Immersion heater - regular use

[Jupiter01]

I have a system boiler with a 300litre Megaflo.
It is set to heat the water once a day for an hour and this is almost always adequate for our household. Only very occasionally over the last 5 years, we have done the extra 1 hour top-up.

The system also has an immersion heater and I was told it’s there for emergencies. I’ve never used it.

With me switching to an Electric Vehicle, I will have a discounted electricity tariff which brings the electricity rate down to 7.5p per KWh. My current gas rate is 6.8p. This has me wondering whether I should use the immersion heater instead of the gas.

Is there any merit in doing this? Is it a sustainable approach or am I going to knacker the immersion heater? Are the savings negligible if I am only using an hour of gas currently?

I would be grateful for any light you can shed on this.

Thanks in advance.

 

[stem]

Electricity is virtually 100% efficient. So, for 1KWh of electricity at a cost of 7.5p you will get 7.5p of heat going into the hot water.

Gas is not so efficient, some heat is lost from the boiler flue. However, with a modern boiler having 90% efficiency (some are much lower) for 1KWh of gas at 6.8p you would get 6.12p worth of heat going into the hot water. Taking the losses into account that makes the cost to get a usable KWh of gas up to 7.55p. So, in this example that would makes the gas 0.05p per KWh more expensive than the electricity.

There will also be some small heat losses from the pipes between the boiler and Megaflo (assuming the pipes are well insulated) but in the winter that will add to the heat going into your home so can be ignored.

When using gas to heat the water, the boiler and pump will also be using a small amount of electricity, but again in the winter when the heating is on they would be running anyway.

We use immersion heaters at work as a sole means of heating hot water, there's just a small kitchen and a shower that gets used about 6 times a week. The immersion elements tend to last about 5 years. The elements don't cost a lot, but with an unvented system they have to be replaced by someone that has a G3 certificate so, it's not a DIY job.

 

[Roddy Young]

If you don't cook with gas you could have the gas cut off and save the standing charge, ie total gas account cost = £ 0
Roddy

 

[fixitflav]

If you don't cook with gas you could have the gas cut off and save the standing charge, ie total gas account cost = £ 0
Roddy

But unless he does a major conversion gas still needed for CH.
One advantage of using gas for HW is the boiler fires up at least once a day, so confident it will be OK when winter comes. If it's left idle for several months you never know.

 

[Rodders53]

1 hour of gas running full bore to reheat a tank --- actually most will reheat in under 30 minutes --- will need the immersion to run for many hours to achieve the same heat input.

e.g. My oil fired boiler and Oso 210 litre cylinder has 23 minutes reheat via the coil, and 175 minutes (5 shy of 3 hours) for the 3kW immersion. It loses nearly 2 kWh per day of that heat input (used to fully dry clothes, sheets and towels in the airing cupboard).

I'd assume an extra 3kW load on top of a 7kW EV charger wouldn't be an issue. EVs must be used where night storage heating is used, I assume and I understand that (some) EV chargers monitor the 'house load' to limit current draw from the grid.

As can be seen from others' numbers the gas vs electric cost is virtually identical. Those diverting solar panel output to 'store' the free* energy may feel different.

* obviously the solar panel install and maintenance costs vs generated electric value calcs need to be done if one is to compare accurately.

 

[stem]

1 hour of gas running full bore to reheat a tank --- actually most will reheat in under 30 minutes --- will need the immersion to run for many hours to achieve the same heat input.

True, but irrelevant to the cost. The energy (KWh) actually being put into heating the water will be the same.

The gas boiler may do it more quickly, but it will use energy at a higher rate whilst doing so.

So for example:

3KW for 1 hour = 3KWh
12KW for 15 minutes = 3KWh

 

[Rodders53]

What is a Kelvin Watt?? kilo not Kelvin get the Units correct, please.

As I stated cost is broadly the same (others had done the sums) ... but that is ONLY if cheap rate electric is available AND the extra 3 kW load doesn't need limiting down due to other use at the same off peak time (washer, dryer, EV).

To me it would appear there's no significant cash benefit and 'wearing out' the backup immersion heater would seem potentially foolish due to cost of such replacement (and if the immersion boss can be loosened for replacement). Especially if a pro has to do the work.
The cost:benefit calculations may differ if solar power could heat the HW via the same immersion element (but that's n/a here I think).

One last cautionary tale:
Having used my emergency backup heater via the original 13A fused connection unit for a many weeks some 5 years back: the darned thing got very, very hot. I quickly replaced it with a 20A switch (protected by the 16A MCB).
IF OP wants to use the immersion to reheat the water at night I'd urge them to check the isolator etc.,. for such overheating potential.

 

[fixitflav]

What is a Kelvin Watt?? kilo not Kelvin get the Units correct, please.

Let's not be too pedantic. I'm a stickler for units myself, but we we all know what was meant. kW for power, kWh for energy.

 

[Rodders53]

Let's not be too pedantic. I'm a stickler for units myself, but we we all know what was meant. kW for power, kWh for energy.

If someone criticises my post I'll fight fire with fire.

 

[fixitflav]

If someone criticises my post I'll fight fire with fire.

What makes you think stem was having a go at you? The way I read #6 he was just pointing out simple facts.

 

[SpecialK]

Yes its fine to do that. If your immersion is not at the bottom of the tank you will not end up with a full tank of hot water.

 

[stem]

My apologies everyone, especially to Rodders53. No Idea why I wrote KWh instead of kWh. I qualified as an electrical engineer in 1984 and have been using kWh ever since....so no idea why I typed KWh, and so many times as well.... must be getting old.

Hopefully the OP will still understand the explanation.

 

[Harry Bloomfield]

A couple of things that have not been mentioned, when doing the sums, is wear and tear. Wear and tear on a boiler, will be much more expensive than that on an immersion heater, and an immersion heater element is much cheaper to replace than a boiler. Running a boiler, also has an electrical cost, to run the pump.

To avoid things seizing up, many boilers these days briefly exercise fans, pumps, and etc., once per day, to check operation. Mine does this.

 

[Jupiter01]

Thanks for the above all.
I will not go down the route of using the immersion heater as the economics of premature replacement don't stack up. I will schedule my hot water process (using gas) to run in the off-peak hours as there is some electricity being used as part of this. I hadn't appreciated this.

 

[Harry Bloomfield]

I will schedule my hot water process (using gas) to run in the off-peak hours as there is some electricity being used as part of this.

I wouldn't bother too much about that - call it 100w for pump, and boiler etc., for one hour run, so at current peak rates (not E7) it would be around 2.6p. It would cost you more than that, for the heat losses from the cylinder, whilst it sits waiting for you to use the hot water.