oil boiler or electric immersion to heat water?

James123_2

Cardew wrote: »

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

quote]

Thanks ... the rising cost of living has meant that I've (sadly) become something of an 'energy anorak'. Before long, I'll be checking the meter each day ... I clearly need to get a life!
I'm sure your figures are correct. Yes, immersion uses 2kwh which is my 20p. A boiler, however efficient, is creating heat some way from the point at which its needed - hence the apparent disparity. (Mine is an A Rated Boiler Idea Icos HE18) It uses 2m3 for a full hour of burn and takes 45 minutes to deliver a full tank (with no heating on). As such, whatever the statistics should imply, it is costing me more for a tank of the hot stuff. The thermostat on the tank is set a a lowly 60 degrees to prevent boiling.
Thanks again for the informative piece ... was interesting!!

Cardew

[quote=James123;A boiler, however efficient, is creating heat some way from the point at which its needed - hence the apparent disparity. (Mine is an A Rated Boiler Idea Icos HE18) It uses 2m3 for a full hour of burn and takes 45 minutes to deliver a full tank (with no heating on). As such, whatever the statistics should imply, it is costing me more for a tank of the hot stuff. The thermostat on the tank is set a a lowly 60 degrees to prevent boiling.
Thanks again for the informative piece ... was interesting!![/quote]

Well you have a very serious fault with your boiler/CHsystem if your measurements are correct.

From the SEDBUK tables your boiler has an efficiency of 90.3%

[FONT=Arial, Helvetica, sans-serif]

[FONT=Arial, Helvetica, sans-serif]Brand:[/FONT][FONT=Arial, Helvetica, sans-serif]Ideal[/FONT][FONT=Arial, Helvetica, sans-serif]Model name: [/FONT][FONT=Arial, Helvetica, sans-serif]icos [/FONT][FONT=Arial, Helvetica, sans-serif]Model qualifier:[/FONT][FONT=Arial, Helvetica, sans-serif]HE18 [/FONT][FONT=Arial, Helvetica, sans-serif]Model Identity no.:[/FONT][FONT=Arial, Helvetica, sans-serif]47-397-84[/FONT][FONT=Arial, Helvetica, sans-serif]First Manufactured:[/FONT][FONT=Arial, Helvetica, sans-serif]2003 [/FONT][FONT=Arial, Helvetica, sans-serif]Last Manufactured:[/FONT][FONT=Arial, Helvetica, sans-serif]Current[/FONT]
Boiler last updated on database 17/Jan/2006

[FONT=Arial, Helvetica, sans-serif]SAP seasonal efficiency:[/FONT]

90.3%

Efficiency band:

[FONT=Arial, Helvetica, sans-serif]Efficiency category:[/FONT]

[FONT=Arial, Helvetica, sans-serif]SEDBUK Certified[/FONT]

[FONT=Arial, Helvetica, sans-serif]SAP equation used:[/FONT]

102

Estimate of Annual Fuel Cost

Power:

[FONT=Arial, Helvetica, sans-serif]modulating 18.2 - 18.2 kW[/FONT]

[FONT=Arial, Helvetica, sans-serif]Main type:[/FONT]

[FONT=Arial, Helvetica, sans-serif]Mounting:[/FONT]

[FONT=Arial, Helvetica, sans-serif]Regular[/FONT]
[FONT=Arial, Helvetica, sans-serif]Wall[/FONT]

[FONT=Arial, Helvetica, sans-serif]Condensing[/FONT]

[FONT=Arial, Helvetica, sans-serif]Fuel:[/FONT]

[FONT=Arial, Helvetica, sans-serif]Exposure:[/FONT]
[FONT=Arial, Helvetica, sans-serif]Flue:[/FONT]

[FONT=Arial, Helvetica, sans-serif]GAS[/FONT]
[FONT=Arial, Helvetica, sans-serif]Indoor[/FONT]
[FONT=Arial, Helvetica, sans-serif]Room-sealed:Fan[/FONT][FONT=Times New Roman, Times, serif]Burner control:[/FONT]
[FONT=Times New Roman, Times, serif]Elec power firing:[/FONT][FONT=Times New Roman, Times, serif]Variable[/FONT]
[FONT=Times New Roman, Times, serif]38 W[/FONT][FONT=Times New Roman, Times, serif]Ignition:[/FONT]
[FONT=Times New Roman, Times, serif]Elec power not firing:[/FONT]
[FONT=Times New Roman, Times, serif]No Permanent Pilot Light[/FONT]
[FONT=Times New Roman, Times, serif]W[/FONT]

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So to produce the same 2kWh as your immersion heater your boiler should use approx 2.2kWh.
Now there will be a little loss in the pipes to the tank but that is tiny.

You are 'losing' over 14kWh and that is a massive amount of heat.
I can only surmise that you have a problem and that heat is going into you CH.

The purpose of my posting is simply in case anyone thinks that your conclusion(immersion is cheaper than gas) is correct.

