The height of summer is perhaps an odd choice of time for a post about heating, however we’ll get to that. Regular readers may recall that our smart heating controls enable the boiler when any radiator demands heat via its smart valve (eTRV) and disables the boiler when the last room is up to temperature. That rather begs the question what happens to the control logic when the batteries go flat. The HomekIt rules are not sufficiently sophisticated for the author to set that behaviour via the program, and I’ve seen no default behaviour described on-line.
Today the inevitable happened and a battery did go so low as to stop operation. The behaviour of the heating was to force the boiler on. Most rooms did not heat up as their own eTRVs recorded them already being sufficiently hot, although the bathrooms and cloakrooms did heat up (they generally lack eTRVs) as indeed did the room with the flat battery (my daughter’s playroom). Other symptoms included an icon in the Home app that would not grey out when disabled like other room eTRVs and the the boiler repeatedly being re-enabled even after manually disabled through the app.
Replacement with fresh batteries immediately restored normal operation.
Although any more heat input is unwelcome on a day as hot as today, we have at least demonstrated that the system is failsafe in a flat battery condition – I’d rather that the system heated up with a flat battery to prevent freezing damage in winter at the cost of some discomfort in summer due to excess heat.
It’s 265 days since my records indicate an earlier battery change, albeit for most of that time the heating hasn’t been on (so no power needed for valve movements). It’s over a year now that I’ve been using Ni-MH cells in these valves. The cells are slightly lower voltage than the recommended cells (1.2 versus 1.5 Volts) so do create spurious low battery warnings, but apart from that they seem to work well with adequate life before recharging. I thus anticipate continuing with their use of a means of reducing battery waste. I’ve had no cell failures to date.