Gas Vs E7 hot water

[jives11]

Well my recent combined gas & elec bill was higher than I thought, and now that both are given in kWh I see my gas consumption seems very high (78 days : 2266 kWh total elec, 9446 kWh total gas) this is for a 5 bed house with 2 adults + 2 teenage boys. Central heating is standard fully pumped (non-condensing though boiler only 5 years old). Hot tank is double insulated. central pump feeds multiple showers.

My house also has E7 and I'm wondering if it would be cheaper to use , say, an hours immersion boost during E7 to reduce the gas burn to heat the HW. We do use a lot of HW as with boys and power showers .

Scottish power costs are:

3.55p per kWh E7 night 8.08p per kWh E7 day 2.65p per kWh Gas

The gas changes a bit due to the calorific value factor.

On paper Gas is cheaper but I think it's more complex than that ?

When heating h/w via gas the pipe works routes hot water from the pump, via the boiler, through the coil in the tank. The pipes route through the upstairs floor over several meters, so heat loss into the house must occur along the length and in transferring between the coil and the water in the tank. Also a number of bathroom radiators appear to be in this circuit, so I can dry towels in summer without needing the full heating on. Now of course all this heat loss does have a secondary benefit in helping to heat the house. the immersion is in the tank so presumably is a more efficient way of heating the water that surrounds it.

The gas system has a strap on stat, so presumably shuts stops heating water as soon as the tank is at temperature. I'm not familiar with immersions but believe they have an integral stat, so theoretically would not over-heat the water either if it's reasonably accurate.

has anyone decided whether E7 is cheaper to heat H/W than a gas fired system ?

[auctions]

Assuming your boiler is roughly 80% efficient, that makes up some of the difference - the immersion heater is virtually 100%. At 2.65p per

0.8kwh, that's 3.31p per kwh for the gas. 3.31/3.55 is about 0.93, so as long as more than 7% of the heat is lost through the radiators and pipework, it's cheaper to use the E7. A

[Owain]

However, the E7 tariff has significantly more expensive daytime electricity, so unless there's the immersion, washer+dryer, and probably at least some storage heating on as well at night, the overall electricity cost is greater with E7 than it would be with a standard tariff.

It's probably cheaper overall to heat the water with gas (lagging those long pipes if wanted - towels on the towel rails will lag them quite nicely) and change to a more competitive electricity tariff, dropping the E7.

Turning down the shower pump slightly would also reduce hot water use.

Owain

[meow2222]

the only time E7 works out cheaper is if youve got an old cast iron exchanger boiler*. These have low efficiency, and a fair bit of heat is stored in the exchanger, reducing efficiency further when cycling. Otherwise gas is the thing.

I think you chose your bill when you put that power shower in.

NT

• I daresay someone can posit an exception, but normally the case.

[jives11]

Thanks to all for the responses, all of which seem reasonable. I think the comment about the power showers is spot on, but it's done now.

For the sake of =A325 I'm tempted to install an immersion timer and do some testing. My boiler is a potterton from about 5 years old and very small, so doubt it has a cast iron exchanger. My bills indicate approx

20% of my elec is in the E7 zone, made up of background + time displaced washing machine, tumble dryer and dishwasher, which I think is around the break even point , last time I checked.

jives11 wrote:

[Dave Liquorice]

Certainly ball park see my post over in the "timed meter" thread Message-ID: .

As for which is cheapest, E7 or gas, it's close, very close. Assuming 75% overall effciency on your gas to HW and 100% for the E7, the price difference is negliable.

[David Hansen]

On 2 Dec 2006 04:43:07 -0800 someone who may be "jives11" wrote this:-

There is generally not much in it financially.

You didn't say what size your hot water cylinder is, but I suspect it is not large enough to cope with daily demand if heated overnight. If that is the case then Economy 7 is no more than a supplementary form of heating that will probably not save money, though it will move consumption from gas to electricity. With a properly sized cylinder in summer gas just becomes a supplementary form of heating, seldom used.

Unless they are very difficult to get at, insulate the flow and return pipes between boiler and cylinder, plus the hot water pipes from the cylinder to taps. Could the bathroom radiators be replaced or supplemented by towel rails?

It sounds as if you don't have Economy 7 specific wiring. In that case you undoubtedly have a short immersion heater. It would be far better to get a long immersion heater to heat the whole cylinder rather than just the top.

Other than bills are there any other problems, like slow heating of water?

[Ian Stirling]

Another wrap of 100-270mm of rockwool round the cylinder isn't a bad plan at all, especially if you happen to have some spare.