Cheapest way to run immersion heater

k_man

Something doesn't seem quite right.
2kWh can heat about 40l of water from 20 (it is warm atm) to 65C.

That's not much for a bath and a shower.

Section62

k_man said:

Something doesn't seem quite right.
2kWh can heat about 40l of water from 20 (it is warm atm) to 65C.

That's not much for a bath and a shower.

People use water in different ways - it is one of those "complications" that wittynamegoeshere maintains don't exist.

If the stored water is at 65C then the user will be blending it with cold to achieve a more acceptable temperature for bathing/showering.  40l of hot water at 65C could give a much larger total volume of blended water at (say) 35C.  It all depends on the preferred temperature, and personal preferences regarding depth of water or flow rate/duration.

The ability to store water at a higher temperature and blend down to the end-use requirement is one of the efficiencies of the stored-heat concept.  The central store (aka hot water cylinder) can keep and distribute water at a temperature above that where legionella is a concern.  Whereas instantaneous heaters may not get up to the 'safe' temperature range, and may never achieve the required temperature/flow rate needed.

This is one of the reasons why instantaneous heaters should be installed as close to the point of use as possible - in other words to minimise the volume of water in the 'danger' zone.  This is - contrary to some assertions - relatively complicated stuff if the system is to be installed and set up correctly.

wittynamegoeshere

Perhaps Harvi has some kind of special energy-multiplying capabilities, or there was just less hot water or at a lower temperature at the end of the whole process, or the bath just wasn't filled.

You definitely can't get more energy out than you put in, however posh your hot water tank.

k_man

Section62 said:

k_man said:

Something doesn't seem quite right.
2kWh can heat about 40l of water from 20 (it is warm atm) to 65C.

That's not much for a bath and a shower.

People use water in different ways - it is one of those "complications" that wittynamegoeshere maintains don't exist.

If the stored water is at 65C then the user will be blending it with cold to achieve a more acceptable temperature for bathing/showering.  40l of hot water at 65C could give a much larger total volume of blended water at (say) 35C.  It all depends on the preferred temperature, and personal preferences regarding depth of water or flow rate/duration.

The ability to store water at a higher temperature and blend down to the end-use requirement is one of the efficiencies of the stored-heat concept.  The central store (aka hot water cylinder) can keep and distribute water at a temperature above that where legionella is a concern.  Whereas instantaneous heaters may not get up to the 'safe' temperature range, and may never achieve the required temperature/flow rate needed.

This is one of the reasons why instantaneous heaters should be installed as close to the point of use as possible - in other words to minimise the volume of water in the 'danger' zone.  This is - contrary to some assertions - relatively complicated stuff if the system is to be installed and set up correctly.

Good point, hadn't thought of the bath and shower being tepid.

Section62

k_man said:

Good point, hadn't thought of the bath and shower being tepid.

NHS Estates use 41C to 44C as the maximum temp for bath fills, 41C for showers.  Possibly too cold for some people, but even at 44C it would still require a significant volume of cold water blended in if the hot supply was at 65C.

k_man

Indeed 40C would mean about 1:1 cold to hot.

So up to 80l of warm water used.
Still seems low for bath and shower, but that does make it more feasible (quick shower, and very shallow bath)

wittynamegoeshere

You can make the maths easier by just ignoring the hot water temperature and treating it as though the immersion heater is heating from cold to the desired temperature.  The outcome is the same however you get there in energy terms.

Anyone know the amount of kWh units needed to raise the temperature of 80L of water from say 15degC to 40?

Not trying to catch anyone out, just trying to work out what a bath actually costs.  I guess nobody's going to be having a cold bath outside of a heatwave though, so a tepid bath isn't really representative of normality.

Getting back more on-topic, I'd be amazed if nobody's ever actually measured the heat loss from a typical hot water cylinder.  Definitely lots in my case, I've checked for myself, but mine's probably at the useless end of the scale.  Presumably some here have much better tanks but I'm sure a representative average is somewhere in the middle.

You'd hope that, in the push for heat pumps and hot water cylinders versus gas combi boilers and/or existing old cylinders that somebody would have actually measured this rather than just having a massive argument among a random bunch on a forum.

Alnat1

I'm sure someone said around 1C an hour heat loss from a decent tank.

Effician

wittynamegoeshere said:

Anyone know the amount of kWh units needed to raise the temperature of 80L of water from say 15degC to 40?

https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

Use the water heater energy calculator.

wittynamegoeshere

1C per hour would be a lot, but is actually pretty much what I see with mine.  But in fact the energy loss would be higher when it's hotter so it wouldn't be linear in temperature terms.

I suppose the answer would be the amount used by the heater to reach its setpoint when no hot water had been used for 24 hours and the ambient temperature was average, say 20 degC wherever the tank is.

If anyone's got any super duper energy monitoring and left their system on while they were away for a couple of days then they should have the answer.

But I guess the problem is that anyone with such a system is also likely to have a better than average cylinder.