Another Grundfos question

[carlos fandango]

Afternoon all,

Having loads of issues with boilers and fires this year.
Battled through all of them.
I’ve just sorted out my latest heating problem which turned out to be faulty Drayton actuator valve.
Since being the cupboard.... I was wondering what the correct setting is for my Grundfos ups3 15-50/65.

Looked up manual, currently on setting 3, there are other settings such as Proportional-pressure mode I, II Constant-pressure mode, I, II ?

The boiler feeds the (hot water) tank via 22mm, but also via the actuator, the central heating rads which are ultimately supplied on 8mm microbore.

Any advice appreciated…………

 

[Johntheo5]

You certainlt shouldn't need the default setting of CC 3 which is over 6M, I would try CC 1 which at over 4M will satisfy most requirements even with 8mm rad tails as long as the mains are 22MM, PP2 might work but unfortunately, as with most Grundfos pumps the head is on the low side especially with such a high capacity pump.
How may rads??.

 

[carlos fandango]

Fifteen rads.
I'll knock down from 3 to 2.
Thank you.

 

[carlos fandango]

Excuse my ignorance.
But the head pressures mentioned;
6M CC3..... 4M CC1.
The head pressure of what to what?

 

[Johntheo5]

On a setting of CC3 (constant curve), the pump will circulate at a flow rate of 1.15m3/h (19.2LPM) at a pressure (head) of over 6M (6.4M) or if you prefer, 0.6bar, the pressure/head will then start falling with increasing demand or based on your pipe sizing, no of rads etc. CC1 only delivers these flow rates at a lower head of ~ 4M (4.1M).

 

[carlos fandango]

So the head pressure (or barg) is generated from the pump?
And does not refer to head pressure of say pump to bolier or pump to header tank?

 

[Johntheo5]

Correct, it should more correctly be called the generated or differential head, its the difference in head between the pump discharge and suction, on a sealed system the expansion vessel is/should be installed close to the pump suction, if the expansion vessel is say at 1.5bar pressure then the above pump discharge pressure will be 1.5+0.6, 2.1bar but the generated head is still only, 2.1-1.5, 0.6bar, 6M.

Re head pressure to say header tank or whatever, it doesn't matter if the header tank is 15M "up" because the pump is circulating through a closed loop and returning to the pump, this elevation makes no difference but practically speaking because of the piping resistance then the head available for circulating through the rads etc is reduced especially on very long pipe runs.

 

[carlos fandango]

Explains alot. Cheers

 

[Johntheo5]

For interest, Pumps are allways specified in head and volumetric flow, can be Feet and GPM (gallons per minute) used in the USA or SI units used in Europe, Meters and M3/hour, the above pump if used as a pump rather than a circulator will pump at a rate of 1.15m3/hour at a head or to a height of ~ 6.3M if filling a tank from another reservoir/tank but if you had a pipe attached to the suction with its end say 7M underground in a well or whatever then that pump (once the suction pipe is primed) will still deliver 1.15m3/hour at a head of 6.3M as the atmospheric pressure (at ~ 10M) will push the water up the pipe into the pump suction, pumps do need a suction head at the inlet of around 1.5M to maybe 3M to prevent cavitation and and loss of pumping but the atmosphere can supply this or it can be provided from a reservoir or another tank, its known as the Net Positive Suction Head, NPSH, when pumping cold water its the atmospheric pressure (head) minus the head loss in the suction pipe due to friction losses.

If you are really bored (or want to be), attached, is the one of the best description of NPSH that I copied from semewhere or other decades ago.