Read lots of posts on here and you will find that people who use gas for HW in the summer use a few kWh a week.

malc_b

The only way to do an accurate comparison would be to to stir the water in the HW cylinder and measure the final temperature. That would tell you the energy in the water after xx minutes of gas/electric. I suspect that the electric is heating the top, usual electric heater position is a top entry and it is probably a short heater that doesn't reach the bottom of the cylinder. The gas heat exchanger is will be in the bottom. The other problem might be that the gas is cutting out or you may have a modulating boiler that can vary the burn rate. Are you just saying xx min = xx gas? For modulating boilers that is not the case. Or if an older type have you watched it run continuously?

James123_2

malc_b wrote: »

The only way to do an accurate comparison would be to to stir the water in the HW cylinder and measure the final temperature. The other problem might be that the gas is cutting out or you may have a modulating boiler that can vary the burn rate. Are you just saying xx min = xx gas? For modulating boilers that is not the case. Or if an older type have you watched it run continuously?

Woow - thanks for all this. Thanks, too, for googling the specs for my boiler. The calculation is simply that of watching & waiting from the point of the boiler firing up to it finally getting to a sufficient temperature that the pump closes down . Obviously a lot depends on the ambient temperature of the tank initially. However, I can assure you that the amount stated is that as used by the boiler to heat the tank. The boiler is brand new and I have been surprised at how it does not seem to be more efficient than my aged Potterton, circa 1990. Obviously, it should be on the ratings as detailed.
Have a good Sunday!

malc_b

Think about this some more I think that you're not comparing like with like in that the electric is just heating the top and the gas is heating all of it, including the bit the electric hasn't heated, hence the gas reads so high.

I put in a new HW cylinder a few years back. It was one of the foamed type. The figures on that (does yours have a label?) were 140l, 2.51kW loss/day. BTW the figure for estimating HW use is 38+25N / day where N is the number of people in the house. 1kW raises 100l by 8.57C. So for say 140l, from 10C-60C, takes 8.75kW. Something's wrong with your 16kW figure as that would be 55% efficiency, unless you have a much bigger cylinder (my 140l cylinder is 1050x450mm hxd).

However, looking at the boiler spec it is a modulating type. The output is 8.8-18.2kW so you can't just say 18.2kW x 45min since it could be running at 8.8kW. You would have to note gas use before and after, if the meter is accurate enough to read such a small use.

Probably the only way to get accurate results would be the wait until summer then run a month using gas and month using electric. Even then you would have difficulty getting the electric figure accurate as the electric thermostat would cut out so you'd over estimated the electric used.

James123_2

Hi, yes, I do have a slightly larger water tank - I think it's 180 litres, which is roughly a third larger than the average. The very minimum it takes in gas to top-up the tank is three quarters of a cubic metre - the 1.5 is taken from cold.
I guess we could end up splitting hairs here, but even on the most conservative assumptions of gas use there still doesn't seem to be much to be gained in firing-up the boiler once a day as opposed to two bursts of the immersion which, as indicated, only heats the top half of the cylinder. (Two bursts are equivalent to a full tank - so my supposition goes)
Clearly it all depends on your electricity tariff - I chase the best deal which is currently Click 6 from BG. Anyway, I found your post fascinating and, on paper, the boiler should certainly be the way to go for stand-alone water heating. I'll review this from time to time, but first indications still make me believe that the immersion is cheaper or, at worst, about the same cost as using the boiler.
Thanks again. I wouldn't have taken such an avid interest had I not lost my job due to ill health in the last year.

Cardew

malc_b wrote: »

I put in a new HW cylinder a few years back. It was one of the foamed type. The figures on that (does yours have a label?) were 140l, 2.51kW loss/day. .

Interesting statistic.

I believe it is mandatory now for all new HW cylinders to have a 30mm foam insulation 'jacket'.

I have seen plenty of figures bandied about that indicate a HW tank at 60C will lose approx 3kWh loss per day. i.e. if no water were run off it would take 3kWh to keep it at 60C.

However I certainly have never heard of a label giving the specification - especially to two decimal places i.e. 2.51kW(I assume that is 2.51kWh?)

Presumably that loss is measured under some sort of standard conditions e.g. HW temp xxC ambient temp xxC?

Does your label give any indication of how such loss is determined - British Standard perhaps?

malc_b

Yep that should have been 2.51kWh (dunno why what I say to myself my fingers don't type?).

The label say:

Part L Compliant
Manufactured to: BS1566, DFG, REF 8

Capacity: 140 litres
Max length of immersion heater: 850mm
Max working pressure of primary heater: 3.5 Bar
Area of heating surface of primary heater: 0.70m^2
Standard heat loss: 2.51(Kw h/24h)


Grade 3
BS1566 2002

If you want I have photos of the label (easier to read it later, especially as I think it ended up at the back). I try to remember to that photos of a lot of my DIY. That way I can check back to see pipe/cable runs etc.

Cardew

Thanks - looked up the British Standard but you have to buy it- and that ain't Money Saving.

However the 2.51 kWh confirms that the figure of 3kWh loss in 24 hours is reasonable.

malc_b

Larger libraries often have BS online service you can use to look at BS standards on one of their PC. Central Bristol does or used to, for example. University libraries do too usually and students can usually get those on their own PC if they are on the Uni network (most Uni have a net login to remote access). Sometimes Uni libraries allow public access. So there are a couple of free routes that you could try to get a look at it.