Vaillant boiler modulating even with on/off thermostat?! Why is this happening?

8 Sep 2023
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United Kingdom
Hi All, I have a Vaillant ecoTEC 637 plus, likely fitted ~2017 mounted as a system boiler, single zone, with a separate water tank for DHW.

The thermostat is a Salus RT500 wired via the 230v RT connection. There is also a Drayton scheduler.

(Small parenthesis: It is also a classic case of oversized boiler, 37 kw whilst I have maybe 17kw of rads and say 3kw for the cylinder. The technical doc says it can modulate down to 6kw and I did manage to set it at this level in d.0 to try. Trying to make the best of a bad situation by somewhat improving controls)

I learned on this forum about the installer mode and ability to take readings (D40/D41 for flow / return flow) and made tests.

When starting from cold, 12 degrees C outside it does this:

1 - 29 flow temp / 25 return flow, turn on, low flame for 5 minutes
2 - After 5 minutes flame is increased, looks like 75% on the display, water heats up quickly, reaches 67/47 in 7 minutes
3 - Boiler gradually reduces the flame, after 11 minutes it's at 68 / 61, flame down at ~25%
4 - It then stays at 69/63 until the room reaches the set temperature and thermostat tells the boiler to turn off.

My questions:

A) Is it modulating? I thought that only Vaillant digital controls via eBus could modulate, but this is a 230v RT connection on a cheap thermostat?
B) Does this mean that if I replace the thermostat by a smart one (Hive, Tado ..) the boiler will keep modulating in the same way as it does now?

Many Thanks
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Basically the boiler modulates on the flow target (setpoint) temperature, when the flow temperature is below the target temp then the boiler will fire at maximum output or whatever its set to in d.0, 17kw in your case?, once it reaches the target temperature it will start modulating down to maintain this temperature, if the boiler minimum output is greater than the heat demand, the flow temperature will continue to rise and the burner will trip at target temp + 5C the circ pump will continue to run and the boiler will not refire until the anti cycle period has elapsed and the flow temperature is target temp - 5C.
on/off roomstats do exactly that, switch on the boiler at the differential setting say 0.5C to 2C below the setpoint temperature and switch off at setpoint temperature, the smart ones can switch the boiler on/off as often as 6 times/hour to maintain very tight romm temperature control.

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