by ericmark »
Tue Jul 02, 2019 10:52 pm
There were units that used solar power to heat domestic hot water, it used the fact that the early solar panels did not have proper metering, but it was assumed as to what they produced, so they would measure the power being pushed into the grid, and with 1 kW immersion heaters they would switch on more or less units to use power rather than export to grid.
You clearly need a hot water store, but size did not matter that much, it could be a under sink heater.
But installation cost is problem, so first is look at existing central heating, can it be improved? EQ-3 Bluetooth Smart radiator TRV heads are around £15, using these you can select when and to what level each room is heated, as you go up in price they can do more, the EvoHome can use the info from each TRV head to control boiler output.
However depends on boiler, there are three ways to control a modern boiler.
1) Wall thermostat off/on.
2) Wall thermostat modulating.
3) TRV modulating.
Some boilers simple don't modulate, but a condensing boiler normally will, it needs to modulate to ensure the return water is cool enough to condense the moisture in flue gases and extract that heat.
However most wall thermostats are off/on, and some have anti-hysteresis software built in, so they start turning off/on before set temperature so they will not over shoot. However every time the boiler turns off, the heat escapes out of the flue, we don't want it to turn off until already running at lowest output, we want the boiler to modulate, i.e. turn down, not off.
Some boilers can use modulating thermostats, OpenTherm is most popular, but some can only modulate using the return water temperature, so theory boiler fires up on full output, all radiators warm up and heat room as required using the TRV. As the valves slowly close more water forced through remaining until the by-pass valve opens, then hot water returned to boiler, once the boiler detects this hot water it turns down, and the output slowly drops from around 28 kW to around 8 kW, once at 8 kW boiler starts to switch off/on, but the water temperature is reduced so less heat lost through flue, however as summer arrives it will never turn completely off, so a wall thermostat is used in a room kept cool with no outside door, or alternative heating, this will switch off whole system in the summer.
But if a off/on thermostat starts to work during the winter, then the modulating system is reset every time it switches off, so more heat lost through the flue.
Theory does not always work, with mothers house the insulation was so good to outside and so poor room to room, although using electronic TRV heads in the main rooms, it saved very little as over night the house did not lose the heat anyway. So setting bedroom to heat 8 pm to 8 am, and living room 7 am to 9 pm, sounds great, but they didn't cool anyway so nothing saved.
