Use of immersion heater in summer v central heating for DHW?

[ericmark]

Remembered to look tonight, used 0.56 kWh today. At that daily use no way will oil be cheaper.

 

[JohnW2]

Doesn't necessarily work that way. .... Let's say we want the top of the tank at 60°C, and the boiler is set up for a max delta-T of 20°C (stanard max for gas boilers). Absent "something additional", to get 60° out of the boiler means putting 40° in - so you have to partially heat the entire tank. ....That "something" can be one of several things : .....

Having read that far. and 'assuming' that you were talking about 'indirect' heating (which is the only sort of boiler-driven DHW system I have personally experienced), I had started mentally rehearsing my response (as reproduced below), but then got to the end of your post and read ...

All of those can allow for only heating part of the tank to a usable temperature - assuming direct as my stores are. It's a bit more complicated if indirect as (as mentioned earlier) you then get complex currents in the tank as hot water round the coil thermosyphons upwards through cooker water.

Anyway, sticking with my initial 'assumption' of indirect heating, although I agree in concept with what you say in the first quote above, I'm not sure that it is necessarily anything like as much of an issue as you seem to imply ....

.... I don't think that anyone is denying that some heating of 'the entire contents of the cylinder' (including the lowest parts) will inevitably occur. However, with an indirect system, although the water retuning from the coil to the boiler cannot be more than 20 C (or whatever) lower than the temp of the water leaving the boiler, the temp of that returning water can be much higher than that of the water in the low parts of the cylinder, particularly if flow rate through the coil is high. Hence, although, for 60 °C 'at the top' the 20 °C (or whatever) limit means that water returning from the coil cannot be cooler than 40 °C, the temp of the water in the lower parts of the cylinder could be much lower than that, couldn't it?

 

[ericmark]

I suppose direct is where the heat is put directly into the hot water store, as with the units at work, with fire tube boilers and injectors to replenish the water used. The old aga had no hot coil, the water which comes out of the tap also when through the side boiler, but most systems have a hot coil, and these can both impart the heat into the cylinder and extract the heat, a system like this allows the hot water to be at cold water pressure, without pressurising the whole cylinder. The main idea is to allow the use of multi heating sources.

However other than the fire tube boiler, there is some pipe work which can loose heat, seem to remember at Sizewell the boiler is heated with high pressure water, that transfers the heat from the reactor. That pipework is the problem, it is where the heat is lost, and to maintain the water at a useable temperature only requires 0.56 kWh, and when so little heat is required, one does not need to loose much for the indirect heating system to loose too much.

 

[Harry Bloomfield]

What does happen is the hot water rises to the top, just as everyone has been posting, however what happens when the heating stops is the continued currents circulate and the 'hot' water gradually warms the cooler water and the junction between hot and cold gradually lowers, both in location and temperature and it gets deeper to, Of course the 'junction' is not a sudden step it is a zone across which is a graduated temperature.

I'm just not seeing that later mixing effect here, apart from when hot water is drawn off - cold water enters at the base, hot exits at the top, so naturally there will be some mixing effect, yet if I draw a bath full of water, and leave the hot tap on, the hot water will quite suddenly change in temperature, to cool/cold.

Feeling the temperature of the hot water, coming out of the hot tap, gives not clues about how much of the cylinder water is hot.

 

[fixitflav]

I don't know whether it's been mentioned, but one reason for using the boiler for HW is that the kit gets regular exercise. If it's off all summer, you might switch CH on in November and find there's a problem, eg pump seized.

 

[SUNRAY]

I'm just not seeing that later mixing effect here, apart from when hot water is drawn off - cold water enters at the base, hot exits at the top, so naturally there will be some mixing effect, yet if I draw a bath full of water, and leave the hot tap on, the hot water will quite suddenly change in temperature, to cool/cold.

Feeling the temperature of the hot water, coming out of the hot tap, gives not clues about how much of the cylinder water is hot.

