Gas or Electric for summer hot water

macman

You are clearly producing more, or hotter, hot water when using gas.

jalexa

gyromain wrote: »

1.845 cu ft per day.
Gas units are converted to kilowatt hours using the following
1.845 x 2.83 x 39.4 x 1.022640 / 3.6 = 58.44kWh
58.44kWh x 3.616p = £2.11 per day!

Your conversion is wrong. 100cu ft (1 on an imperial meter ) is approx 32kWhrs.

So if "1.845cu ft per day" is right then the usage is 0.58kWhrs. Which I don't believe any more than I would believe 184 cu ft per day which would heat the entire house on a cool day.

HappyMJ

gyromain wrote: »

Over the last 2 days of summer weather I have measured how much gas I have used heating hot water only. Over the 2 days, gas used 3.69 cu ft - 1.845 cu ft per day.
Gas units are converted to kilowatt hours using the following
1.845 x 2.83 x 39.4 x 1.022640 / 3.6 = 58.44kWh
58.44kWh x 3.616p = £2.11 per day!

Immersion heater 3kW takes 2.5 hours to heat tank from cold.
3 x 2.5 = 7.5 kWh x 6.149p E7 = £0.46
When using the immersion heater we only occasionally run out of E7 heated water and have use day units on the boost button.

I must switch the immersion heater on.

I know has boilers aren't efficient at heating water in a cylinder but that is just a bit too inefficient. My figures for the hot water are 30 minutes of gas boiler per day heats the full cylinder back up to temperature (not from cold). It's a 14.65kW input boiler so that is 7.3kWh. The electric immersion uses about 4kWh of electric per night to top up the heat from the previous night. This implies the boiler is about 55% efficient at heating the water which tallies with the efficiency ratings on SEDBUK for hot water. My primary gas rate is 7.173p (I have never used enough gas in summer to hit the secondary rate) and my E7 rate is 4.49p.

Gas = 52p per day.
E7 heated water = 18p per night.

So for me and only me E7 heated water is cheaper. For 90% of households the gas is cheaper.

gyromain

The gas meter readings were made at the same time of day.
21 May - 3797.47
22 May - 3799.58
23 May - 3801.16

The conversion factor calculation was copied from a BG bill.

The house is three storeys high. The gas boiler is in the basement and the hot water tank is on the top floor. They are connected via 22mm pipe using convection.

Pincher

It takes 1 kCal to raise 1kg (~1 litre) of water by 1 degree C.
Let us say you have a 180 litre cylinder,
which you are heating from 15 degrees to 65, raising 180 litres by 50 degrees,

you will need 180 x 50 = 9,000kCal

1,000 kCal = 1.17kWh

So 9,000 kCal = 1.17 x 9 = 10.53kWh

Assuming you are heating from lukewarm water, 7.5kWh is reasonable.

Assuming your boiler is 75% efficient, you would use 10kWh in gas to heat water by 7.5kWh.

10kWh of gas at 3.616p is 36.16p

So it should be:

Gas 36p vs. E7 46p.

I assume you are cooking by gas, and topping up the hot water during the day, to account for the other 48kWh.

Gloomendoom

HappyMJ wrote: »

So for me and only me E7 heated water is cheaper. For 90% of households the gas is cheaper.

It's not just you. Our gas boiler gets turned off in the summer and the immersion heater used instead. I did a comparison over two summers a few years ago and IIRC electricity costs about a quarter of the price of gas.

dogshome

Have a look at the Hot Water tank - you will see that the feeder pipe from the boiler enters the tank about 2/3rds down from the top, so when you heat water from the boiler it heats a lot of water - which is what you need when you have a family all wanting to shower within a short time period

Your immersion heater is mounted on the top of the tank and heats less water, which is great for just one persons shower (Some immersion heaters have two elements, one long and one short and you can choose which one to use )

macman

Gloomendoom wrote: »

It's not just you. Our gas boiler gets turned off in the summer and the immersion heater used instead. I did a comparison over two summers a few years ago and IIRC electricity costs about a quarter of the price of gas.

No. electricity cost 3 times as much as gas! Even E7 cheap rate is still more than gas per kWH. Even an inefficient old boiler will not cancel that out.

HappyMJ

macman wrote: »

No. electricity cost 3 times as much as gas! Even E7 cheap rate is still more than gas per kWH. Even an inefficient old boiler will not cancel that out.

Not in the summer though when only the primary rate is used. I get 4572kWh of gas at the primary rate. Over 6 months between May and November when the meter is read and the bill produced I do not use more than 2286kWh of gas so I am only charged primary rate units on all the gas used. That is as I stated 7.173p/kWh and E7 electric is as I stated 4.49p/kWh.

Gloomendoom

macman wrote: »

No. electricity cost 3 times as much as gas! Even E7 cheap rate is still more than gas per kWH. Even an inefficient old boiler will not cancel that out.

I beg to differ! My bills prove otherwise. In the summer my gas costs just over 2.3 times more than my E7 per KW. This, combined with the heat losses from the very long pipe runs from the boiler with my particular installation, make electricity a clear winner. Every KW of it is used to heat the water.