John Carroll

Yes, this flat out firing is very hard to explain thermodynamically. For example, with a flowrate of your now 87%, 18.71LPM, then the rads will output 16.33kW(45% boiler output) under steady state conditions with flow/return/dT temperatures of 60C/47.57C/12.43C, assuming 20C room temperature, if the boiler firing is then increased to 36kW then the dT at that flowrate of 18.71LPM has to be, 36*860/60/18.71, 27.58C, so the flowtemp should/must then increase almost instantly to, 47.57+27.58, 75.15C, there will obviously be a delay in the sensors reading this correctly as they are surface mounted but surely 4 or 5 minutes should be well more than adequate time for this? also the boiler fires at 100% output for well beyond this period so a mystery as to why you are not seeing anything remotely approaching 75C., the sky's the limit then up to at least 100C, see below. The rad output theoretically/thermodynamically can output 36 kW but would require flow/return temps of ~ 100C/73C, (even assuming a 25C room temp), but I just can't see this happening. Don't know anything really re insulation, I just use the cheap as chips lengths.

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.

Overall, not too bad really, 25 minutes from a cold 15.6C to 60C Target/flowtemp in 25 minutes isn't outrageously slow IMO.

It's more than likely that it won't or shouldn't be at 100% firing at target temp so best window is well before it reaches target temp as it should then be firing at 100%.

As above, more or less, get the boiler to fire at 100% for say 4/5 minutes, during that time take two gas meter readings exactly 3 minutes apart, should then get a fairly accurate feel for the actual boier output in kW. If the output is 36kW, then the flow/return dT should be 23.9C until the target flowtemp of 65.5C is reached, assuming a flowrate of 21.6LPM. Target temperature of 65.5C should be reached when the return temperature rises to, 65.5-23.9. 41.6C Maybe clamp the pump minimum output to 95% until all tests completed. Post back.

That's the primary flow John, not the secondary, DHW flow, strangely enough, there didn't appear to have been any restriction to this in the old PHEX.

For info only. Came across some data recently from a WB Greenstar 36CDI Combi that had its PHEX renewed due to very poor DHW performance, the primary flowrate increased from 8.2LPM to 23.0LPM with DHW back to normal so seems a bit strange that (even though the primary HEX was renewed in the above case) that there was no increase in the flowrate, obviously the system&rads are in series with this so a major factor in the head loss, still, one might have expected some increase in the flowrate?, wonder was it just some form of scaling or perhaps nothing wrong with the HEX and reattaching (IF) the flowtemp sensor to the HEX made a proper connection, it would point to the former though, highly insulating scale of some form or other?.

Anyone can do their own calcs from the attached. Condensing Calcs Extract ET Rev0.xlsx

I think the above anti cycling time is primarily designed to enable the boiler to refire after burner cut out due to the heat load being less than the boiler's minimum output, Systems like Evohome make a virtue of switching the boiler on/off but you can limit the cycles/hour to a max of 6 I think, presume the boiler just does a pump overrun then at the end of each switch off period and the circ pump then stay off until the next Evohome requested On period?

I think they are exactly the same as the Vaillant ones. The Set maximum burner anticycle time determines the actual anticycle time. The default setting of 20C means the maximum anticycle time is 20 minutes in the very unlikely event that the target/flow temp is 20C, if (for the same 20C setting) the target/flowtemp is 60C then the anticycle time is only 6 minutes which makes sense because the return&flow temperatures will fall quite alot with the circ pump continuing to run so gives the boiler better leeway to get away on refiring and prevent the burner to trip again at targettemp+5C, if you find 6 minutes is still insufficient then you can change the anticycle time to say 30C which gives a anticycle time of 9 minutes at a target/flow temp of 60C which might then allow the boiler to get away in one refiring, and so on, clever enough if using outside temperature compensation or the like. Vaillant were/are? notorious for maintaing ignition conditions for 60 seconds or so after refiring before allowing modulation from ~ 65% firing, most boilers will allow modulation after 10 secs or less after flame detection or whatever. Do Vaillant own Glowworm?? Vaillant anticycle times

I suppose so, you can play around with it. It would be interesting to see what the pump modulation goes to and the other data if you remove the minimum clamp. Did the engineer renew anything else apart from the HEX?, you mentioned that he might renew some control card or other??.