I understand the difficulty but let's pretend there is a distinct thermal barrier (Imagine there is a real copper barrier if it helps) the hot water sits on a surface of a lower temperature. By simple thermal conductivity:
The bottom of the hot water will cool and the top of the cold water will warm.
Now imagine the cooler hot forms a barrier with the hot and the same process will occur so the cooler will warm more and the bottom of the hot will cool.
Simultaneously the original barrier is therefore warmer so warms the cold layer.

Hopefully it can be seen the distinct barrier is becoming wider with a temperature gradient between the hot and cold.

Eventually the barrier will disappear altogether to the point the top becomes warm rather than hot and further down becomes warm (but not as warm as the top) rather than cold.

Chances are heat loss will contribute and likely be blamed.


If that doesn't make sense; think about suspending a hot body in cold water, the body will cool and the water will warm and the depth of the warming will depend on the heat loss from the body into the water (heat transfer) until there is not heat left to transfer.


Irtonically when one wishes to only heat the top of a vessel it would be desirable to have a definite stratification issue to prevent the lower water being warmed or a taller thinner vessel.

 

[Harry Bloomfield]

I don't know whether it's been mentioned, but one reason for using the boiler for HW is that the kit gets regular exercise. If it's off all summer, you might switch CH on in November and find there's a problem, eg pump seized.

Not in my case - the pump is controlled by the boiler, for overrun. The boiler has a built-in what seems to be a daily exercise routine, to run its fan, pump, and perhaps other things.

 

[fixitflav]

Not in my case - the pump is controlled by the boiler, for overrun. The boiler has a built-in what seems to be a daily exercise routine, to run its fan, pump, and perhaps other things.

OK but I doubt whether Eric's oil boiler is that sophisticated.

 

[Harry Bloomfield]

OK but I doubt whether Eric's oil boiler is that sophisticated.

I;m sure Eric would be able to cobble something together.

 

[ericmark]

I;m sure Eric would be able to cobble something together.

I am sure I could, if there was a problem, however the only problem is I had wrongly assumed oil was cheaper than grid electric I again have read how much used today and shows 0.55 kWh, and at that consumption simply not worth doing anything about it.

 

[SimonH2]

I suppose direct is where the heat is put directly into the hot water store

Yes

... but most systems have a hot coil, and these can both impart the heat into the cylinder and extract the heat, a system like this View attachment 353885 allows the hot water to be at cold water pressure, without pressurising the whole cylinder. The main idea is to allow the use of multi heating sources.

Indeed - and that's what I have installed, but currently only have a gas boiler (and immersion heater for backup).

The other benefit is the the store acts as a neutral point that decouples not just the heat sources from each other, but also the geat loads bith from each other and the heat sources.
So I gave the CH fully TRV and with a midulating pump - allowing flow rates tgat would make just about any boiler trip on either flow rate, flow temp., or flow/return delta-T. Makes the system very quiet.
Similarly, DHW has consitent temp. regardless of flow rate - unlike a combi.

However other than the fire tube boiler, there is some pipe work which can loose heat, seem to remember at Sizewell the boiler is heated with high pressure water, that transfers the heat from the reactor. That pipework is the problem, it is where the heat is lost, and to maintain the water at a useable temperature only requires 0.56 kWh, and when so little heat is required, one does not need to loose much for the indirect heating system to loose too much.

When I out the store in the flat, the flat was empty so I could take measurements. With no loads and heated by the immersion, the standing losses were 80W, or just under 2kWHr/day. By contrast, tge cimbi next door could burn double that in it's "keep the HE warm for instant hit water" non-eco mode.

 

[ericmark]

When I out the store in the flat, the flat was empty so I could take measurements. With no loads and heated by the immersion, the standing losses were 80W, or just under 2kWHr/day. By contrast, tge cimbi next door could burn double that in it's "keep the HE warm for instant hit water" non-eco mode.

This is interesting, likely mine is better due to airing cupboard being stuffed full. So you are using around 3 times what I am using if that is the iboost+ gives an accurate reading.

I am lucky in that the boiler is a system type, as once one goes to combi the cost to return to system is rather high. I feel the combi is a con with oil, it is simply a smaller cylinder built into the boiler, as it can't modulate, it is either off or on.

At 9 pm looked again, 0.69 kWh, still very low use.