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