Central heating pump sizing

[John.g]

That would be pretty correct, all very large centrifugal pumps are started with the discharge valve shut to avoid the motor tripping on overload, the discharge vale is then opened gradually.

 

[SJB060685]

Page 25 shows two graphs the top one is the normal pump curves and I am just assuming that you have choosen the pump to run on setting CP3 and just suppose that your system will circulate 16.67 LPM at this setting of 4.8m. if you look vertically downwards you will see another graph of power vs flow for all the different pump settings, you have choosen CP3 and because the system is circulating at 16.67 LPM then the pump power required is 35W, if the system, say boiler Hx starts getting dirty/fouled then the pump speed will reduce to maintain this constant head of 4.8M and if it got completely blocked the pump will still maintain 4.8m but with no flow and still require 16.5W. If you had a smaller system in that even when spotelessely clean only circulates 8.3 LPM (0.5m3/hr) then you can see that the pump only requires ~ 25.5W at the constant pressure of 4.5M.

If you take my system as a example which circulates ~ 15 LPM (0.9m3/hr) @ 3.5M.***...I could set the pump to CP2 which is constant pressure at 3.8M, my system would then circulate 15.7 LPM (0.94 m3/hr) @ 3.8M.
with a power demand of 26W.

I woud suggest CP2 (3.8M)as a reasonable starting point, it can very quickly be changed especially with all the info that will be displayed on this pump.

Thanks again John. I've checked everything against the charts and did the maths for your bottom paragraph and it all adds up. Thanks for taking the time

 

[John.g]

I'll go through another two settings, constant curve (CC) and the most interesting one of all, proportional pressure (PP) control, based on my own system, tomorrow.

 

[SJB060685]

I'll go through another two settings, constant curve (CC) and the most interesting one of all, proportional pressure (PP) control, based on my own system, tomorrow.

Please do. I have a basic understanding i believe but I'm sure you'll teach me otherwise lol

 

[John.g]

Ok, at tremendous expense here are a few numbers/attachments based on my requirements of 15 LPM @ 3.5M head and also based on the TRVs closing down with flow demand as low as 5 LPM.

speed2 will give 16 LPM@4M and 29.5W
speed2 will give 5 [email protected] and 27.5W

CP2 will give 15.9 [email protected] and 26W
CP2 will give 5 [email protected] and 17W

PP3 will give 16.8 [email protected] and 30W
PP3 will give 4.5 [email protected] and 12W

PP2 would almost do the job, [email protected] and (only) 12.2W.

 

[John.g]

Don't seem to be able to attach other file above so attached it here, but can't.
Its zipped excel file so don't know why not.

 

[SJB060685]

Thanks John. Could you give me a quick explanation from your view of CC, CP and PP please.

 

[John.g]

CC or constant curve or fixed speed, the pump runs at a constant speed, the faster the speed the greater the head, all pumps have 3 CCs to cater for different size systems, the problem is the power consumption is almost constant because as the heating (flow) demand goes down due to zone vale closing or TRVs throttling in the pump head increases and the pump efficiency falls off as you can see by the tiny differences in the power consumptions between 15 LPM and 5 LPM.
CP or constant pressure is one of the best controls, the head stays constant due to the pump speed changing up/down to maintain the same head for different flowrates and gives big energy savings like above. 17W vs 26W.
PP or proportional pressure control can give the biggest savings of all as the pump (speed) and head decreases as the flow demand decreases, unfortunately because of the limited number of settings one sometimes has to select a higher setting than necessary like above, where PP2 falls just short of the required flowrate but only consumes 12W, the reason I choose the Wilo is that the PP settings can be incrementally changed in 0.1M stages to give any flowrate required.
Also PP control reduces the (speed) head as the flow decreases, and vica versa unlike the CC settings.

 

[SJB060685]

Thanks John for a clear explanation. One last stupid question. I take it long before the introduction of erp products and modulating pumps all circulating pumps were just CC?

 

[John.g]

Correct,