by ericmark »
Sat Nov 26, 2022 6:02 pm
There are broadly speaking two types of boiler, non modulating and modulating, and they need very diffrent control systems.
So non modulating the thermostat has a mark/space ratio and as it approaches the target temperature it starts to switch off/on and slowly increases off time so it will not over shoot.
However do that with a modulating boiler, and it actually messes it up, as every time the boiler is switched off, (with an exteria thermostat) when it switches on again it does so flat out.
There are two ways to control a modulating boiler, one with the TRV's and two with a modulating wall thermostat, however the former has a problem.
When all the TRV close the by-pass valve opens hot water returns to boiler and it turns off, but as the boilers cool it switches on again, so it will continuously cycle, anti cycle software can reduce it, but not stop it.
So the wall on/off thermostat it put in a room, normally kept cool, on the ground floor, with no alternative heating and no outside doors.
In real terms the room often never exists, so the TRV and wall thermostat are set so in Winter the wall thermostat never opens, only as summer approaches will it get warm enough to open.
The best option is TRV's linked to the main thermostat, but it gets costly.
Using electronic TRV's one can get better control, I got eQ-3 heads for £15 back in 2019, there is also the Terrier i30, not linked, but electronic.
The linked ones start at around £35 each, and good ones like Drayton Wiser more like £55 each so it is a case of are they worth the money?
I made a mistake, I got Energenie TRV's and Nest Gen 3 having been told they worked together, however this was not true, when Nest was taken over by Google support was withdrawn.
