Oil vs Elec - Heating water

[Mark]

I realise this has been aired before but didn't really get the full answer I was hoping for. Any input very welcome:

What's the cheapest way to heat my water during the summer? I've a regular oil fired boiler system that heats water in a hot water cylinder and also does the central heating when required.

My oil is about 38p/l, electricity roughly 5p/kWh plus a standing charge but that's kinda irrelevant since I'll pay that either way.

I've read lots of heated (sorry) discussions about efficiency of heating the water tank with the oil burner vs. the electrical immersion heater. I'm happy to assume that electrical is more efficient than oil method but not by a massive amount. Obviously the oil method has to heat the pipes, loses some energy out the flue and needs electricity itself to drive the pump.

I guess the question is roughly how much energy does an average oil boiler get from a litre of oil?

The interweb figures I've found suggest 11.5kWh from a litre of oil, or

3.3p/kWh.

So oil energy is roughly 2/3 the cost of electricity energy.

So my oil boiler would have to be better than 66% efficient at converting the energy from the oil into energy directly heating the water?

It sounds like oil method should be better, anyone feel like wading in?!

Mark.

[meow2222]

sounds about right

NT

[Dave Plowman (News)]

Since you've done the sums and have answered yourself I'll add a rider. Switching off a boiler fed system totally during the summer months often results in seized components - valves and pumps, etc. Running it every day for hot water helps prevent this. So it's even more heavily weighted on using the boiler.

[David Hansen]

On 07 Aug 2006 12:35:49 GMT someone who may be Mark wrote this:-

Solar.

Zero energy cost. Obviously one has to pay for the capital cost, but that was excluded from your figures for oil and electric.

Ignoring the standing losses in the cylinder (which will be similar for oil, electric, or solar) electricity is 100% efficient (assuming a suitably sized cable). Oil is not.

It sounds like you assume that cheaper is the same as better. If so then it is probably better to heat the water on White Meter electricity (they have another term for it in England and Wales) in the summer. Assuming your cylinder is properly lagged and sized then it should provide enough water for a day's use.

Topping up the water may well be better done by oil, rather then full price electricity.

[Owain]

Except in the very hottest weather, I put the central heating on for 5 mins a day for this very reason.

Owain

[Huge]

OTOH, in our case, the waste heat kicked out by the (admittedly elderly) boiler and its associated pipework is unwanted in the summer, so we switch it off and use 'leccy, then.

I think we'll be looking at a new kitchen next year & we'll probably get the boiler changed and have the heating/hot water niggles sorted at the same time.

Anyone got any opinions of any kind on (i) solar water heating & (ii) heat pumps (especially submersible ones - we have a well it could go down.)

[The Natural Philosopher]

Use the oil.

More than the average power station does.

Yes, it is, but don;t bust a gut over it.

In winter we burn getting on for 500 quid of oil a month, in spring and autumn we burn maybe 200 a month. In the three summer months the boiler fires up generally twice a day for about half an hour. i.e. about 10 unit of electricity (we have a 10KW boiler)..domestic water heating is the least of MY worries, cost wise.

[Nick]

Last time I looked I thought electricity was approx 7p per kWH, and we got approx 10 kWH out of a litre of 28 second oil.

I amnot sure if that is net or gross - assuming gross, that's about 8 kWH into the water and say 7 kWH into the hot tank.

If oil is 38 p / litre, that makes a kWH from oil cost about 5.6 p.... I suggest you re-visit the Electricity bill and check that price per unit.

Please let us know - I am sure my standard rate is around 7 to 8 p / unit

Nick

[Mark]

I grabbed the figure from my last bill from Scottish Power which showed

4.89p/kWh plus a 17.57p/day standing charge. 5% VAT needs to be added and I got a modest reduction for taking gas & electric and being on an internet tariff. I think it was capped too. I've since moved (hence the talk about oil) so have lost the capped rate benefit.

Maybe you're on a all-in rate without the standing charge? The equivalent price for my last bill if it was without standing charge would have been

6.26p/kWh before vat.

I need to get out more...

[Mark]

Thanks all for input. I'll stick with firing up the boiler for 40 mins a day which seems to cover the days needs. Solar panels and wind turbines are likely to become a more serious proposition the way energy prices are going.

[Dave Liquorice]

That's good if still current. Our Scottish Power, NORWEB area, online, single rate, electric only: 7.191p/kWh 12.39p/day standing charge. Just gone up (again) on the 10th July from 6.85p/kWh. Only have to use about 2 units/day to make up the standing charge increase by the cheaper unit cost, not a problem we use 22 units/day...

I guess the higher standing charge is because of the combined gas/electric aspect or do you have a seperate gas standing charge?

They normally only increase the first X units above the standard rate, once the standing charge has been recovered the cost/unit is the standard rate.

[Nick]

and geothermal if you have a bit of land and don't mind doing the trench yourself

The rest of the kit is really not too expensive, although under floor heating would make it more efficient, else radiators would have to be a lot bigger

Nick

[meow2222]

Theyre already an easy payback if you understand how to design to maximise return/cost ratio, and treat it as a hobby ie value your labour at zero.

NT

[The Natural Philosopher]

More like 10-13p

[The Natural Philosopher]

Here is a good site to answer these questions

I query their 90% efficiency figures for a boiler. Probably

60-70% nearer the mark.

[David Hansen]

On Tue, 08 Aug 2006 11:57:39 +0100 someone who may be The Natural Philosopher wrote this:-

There are two major flaws with the figures they present:

1) they have not included a figure for off-peak electricity. It is undoubtedly cheaper to heat water using off-peak electricity rather then run an old boiler with a long pipe run to the cylinder.

2) it is not just the efficiency of the boiler. A long run of uninsulated pipes between boiler and cylinder will lose a fair amount of heat and thus reduce system efficiency.

[The Natural Philosopher]

>>> However I query their 90% efficiency figures for a boiler. Probably

Points well made and accepted. HOWEVER I suspect that even off peak electricity is barely a match for a decent oil burner, and many of peak charges include standing charges as well.

As far as the long runs go, if the pipes are well lagged its not a huge loss for the very short periods that the boiler is in fact heating water.

[David Hansen]

On Tue, 08 Aug 2006 13:42:13 +0100 someone who may be The Natural Philosopher wrote this:-

It is a matter of the specific against the general. It's easy to construct systems under various charging regimes where each is better in terms of various costs. However, for the typical systems seen in houses I don't think off-peak water heating is that expensive. I have done the sums a few times.

[The Natural Philosopher]

Trouble is after a day of washing up, and laundry, there is nothing left for an evening shower or bath.

[David Hansen]

On Wed, 09 Aug 2006 11:01:56 +0100 someone who may be The Natural Philosopher wrote this:-

Then the hot water storage has not been sized properly.

Anyway, washing up and laundry can usually be done overnight. In summer the off-peak period runs until 08:00-08:30 depending on meter and location. This means breakfast, washing and so on can be done and there is still time for the storage to be warmed up before the end of the off-peak period.