out. That's not the kind of load I personally would want to leave on unattended for any length of time, at least not without some periodic inspection of likely failure points.

[dom]

This is rubbish.

Rate of heat (energy) loss with time is not uniform. The hotter it is, the faster it looses energy.

It is ALWAYS more economic to re-heat when needed, than to try to maintain a constant temperature.

[Andy Burns]

Except (as in this case) when the cost of power is not uniform.

However this only leaves the O/P looking for edge-cases and requires knowledge of the departure time and remaining hot water at that time, rate of heat loss, expected return time and time that a full tank of hot is next required ...

[harryagain]

Tch tch. Very foolish statement. All hangs upon how well insulated his tank is which we don't know so the

20deg asumption is nonsense. The manufacturers will have this data, described as "standing loss". So the answer is as long as a piece of string.

Any heat "lost" however goes to heat the house.

So long as there is frost protection for the house, probably best to turn it off.

The greatest danger to your house while absent is pipe freezing because no water is being drawn. So turn the water off at the mains. Won't stop freezing but will limit water damage if it does.

Get a neighbour to keep an eye on things. Make the house look occupied, maybe a table light/radio on a timer?

[Terry Fields]

Immersions can fail in a number of ways, and personally I'd switch it off when going away as a matter of routine, even if only overnight. Ditto mains water.

When the DHW tank is at max temp, heat losses will be highest, but these will fall as the temperature falls. After a week you'll find there is still warm-ish water in the tank, perhaps enough to have a cool shower.

I don't know what sort of tank you have, but if you can add insulation to it, *including the pipe-work*, you'll benefit from lower heat losses both in daily use and when you're away.

Let's say you come home after a week, and the DHW temp has fallen to

25C, and you want a shower. Tank is 100 litres with a 3kW immersion. Warm shower is 40C. This implies turning off the immersion when it reached 40C.

The energy requirement is 100 x (40-25) x 2000 = 3MJ or 5/6ths of a kWh, which at 20p/kWh will be 17p and take about 17 minutes. I suggest that as part of the cost of your travel and stay away expenses, 17p is negligible and not worth taking into account.

Hence: turn off immersion and mains water, reheat water as necessary on return, and add lagging/insulation to pipes and tank.

[chris French]

In message , Roger Tonkin writes

Why obviously, to me it obviously isn't sensible to heat it when it won't be used.

Sounds like a common misconception here like the one about turning off the central heating/keeping it on.

Nope, it's never more cost effective keeping the cylinder hot, as opposed to letting it cool down. It's not that hard to do calculations for this, the rate of loss from the tank is known (or can be found out) and the rate of input is known.

As a demonstration.

Your tank will lose some heat, say X (and the hotter is the more heat it loses proportionally) over a given period of time before the immersion comes on again., and it will take Y to heat it back up again. heat it everyday and unused it will use Y everyday.

Turn off the immersion and the second day it will cool some more, but not as much as on day 1, so it won't be twice as cold (for the reason above) so to heat it up would take X + say 0.75 x Y (I've no idea of the real figure, its the principle).

on day 3 it would cool even less, probably by now it has lost a lor of it's heat. so it might take an extra say 0.5 x Y to raise the temp back up to full.

So 3 days left on would be 3Y, 3 days left off would be say 2.5 Y to reheat at the end.

(ok reheating it on return at peak rate if you need HW straight away adds an extra complication)