Immersion Heater during Off Peak

I've recently had a 9.6kW battery and 3.5kW solar system installed and it's gotten me thinking about various different money saving approaches.
Our off peak unit rate for electricity is 6.9p 00:00 - 07:00 and our gas unit rate is 5.65p

I'm wondering whether timing our immersion heater to heat the water during the off peak period makes sense rather than using the gas system boiler.

With an assumption that the immersion heater is pretty much 100% efficiency for converting electricity to heat the gas boiler would need to be less than about 82% efficient for it to make financial sense.

Are my assumptions here correct? Anything else for consideration? (burning less fossil fuel is one such consideration)

One thing you need to check is where abouts in your tank the immersion element is located. Mine is half way up, so if I use the Immersion only, I just get half a tank of hot water which is not enough to last the day.

R0ss I'm wondering whether timing our immersion heater to heat the water during the off peak period makes sense rather than using the gas system boiler.

I came to the same conclusion. I figured boiler probably was very inefficient
One consideration is that, if you are in a hard water area, the immersion will probably fail sooner or later.

R0ss I do this. The main saving for us comes from being able to run the boiler at a lower temperature for central heating.

R0ss Anything else for consideration?

I guess one other issue is, unless your boiler has an automated circulation feature, your heating system sitting idle all summer might not be ideal.

I realised rather late that the motorised valve remained in the heating rather than hot water, which consumes power to maintain. (Interesting design feature that the water position is typically the stable one.)

My calculations showed it cost almost 12p per unit to heat the water with Gas so if rates are below this amount I use the immersion heater. My system is roughly 50% efficient when heating the water due to the amount of heat left in the pipework and boiler after the heat cycle (all lost) and the 85-90% efficiency of the boiler itself so the 5.65p gets multiplied by 2.

My tank in a 150l Megaflo which usefully stores a lot more than just the top half when heated by the immersion.

I use a controller that reduces the power from 3KW to 2KW to try to make the element last longer as we are in a hard water area.

I set my gas tank stat slightly lower (55oC) than my electric tank stat (60oC). I also only heat the water by gas once a day between 6 and 7am. This means that if the tank is heated by electric before 6am, as would happen during cheap energy slots on Agile, the gas doesn't come on. But if I don't heat with electric I still get a hot tank once a day on the gas. No problems then with an idle pump/valve system through the summer.

I can get caught out when family are here and we get to 4 people who've showered one after the other and drained all the hot water so then have to add extra times into the gas timer.

Windy Miller My system is roughly 50% efficient when heating the water due to the amount of heat left in the pipework and boiler after the heat cycle (all lost)

Another factor I considered was that, during the summer, as you say, all that residual heat is wasted. During heating season, though, it will probably get used anyway.

Windy Miller

I use a controller that reduces the power from 3KW to 2KW to try to make the element last longer as we are in a hard water area.

Good idea, I will look into doing that

Avenir Make sure it is 16A capable as light dimmers won't be powerful enough. I made my own using this module in a diecast box to act as a heatsink.

https://cpc.farnell.com/kemo-electronic/m028n/module-power-control-4000va/dp/HK01157

https://community.givenergy.cloud/d/4047-givenergy-smart-plug-nearly-burned-down-my-house/91

Windy Miller I have an Eddi, so this is pretty straightforward

I have a Telsa controlled immersion heater and only use it when really cheap agile rates.

In the winter if you are heating rads it really doesn't make much more difference to heat your H/W tank too.

M_J In my case, we have oversized rads and no weather comp on the boiler; doing the hot water on overnight immersion means we can run the heating at 40C, rather than _65C. That makes a decent difference in running costs.

I think the main problem with using the immersion heater when you have gas is that its not going to heat the whole tank and its going to take a lot longer to heat what it does because there's only one element.

For example our tank has a 3kW element (electric) and an 18kW coil (gas). Do the maths yourselves.

BTW you don't need to run H/W to 65. There is an argument to once in a while run at 60 to sanitise it.

Vestas That's very true. Our gas boiler is 8.3KW and doesn't short cycle when heating the tank. It typically takes 45 minutes each day (6.25KWh) and is putting in around 3-4KWh. When on electric the immersion (at 2KW) needs to be on for 2 hours to put in the same amount of energy. 4 agile 30 minute slots need to be below 12p for this to be economical but last night the 4 best slots averaged -1.5p so we were paid 6p to heat our tank with the immersion 😃

Vestas I think the main problem with using the immersion heater when you have gas is that its not going to heat the whole tank and its going to take a lot longer to heat what it does because there's only one element.

In our case, we only needed half a tank of water, and since it was heating overnight on cheap electricity, I wasn't in any hurry.

