heating water: oil or economy seven?

[Fred]

Hi,

Our DHW can be heated by the boiler (oil fired) or by an immersion heater on economy seven. I'm never sure which is the cheapest way. So that I don't have to post every time the price of fuel/electricity goes up, can anyone explain how I can work it out?

The system was already installed when we moved in, so I don't know what make the cylinder/immersion/etc. is, nor do I know whether it has been there 3 years or 33 years!

Is it safe to assume that all 3kW immersion heaters are the same? Do you assume 100% efficiency?

If so, is it a simple calculation of working out how many much water the cylinder holds and then multiplying that by the temperature difference of the water and 4.2?

The water is heated to 60C but I don't know what the temperature of the incoming water is until I get a thermometer out.

So energy (J) = volume of cylinder in ml x (60 - incoming temp) x 4.2

If the immersion is 3kW, work out how long the immersion heater needs to be on to give that number of Joules heat and that way work out how many kWh of electric I use?

That's more straightforward than the boiler calculation. I don't really know where to begin with that. It's oil fired (kerosene if that makes any difference) and that is only a couple of years old, so it's a condensing model.

If the boiler had a fixed output of so many kW, I suppose I could follow the calculation I did for electricity but don't all boilers modulate now? So I wouldn't know what the output was.

The other issue is heat loss. The boiler is on the far side of the kitchen, so the pipes have to go up the wall and across the ceiling, under the landing, to the airing cupboard. So I guess there's anything between 6m and 8m of pipe (all 22mm; some copper, some polybutylene). I have tried to lag this as much as possible.

I used the "economy" pipe wrap. This claims to reduce heat loss by 70% but that still means I could be losing a third. I didn't use the thicker "bylaw" insulation; that claims to reduce loses by over 75% but doesn't say how much over 75%. If it is a case of 70% vs. 75%, does that 5% make much difference? I think I went with the economy version not just because it was cheaper but because I don't think there was room to use that thicker stuff.

Once the hot water gets to the cylinder, I don't know anything about what is inside. Would I be right to think that some cylinders have more efficient heat exchangers than others?

I guess people with gas boilers have an easy way to do this: read the meter before and after heating the water! If only I had an oil meter, I could find out how many litres were used at x pence per litre. I have seen some oil meters in BES but they are over a hundred pounds, which is an expensive way of finding out!

One reason for asking is I'm wondering whether to abandon economy seven. We try to run the dishwasher and washing machine overnight to take advantage of it, but I imagine it is the immersion heater that is the biggest load. If that was gone, I could switch to a traditional tariff and enjoy a lower rate during the day.

Economy seven was ok before we had children but now it's harder to schedule washing to be on overnight as the washing machine and dishwasher need to be on much more frequently now!

TIA

[Owain]

I think 98% efficiency actually, to allow for thermal losses from the cylinder. But that will be the same for oil or electricity, so can be ignored

Will be the same for oil and electricity, so can be ignored

Will be the same for oil and electricity, so can be ignored

Will be the same for oil and electricity, so can be ignored. Remember you do not need the actual cost in =A3 to heat the water, only the relative costs of doing so,

But the efficiency should (nominally) be the same regardless of output. It would take 3x as long to heat the water with a 1kW immersion as with a 3kW immersion, but the efficiency and the total cost would be (about) the same.

No, because it is 5% of the heat loss, not 5% of the total energy, and in winter this will contribute to the house heat anyway.

If the heat isn't transferred to teh stored hot water it just goes back to the boiler and circulates round.

You can assume your immersion heater is as near 100% efficient as dammit. So price per kWh on electric bill =3D price per kWh used.

You need to get the price per kWh of your oil [1], divide by 0.8 if your comdensing oil boiler is nominally 80% efficient, to get the price per kWh used.

Owain

[1] 10.27 kWh/litre or 10.22 kWh/litre according to footnote 1 at Other calculators will use different figures. So if you're paying 60p/litre it's about 7p/kWH going into the boiler. Divide by efficiency to get the cost of useful heat coming out of the boiler.

[The Natural Philosopher]

Very roughly 1/10th of a liter of oil is one unit of electricity.

Which puts the break even price about 4.7p per unit electricity.

[Dave Liquorice]

Bung make and model of boiler into google and see if you can find out if it is a modulating boiler. I have a sneaky feeling that oil boilers don't modulate as it's not quite so easy to turn high pressure oil down and maintain the spray pattern as it is to just close a gas valve a bit...

10kWh/litre is near enough for us.

Fepping heck I hope heating oil isn't that price! Low 40's p/l should be more like it. so the cost of energy input to the boiler is 4 to

5p/kwHr.

When you find the boiler data you'll probably find an effciency as well. Bear in mind that might be marketing best case so it might be worth knocking 5% of it for the real world. B-) So lets say 6p/kWhr for the energy out of the boiler.

I suspect that the night rate E7 electricity will be less than

6p/kWhr, but is the water ever heated at day rate rate which may well be more like 12p/kWHr... This balance between day/night rate and when the water is actually heated is the hard bit.

[Chris J Dixon]

With a tank which came with reasonably thick foam insulation, is it practical or worthwhile to add more?

Chris

[The Natural Philosopher]

not a lot, no.

[Fred]

Thanks. Is the same true of a gas boiler? Only in my limited experience gas users don't seem to have/use immersion heaters, they use the boiler. the only people I know with economy 7 are people without gas.

Lots of interesting ideas, thanks. I'm not sure about the electric shower: sometime quality of shower is more important than quantity of fuel used to heat water, or perhaps I have just been unlucky with the electric showers I have had (poor flow)?

Under sink heaters would be a good idea regardless, because I think most people have a few seconds when the water flows cold before they get any hot out and under sink heaters would remove this. However if the object of this exercise is to save money, I don't think the fuel savings would outweigh the cost of several under sink heaters. Aren't they about £100 each?

I'm in a hard water area. I thought under sink heaters did not like hard water and the elements needed changing annually?

Thanks again.

[Fred]

Thanks for all your help. IIRC it said something like 15-21kW output, so I assumed it modulated between the two limits but I will go away and look it up properly. Thanks for all your help.

[Fred]

Thanks everyone for your replies. A few of them have mentioned litres and that's the problem I have. I don't have an oil meter. If only I did, it would be so simple!

Thanks again.

[GAP]

Someone on this group previously suggested that if you recorded how long the boiler was firing for, then some mathematics on the jet size would reveal how much oil you were using. I quite liked this idea, but I couldn't easily find an hour meter that I could recalibrate (using the aforesaid maths) to read directly in litres.

[alexander.keys1]

HARRY ENFIELD: You don't wanna do that...

...that cupboard is going to be a fire risk, the lagging will insulate the electric cables too, and may attack the sheath chemically, especially if the cables get hot, which the ones supplying the IH will do, as they are carrying a fairly heavy current for long periods.

[Dave Liquorice]

You don't need one. Read your electric meter or do the maths on the heaters consumption against time and divide the number of electric units by ten to get the number of litres of oil equivilant. Then correct that number by the boilers effciency to get the number of litres oil input. It'll be an approximation but close enough.

[Fred]

Thank you very much. Of course you are right, I don't need to know how many kW of oil the boiler is actually using, if I know how many kW of electric are used, I only need to know how much the same number of kW of oil would cost (after efficiency is taken into consideration). Thanks again.