Is it cheaper to heat water with immersion or gas?

Snookie12cat

I have been using gas to heat my water and its coming in at roughly £20 a month to do this. I have it come on 20 mins in the morning and evening. My boiler is 20 years old and is not the most efficient as its costing me 70p an hour to heat my house...

Today I got thinking and tested the immersion on my tank and it runs at 3kw an hour or 60p an hour currently. Would it not be cheaper to heat it using electric for the same amount of time as I currently do or does it work out very much the same?

Is there something I am not considering here?

Astria

If your immersion heater is 3KW and you run it for an hour that would surely be 3x34 = £1, not 60p?

Gas is currently 3xcheaper than electric at 10p/kWh, so it would need to use 10KW to get the same price.

I very much doubt it's use that amount of gas to heat your water.

How many hours did you have your heating on for the 70p test? There's lots of factors in that such as starting temperature, expected temperature, temperature of the water, surface area of radiators, etc.

Snookie12cat

I am on a good tariff and so my electric is 19p and my gas 4p. I always thought it was more expensive to use electric, but at 60p an hour as specified by my smart meter and knowing how much I pay to heat my water each month, it does not appear gas is significantly cheaper - definitely not 5xs cheaper. 

I put the heating on for an hour and the smart reader told me i spent 68p. I have therefore used that as the benchmark for cost and it does not seem far out. If I have it on longer, its about that price each hour. 

My smart meter says I use between 10 - 18kw a day heating my HW...

chris_n

Your boiler will heat a lot more water in 20 mins than the immersion heater.

[Deleted User]

It is not a simple 'like for like' equation.

The major factor is the difference in cost per unit for gas and electricity.

Other factors include the efficiency of your boiler (note; most condensing boilers do not condense when heating a cylinder); the length of the pipe run between the boiler and the cylinder, and the required cylinder temperature.

I re-heat my cylinder to 65C using surplus solar energy and an immersion heater. Yesterday, it took 3.3kWh of electricity to re-heat a 216L unvented cylinder. Had I elected for gas, this would have been in excess of 8kWh.

When a gas boiler first comes on, the temperature of the flow water is less than the cylinder temperature. For a while, heat is extracted from the cylinder into the flow water. To re-heat a cylinder to 65C, the boiler set temperature must be higher or the cylinder will never heat up to the required temperature. A 70C flow temperature on a relatively short pipe run means that the flow return temperature will invariably be above the condensing temperature of 56C once the boiler has reached its set temperature. Some heat will also be lost from the pipe run as the pipes act as mini radiators. All these factors reduce the efficiency of gas re-heating.

Another factor in my decision to use the immersion heater is that I can run my system boiler at a low temperature. Presently, I am running it at 55C (1C below the condensing temperature). In the 3 1.2 hours since the 24kW boiler came on, I have used 13kWh of gas.

My advice run your own tests. It is important to pick 2 typical days where the hot water used is roughly the same. The more tests that you do, the more informed the result will be.

Snookie12cat

I suppose the question is. Is it more efficient (as in quicker) to heat water with electric than gas? If it is, price aside, electric might win out? 

Snookie12cat

chris_n said:

Your boiler will heat a lot more water in 20 mins than the immersion heater.

Ok, thats kind of what I wanted to know! It will be more pricey then because it needs to be on longer.

RobM99

As an aside, is your cylinder insulated?

Snookie12cat

[Deleted User] said:

It is not a simple 'like for like' equation.

The major factor is the difference in cost per unit for gas and electricity.

Other factors include the efficiency of your boiler (note; most condensing boilers do not condense when heating a cylinder); the length of the pipe run between the boiler and the cylinder, and the required cylinder temperature.

I re-heat my cylinder to 65C using surplus solar energy and an immersion heater. Yesterday, it took 3.3kWh of electricity to re-heat a 216L unvented cylinder. Had I elected for gas, this would have been in excess of 8kWh.

When a gas boiler first comes on, the temperature of the flow water is less than the cylinder temperature. For a while, heat is extracted from the cylinder into the flow water. To re-heat a cylinder to 65C, the boiler set temperature must be higher or the cylinder will never heat up to the required temperature. A 70C flow temperature on a relatively short pipe run means that the flow return temperature will invariably be above the condensing temperature of 56C once the boiler has reached its set temperature. Some heat will also be lost from the pipe run as the pipes act as mini radiators. All these factors reduce the efficiency of gas re-heating.

Another factor in my decision to use the immersion heater is that I can run my system boiler at a low temperature. Presently, I am running it at 55C (1C below the condensing temperature). In the 3 1.2 hours since the 24kW boiler came on, I have used 13kWh of gas.

My advice run your own tests. It is important to pick 2 typical days where the hot water used is roughly the same. The more tests that you do, the more informed the result will be.

@[Deleted User]
This is very very useful. 
I did turn down the boiler temp and the hot water would not heat up and this would explain it. The boiler is downstairs and the tank upstairs, so not too close to each other.

What does running it at 55C do? Why do you not want it to condensate? Are the rads still adequately hot?

How long did you need the immersion on to heat the entire tank?

Snookie12cat

RobM99 said:

As an aside, is your cylinder insulated?

Yeah, brand new Megalow Eco so it should be the best it will ever be right now. 

Alnat1

It might be better to have one longer period for heating the water by gas, rather than 2 short ones. Your brand new tank should lose very little heat, so the water will stay hot for 24 hours if not used.

We heat ours for 40 minutes each morning, does a couple of showers, hubby's shave and a bit over. It used around 3.5kWh over summer, now around 5kWh I think, as the incoming water is colder. Harder to tell as the heating's on too