John Carroll

If you are happy enough with that overshoot then why not leave well enough alone, you probably may not have even noticed it if you hadn't had the other big problem. But no way IMO should the there be a offset of 2.5C between the target and flow temperature once steady state conditions are achieved, these controllers are/should be PID, where the "I", (integral) will achieve zero deviation between the target and flow temperatures especially under these benign conditions, you might see + or - 0.1/0.2C deviation now and then, the colder day/pipes etc has nothing to do with the control, the controller is just looking at the process variable which is the flow temperature in this case.

Happy New Year to all. The max rad output seems to be ~ 18kW with the present target/flow temp of 60C and because the boiler isn't range rated then at times it is inputting 15kW more than required which will lead to overshoot but IMO should still modulate down before targettemp+6C and burner trip? is reached, maybe range rate the boiler to say 20kW, 56% and see what effect this has on heat up from cold and overshoot temp. Eventually, if required, to speed up the heating period from cold, you can keep increasing the range rating until it doesn't overshoot by say targettemp+4C and leave it like that. I would also leave the pump speed high at 90/100%.

I have a feeling that it all revolves around the flowtemperature measurement or what the boiler's PCB perceives the actual flowtemp is, even though the boiler does eventually return to "normal" and your measured readings correspond to the displayed reading. If, for whatever reason, the flowtemp is sometimes perceived to be as much as 5C below the targettemp then the burner output will be outputting as much as +7.53kW (21%) more than that required to satisfy the actual targettemp, even a error of 2/3C is a substantial increase in firing of ~ 3.8kW (10.6%).

I have a fair idea I think of how those curves "work" A (&B) are just the installed pump curves, A being the one at 100% speed. All the dotted lines are the pump head remaining (residual) after the head loss through the boiler, mainly the HEX, so we'll call it that, HEX loss = boiler loss and RAD (+pipes etc) loss = residual head required or remaining. A 36kW boiler at 20C flow/returm dT requires a flowrate of 25.8LPM,1548LPH, so a flowrate of 25LPM, 1500LPH will give a dT of 20C with the boiler at 34.9kW output so we'll use that 1500LPH number. You can see that the pump head at 1500LPH is 650mb and the residual head (on 6,) is 400mb so HEX loss=650-400,250mb and the Rad loss is 400mb. If you use your "average" of 20.0LPM,1200LPH then, by calculation, the HEX loss is now, 250*sq(1200/1500), 160mb and the RAD loss is 400*sq(1200/1500), 256mb, so total (pump) head required is 160+256, 416mb (4.16M) the pump will deliver 1500LPH at 6.50M so at the required head of 4.16M should reduce its speed to ~ 100*sqroot(4.16/6.5), 80%, I havn't the exact pump curve but the above is the gist of how I think this works, of course if your RADs/pipework has sludge then the pump may still continue to run at 100%

Can, if you have "pump setting proportional to thermal output" take a few sets of readings in this mode, to see what effect it has, if any, on the targettemp/flowtemp dT.

Irrespective of any of those pump settings then, as stated previously, (especially) under steady state conditions the target&flow temperatures should be almost identical, as long as the minimum boiler output is less than the heat (rad) output. The flow/return dT increases (correctly) with decreasing pump head, does this have any effect on the targettemp/flowtemp dT?. You said, I think, in one post that the boiler HEX only was replaced but a (the) card wasn't because it was for a older type boiler but in a later post you said, again I think, that a card was replaced??. Did WB advise you to try these different pump settings after the senior engineers visit??. I certainly wouldn't be happy with the above "performance" so back to WB I would go.

@EinTopaz, any update on that pesky boiler of yours?

The 6.4kW vs 3.7kW looks like a "true" 3 phase (vs single phase) supply to me because 6.4/3.7 = 1.7297., almost sqroot of 3? Ignoring the PF, Power, kW (3phase) = VI*1.732, so 6.4kW requires a phase to neutral current of, 6.4*1000/230/1.732, 16.07A. Power, kW (single phase) is VI, so 3.7kW requires 3.7*1000/230, 16.09A, almost the same as the 3 phase (obviously?).

May be something as simple as this??.

I would suggest, under steady state running, get the flow/return dT to less than 20C, say 18C, assuming the target & flow temps are equal, take readings, then slowly decrease the pump speed, (you can use your max clamping method to achieve this), the dT will keep rising, see if the flowtemp starts to exceed the target temp as the dT increases, keep reducing the pump speed until the flowtemp exceeds the target temp by 5.5C (trip point?), note all readings. Before doing, (if) I would increase the anti cycle time to 5/6 minutes again.

From some horse's mouth re cycling and boiler efficiency degradation.

It doesn't matter what kind of fancy controls or otherwise it may have, maintaining a alleged targettemp+5.5C for 20 minutes smacks surely of some boiler control problem, especially since the boiler minimum output is way below the steady state rad demand.

Can you post a E link to a exact copy of your paper MIs?.

No way should the boiler behave like this, under steady running conditions, like above, the only reason that the flowtemperature exceeds the targettemp up to the burner trip point and recycle is when/if the minimum boiler output is > than the rad(s) demand, can you repost that thumbimage above or just take a screenshot as I can't open it. OK see it now, above, I don't think that has anything to do with the burner tripping when the flowtemp exceeds the targettemp, its to do with the anticycle time, on at least some of these WBs you have two parameters for setting the anticycle time..... a time (minutes) and a (negative temperature hysteresis) where the "longest" one wins, it goes something like this, if the time is set to say 5 minutes and the hysteresis set to (-) 6C, if, after burner cut off, the 5 minutes time has elapsed but the flowtemp hasn't reac hed targettemp -6C then the burner will not refire until the flowtemp does reach targettemp -6C, if the flowtemp had reached targettemp - 6C, then the burner will not refire until the 5 minute time has elapsed, or something like that. Most burners will trip at targettemp+5C.

Happy Christmas to all. I have a 20kW oil fired boiler that fires either flat out or not at all, with ~ 15kw of rads & ~ 85L system contents, and a circ pump flowrate of a measured (smart pump) of 14.5LPM. the flow/return dT is just under 20C, (19.8C) If I fire that up from a cold water system contents of 15C, it takes 25/35 minutes to reach its target temp of 65C and burner cut out, this means the return temp is 45C when this happens so a return temp rise of 45-15, 30C, in 30 minutes or 1 degC/minute return temperature rise, so depending on the rad output to boiler output ratio, 75% in my case, probably fairly typical of most oil fired systems, gas boilers, especially the WB ith its slow ramp up might have a typical return temp rise of a bit less than this.