Originally posted by StephenC
View Post
The first X percent of the pin travel (typically 30-40%) is effectively "lost travel" because the valve remains non-flowing until some threshold of pin movement is reached. The water flow then starts to increase quite rapidly over only about 20% of the movement and then is typically flowing fully after another 20%.
There is also a non linear relationship between the heat demand sent from controller to the BDR91/OpenTherm bridge, vs the heat demand sent from an HR92 to controller. For example a heat demand of 20% from an HR92 usually means the valve is still closed, whereas a 20% heat demand sent to a BDR91 would in fact fire the boiler for 20% of the time.
My suggestion for checking the pin position was not in relation to overshoots, but in relation to what you were saying about the boiler cycling on when all radiators were cold. If you can hear the hiss from a radiator, try making 0.5 degree adjustments around the current temperature directly on the HR92 until you find the point where the valve JUST starts flowing slightly. Then check the valve position. If it's between about 30 and 40 that's fine, however if its say 60 or over then it would probably cause the boiler to come on when the valve is not quite open yet.
So a newer/different valve and more appropriate valve open percentages does not seem to massively affect overshoots for me though.
Reducing flow temperature to only what you need is the easiest solution but it only works if the radiator sizings in all rooms are correctly balanced with respect to other rooms, (otherwise if you need a high flow temp for a room with an under-performing radiator other rooms may overshoot) and if you tweak the flow temperature with changes in outdoor temperature. (Or let a weather compensation system do it for you) I tend to adjust my flow temperature based on the weather - about 60 in summer, 65 in spring/autumn or mild weather and 70-75 in winter, which is what works for my radiators.
Now this is just a small sample and not very practical to keep monitoring other than spot checks.
Of course if most radiators are within 0.5 degrees and valve slightly (or fully as some of mine seem to be) open, and then just one zone/HR92 calls for max heat, it's going to throw the other zones off by sending through significantly hotter water through than was requested, and expected. This will result in overshoots as I am seeing.
If your maximum flow temperature is appropriately set the overshoot will be minor (say 0.5 degrees) before the other HR92's figure out what happened and start throttling back, however if the flow temperature is way too high you may well see a 1-1.5 degree overshoot in other rooms caused by a room that was previously off being scheduled to come on to several degrees above it's current temperature.
Comment