Immersion heater or Oil boiler?

D00gie72

Hello, Hope someone on here has more experience than me. I've just moved into a new house. It has an oil fired boiler and an immersion heater with a timer switch. The property also has economy electricity over night. The question is - would it be cheaper to heat my water with the oil boiler or use the immersion heater using the economy overnight electricity. My washing machine is just cold water fed - and the shower in the house is a power shower so we really only need hot water in the house for washing up brief washes and the occasional (and I mean occasional bath).

Any thoughts would be appreciated. Thanks.

Smiley_Dan

There are a number of things to consider. Where is the immersion coil positioned in the cylinder? This affects how much water is heated and thus how much energy it requires and therefore how much it will cost.

The other major thing is how much your E7 rate is.

There are other things that matter: how much water you actually need, the temp the immersion thermostat is set to, the cost you can purchase oil at, the efficiency of your oil boiler and the length of runs between the boiler and the cylinder.

Normally, E7 would be cheaper than oil overall because even if the £/kwh is higher, boilers are notoriously inefficient at heating cylinders and you may not even need all that water.

If you were using daytime rate then the other variables would come into consideration.

But the real answer is: "it depends". You can always take meter readings and gauge it yourself.

matelodave

You would probably be better off heating the water with the immersion heater for an hour or so before you get up during the off-peak rate. Providing that you've got a properly insulated tank the water should stay hot enough to see you through the day - putting another jacket over the tank will help, as will some pipe insulation over the pipes leading to & from the tank.

It all comes down to the cost of your off peak electricity, the cost of oil, the efficiency of your boiler and how much hot water you use. Although oil might be slightly cheaper per kwh you'll lose heat via the pipework between it and the tank as well as up the chimney and it will use electricity running the pump & boiler so it's not so clear.

Try to use your electric shower during the off peak time to minimise the cost and don't stand in it too long either. Using other stuff like the washing machine, tumble dryers and dishwashers during the off peak time will also keep your costs down.

Cardew

I would shut down oil boiler for the summer and use immersion.

Robwiz

+1 with Cardew. I have found it worth doing that with a gas boiler too.

I'd be tempted to leave the immersion switched off most of the time and use the kettle for washing up and I'd wash in cold water (which in the summer should be close to room temperature anyway) unless the distance from the cylinder to the taps is so short that you won't be wasting much hot water in the pipes.

Just power up the immersion using E7 the night before you want a bath.

matelodave

For those who are interested there was an Energy saving Trust trial on boilers and efficiencies a few years ago which came to the conclusion that boilers aren't quite as efficient as specified when they are incorporated into systems. Contributing factors were heat losses in the pipework especially in the summer where it didn't contribute to house heating. Heat loss from the hot water tank and the plumbing surrounding it which helps dissipate heat into the house. Short or small draw off of hot water (such as hand washing or rinsing stuff) wasted a lot of heat compared with say filling a bath. So on balance either shut the boiler off in the summer as Cardew suggests or do what robwiz suggests and only heat water when you want it.


see here for chapter and verse http://www.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report

J_B

D00gie72 wrote: »

and the shower in the house is a power shower

We are all assuming that the water in this is heated as it goes through, but some power showers use hot water from the hot water cylinder - can you confirm?

Robwiz

Thanks for the link to the EST report, Dave. The discussion of summer water heating on p81 caught my eye:

'For summer months, the regular boilers were efficient for generating DHW to the cylinder (average heat efficiency 81%) but recorded heat delivered to taps was much lower. The average efficiency of heat delivered to the taps is only 38%, and much lower where little DHW is used (range 13% to 65%).'

This confirms my empirical measurement for my gas boiler. For an average efficiency of 38%, the lower unit cost of gas is cancelled out (about 3:1 per kWh) and in 'worst case' situations, e.g. 13%, electricity could be significantly cheaper.

Our kitchen hot tap has to discharge 10 litres before it runs hot enough for washing up. So every time we wash up in the sink it costs 15 litres of water, approximately 10% of our cylinder's capacity. Which is why we boil the kettle!

Cardew

J_B wrote: »

We are all assuming that the water in this is heated as it goes through, but some power showers use hot water from the hot water cylinder - can you confirm?

Good point!


Loads of people on MSE incorrectly refer to an electric shower as a power shower.


An electric shower heats cold water within the shower.


A power shower is supplied with hot water from the Hot water tank, mixes this with cold water and pumps the water at high pressure to the spray.


My reading of the OP's post is that he has an electric shower rather than a 'power shower'.

Smiley_Dan

J_B wrote: »

We are all assuming that the water in this is heated as it goes through, but some power showers use hot water from the hot water cylinder - can you confirm?

Yes, I just followed his question, I didn't think that he might just mean an inline heater. He said "power shower" so I assumed it came from a hot tank (and cold tank).

Re-reading what he wrote it looks like it isn't a power shower because he implies the shower doesn't draw from the hot tank.

TickersPlaysPop

matelodave wrote: »

For those who are interested there was an Energy saving Trust trial on boilers and efficiencies a few years ago which came to the conclusion that boilers aren't quite as efficient as specified when they are incorporated into systems. Contributing factors were heat losses in the pipework especially in the summer where it didn't contribute to house heating. Heat loss from the hot water tank and the plumbing surrounding it which helps dissipate heat into the house. Short or small draw off of hot water (such as hand washing or rinsing stuff) wasted a lot of heat compared with say filling a bath. So on balance either shut the boiler off in the summer as Cardew suggests or do what robwiz suggests and only heat water when you want it.

see here for chapter and verse http://www.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report

No need to read the link, it's just common sense isn't it?!