Immersion or Gas for summer hot water?

[RandomGrinch]

I always wondered if those pass-through-plug power meters that do do kWh were just internally a CT clamp around the passed through live..

A nice teardown of one here :

 

[fourtytwo]

It might be worth you thinking about solar panels depending on how much hot water you use. We had an old oil boiler in our last house, method of measuring oil consumption was tape measure stuck up side of sight glass on side of tank and a spreadsheet. We had a 1Kw PV array that was enough on most days to provide enough hot water (via the immersion) and only had to run the boiler if we got more than a few cloudy days in a row. As can be seen we ran oil free for quite a few months most summers. Vertical axis is Litres/day, PV was initially 500W installed late 2016 then extended to 1Kw mid 2019. We moved out in mid 2022 hence end of figures.

 

[robinbanks]

I'd love to go solar.. I only need a 40kw array to cover my winter usage!

 

[fourtytwo]

40kw, how do work that out, are you talking about your entire household heating load ?
I was replying to your query regarding summer hot water, to expect PV to take on the whole heating load is crazy especially as winter production is markedly down on summer, be realistic or perhaps you were joking

 

[robinbanks]

PVGIS for my location and existing whole house usage.. I wasn't a joke, but the post was tongue in cheek!

 

[ericmark]

I use a computer linked energy monitor it does not show power factor, but it does form a graph of usage. I also have a non linked, and one which includes a switch, but to use it would need the immersion turning to plug and socket.

I have considered a under sink unit, a unit like this would likely remove the need for the immersion or boiler to be on at all in the summer. But it only serves one tap, and my upper floor bathroom does not have a direct water supply, both hot and cold come from the header tank, and I have four locations where I would like hot water in the summer.

It would likely save money as no need to run off cold water, but also takes up room, it still has an immersion heater inside, but only heats 10 litres of water, needs 2 kW, and since directly under the sink it supplies, does not heat up pipes.

But is it worth £100 plus to fit one, and loose the cupboard space?

 

[robinbanks]

But is it worth £100 plus to fit one

How long does it take for the sum costs of standing losses of a tank of hot water, pipes to the tap, plus a need to run off X litres of cold down the plug hole before the hot arrives to breach £100? And what's the convenience of instant hot water worth versus the inconvenience of losing under-sink cupboard space?

 

[fourtytwo]

plus a need to run off X litres of cold down the plug hole before the hot arrives

Plug hole, never, goes in a bucket for later use in the loo

 

[dishman]

The, very rough, experiment is complete.
With the "warm days" for the last two days. I decided to do an experiment. I turned off heating and hotwater via the boiler and only used an immersion heater set so that the hot water was "hot but touchable", about 45degrees (Yes, I know I should be hitting 60 for legionaries). With the immersion on pretty much all day, relying on it's own thermostat to keep the top 1/4 of the tank hot, it used in total an extra 7 to 8 KWH above my baseline daily use.

From my records my gas boiler would use about 25KWH, heating the whole tank (100L) twice in a day to a similar temperature. There would be extra "losses" here from heating the whole tank rather than (approximately) 25litres. In addition, with it being a gravity circulation on the hot water, there is no valve preventing some losses to the nearest radiator (unless I turned them off). There is also the permanent boiler pilot light.

So, the idea was, taking into account the extra inefficiency of my old boiler, what would come out cheapest.

Well, based on the above. Using the immersion would cost me (rounded up) £2.98 to leave the immersion on all day,

The Gas boiler using 25kwh per day (including losses) comes out at £2.76.

Close!

 

[ianmcd]

But you are not comparing apples with apples, if you want to know the true figure you need to heat the whole tank with electric to compare it with gas which is heating the whole tank, what is your HW usage if only the top section is heated and is adequate ?

 

[dishman]

This is true, but at the same time, what the immersion heats is more than adequate, as you say (I should have confirmed that). I was already keeping in mind the difference between how much they heat in my "experiment".

So indeed, while "like for like" the electric would indeed be far more, it is compensated somewhat for heating exactly what I need with less wasteful losses (pilot light, and circulation radiator losses and boiler age).

I would expect the gap to a modern boiler to be much wider.

But you are not comparing apples with apples, if you want to know the true figure you need to heat the whole tank with electric to compare it with gas which is heating the whole tank, what is your HW usage if only the top section is heated and is adequate ?

 

[Johntheo5]

The, very rough, experiment is complete.
With the "warm days" for the last two days. I decided to do an experiment. I turned off heating and hotwater via the boiler and only used an immersion heater set so that the hot water was "hot but touchable", about 45degrees (Yes, I know I should be hitting 60 for legionaries). With the immersion on pretty much all day, relying on it's own thermostat to keep the top 1/4 of the tank hot, it used in total an extra 7 to 8 KWH above my baseline daily use.

From my records my gas boiler would use about 25KWH, heating the whole tank (100L) twice in a day to a similar temperature. There would be extra "losses" here from heating the whole tank rather than (approximately) 25litres. In addition, with it being a gravity circulation on the hot water, there is no valve preventing some losses to the nearest radiator (unless I turned them off). There is also the permanent boiler pilot light.

So, the idea was, taking into account the extra inefficiency of my old boiler, what would come out cheapest.

Well, based on the above. Using the immersion would cost me (rounded up) £2.98 to leave the immersion on all day,

The Gas boiler using 25kwh per day (including losses) comes out at £2.76.

Close!

If you use 25kwh/day of gas and assuming you are heating water from 10C to 45C, and assuming a 85% boiler efficiency then this equates to heating and using 522 litres/day, very unlikely even if you are loosing a max of 2kwh/day through the cylinder then you are heating and using well over 500 LPD.
To heat 200 LPD from 10C to 45C at a boiler efficiency of 85% and allowing 2kwh for cylinder losses will require 11.6kwh

 

[ianmcd]

So what are you actually using the HW for , if it is only face washing/shaving and dishwashing , just boile a kettle and use that, much cheaper than pre heating any part of the HW cylinder

 

[dishman]

Indeed, if I really wanted to cut it to the bone I could turn off the immersion for the most of the day and only put it on for baths/showers. The reheat time is pretty quick.

@Johntheo5 I imagine the boiler efficiency of this particular model is below 60%. Potentially nearer 50%. Idon't want to imagine it being any worse than that. But, I can only go by my very regular records of use and cost.

 

[Johntheo5]

Using 50% efficiency then you are using/loosing ~ 258 LPD.
Using 8kwe (say 6 kwe effective) electric immersion then the above becomes 147LPD enough for ~ 3 persons usage per day