That’s correct, the ‘minimum on time’ is applicable when the thermostat is using time proportional control in the proportional control band (the 1.5C around the set point). If you increase the on time you are making the control mechanism coarser and the temperature will fluctuate somewhat more. For instance, if the output demand is 10% then, with a minimum on time of 5 minutes the system will be on for 5 minutes, off for 45 minutes. Whereas in the standard settings, the system would be on for 1 minute then off for 9 minutes.
You are quite right, I should have referred to my copy of the OT protocol. The controller calculates the required water temperature and sends this info to the boiler. How the boiler achieves this temperature is left up to the boiler. The controller can interrogate the boiler for the current modulation, which is given as a percentage (0%=minimum, 100%=maximum).
It's more than that. OT not only defines how the messages are transmitted but what messages can be transmitted. The vast majority of the messages are requests by the controller for data, i.e read messages, from the boiler, which depend on the sensors in the boiler. The only write message is the required water temperature.Dan_Robinson said:
At last! Someone with the experience I asked for. What kept you so long?
But that's 50% of the 10 minute cycle time. Do you change the Proportional Bandwidth to prevent overshoot?
I agree, having watched the nF parameter while someone turned the thermostat up. According to a email from Remeha the startup speed (P20 = 25 or 28 ) is used while the boiler is igniting to ensure reliable ignition.
I would have thought it was required to maintain the required flow temperature. The differential depends on the flow rate, which is controlled by the pump speed and pressure loss round the circuit.
Presumably this was because you were wiring up as a standard Y or S plan.
Do you mean the code to get into the "installer only" parameters?
I was querying this as well. The whole purpose of reducing the on time as the temperature approaches target is to reduce the amount of overshoot. Setting a minimum on time of 5 minutes is likely to increase the amount of overshoot. Increasing the Proportional Bandwidth should reduce this.
If, with a minimum on-time of 5 minutes and a cycle rate of 6, the cycle period remained as 10 minutes, then the system output level can go no lower than 50%, which is going to be too high to maintain a set temp in most conditions. The CM control must increase the cycle period if a higher minimum on-time is selected.
The purpose of reducing on time is to reduce the system output level to the point where it balances the heat loss from the building and thus maintains the set temperature. The rate at which the output level changes in the proportional band (system output proportional to temperature error) reflects the gain of the control system and will affect the overshoot during a warm-up period. I agree that increasing the minimum on time is likely to increase overshoot because the effect is to increase the time constant of the control system (longer cycle period).
Presumably these are based on your observations and timings. If so, were you relying on the displayed figure for the temperature?
You may have checked it many times, but there is still the 0.5C steps on the display to take into consideration.
So it's not wired up to a boiler? If that's the case, how do you know when the stat has turned the boiler on or off?
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