Heating water in electric immersion heater with boost function only

NSG666

to_be_FTB said:

Hi, I have a cylinder immersion heater and E7 tariff. I don't use much hot water at home atm so I decreased the heating time during the night tariff to two hours. I tried to decrease it more, but then the water was cold in the morning. I was wondering whether I could just heat the water during the night tariff with the top boost heater only to get a smaller amount of hot water (ie set the programmable heating time to 0, and programmable boost to 2 hours during the night). Has anyone tried that? Do would the water remain hot or would it mix with the cold water at the bottom of the boiler? Any other potential issues with this solution?

* edited to correct: a tank/cylinder, not a boiler

Give it a try and see what happens - it won't damage anything and the worst that can happen is you've no hot water the next day and you need to use the boost during peak times. If that happens you can go back to your current settings - job done.

to_be_FTB

Give it a try and see what happens - it won't damage anything and the worst that can happen is you've no hot water the next day and you need to use the boost during peak times. If that happens you can go back to your current settings - job done.

Thanks, I guess this was my main worry - can using the boost function only damage the heating elements/cylinder/something, but it seems from your answers this is unlikely, so I will go ahead and play with the settings. 

to_be_FTB

Le_Kirk said:

If it is true Economy 7, the system only allows off-peak electricity through as there is a relay/switch in your meter that switches between two separate circuits, so putting the boost on during off-peak hours would achieve nothing as there is no electricity available on that particular circuit. 

Interesting. I definitely have no electricity coming to the storage heaters during peak hours, but I need to double check how the boiler is connected...

Section62

to_be_FTB said:

Thanks, I guess this was my main worry - can using the boost function only damage the heating elements/cylinder/something, but it seems from your answers this is unlikely, so I will go ahead and play with the settings. 

I've done exactly what you want to do, for the same reason as Cornucopia - that I had a dual element and the main one failed so I switched over to using just the boost as the 'main' element which was sufficient to meet my hot water needs.

In operation there is no risk of damaging the element(s) or cylinder, but in setting it up there is a very real risk of getting something wrong with the wiring which could do damage to the system, or to you.


I used the smaller element for about two years before deciding to change the element to a new larger one.  The reason being that the risk you do run with not heating the whole contents of the cylinder is having some of the water always at a temperature where legionella bacteria thrive. To reduce that risk I had turned the thermostat on the element up a bit, so was using more electricity than strictly necessary to heat the smaller volume of water.

Provided your cylinder is properly lagged the heat loss in a 24 hour period isn't that great, especially if you are only using a small volume of hot water.  I've found the energy saving from using the boost element only vs the normal element to be relatively small, when taking all the other factors into account.  So whilst it should practically work for you, don't necessarily expect a big saving on your bill.  You may be able to save more by improving the insulation/lagging on the hot water pipes and cylinder.

Section62

grumbler said:

I know very little about E7, but is it really worth having it and heat water with electricity  when you have a gas boiler and gas is about 5 times cheaper than electricity?

It's a moot point now the OP has confirmed they don't have a gas boiler, but IMV the answer to that is 'yes', it could be.

The kWh cost of gas is cheaper than electricity, but by how much depends on the tariff.  My E7 night rate kWh cost is 2.88 times the gas kWh rate (5 times sounds really expensive).

But whereas the immersion heater is close to 100% efficient, no gas boiler can match that. Then there is the efficiency of the system to consider, with a length of primary circulation pipework to heat up and with its own heat loss when working. Plus the cost of electricity for running the boiler, pump, thermostats and control valves.

It also makes for an unfair comparison if you are ignoring the wear and tear on a gas heating system, though estimating the cost of those is quite complicated.


The equation for which energy source works out cheaper overall will vary depending on the amount of hot water you use, whether it is winter or summer, whether you have the heating on all year, the amount of lagging and pipe insulation you have etc etc.

But I've done my own experiments and found heating my hot water using E7 electric (in the summer) could work out cheaper than heating it with my gas system boiler.

Moreover, with what's happening with energy prices, and government plans to phase out gas CH, the equation is inevitably going to tip further in a favourable direction for TOU electricity.