Another way to look at OpenTherm low load low/off control is that Evohome has no idea what the lowest flow setpoint is. It can read the Max CH water setpoint (which is often wrong e.g. Intergas always reads 90C) but there is no minimum reading.
Furthermore, the boiler minimum supply temperature can be changed via boiler parameters (e.g. Intergas 10C-60C) and OT response (e.g. 0=Ignore OT demand if the setpoint for the flow temperature is below minimum, 1=Accept OT demand if the setpoint for the flow temperature is below the minimum).
So I think Evohome may try to determine a working minimum setpoint that actually heats by monitoring the boiler water temperature reading, flame status.
Theoretically if the minimum setpoint was high, or misconfigured to match the boiler's max CH flow temperature, I guess the behaviour becomes more like TPI with on/off periods to try to reduce the output.
With my setup Evohome is running as low as 26C flow setpoint when it starts employing low/off control.
Furthermore, the boiler minimum supply temperature can be changed via boiler parameters (e.g. Intergas 10C-60C) and OT response (e.g. 0=Ignore OT demand if the setpoint for the flow temperature is below minimum, 1=Accept OT demand if the setpoint for the flow temperature is below the minimum).
So I think Evohome may try to determine a working minimum setpoint that actually heats by monitoring the boiler water temperature reading, flame status.
Theoretically if the minimum setpoint was high, or misconfigured to match the boiler's max CH flow temperature, I guess the behaviour becomes more like TPI with on/off periods to try to reduce the output.
With my setup Evohome is running as low as 26C flow setpoint when it starts employing low/off control.
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