# heating water by gas or electric immersion



## Ndiddy (13 Feb 2020)

Hi, 
We had our boiler and hot water tank replaced a year ago and at the same time got dual controls and rad valves.  I was always told that with a separate  hot water and heating function, it was cheaper to heat water by the gas boiler rather than the electric immersion and just wondered if this was actually true?
Thanks


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## Brendan Burgess (13 Feb 2020)

I had my boiler serviced recently and the guy told me the same that I should use the gas instead of the electricity. 
Brendan


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## Leo (13 Feb 2020)

It could cost 5 times as much to heat water via electricity, depending on the exact rates you are paying.


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## Buddyboy (13 Feb 2020)

And I always set my gas boiler to heat (only) my water during the summer, (1/2 an hour in the morning and evening does it).  I think a boiler that is fired up every day will be better than one that sits unused all summer.

And I concur with above, it is cheaper to heat water by gas rather than electricity.


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## Frank (4 Mar 2020)

make sure your cylinder is well insulated.


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## deco11 (29 Sep 2022)

Hi, with the huge increase in gas prices, do you think it's cheaper now to use the immersion to heat the water tank?


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## ClubMan (30 Sep 2022)

Leo said:


> It could cost 5 times as much to heat water via electricity, depending on the exact rates you are paying.


The unit (kWh) cost is only one factor.
The efficiency of each option is another for example.
Heating water using electricity is actually generally more efficient than heating it using gas.
It may be that heating water using gas is generally cheaper than heating it using electricity, but that link alone doesn't prove that hypothesis.


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## Leo (30 Sep 2022)

ClubMan said:


> The unit (kWh) cost is only one factor.
> The efficiency of each option is another for example.


Absolutely, and the SEAI methodology takes that into consideration.  

The pdf at that link details the assumptions they are making about appliance efficiency, etc.. See the efficiencies table to the right of the second page.


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## Leo (30 Sep 2022)

deco11 said:


> Hi, with the huge increase in gas prices, do you think it's cheaper now to use the immersion to heat the water tank?


The SEAI table linked above give the view as of July 1st. Electricity prices are also increasing, but I don't think there has been a 300% plus increase in gas prices that would make electric immersion heating cheaper.


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## odyssey06 (30 Sep 2022)

If you have an efficient boiler, which you'll be using to heat radiators, then the extra cost of heating water is different to just running boiler to heat water along. So you need to factor that in also.


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## AJAM (30 Sep 2022)

Leo said:


> The SEAI table linked above


Actually that SEAI table shows that it can be cheaper to heat hot water with electricity if you are using the night rate.
The table shows that the cost per KWh at 100% efficiency for Night time electricity is 11.54 cent. Resistive heaters are 100% efficient.
However the maximum efficiency you could hope for for gas would be 90% and that's being very generous, i.e. assuming your running your condensing boiler at a temperature low enough for the condensing tech to kick in and that your not losing any heat/efficiency moving hot water from the boiler to the the tank.  
*The table shows the the cost for gas at 80% efficiency or lower, is higher than the night electric rate of 11.54 cent.*
There is a very good chance that the efficiency of most boilers combined with the heat losses between boiler and tank are well below 80%.
Also the table shows that the night rate beats oil in every scenario.


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## newirishman (30 Sep 2022)

AJAM said:


> Actually that SEAI table shows that it can be cheaper to heat hot water with electricity if you are using the night rate.
> The table shows that the cost per KWh at 100% efficiency for Night time electricity is 11.54 cent. Resistive heaters are 100% efficient.
> However the maximum efficiency you could hope for for gas would be 90% and that's being very generous, i.e. assuming your running your condensing boiler at a temperature low enough for the condensing tech to kick in and that your not losing any heat/efficiency moving hot water from the boiler to the the tank.
> *The table shows the the cost for gas at 80% efficiency or lower, is higher than the night electric rate of 11.54 cent.*
> ...


maybe this works if you use power showers with cold feed only. otherwise you’ll run out of hot water by evening, most days,


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## Leo (30 Sep 2022)

odyssey06 said:


> If you have an efficient boiler, which you'll be using to heat radiators, then the extra cost of heating water is different to just running boiler to heat water along. So you need to factor that in also.


In reality there will really be little difference in the costs there. You're still consuming more gas at X% efficiency to provide a set amount of energy to the water tank. The proviso would be where the boiler might short-cycle more due to lower water quantity in the hot water circuit and the time taken to transfer that to the tank contents.


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## Leo (30 Sep 2022)

AJAM said:


> Actually that SEAI table shows that it can be cheaper to heat hot water with electricity if you are using the night rate


Good point, I was just using standard rates. I believe that day/night meters are a lot less prevalent among those with gas heating.


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