Residual Pump Pressure

[Johntheo5]

I’ve been asked numerous times to explain the residual pump curves published in some Vaillant Ecotec Boilers MIs, I’ve looked at them several times but afraid I can’t make any sense of them. Someone on here might explain them or/and post similar curves from their (Vaillant) HP circ pump curves, if available.

The first screenshot shows my calcs based on a typical 7M circ pump, second screenshot is the Vaillants and the third is unknown but does make sense.

The Vaillant’s residual Head default setting is 250mB, 2.5M. At this setting, 100% pump speed, mine shows a flowrate of 1200LPH (20.0LPM), unknown’s 1300LPH & vaillant’s 1230LPH, much of a much. At 70% pump speed (72% for unknown’s but won’t have any big effect vs 70%), mine = 540LPH (9.0LPM), unknown = 620LPH (10.3LPM), again not hughely different and probably explained by pump differences but Vaillant = 1150LPH (19.2LPM), can’t figure this out but one would think Vaillant have some basis for publishing these figures??.

 

[fixitflav]

I’ve been asked numerous times to explain the residual pump curves published in some Vaillant Ecotec Boilers MIs, I’ve looked at them several times but afraid I can’t make any sense of them. Someone on here might explain them or/and post similar curves from their (Vaillant) HP circ pump curves, if available.

The first screenshot shows my calcs based on a typical 7M circ pump, second screenshot is the Vaillants and the third is unknown but does make sense.

IIRC this was discussed a while back. Can you find that thread, to save thinking it out from scratch?

I can't understand the Vaillant curves either, in partic the horizontal part. I assume residual head curve should show the bare pump Q/H curve, minus the boiler internal losses, varying as Q^2. Again IIRC some manufacturers' curves follow that logic.

Your 1st screenshot - does it include internal loss?

 

[Johntheo5]

IIRC this was discussed a while back. Can you find that thread, to save thinking it out from scratch?

I can't understand the Vaillant curves either, in partic the horizontal part. I assume residual head curve should show the bare pump Q/H curve, minus the boiler internal losses, varying as Q^2. Again IIRC some manufacturers' curves follow that logic.

Your 1st screenshot - does it include internal loss?

No, its a bit generic but still I would think quite accurate, I just based it on a (Vaillant) HEX dP of 3.1M at 20LPM and a (my) rads losses of 3.44M at 20LPM, the rad calcs obviously have no effect on the residual loss, have a look at the spreadsheet which I put together some time ago and am now using this as "day zero" of trying to understand the Vaillant's curves, to no avail!.

 

[fixitflav]

There's this thread https://www.diynot.com/diy/threads/valiant-vs-viessman-heat-only-boiler.609063/post-5452724, which also contains a ref to a long discussion

Several threads came up when I searched for residual head

 

[Johntheo5]

Another reason I configured this spreadsheet is that someone with the latest Vaillant 624 couldn't get his boiler to control on flow/return temperature spread dT, settable between 10C and 20C, it doesn't, by design, allow the pump speed to fall any further once the residual head falls to 100mB, the pump speed was remaining at ~ 75/80%,. Based on my rads head loss, the speed would have to drop to 55% to reach 100mb, the previous 624 had a similar pump setting "Auto" which presumably is flow/return dT control to 20C (not settable), and implied that a similar 100mB residual head applied yet I know of at least two that allow the pump speed to fall to 15% to give ~ 4 or 5LPM at a head of ~ 20mB.