John Carroll

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?.