hot water serviced by electric immersion heater - what does this mean?

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PigletJohn, yet another 'ignorant' question to you, or anyone else who wants to reply.

What does the above means - if there is an older non-combi boiler for the heating in a house, but water is not by boiler, is this a normal thing (and is the immersion heater in some way dependant on the boiler or is it a totally separate system? ). From what I've read so far on here from PJ, they usually co-exist. If they are not linked at all, how reliable is immersion heater, and how expensive to replace if it goes wrong - again would it involve also replacing the boiler?
Also are these heaters very small? surveyor couldn't see it - I assume it's in the loft conversion but the closed-in voids aer tiny there - how could he not see it there if opened? If it's not reachable in the converted loft, isn't that a problem?
Thanks!

PJ, just checked and it's wrapped in a pale greeny-greyish looking insulation covering. So I'm guessing it's quite old and inefficient?

Do you mean stiff plastic foam?

Yes it's stiff and looks foamish so is probably plastic.

It's not especially old. Maybe ten years, I can't remember. They were yellow before green. Look for a red insulating cylinder jacket to put over the top, it will reduce heat loss still more, and Climaflex or similar pipe lagging on the hot pipes, they are probably 22mm. Don't insulate over the cap or cable of the immersion heater, if it has one, and let the thermostat near the bottom of the cylinder poke out as it might one day need adjusting.

The later blue ones also heat up faster.

P.s.

The oldest cylinder I have seen in daily use was 55 years old, and still had the original GEC immersion heater in it (no longer used) with the instructions nearby. Using the boiler, it was very slow to heat by modern standards.

Mostly they have been replaced due to leaky joints by then.

I put two jackets on it and converted to pumped and controlled, the gas bills went down and it heated faster.

Thanks PigletJohn!

I'll definitely look for a red insulating jacket for the cylinder, and also it's a good idea to pipe lag the pipes .... do you just mean the ones near the immersion heater, or could I lag them all over the house .... there seem to be exposed ones all over the place.

(Sorry for the hijack OP)

The hot pipes around and between the boiler and cylinder need insulating to save energy. The pipes to hot taps benefit from it so that the first water out if the taps stays hot for longer. Pipes in unheated areas like lofts and under ground floor need insulating to save energy and protect from freezing. Pipes to individual radiators are not so important because their waste heat mostly goes to help warm the room.

Ok ... that makes sense. My pipes to kitchen hot taps are exposed, as are the pipes from the back boiler in the kitchen fireplace to the immersion heater, so they can be insulated.

I've just had a thought ... if I put this jacket on the cylinder AND line the pipes near it, might that have the negative effect of the airing cupboard not being an airing cupboard any more? If everything's so well insulated it might not be as warm as it is now.

Thanks PigletJohn for all your helpful advice!

with luck, it will be fairly cool in summer, when you will not be using the boiler much. If the pump and 3-way valve are by the cylinder, there will be extra waste heat from them and the heating pipes during winter.

IIRC a modern HW cylinder, fully hot and insulated, leaks about 1.5KWh of heat per 24-hour day, which will keep an airing cupboard warm, but there should not be a gush of hot air when you open the door. Judicious use of the timer will reduce that very slightly. There will be some extra from hot pipes, but this will mostly be in winter, when it just helps to heat the house.

If heated by gas, the heat wastage will cost you about 6p per day. Electricity could be about 18p.

it looks like the "minimum" heat loss from a cylinder is about 2.1kWh per day, which I will assume is for a modern, well-insulated one.

There is a table here although the incompetent half-wit who wrote it uses the term "tank" when it should be "cylinder"

nope, the table on here (page 3) for a white one of 150 litre capacity claims a loss of 1.38kWh/day, which is nearer what I had in mind. Again it is incompetently written because it says "kW per 24 hours" which cannot possibly be correct, it must be kWh.

Why are there so many halfwits writing about energy saving?

no prob Typical, all very useful to read.

Thanks again PigletJohn ... hoping to keep my bills down come summer when I'm using the back boiler less, so hopefully with all your advice that will happen!