oil boiler or electric immersion to heat water?

kathykate_2

We have an electric immersion heater which we never use. In future, when only heating water, will it be more economical to use the oil boiler to do so, or the electric immersion heater?

Cheers!

malc_b

Electric will give 100% to the water. Oil will depend on the efficiency of the boiler. You can work out. Find how much oil the boiler says it uses and its output in kW, in the manual usually. Then you know for 1hour that xx liters of oil gives you xx kWhrs and you can compare that with the price of electric.

For example, my boiler say 84% efficient, 28sec oil is 10.35KWhr/l so 8.69 l/kWhr. 50p/l =5.75p/kWhr, 55p/l = 6.33p/kWhr

kathykate_2

Oh thanks Mac B. Now, our boiler is ancient, came with the house, boiler man reckons about 25 years old. No documents came with it. However, boiler man says it's solid as a rock, but we did have to get the burner replaced in '03. Riello burner. I have a booklet for that but am finding it difficult to read.
There is something which says thermal power - output 19-35kW - 1.6-1.3 kg/h
Fuel - Gas oil 35 sec max viscosity 6 mm2/s at 20 degree C Keorsene 28 sec

Are those the figures I need? Cant find anything about efficiency percentage!

As I've just ordered 800 L of oil I'm determined to use it wisely!

malc_b

Which oil do you burn? Kerosene (28sec) or Gas oil (35sec). The 10.35 kWhr/l is for Kerosene. Does the boiler have a name/manufacturer? Efficiency is more the boiler than the burner head. You could try looking your boiler up on http://www.sedbuk.com/ .

kathykate_2

Boiler has Perrymatic written on front, and I think it's Kerosene we burn, to be honest I didn't realise I had a choice, I'm rubbish aren't I!!??:rotfl:

malc_b

If you go to http://www.sedbuk.com , click [FONT=Arial, Helvetica, sans-serif]View the Boiler Efficiency Database[/FONT] then click oil boilers button, then on next page click refine search and then enter perrymatic in the model name it will find your boiler. It should return a list of models of different sizes. All of them are rated at 65%. That may be a bit pessimistic since it looks like the model is so old they are just lumping it in the old model set. Or it could be optimistic.

New oil boilers will be 85% non-condensing, 95% condensing. That would save you 25% or 30% of your oil so you may want to weigh up the cost of oil per year against a new boiler. You may have to have a condensing although in some cases you are still allowed a non-condensing. A few years back I actually went for non condensing the reasons being that condensing - 1. need more radiator area otherwise they are same % as non condensing; 2. can put out smelly fog (and my flue would dump this into next door). Of course oil was cheaper then so 10% didn't seem so much.

Waldir

I am not sure the following is true, but that's what I started experiencing with an oil&immersion heaters:

When heating the water with the oil boiler, the water circulates in the pipes and the whole tank gets heated.
When heating with the immersion heater, it starts by heating the water on top first.

So if we just want to take 1 shower, we can leave the immersion heater for just 15 minutes, it doesn't heat the whole tank, but produces enough hot water (the water on top of the tank is the one that gets used first) for 15p of electricity.
I measured that it costs 69p to heat my tank with oil, and it would cost 79p to heat the full tank with electricity.

Conclusion: it would cost more to heat the full tank with electricity than with oil, but using electricity allows us to heat only a part of the tank, making it cheaper.

owned_by_2_Siamese

Hi - still do the calcs every so often ! Oil for me is cheaper than electric !
Have saved more by only haing the oil water heating for 2 fifteen minute periods a day - water is hot enough for 2-3 showers or just enough for a bath - we boost it if we need more but as we never used all the hot water when it was on for hours this seems to wok for us

malc_b

Waldir wrote: »

I am not sure the following is true, but that's what I started experiencing with an oil&immersion heaters:

When heating the water with the oil boiler, the water circulates in the pipes and the whole tank gets heated.
When heating with the immersion heater, it starts by heating the water on top first.

Oil will heat the water with tubes running around inside the tank in usually the bottom 1/3 so it will spread the heat around more and heat the bottom. Electric elements usually enter at the top and will heat down their length. Electric elements come in various lengths the longer they are the more expensive so there is a tendency to fit ones a bit short. Also, as hot water rises both types will produce hot water at the top. But electric heats the top too so you will get the top hotter quicker. And since electric is less kW than oil (oil burners are >10kW, electric are 3kW) the thermal convection will be less so the water will mix less. All of which says that what you see is what I'd expect to happen to. But for it to be of any use you'd want to just heat the minimum water, otherwise the (just) cheaper cost of oil wins.

BTW you can get dual electric heaters with a short and long element. You use the long to heat the full tank and the short to heat just the top. Some tanks have 2 separate elements coming in at the side (rather than top fit). The bottom heats the entire tank and the top boosts the top. You usually find these on economy 7 where the tank is much larger.

James123_2

I've been having the same argument with myself over whether to use the gas boiler to heat water, or to stick with the immersion. The only true way I could compare was to see how much gas the boiler consumed to heat-up the tank with the cost of the same for electricity. I assumed gas would be cheaper, but this was certainly not so by quite a wider margin. I guess that this is because gas increases have outstripped those of electricity by quite a wide margin.
Gas used 1.5 cubic metres which is roughly 70p
Electric was on for 40 minutes to heat-up with a 3kw element at 9p a unit - roughly 20p. OK, the electricity doesn't heat all the tank but, even with a twice-daily cycle to compensate, it still appears to be cheaper! So, I'm saving the gas for heating only!
AND all the above using a new condensing boiler - which uses no less gas than my old unit.

Cardew

James123 wrote: »

Gas used 1.5 cubic metres which is roughly 70p
Electric was on for 40 minutes to heat-up with a 3kw element at 9p a unit - roughly 20p. OK, the electricity doesn't heat all the tank but, even with a twice-daily cycle to compensate, it still appears to be cheaper! So, I'm saving the gas for heating only!
AND all the above using a new condensing boiler - which uses no less gas than my old unit.

I am sorry, but those figures are simply not supportable.

1.5 cubic metres is approx 16 kWh.

40 mins with a 3 kW element is 2kWh

A kWh is a kWh - so 2 kwh produced by gas is exactly the same as 2kWh produced by electricity.

The only variable is the efficiency of the boiler which is normally between 65% for an old boiler to 90% for a modern boiler. so to produce the same heating effect as 2kWh of electricity, it might take up to 3.5 kWh of gas.

So gas is always cheaper than electricity on a daytime tariff to heat water.

You 16 kWh of gas must have gone somewhere else - it didn't go to your tank or it would have been boiling away.