Octopus Agile immersion heated hot water cylinder vs gas combi boiler for domestic hot water

antivirtel

Hi, 

I've just bought a house and this house has a gas combi boiler (installed Nov 2023, it's a Glowworm Energy 7 30C combi boiler) and it has a backup immersion heater hot water cylinder (installed ages ago, ~15 years ago - it doesn't have the coil in it/it's not plumbed in, so the gas boiler can't heat that). There are 2 valves to turn and switch off domestic hot water (DHW) mode of the gas boiler to switch between the 2 domestic hot water sources.

When the electricity with Octopus Agile is 0 p/kWh or goes to the negatives, it's a no-brainer to switch to the cylinder. However, I would like to calculate or measure the actual threshold, at what price would it worth switching to the cylinder. 

Given the gas combi boilers are not 100% efficient (~90%, right?), but even though the cylinder with immersion heater is 100% efficient in theory, there will be a loss of heat eventually on the unused water in the tank (when it's unused), whereas the gas will only be heating the required amount of hot water (as it's not heating the water tank with that classic coil, but directly the water coming to the taps).
The gas tariff (Flexible Octopus) is 5.40 p/kWh currently here.

(The gas boiler is in Eco mode for DHW, so it doesn't keep the heating element warm all day all night wasting gas - what a stupid implementation anyway. Why would I want to keep it warm during the night, no chance I use hot water then?! It's nice to have it during the day, but there's no timer for it, so it does it all the time otherwise...)

I have a smart switch in front of the cylinder, so I can just set the exact times when it should come on from my phone, making it very convenient. The cylinder has that fiber glass insulation cylinder jacket (eg. https://www.diy.com/departments/mangers-4-piece-cylinder-water-tank-jacket-h-1067mm-w-450mm-t-80mm/26876_BQ.prd ). So it keeps the hot water warm as long as possible. 

Is it as easy as <4.86 p/kWh (gas price x efficiency @ 90%)? - This ignoring the unused, lost hot water, as that's hard to predict - but in my previous house I only had electricity, gas was uninstalled, so I kind of could predict how much time I need the cylinder on per shower and a bit of reserve for the occasional tap use - so I could set it up according to how many showers I need on that specific day - only takes a minute with the smart switch (have a dishwasher, so don't need much more hot water for the taps).

So since I can't just reheat the same hot water tank with the 2 methods, what would be the best way to measure/test this scenario? Is there a formula for the this - or is it just the above?

Thank you! 

Reed_Richards

Please carry on.

MP1995

When agile is 5-6p or lower heat with immersion (depending on what you pay for your gas)

Just guessing at the question.

I could also say depends how old your gas boiler is as it could be anywhere from 70-95% efficient where as electricity is 100% efficient,

antivirtel

Reed_Richards said:

Please carry on.

Yeah, it's done now, sorry accidently pressed the post before I finished writing, there was no delete button at all. 

antivirtel

MP1995 said:

When agile is 5-6p or lower heat with immersion (depending on what you pay for your gas)

Just guessing at the question.

I could also say depends how old your gas boiler is as it could be anywhere from 70-95% efficient where as electricity is 100% efficient,

Yeah, it's finished now, you can see that it's an almost brand new boiler. Not too sure, is the efficiency measured for these gas boilers, so I can find it in the has boiler's technical data somewhere? 

lohr500

As well as the smart timer, aren't you going to need to turn the two valves to switch over the hot water supply every time you decide to use electricity?

And unless Agile is regularly (Like every day or so) less than 5-6p won't you be wasting a lot of electricity reheating the full contents of the immersion tank from cold every time you switch over?  (Or for the guestimate of how long you think the immersion heater will need to be on to provide you with the hot water you think you will need).

Personally I don't think it is makes sense until the point where gas pricing is generally higher than Agile for a prolonged period.

antivirtel

lohr500 said:

As well as the smart timer, aren't you going to need to turn the two valves to switch over the hot water supply every time you decide to use electricity?

Just to reheat the cylinder, there's nothing else to do, just schedule the time on the smart switch. I can turn the valves before the shower. If I know the threshold, I will see/I can probably tell how often is usually cheap than that, then I'm thinking of getting a motorised valve, so my wife doesn't go mad switching the 2.

lohr500 said:
And unless Agile is regularly (Like every day or so) less than 5-6p won't you be wasting a lot of electricity reheating the full contents of the immersion tank from cold every time you switch over? (Or for the guestimate of how long you think the immersion heater will need to be on to provide you with the hot water you think you will need).

