Hi All,
Can one of the Honeywell guys (or anyone who knows for certain) explain the algorithm behind the "minimum on time" system setting ?
Yes, it's fairly obvious that minimum on time is the minimum amount of time that the boiler will be fired in each TPI cycle, to protect the boiler against damage or inefficient operation. What is not stated however is what happens when the demand is above zero but below the set threshold.
For example say the system is set to the default of 6 cycles per hour giving a 10 minute cycle, and minimum on time of 1 minute. This means the minimum duty cycle that the boiler will be fired at is 10%.
The total boiler demand seems to be the combination of all the different zones. Eg, each zone stat sends a demand figure from 0 to 100% to the main controller (based on how much heat it thinks it needs to reach/maintain the set point) which then aggregates them (sums and truncates to 100% perhaps?) and then sends the duty cycle to the boiler relay.
So what happens if the total system demand is below that which the minimum cycle time would allow for - for example if the demand is 5% due to only one zone being active and needing a small amount of heat to maintain its set point.
Will the boiler not fire at all until a demand of 10% is required, or will any demand from 1% to 10% cause the boiler to fire at a 10% duty cycle ? (with 0% not firing at all)
The reason I ask is I have noticed an interesting/annoying phenomenon on our system. In the early evening multiple zones are active and during this time the boiler is typically operating at roughly 20-30% duty cycle (estimated by timing the BRD91 cycle time) and during this time the living room maintains a perfectly stable 21 degrees.
At 8pm all the other zones in the house go off but the living room zone remains at 21 degrees until 11pm. Roughly half an hour after the other zones go off the temperature in the living room starts to drop significantly due to the boiler not firing sufficiently (or at all, hard to tell without watching it constantly) and the flow temperature dropping way down...
The temperature will typically drop as low as 20 degrees before the system makes any attempt to rectify the drop in temperature - checking the valve position (option 10 on the HR92) shows that the valve is fully open, eg >80% which if the boiler was running would warm the room back up again easily, however the boiler is either not running at all or only at the minimum 10% duty cycle.
Once the boiler does eventually come on enough to heat the flow temperature back up the radiator gets hot and the room then overshoots the set point, usually by half a degree, sometimes by a full degree. This is after the room had been previously stable within half a degree for hours on end.
The overshoot I think is caused by "integral windup" - the HR92 knows the temperature is too low and progressively opens more and more but because the flow temperature is cold it has no real effect. The longer the room is below temperature with the valve open the more the algorithm increases the integral which means when the flow finally heats up it overshoots while the integral settles down again.
To see if it helps, I have increased minimum on time to 2 minutes. Whether this helps or makes things worse, depends on whether boiler demands below the minimum on time are completely ignored, or trigger the minimum on time, hence this post posing this question.
If demands below the minimum on time are just ignored, increasing the minimum on time will actually make this problem worse because now the temperature in the living room zone will have to be even lower before sufficient heat demand is generated by the zone stat to trigger the boiler to be fired for the minimum on time.
However the other possibility is that the boiler was firing at a 1 minute / 10% duty cycle, but that is simply not enough warmup time for my boiler to generate any usable increase in flow temperature. Thus increasing it to 2 minutes (or maybe 3) would mean that even if the actual demand is very low (say 5% to maintain the temperature in one zone) the boiler would fire on a 20% duty cycle and thus heat the flow quick enough to maintain the temperature in the room when the demand from the other zones goes away...
Anyone else notice this phenomenon when all but one zone shuts off ?
Edit: Here is a graph from today, showing that the temperature came quickly up to the 8am set point in the morning without any overshoot and apart from one small blip remained dead on 21 all day until 8pm - a remarkable feat when the door was open some of the time, closed some of the time, people were coming and going and so on... Then after 8pm when the other zones went off the temperature drops a full degree while the radiator goes cold due to insufficient firing of the boiler until it eventually wakes up and starts firing the boiler enough to heat the radiator up again.
