Bart wrote:Thank you for confirming the way for operation.
I still convinced that there is no reason to not open valves completely and regulate zone temperature by controlling water temperature if that zone is the only one requesting heat (or even opened 100% for the zone requesting most heat, if more zones request heating).
I posted this image over on a
thread in automated home recently where I measured the relationship between the HR92 valve pin position percentage (measured by option 10 in the HR92 menu) and the heat demand sent to the controller:
Heat demand sent to the controller is always proportional to the radiator valve pin position although as shown in my graph it is not a straight line. Below 30% pin position no heat demand is sent - this is because most valve bodies only start to open and allow water to flow at about 30% pin travel, because for the first 30% of the pin travel the rubber seal in the valve body is decompressing but still sealing. (This percentage can differ on some makes of valve though, which can cause some minor issues)
This makes sense because you don't want to call for heat when the valve is not open enough to allow any water to flow. Similarly, most valve bodies are "fully flowing" by about an indicated 70% pin position, with further pin movement not significantly increasing the flow of water. Only from about 30-70% pin travel is there progressive, variable control of the water flow, outside this range it is either not flowing at all or fully flowing.
You'll notice its a straight shallow line between 30% and 70% and a much steeper line from 70% to 100%. The reason this is probably done is so that in the 30-70% valve position range where the valve has the most direct control over water flow and therefore radiator temperature, the heat demand sent to the controller remains relatively low - only reaching 30% heat demand (30% TPI duty cycle for a BDR91) when the valve pin is at 70% - fully flowing.
So if the amount of heat required is small the heat demand sent to the boiler remains relatively low and the water flow is controlled to precisely control the temperature. However if the temperature can't be maintained with the "full flow" of a 70% open valve, the valve opens further - this doesn't result in any significant increase in flow, however the steeper line on the graph means that it calls for a lot more heat from the boiler resulting in a larger increase in flow temperature.
So the way the system is designed is that it does in fact try to get as high a radiator panel temperature as it can from a low heat demand (<30%) and mostly opens the valve before it starts calling for a higher heat demand.
The problem with your suggestion of fully opening the valve and keeping the heat demand really low when only one zone is active and requires a small amount of heat is that as soon as another zone comes online and causes a large (perhaps 100%) heat demand the flow temperature sky rockets and the valve that was previously fully open now causes that radiator to quickly heat up and the room temperature would massively overshoot.
In your picture you show a heat demand of 29% - which means the radiator valve pin position is at about 70%, so that radiator valve is almost fully flowing, however because it is at 70% and not 100% pin position if another room comes online and causes the flow temperature to increase and results in a small overshoot the valve will quickly close down towards 30% to minimise overshoot, and because it is within the 30-70% range where it has precise control of the water flow it can do so. If it was at 100% to begin with the first 30% of closing the valve would be "lost travel" where no reduction in water flow occurred and there would be a big room temperature overshoot. By the time it closed the valve enough it would be too late to avoid the overshoot.
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