It is quite good nowadays, I had a lot of minus the last few weeks and we're reaching the ~5 p/kWh 2-3 times a week, so that's why this is a good question.

Yeah, you're right, reheating the full cold tank will cost more than being always on the cylinder.

In the old house if the heating element was the same as this (the cylinder looks the same), it needed 1h for 1 (long) shower that day, and 1.5h for 2 (long showers). So at around 2.7 kW power draw it would be 2.7 kWh for 1, and 4.05 kWh for 2 showers.

I know it's not really scientific, but on gas 1 shower used 2.25 kWh of gas (last I had one), so if we compare 12 p/shower on gas, would need the electricity to be <5.9 p/kWh to be cheaper (if my logic and maths is correct).

lohr500 said:
Personally I don't think it is makes sense until the point where gas pricing is generally higher than Agile for a prolonged period. 

Yeah, I see what you mean. The good news is though, that when it's a cheap period, it usually in a cluster (due to the storms and other weather stuff playing with the wind turbines), so sometimes you get a 4-5 days of minus in a row, or similarly lower prices, so that's the interesting part. 

FreeBear

https://www.omnicalculator.com/physics/water-heating - This will give you an idea of how much energy is needed to heat water.

As an example, I had a bath last night. incoming water was 21.1°C, heated to 44°C, and used 120l - According to the Loop app, 3.8kWh was used, and the calculator indicates 3.5kWh. In the winter months when the water temperature drops to 10°C, energy usage would increase to around 5kWh (also like hotter water).

With an old copper DHW cylinder with a jacket, heat loss would be around 4kWh per day (depending on ambient and water temperatures). On top of that, you need to factor in the energy requirements to get a tank (say 180l) of water up to temperature - Even without the back of fag packet calculations, a combi boiler is going to use less energy than a poorly insulated DHW tank as there is no daily heat loss.

If you can heat a tank of water at zero or minimal cost, there are savings to be had, but with a combi, you get hot water at mains pressure - Something that you won't get with a vented DHW tank unless there is a booster pump attached (another running cost that needs to be factored in).

Personally, I don't see there is much to be gained in having two sources of hot water once you've factored in the complexities of plumbing and the different flow rates - What could work is an unvented tank heated when electricity prices plunge. Feed this water through the combi boiler to boost the temperature if needed, and you get the benefit of constant water pressure & flow with minimal plumbing modifications (and no zone valves to fail) - The assumption here is that the tank & boiler are near each other..

Scot_39

Its not a simple balance. 

And like the boiler has an efficiency - so does the tank and associated pipe work - as they have inherent energy losses too.

A modern well insulated tank - when I checked the spec sheets for a common UK brand - for vented direct (immersion het) thevdata sheet loses typically between 1-2 kWh per day from their c 100l upto near 300l models - based on heating to 60C.
But thats just the tank - and the water heat leaks into the adjacent pipework - and in many installs that is often left uninsulated.

My small c100-120l tank (*) - takes about 7kWh - just over 2 hrs on a 3kW heating element - to heat that volume from cold to 60C depending on assumed incoming temp. Larger tanks upto 5 hrs with just 1 at 3kW - some have 2 for an e7 main/ daytime boost - or simply high low volume demand.

If was unvented I'd drop that 60C a bit - 55C (but if renting you shouldn't - as it risks fouling up landlords legionella obligations). 

But with a direct flow (no tank) many set HW on combis at 50C - a few posters here even lower - as not storing water.  

And even that's too hot for many. 

So in care locations (and many businesses have followed - forced to or risk averse / voluntarily ?) thermostat mixer valves (TMV) just ahead of taps and showers etc are madatory and used to lower below (to?) iirc 44C. The need under HSE to store (60C min) and distribute at higher temps up to those local valves remain in place. And similar domestic regulations for new  - but iirc at 48C max for baths (and bidet if have one). 

My tank doesn't feed my shower (* you would I think need a far bigger tank if it did ) - and I am not big water user otherwise on many days -  so losses a non trivial share of immersion energy draw - maybe 3 kWh per day (more obviously if take a now rare bath or wash car myself etc). 

My tank an older design  with "foam" type insulation and although I could fit a secondary jacket and lag a couple of m of hot outflow pipework thats hot to the touch to save a bit  - or fit a seperate timer -  a warm linen cupboard is quite a useful thing.  

There may in some cases also be a longer pipe run from one or the other too - additional heat loss from pipes when filled with hot - and a bit more water if metered too as run off more cold before hot enough for use.