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Is immersion heating as efficient as the gas boiler for heating the water? I'm trying to work our solar savings...

Forum|Forum|3 years ago
March 12, 2023
11 replies
4974 views

We have recently had solar pv fitted, with iBoost on the hot water tank to use any generation excess.

I am trying to get a figure for the savings made by using the solar pv to heat the hot water rather than the central heating gas boiler. There is a 3-way valve to control rads & hot water separately. This raises the question:

Does 1 kWh of immersion heating produce roughly the same amount of hot water as 1kWh of a gas boiler heating the water? I realise it will not be identical due to efficiencies and losses, but are they similar?

I hope to be able to compare costs of heating hot water by pv or gas, ie loss of SEG payment vs gas rate.

Then I will know the savings to be made.

Best answer by mavison

@Jonathanprentice You can regard the immersion with iBoost as another energy storage device like the battery - they both can store surplus PV power. The difference is that the energy stored in the battery is used as electricity, the energy in the immersion is used as hot water. When neither battery nor immersion are full, any surplus PV may first go to one or the other, depending on setup. Here it first goes to the battery, but when 80%? plus some also goes to the immersion. Only when they are both full is any significant surplus amout exported.

M.isterW

Rank 6

The immersion heater is 100% efficient so 1kWh of electricity will create 1kWh of heat.

Your gas boiler is probably about 90% efficient (possibly a bit less, depending what temperature you have your hot water) so 1kWh of gas creates 0.9kWh of heat.

1992 4 bed detached house. Solar PV + battery, Zappi EV charger, Mitsubishi Ecodan ASHP and i-life2 fan assisted radiators.... No gas 😊

M.isterW

Rank 6

The immersion heater is 100% efficient so 1kWh of electricity will create 1kWh of heat.

Your gas boiler is probably about 90% efficient (possibly a bit less, depending what temperature you have your hot water) so 1kWh of gas creates 0.9kWh of heat.

I've just realised, you're going to lose some heat in the pipes so your gas efficiency will be less. You might be better using 85%.

1992 4 bed detached house. Solar PV + battery, Zappi EV charger, Mitsubishi Ecodan ASHP and i-life2 fan assisted radiators.... No gas 😊

M

mavisonAuthor

Rank 2

@M.isterW Thanks - I thought it would be something like that.

J

Jonathanprentice

Rank 5

Hi there. I also thought of this idea. I have 8x415watt panels and 5.8 kWh battery. However my theory is that if u use the iboost on the immersion heater it will run at 3kw all the time it’s on, I think. Therefore surely you’ll need a high solar system to cope with the 3kw your drawing from either a battery or solar panels at any time immersion is running.

I decided against it am prepared to use normal boiler for the moment.

it is a good idea but feel it has its drawbacks.

M

mavisonAuthor

Rank 2

@jonathanprentice: No, the iBoost only normally uses *surplus* PV energy that would otherwise be exported to the grid. If there is 1kw surplus it will just use that. If there is 5kw surplus it will use that ... but probably limited by your immersion heater(s). It varies the power taken minute by minute. If you turn boost on, it will then use the immersion max kw from anywhere - solar, battery or grid (probably in that order). Seems to work very well - our gas water heating has been off for months ... but will be on when PV output drops in winter months.

J

Jonathanprentice

Rank 5

We’ll that’s my misconception squashed. Thanks. I learnt more from that last post than on iboosts website. Thanks again. Next year may look into this.

M

mavisonAuthorSolved

Rank 2

@Jonathanprentice You can regard the immersion with iBoost as another energy storage device like the battery - they both can store surplus PV power. The difference is that the energy stored in the battery is used as electricity, the energy in the immersion is used as hot water. When neither battery nor immersion are full, any surplus PV may first go to one or the other, depending on setup. Here it first goes to the battery, but when 80%? plus some also goes to the immersion. Only when they are both full is any significant surplus amout exported.

J

Jonathanprentice

Rank 5

Great thanks for the help. Just out of curiosity when I do go ahead which connection do I add the wireless sensor clamp. See photo

M

mavisonAuthor

Rank 2

See https://www.marlec.co.uk/solar-iboost-downloads/ where you can download from the Technical Support area the User Manual and the Installation Instructions. You should find details in there.

Is that another CT clamp I see there already?