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.

Spreadsheet attached, is easier to use.

 

[fixitflav]

Is it possible to adjust the 100mb setpoint, perhaps Vaillant can help?

If on falling heat demand the pump reaches its allowed minimum head (hence minimum flow), the dT falls. Does it maintain flow temperature, so return temperature rises, or maintain return temperature, so flow temperature falls? The latter is better for condensation, if that’s the case, is there a problem?

How is the boiler output modulation controlled, does it get an analogue signal from the roomstat? I’ve never had a modulating boiler and haven’t gone into it.

 

[Johntheo5]

Is it possible to adjust the 100mb setpoint, perhaps Vaillant can help.

If on falling heat demand reaches its allowed minimum head (hence minimum flow), the dT falls. Does it maintain flow temperature, so return temperature rises, or maintain return temperature, so flow temperature falls? The latter is better for condensation, if that’s the case, is there a problem?

How is the boiler output modulation controlled, does it get an analogue signal from the roomstat? I’ve never had a modulating boiler and haven’t gone into it.

The available flowrate at 15% output based on a Vaillant boiler's HEX and my rads/system dT is (only) 2.8LPM, depending on the number of rads and output you can haveanything up to 5.8kW with 9.5/11.5 "kW of rads", say 6 or 7 rads but the (residual) head available is only ~ 0.07M, 7mB, not a hope IMO of balanced rads with that head, my rads regularly run with a flowrate of this ~ 2.8LPM but thats because I maintain a head of ~ 2.5M and the TRVs just throttle down the flowrate to give that same 5.8kW output, no problems with this obviously because I run the pump at ~ 60% speed which will give 2.4M residual head at that 2.8LPM, see below. You will get away IMO with say maybe 3 or 4 rads, 5kW output to give ~ 3.3kW with flow/return/dT, 65C/48C/17C (at 2.8LPM) butthats about it?.

Most boilers modulate just using the flowtemperature, some. like WB, don't even have a return sensor.

 

[fixitflav]

I'm not clear what your problem is. If it's just that the pump doesn't turn down below a certain point so (assuming the boiler is controlled on flow temperature) you lose some condensation, maybe you just have to live with it.

 

[Johntheo5]

I'm not clear what your problem is. If it's just that the pump doesn't turn down below a certain point so (assuming the boiler is controlled on flow temperature) you lose some condensation, maybe you just have to live with it.

The problem is that its turning down far too much, it should only be turning down on Auto to a speed that doesn't result in the residual head falling below 100mb, you can see from the attached, this corresponds to ~ 50% ("Minimum with Auto"), with my system corresponds to a flowrate of 10.5LPM at 100mb. 1.0M., speed should not fall below 56%.

 

[fixitflav]

I’m getting lost here! You said in #5 "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%"
Am I right thinking the pump speed modulates to try to achieve a setpoint dT? I assume the varying boiler output causes change in dT, boiler output being determined by an analogue signal from the roomstat (as I asked in #6) but I could be wrong.

Do you mean it turns down so much it affects the system, eg balancing the rads?

 

[Johntheo5]

On one boiler, the latest version, the pump wasn't reducing speed low enough to get a fairly modest set dT of 15C, on the other boiler, (previous model), the pump speed was/is reducing far too low to 15%, to give a residual head of ~ 0.07M, 7mb, which is far too low to give a balanced flow to the rads.
Any type if roomstats will either switch the boiler on or off or analogue ones may change the boiler target flowtemperature, the boiler output then reacts to this, either increasing or decreasing it's output, if the target flow temperature is steady the actual flowtemperature will tend to rise and fall depending on the outside air temperature causing the boiler output to increase or decrease to maintain the target temperature.