Residual heat wasted: ch_enable=false during PWM OFF stops pump while boiler water is still ~35°C

[FCygan]

Setup

• SAT v4.2.0 with DIYLESS OpenTherm thermostat (ESPHome)
• De Dietrich INIDENS 20/24 MI condensing boiler
• Climate entity: climate.diyless_thermostat

Observed behavior

When SAT transitions to PWM OFF phase (setpoint drops below COLD_SETPOINT of 28.2°C), or when heating demand ends entirely, the thermostat reports:

• t_set = 10 °C (MINIMUM_SETPOINT)
• ch_enable = off
• Boiler flow temperature = ~35 °C (still warm from previous cycle)
• Pump = stopped immediately

Problem

Setting ch_enable=false causes the boiler to stop the pump immediately. Residual heat in the heat exchanger (~35 °C) dissipates through the casing and flue instead of being delivered to radiators.

This happens in two scenarios with different impact:

1. PWM cycling (frequent short on/off) — small heat loss per cycle, but it adds up across many daily cycles
2. Long idle periods (e.g. night setback, room at target) — significant heat loss as ~35 °C water sits in the boiler for extended time

Looking at the code, t_set is already set to MINIMUM_SETPOINT (10 °C) during OFF phases (climate.py:981). This value would be perfect for keeping the pump running without firing the burner — but ch_enable=false overrides it and kills the pump, making the 10 °C setpoint meaningless.

Suggested improvement

Add a configurable pump post-circulation delay to the PWM OFF / idle transition:

1. When SAT ends a heating demand or enters PWM OFF, keep ch_enable=true with t_set=10 °C
2. The boiler won't fire (water temp is already above 10 °C), but the pump keeps running and pushes residual heat to radiators
3. After a configurable timeout (e.g. 1-5 minutes) or when flow temp approaches return temp (indicating residual heat has been delivered), set ch_enable=false

This approach is already used by other OpenTherm implementations — e.g. rsciriano/ESPHome-OpenTherm explicitly idles at 10 °C to keep the pump running.

For PWM cycling specifically, an even simpler approach: just never set ch_enable=false during PWM OFF — keep it true with t_set=10 °C throughout. The pump keeps running between PWM pulses, the boiler doesn't fire, and the next PWM ON phase starts with warm water already circulating.

Question

Would a configurable pump post-circulation option be considered? Or alternatively, could the PWM OFF phase keep ch_enable=true with t_set=MINIMUM_SETPOINT instead of disabling CH entirely?

[sergeantd83]

Hi, thanks for the recommendation. The situation is a bit more complicated and highly boiler-specific.

Not all boilers are capable of running their circulation pump with t_set = 10°C.
All boilers immediately stop the pump when ch_enable is set to false. When ch_enable is true and t_set = 10°C, most boilers still do not maintain circulation because the control setpoint is below their internal minimum operating threshold ( MsgID49 LB ).

Because OpenTherm pump behavior varies between manufacturers, relying on t_set = 10°C to guarantee post-circulation would result in inconsistent behavior across installations.