Water cylinder: oil boiler or immersion?

[Allan Gould]

What's the current thinking on oil vs electricity for heating hot water?

My Mum has got to replace her hot water cylinder (it started leaking over the weekend), and we're discussing whether to get an immersion (with thermostat) included. She has a new-ish oil-fired boiler which does heating and hot water, in a large old house, and she lives on her own. In the summer months, she doesn't use the heating, so only needs to put the boiler on to heat the hot water tank periodically. I suggested an immersion might do the job better in the summer months.

I know there are factors that a full answer to this query would involve, such as boiler efficiency and energy conversion, also the lifespan effects of running the boiler running though the summer vs immersion replacement costs, and the lagging on the tank, but...

to heat the hot water tank in summer, would be it better/cheaper to:

run the boiler as needed (using oil) or use an immersion (using electricity; she doesn't have Economy7 or equivalent tariff)

(if an immersion heater is viable, how much would an immersion typically add to the cost of the new cylinder?)

The last time this topic seemed to get an airing in this ng was 2002, so I hope it's OK to ask again.

TIA

Allan

[Ian Stirling]

Pretty much the same as always.

Oil is maybe half the price of electricity, so it has to be fairly inefficient. Running the thing on the thermostat 24*7 is going to use a moderately large amount of oil, running it on the thermostat 1 hour a day isn't a bad plan.

Adding a wrap of 200mm loft insulation around the cylinder is always a good plan.

[mcbrien410]

1. A new hot water cylinder will have foam insulation already bonded to it, however, you can get them without.

1. Buy a HW cylinder with an indirect heating loop in it that also has the large threaded hole in the top for an immersion heater element.

I think the immersion heating elements that are used in the tops of cylinders are 27 inches in length. This means that only to top 27 inches of water will be hot. This ought to be enough for a single person.

The indirect heating loop is plumbed into the central heating system. As this heating loop is nearer the bottom of the HW cylinder than the immersion it will heat more of the cylinder.

So, in the summer you just use the immersion which can be timed to coincide with the Off Peak electricity. My off peak starts at midnight and goes off at 0800 hours. My immersion switches on at 0500 and off at

0800.

Chris.

[Ian Stirling]

However, 200mm of extra insulation will well over halve the current heat loss.

This may 'only' be 50W, but 50W*24*7 pays off a small roll of insulation in 3 months or so.

[Owain]

You might as well get one with an immersion; it won't cost much more, and is a very useful back-up if the boiler breaks down.

Owain

[Dave Liquorice]

Well it might a bit but as a percentage of the total bill not a lot. What may add significantly to the cost is the supply wiring for the immersion if it doesn't already exist.

Agreed. As this is for a single person living alone, during the summer I'd be tempted to use the immersion "on demand" assuming the reheat time from cool to hot was reasonable and the tank is very well insulated. At least a good cylinder jacket on top of the expanded foam insulation.

Only heating what you want is a *very* good way of saving energy. When the timeswitch on the heating/HW broke when I was living on my own in a flat the gas bill dropped in half by using the "on demand" system instead of heating the HW morning and evening on the timer. This was a fairly quick system, it would heat from cold to hot in 30mins and stay hot enough (just) for hand/face washing for the best part of 48hrs.

Letting the oil boiler sit without excercise for long periods may not be a good idea, I'd fire it up HW and CH for an hour or two every month. Rather than wait until you want it in anger and find that it doesn't work...