Can one of the Honeywell guys (or anyone who knows for certain) explain the algorithm behind the "minimum on time" system setting ?
Yes, it's fairly obvious that minimum on time is the minimum amount of time that the boiler will be fired in each TPI cycle, to protect the boiler against damage or inefficient operation. What is not stated however is what happens when the demand is above zero but below the set threshold.
For example say the system is set to the default of 6 cycles per hour giving a 10 minute cycle, and minimum on time of 1 minute. This means the minimum duty cycle that the boiler will be fired at is 10%.
The total boiler demand seems to be the combination of all the different zones. Eg, each zone stat sends a demand figure from 0 to 100% to the main controller (based on how much heat it thinks it needs to reach/maintain the set point) which then aggregates them (sums and truncates to 100% perhaps?) and then sends the duty cycle to the boiler relay.
So what happens if the total system demand is below that which the minimum cycle time would allow for - for example if the demand is 5% due to only one zone being active and needing a small amount of heat to maintain its set point.
Will the boiler not fire at all until a demand of 10% is required, or will any demand from 1% to 10% cause the boiler to fire at a 10% duty cycle ? (with 0% not firing at all)
The reason I ask is I have noticed an interesting/annoying phenomenon on our system. In the early evening multiple zones are active and during this time the boiler is typically operating at roughly 20-30% duty cycle (estimated by timing the BRD91 cycle time) and during this time the living room maintains a perfectly stable 21 degrees.
At 8pm all the other zones in the house go off but the living room zone remains at 21 degrees until 11pm. Roughly half an hour after the other zones go off the temperature in the living room starts to drop significantly due to the boiler not firing sufficiently (or at all, hard to tell without watching it constantly) and the flow temperature dropping way down...
The temperature will typically drop as low as 20 degrees before the system makes any attempt to rectify the drop in temperature - checking the valve position (option 10 on the HR92) shows that the valve is fully open, eg >80% which if the boiler was running would warm the room back up again easily, however the boiler is either not running at all or only at the minimum 10% duty cycle.
Once the boiler does eventually come on enough to heat the flow temperature back up the radiator gets hot and the room then overshoots the set point, usually by half a degree, sometimes by a full degree. This is after the room had been previously stable within half a degree for hours on end.
The overshoot I think is caused by "integral windup" - the HR92 knows the temperature is too low and progressively opens more and more but because the flow temperature is cold it has no real effect. The longer the room is below temperature with the valve open the more the algorithm increases the integral which means when the flow finally heats up it overshoots while the integral settles down again.
To see if it helps, I have increased minimum on time to 2 minutes. Whether this helps or makes things worse, depends on whether boiler demands below the minimum on time are completely ignored, or trigger the minimum on time, hence this post posing this question.
If demands below the minimum on time are just ignored, increasing the minimum on time will actually make this problem worse because now the temperature in the living room zone will have to be even lower before sufficient heat demand is generated by the zone stat to trigger the boiler to be fired for the minimum on time.
However the other possibility is that the boiler was firing at a 1 minute / 10% duty cycle, but that is simply not enough warmup time for my boiler to generate any usable increase in flow temperature. Thus increasing it to 2 minutes (or maybe 3) would mean that even if the actual demand is very low (say 5% to maintain the temperature in one zone) the boiler would fire on a 20% duty cycle and thus heat the flow quick enough to maintain the temperature in the room when the demand from the other zones goes away...
Anyone else notice this phenomenon when all but one zone shuts off ?
Edit: Here is a graph from today, showing that the temperature came quickly up to the 8am set point in the morning without any overshoot and apart from one small blip remained dead on 21 all day until 8pm - a remarkable feat when the door was open some of the time, closed some of the time, people were coming and going and so on... Then after 8pm when the other zones went off the temperature drops a full degree while the radiator goes cold due to insufficient firing of the boiler until it eventually wakes up and starts firing the boiler enough to heat the radiator up again.
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