Our immersion electric water heater used to be smart – it was controlled by a diverter that automatically used excess solar power. Unfortunately the diverter and the battery would not play nice together so the clever solar diverter became a dumb timer. Worse it was a dumb timer that did not integrate with Home Assistant – it was controlled by the SolarEdge app – so no possibility to automate it beyond a timer, no possibility to exploit all the other information brought together in Home Assistant.
Controlling an immersion heater is easy – simply turn its power on or off – but it is a 3 kW load that could be running for a couple of hours. Switching could be done with a smart plug but even though some are specified for 13A/3kW operation they are small devices that can get warm – for us not worth the risk of using one in such a demanding application. Contactors are purpose built devices for switching large loads – they fit in a standard consumer unit, are fail safe and proven over many years. The contactor takes a 230V control signal and when the control signal is present it switches on, take away the control and it switches off. The control signal needs only a mA or two so can be safely provided from a smart plug. The solar diverter was duly replaced with a contactor controlled by a smart plug running Tasmota software
Tasmota? Tasmota is open source software for smart devices that works locally over the house network – it does not need to talk to a server somewhere over the internet. Almost all smart plugs will not work locally – their apps require data to leave the local network and travel over the internet, commands are sent back over the internet and via the house network to the device. It’s unnecessary, could be a security risk and is wasteful for most applications. These smart plugs come with Tasmota software pre-installed – they also have an on/off button which can provide manual control if required.
The new immersion controller works well and can be integrated into the Home Assistant environment for much smarter control.
For legionella control the actual water temperature can be used to turn off the immersion when 60C is reached. Yes, the immersion has a built in thermostat that will turn the heater off once a preset temperature is exceeded but it’s a manual adjustment that takes a bit of trial and error to set correctly and it can’t be easily changed. Using the actual tank temperature is just so much better. The legionella control routine can also be triggered immediately after the heat pump water heating finishes so minimising the power used. In winter this all happens automatically overnight using cheap electricity.
Above is our Hot Water dashboard in Home Assistant. The ‘Tank temp – set/actual’ is control and monitoring for the Heat Pump. Immersion Boost uses the immersion heater to heat the water to the user set temperature then turns the immersion off. A future development may look at the predicted solar production, the state of the battery and automatically heat the water when solar is plentiful.
A last question – why use the immersion when the heat pump is more efficient? It’s a great question that we’d like to be able to definitively answer. Physics says that raising 250 litres of water (the size of our hot water tank) by 5C will use around 1.5 kWh of energy. That’s in line with our monthly hot water energy consumption as reported by the heat pump – in winter heat pump runs overnight on cheap rate electricity to heat water. In summer though there’s plenty of free electricity from solar and the heat pump is idle except for hot water – don’t know if firing up the heat pump from cold just for 30 mins to heat water is efficient – there’s the pump to run and lots of piping and water it needs to heat up that the immersion doesn’t.
The reality is we don’t use much hot water so it probably doesn’t matter either way. Say the immersion uses 1 kWh more than the heat pump a day – in summer that means we export 1 kWh less, missing out on 4.1p – a little over £1 a month. Not really worth thinking about…
…but may try an experiment or two over summer to know for sure – can’t resist!