Boiler Return Temperature Control

[John.g]

I have rarely if ever seen mention of this form of control being used, which Valliant, for one, have available for years, is there a reason for this?

 

[Deleted member 120897]

I wasn’t aware of this, I’m guessing the need is for lower temperatures required for commissioning? Vaillant Ecotec Pro has flow and return temperature accessible via the diagnostic menu, which others may too?

 

[Chuck]

If you used 'return temperature' directly as the primary form of control, the long time-lag between changing the boiler output and seeing a response on the return temperature would result in a system that is unstable, i.e. the boiler would short-cycle.

Modern domestic boilers often do, however, modulate to keep the difference between the flow and return temperatures to a fixed amount, e.g. 20degC subject to not exceeding the specified maximum flow temperature. They may also wrap the primary control loop in a secondary control loop that monitors return temperature to ensure that condensing occurs when possible. The algorithms used are proprietory so you need to be pretty determined (and have a data-logger and some thermocouples) to figure out exactly what strategies are being used by a given manufacturer.

 

[John.g]

I don't think the control is direct as someone on here (Vaillant Ecotec 418) briefly c/o to Return Temp control, it had been set to Flow control with a SP of 70C which gave a return temp of 55C, the target temp was then set to 55C on c/o and the flow/return temps then fell to 47C/37C, with a observed d.05 of 36C which is the "Determined heating flow set target temperature", presumably, this was now the calculated return temperature required?. Unfortunately, as there were more pressing problems at the time, the test was not continued, maybe there's a volunteer or two out there?.