At what point does immersion beat boiler on costs?

potts8

We currently have a recently installed oil fired boiler(Greenstar Danesmoor) which we use to heat our water however I'm also looking to switch my energy tariff to Octopus GO which will allow us access to elec at approx 5p KWh.

Has anyone done the calculations for the tipping point of it becoming more beneficial to switch to an immersion?

brewerdave

Don't need to do any maths - provided that you ONLY heat water in the cheap Octopus period, the immersion heater will definitely win over oil (and the way gas prices are going it'll probably beat a gas boiler)

QrizB

Here's how to calculate it.

• You need to know the cost of your oil per litre (or gallon, or pint, or bushel, as the case may be).
• You then need to know the number of kilowatt-hours of energy in a litre (or gallon etc). Fortunately this is fairly standard and easily Googled, eg. table here. It's around 10.
• Divide the price of a litre by the kwh-per-litre to get the raw energy cost of your oil, in pence per kWh.
• you then need to asjust for the efficiency of your boiler. This is likely to be around 90% for a modern boiler, so divide the price per kWh by 0.9 (or multiply by 1.1, if that's easier).

Eg. if oil is 50p/litre, and 10kWh/litre, that's 5p/kWh. Adjusting for boiler efficiency, that becomes 5.5p/kWh.

Electricity is priced in pence per kWh and immersion heaters are 100% efficient so there's no maths involved. If electricity is 5p/kWh, that's what it costs.

In my example electricity is slightly cheaper than oil.

potts8

brewerdave said:

Don't need to do any maths - provided that you ONLY heat water in the cheap Octopus period, the immersion heater will definitely win over oil (and the way gas prices are going it'll probably beat a gas boiler)

Thanks, this aligns to what I was thinking. In the current climate it seems best to heat via immersion assuming you have cheap rates at night during the period where you have no central heating on. When the time comes when you do have the heating on then it may blur the lines in what's more economical if you are also heating the house at the same time as the water cylinder?

brewerdave

potts8 said:

brewerdave said:

Don't need to do any maths - provided that you ONLY heat water in the cheap Octopus period, the immersion heater will definitely win over oil (and the way gas prices are going it'll probably beat a gas boiler)

Thanks, this aligns to what I was thinking. In the current climate it seems best to heat via immersion assuming you have cheap rates at night during the period where you have no central heating on. When the time comes when you do have the heating on then it may blur the lines in what's more economical if you are also heating the house at the same time as the water cylinder?

Not really ,because you will need to use the same amount of oil to produce the hot water whenever you heat it up. Obviously once you have used the hot water produced overnight by the immersion then it'll be cheaper to heat more in the day using oil.
Possibly with a brand new oil boiler you may get near to the  5p/kwh of the Octopus leccy - altho I doubt it. We reckoned a relative's oil system was more like 10p/kwh some time ago.

[Deleted User]

A factor that needs to be taken into account is the length of the pipe run from the boiler to the HW cylinder. Unless this is lagged, it is to all intents and purposes a radiator. An immersion heater doesn’t have this constraint. FWiW, I use solar diversion to heat my HW cylinder in the summer. The average consumption is about 4kWs. Heating by gas is closer to 7kWhs. Even though the two thermostats read the same temperature, I sense that the gas powered coil heats up more of the cylinder than does the immersion heater.

coffeehound

The situation is complicated by the condensing function of the boiler.  Once the water in the tank passes a certain temperature, the return flow to the boiler will also get warmer to the point where the efficiency falls away.  So Go and Go faster are a good bet for water heating. 

tux900

Dolor said:

I sense that the gas powered coil heats up more of the cylinder than does the immersion heater.

That's a good point and indeed an immersion element is typically some way up the side of the cylinder whereas the heating coil extends down nearer the bottom hence is heating up a greater volume of water.

coffeehound said:

The situation is complicated by the condensing function of the boiler.  Once the water in the tank passes a certain temperature, the return flow to the boiler will also get warmer to the point where the efficiency falls away. 

And to add to that, many boiler controls these days will demand that the boiler runs flat out during hot water reheat in order to maximise the heat transfer to the cylinder and reduce reheat times. In doing so the boiler is likely not condensing for a good chunk of the time and hence won't be achieving the headline efficiency figures.

All said and done, however, I suspect that hot water heating costs are a fairly small proportion of overall energy costs for most people. Every case will be different of course, but I know that our gas usage during non-heating periods (which means just hot water heating and some hob usage) is less than 5kWh/day and so there's a limit to how much savings there are to be made in absolute terms. It all adds up of course.

lohr500

It may not be a constraint depending on the size of your immersion heater, size of tank and how far down the tank the immersion heater reaches, but with a large tank and a smaller 3kW heater, you may not be able to heat it up fully within the 4 hour Octopus Go off peak window. 

There is a calculator here that shows the time taken, based on water volume and heater size.

 https://www.sust-it.net/immersion-heater-energy-calculator.php

I couldn't switch to the Octopus Go or Go Faster tariffs because we need a longer off peak period to heat up our Electric AGA, so I am using our new oil boiler to heat the water. Last oil fill in August cost £0.41 per litre including VAT, so £0.041 per kw/h. Even with boiler efficiency losses, I still think it is more economical right now to use the oil boiler over electricity even at £0.05 per kw/h. Obviously if kerosene prices rocket, then the story may be different.

The other advantage of using the boiler, is that we don't have to worry about running out of hot water later on in the day or evening. I guess this could have been solved by installing a larger immersion tank, but space is limited.

Rodders53

My latest fill of oil was @ 4.53 p per kWh inc 5% vat (41.7p/l exc).  Bedfordshire.  Oil prices vary a lot with location, so OP needs to check their price.

My HW is set to be on for circa 1 hour in morning and early evening.  I keep meaning to set it to once a day, but to be frank the oil used over the summer has been so low I had to wait as long as possible to get a nearly 1000 litres in the tank.  (Probably using around 100 litres or so in 4 months).

Using off peak electricity at 5.00 p inc VAT would be OK if the tank is well enough insulated and large enough to remain stored for use when needed.

Our tank has a reheat spec of 28 mins on 26kW through the coil; or 202 minutes using the 3kW immersion (i.e. almost 3 1/2 hours)... It also loses just over 2kW of heat every 24 hours.  OP needs to study their tank specifications carefully to do a calculation.

In the 5 years we've been on oil the fills have varied between 3.40 and 5.16 pence per kWh.

So at the present imho, its a 50:50 and no clear winner.  Both E7 and oil price changes in the future may change that of course.

coffeehound

lohr500 said:

It may not be a constraint depending on the size of your immersion heater, size of tank and how far down the tank the immersion heater reaches, but with a large tank and a smaller 3kW heater, you may not be able to heat it up fully within the 4 hour Octopus Go off peak window. 

In theory (and assuming my maths is right) a 250-litre cylinder could be heated from 20 to 60 ℃ in four hours, so most will be fine with the four or five hour off-peak tariffs.  Also the four hour one is at 4.5p IIRC?  I suppose those with a large heat store that is maintained at a higher temperature than 60 could struggle.  

A different approach would be to heat up to say 45º with the boiler, and then top-up to 60 from there with electric, thereby reducing the time the boiler spends running at low efficiency.  From the data sheets, it seems that the latest boilers give efficiency in the mid-to-high 80s when out of condensing mode.