M_J that’s a good point. The temp setting on my tank is not supposed to be user serviceable, nor does it tell you what the temp actually is – but I could have a go at turning it down a bit.

https://www.teslauk.com/product/7795/t-smart-thermostat

This can be retro fitted to most H/W tanks and means you can program it to make use of the cheapest 30 min slots via the app and a mobile phone.

This is very useful when we have minus rated agile slots...

M_J I was surprised to see Tesla were stocking such items until I realised that Tesla UK isn't anything to do with the Battery/Car/etc. maker.

I also heat the hot water vai an EDDI whilst on Intelligent Octopus Go in the winter.

The main hot water heating is during the off peak hours plus a boost at 16:30 - 17:00 from the battery (that was charged overnight at 7p / kWh)
On average I use 2.5 - 3.0kWh via the EDDI so it costs me about 21p at 7p/kWh (or 28p if I consider the afternoon booost as a loss of export capability).

Using Gas I use about 8kWh per day to heat the water so it would cost me about 40p. (Octopus Tracker Gas).

As said above, the immersion heater only heats the top quarter of the hot water cyclinder, this is normally plenty for just two of us unless someone wants a bath rather than a shower.

I have an AC3.0 inverter which is limited to 3.0kW discharge, so via the EDDI I limit the hot water heating to 2.0kW.

During the summer months I am on Intelligent Octopus Flux. Electricity. The balance then moves in favour of using gas to heat the water.

Heating just the top of the tank with the immersion heater is a problem with many standard cylinders as the element is inserted through the hole at the top and is often quite short. A longer element might help (as you can get different lengths). One of the many attractions for me of the Megaflo system was that the immersion element is mounted low down and therefore heats from the bottom of the tank giving me very similar hot water storage to the indirect coils from the gas boiler which are also at the bottom of the tank. Coupled with the increased cylinder size, the excellent boiler coupling (lots of coils), the mains pressure hot water, separate gas and electric thermostats (2 different temperatures), slimline (and fat) cylinders which fit in a tighter space and improved insulation I swapped out my old tank even before I had solar/batteries for much better convenience. But they must be installed carefully as mains pressure hot water could be a considerable hazard if it bursts. If you use a plumber they must have the right ticket for this and use appropriate pressure reduction and relief valves in the right places.

Vestas Depends on where your thermostat / element is.

My immersion will heat the whole tank (as you say takes ages though) as it is right at the bottom and has an angled form so I would suggest the element isn't far off touching the floor.

M_J Why don't they display prices? Is there a bartering system in place?

Ra huh? Not sure what you mean? That's the manufacturer site look at say Screwfix for the retail price or do a Google search for prices?

Heat half the tank on off-peak over night - sufficient for most of the day and then boost in the late afternoon using gas (same price day or night), sufficient for your evening use. Result is lower losses from fully heated tank, hotter evening water (without having to raise water temperature during the night) and ability to respond to any changes in usage day to day.

For those complaining about the immersion element not giving a FULL tank of hot water, fit a recirculation (de-stratification) pump, I did and now get a full tank of hot water (if I require it)
Mine is plugged into a smart plug and integrated into Home Assistant
I bought a stainless steel pump from Amazon uk for £65

Graham_Harrison Won't help in most instances if mains gas is/was the primary heat source. Reason is that in most tanks the electric element (and more importantly the thermostat) will be half way up the tank. Edit - and if you're fitting a pump to get around that then may as well get a new tank.

Modern tanks (like the megaflow ones fitted in new builds) have the element and thermostat further down so its not so much of an issue. However this is a historical problem where the electric element was very much intended as an emergency only function when your gas boiler had failed.

Vestas Won't help in most instances
I can't understand why you think that

I did actually replace my tank 2 years ago with a bigger 1 (OLD - 92 litres, NEW - 142 litres), still a fairly standard tank but bigger with 2 immersion bosses an extra coil (pre-heats the cold water feed to my electric shower) and integrated cold tank (because of the cold tank the immersion bosses are in the side).
A year later I fitted the pump which pulls cold water from the very bottom (a low point drain) and circulatesit to the top.
Both my immersions are hooked up to a MyEnergi Eddi
I did a lot of reasearch before fitting my pump and see no reason why a de-strat pump wouldn't work on majority of hot water cylinders.
I have smart temperature sensors located at 3 positions on my tank and so can see exactly what effect the pump has.
A 3 minute run is long enough to ballance the temperature from top to bottom

I have a 21 yr old megaflo and its element is nearer the bottom. I have a Tesla thermostat to use it during free power up sessions.

My old thermostat used to trip, so I had to replace it anyway.

generally a no brainer. 24/7 timer for E7 saves me in region of £1 par day!!!!!!!!!!!!!!!! Even most efficient boilers are nowhere near as efficient as they claim. immersion is 100% efficient.