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Author Topic: Immersion Heater Control  (Read 691 times)
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« on: January 10, 2017, 10:11:45 PM »

I've put this under PV as it at least partially relates to use of solar power but feel free to move it if somewhere else is more appropriate.

I have been happily heating my domestic hot water with solar hot water in summer and a back burner on the log burning stove in winter, using the immersion heater occasionally when the others are not available.
Due to an asthma diagnosis in the family I probably need to stop using the wood burning stove and the ASHP is taking over all the heating of the house in winter.
There is significant PV generated electricity even in winter - 50kWp installed of which 22kWp is on the phase used for the house.
Electricity exports are sold at 5p/kWh.
The existing immersion heater is too short to heat enough water for a decent bath.

I am looking at 4 options:
1) Simply replace the immersion with a longer one - cheap and simple but means too much water will be heated quite often. Electricity will often be imported at 13p/kWh.
2) Replace the immersion with a longer one and add a solar power diverter - as1) but much of the electricity will be self generated at a loss of income of 5p/kWh.
3) Replace the immersion with a dual element unit which can heat either just the top of the tank or the bulk of it - as 1) but avoiding heating too much water as often and needs an additional cable from the consumer unit and an MCB which is quite easy to do.
4) As 3) but with a solar power diverter to sequentially heat the two immersions.

My inclination is towards option 4 but there are some difficulties.

If I use a solar power diverter such as the iBoost or Immersun they are basically designed to dump all excess power into a load until demand is met. To avoid the unit operating when solar hot water is being generated and to avoid heating too much water and unnecessary standing losses due to heating the tank too soon I want to nominate when either of the immersion elements is turned on, use any available PV and then the "boost" facility (using imported electricity) if needs are not met. I could do this simply by using isolators in the cables between the diverter and each immersion element and switching on the immersions when I wanted to use them but some suppliers (iBoost) prohibit this.
The clamp sensor needs to be on the other side of the drive so should be wireless. The solar power diverter could be mounted either by the consumer unit which is about 5m and one wall from the sensor but is within about 150mm of an inverter and means pulling an extra cable to the consumer unit; or could be in the immersion cupboard which is about 20m and 4 walls away from the sensor.
I don't need any of the add on features like timers, fancy displays or complicated programming options.
So any recommendations on a cheap and simple solar diverter for this duty with wireless sensor and where it should be mounted?


Schuco solar hot water - 3300kWh/annum, 16 BP 4175N PV panels - 2.8kWp, log burner and back boiler and 18 Ying Li 235 PV panels - 4.2kWp, 42kW ground mount PV, 9kW Panasonic ASHP, 40kWh Nissan Leaf
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