rvicev avatar image
rvicev asked

Dumpload – Hot Water – Legionella, Efficiency ?

I am still in the planning phase of an off-grid system (MP II + AC-coupled Fronius or Fimer + LiFePO4 battery), reading for months now all relevant infos.

Using a hot water boiler as a dump load in case of excess PV inverter power sounds attractive, otherwise that power would just be lost (because the MP II would throttle down the solar PV inverter when the batteries are full).

But it does raise a few issues :

a) Legionella

Most water boilers (at least in France) take a long time to heat up, depending on volume of the boiler and capacity of the heating element (anywhere from 1,5 up to more than 5 hours).

During summer this may not be an issue : provided the solar system is correctly dimensioned there will be plenty of excess PV inverter power over the day and the water heats up to the temperature configured on the water boiler (typically a mechanical thermostat in simple boilers), say 55+ C.

However during winter this is rather unlikely and water may be heated up to e.g. only 35 C. That´s a perfect temperature for legionnella bacteria colonies to grow (growth range : 25 – 45 C). You won´t be taking a shower at 35 C, so you will need some means to increase the temperature (e.g. gas heater). Unfortunately, while showering the water temperature gets increased to e.g. 55C in the gas heater for only a very short time, not long enough for legionella bacteria to die, which takes 5-6 hours @ 55C and still more than 30 minutes @ 60C).

You would have to heat up the water to 70 C for the legionella bacteria to die instantly. But that takes a lot of extra gas ….. and so you would loose the energy saved by means of the dump load, possibly even more.

Legionella is not wide spread in western Europe, but if not recognised and treated early on potentially very dangerous.

All this makes me wonder if a boiler dump load is a sensible solution. Any comments ?

b) Efficiency : AC Thor, SoC

Heating water electrically is not very efficient, but who cares about efficiency if that electricity would otherwise be lost ….

The My-PV ´AC Thor´ is a technically clever solution, but rather expensive (because you also pay for a lot more functions which you will never use in a ´dump load only´ scenario, a fancy LCD which you will never look at again once the system is correctly up and running, data logging, etc). IMHO technically it may be the best, but no pay back in a reasonable time frame.

Switching on the boiler via the relay built into the MultiPlus II based on SoC is a standard feature at no extra cost, except for boiler, piping, cabling and RCD. But it will eat some battery capacity and if for example you switch on at 99% SoC and off at 95% SoC it will likely take a very long time for the boiler to warm up (switching on-off-on-off- ….), with the potential risk of the (literally) tiny problem mentioned under a) …..

All this makes me wonder if it is not far better to save the money otherwise spent on a dump load solution and put it in the budget for larger battery capacity.

Any thoughts / comments ?

Hot Water Diversionbalanced ac-load
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1 Answer
Jason - UK avatar image
Jason - UK answered ·

@rvicev You would be better in having a thermal store. A thermal store stores non-potable water at temperature between 45*C and 90*C. You then have a heat exchanger to transfer the heat energy from the non-potable water to main cold water, potable water to heat it up then use a thermostatic mixer to provide a constant temperature.

Several benefits of a thermal store.

  • Variable temperature to allow better utilisation of different heat sources without the concern of legionella.
  • No actual storage of potable water therefore no need to comply with local regulation regarding the storage of domestic hot water.
  • Thermal store allow a greater amount of heat energy to be store than a hot water cylinder as you can heat a thermal store up to 90*C, ideal for solar thermal.
  • You could also use the thermal store to provide heat energy to a wet heating system as well.
  • Multiple energy sources can input their heat into a thermal store - heat pump, solid fuel boiler, gas boiler, solar thermal, high temperature immersion heaters.

If for example you have a thermal store operating at 60*C but you want to dump extra energy into it, you can but also if you run short of energy and the temperature drops down to 40*C, its still useable without the risk of legionella.

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rvicev avatar image rvicev commented ·

But a thermal store is a complete heating concept that has to be taken into account during construction of a building. I guess the My-PV products are aimed at this also. You´re also talking about a fundamentally different investment .... Besides, heating up (non-potable) water and then transferring that heat via an exchanger to main cold water will not increase exactly efficiency, on the contrary.

My thread was rather aimed at people who use standard boilers as dump loads, since I came across several people who appear to be doing so on this Victron Community forum.

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