Replaced CH Pump Primaflow CP5 -> Short Cycling

[Pedro202]

Morning and Happy New Year everyone!

As is typical over the Christmas period my Central heating circulating pump died on new years eve. It had sounded like a bag of spanners for a while so i wasnt totally surprised. But its timing was impeccable.

The pump i replaced was a Primaflow CP5 (GPD 25-5S). The only one i could collect locally was a Grundfos UPS3 15-50/65. Replacement was fine and heating and hot water are working. Ive noted that to start with when my heating fires my oil fired boiler is short cycling (on 20s off 2-3s on 20s etc) for a while and will eventually stop and operate normally. I couldnt find ANY details on the flow curves of the old CP5 so im not sure how much of a replacement the UPS3 was. The old pump was running in flow curve III. So for now the new one is in flow curve III also.

Have i made a mistake with this pump? Do i just need to tune the flow lower perhaps? Worth mentioning that i have a back boiler fire also so i have 2 CH pumps and my Oil fired pump is on the return not the feed side of the boiler it would seem.

Thanks all

 

[John.g]

Hi Pedro, and many happy returns.

No, you havn't made any fatal mistake even though I wouldn't be a great fan of the UPS3 choice of pump curves.
The UPS3 comes set to CC3 (constant curve) and grundfos recommend to run at this setting for 10 minutes or so to get rid of any air!!, but running at this setting (6.4M) continuously is almost guaranteed trouble especially if you have a vented system. I would, straight away suggest changing over to CP1 which means the pump will develop/run at a constant head of 3M. Also vent all your rads etc with pump off before change over to CP1 and see how you get on.

It's in the attachment but just press the setting button for > 3 secs and then set to CP1 (3M), we can look at the PP curves later to see if suitable. Don't be put off by any flashing LEDs in setting up.

There is also the remote possibility that you have a dud UPS3 which has been noted on here more than once before.

 

[Pedro202]

Hi John,

Thanks for the early reply. Hope your new years was peaceful. I have this morning after posting knocked it over to CPII (from memory) and it seems to be fairing better. Ill drop it to CPI and test for some of the day.

Pete

 

[John.g]

CP2 is a constant 4.5M head, if you change to CP1 (3M) then you will get 82% of the CP2 flow rate which might be beneficial as it will give a lower boiler return temperature and a % or two Boiler efficiency gain.
Give us a post back please when you settle on you final setting.

 

[John.g]

Found curves (below) which displays pump power as well (very handy for trouble shooting) similar to your old pump, assuming your flow requirements at 1M3/hr (16.7LPM) then the pump head was 3.8M on CC3 and 4.2M if flow rate is 0.8M3/hr (13.3LPM), your present UPS3 setting of CP2 (4.5M) is probably a good choice IMO, actually, CC1 on the UPS3 is almost identical to CC3 on the old pump so any if problems with CP2 or CP1 suggest this setting.

 

[Pedro202]

You are a finer man than I. I couldnt find anything on this old pump. Heating is due to fire again after fully cooling down soon so i will report back on its performance then.

 

[John.g]

Most other manufacturers 5M pumps are much of a much but 5M or even 6M pumps are now becoming history as quite a lot of the new A rated "Smart" pumps are designated 4M to 7M, not many are even designated 6M. UPS pumps generally have greater outputs than the others but very unfortunately their proportional pressure (PP) settings are even now, still too low. All makes of A rated pumps do not have true CC curves like the old traditional (3) fixed speed pumps, the UPS3 is particularly "bad" as they are essentially constant pressure (CP) curves for normal household circulating rate requirements which makes it almost impossible to set up a automatic Bypass Valve (AAV) properly, if installed.
I have a 3 year old Wilo Yonos Pico which gives excellent control to suit any situation as you can incrementally change the CP & PP settings in 0.1M steps anywhere between 0.5M & 5.5M. Mine is presently set to a PP setting of 4.4M which gives ~ 3.3M head at my full flow rate requirement of ~ 0.8M3/hr (13.5LPM) with ~ 22W power demand which can fall as low as 14/16W when the TRVs close in as the rooms heat up.

Primaflow pump below is not yours but pump curves will be similar IMO.

 

[Pedro202]

Thanks again John.

Glad to report that from fire up to rads all reaching temp it only cycled 1 or maybe 2 times. Which is likely the boilers internal stat just controlling as normal. CP2 seems to be providing the right setup right now. I will try CP1 in the coming days aswell to try the efficiency savings but overall i think we are sorted. Thanks for your help. My rads are overall much hotter than on the old pump so it was either very knackered to start with or it was too fast for efficient heat transfer. Every day is a learning day!