Central heating pump sizing

[John.g]

still can't attach zipped file.

 

[Reddwarf4ever]

Haven’t installed my new DAB EVOSTA 3 Replacement Grundfos Circulating Heating Pump 21412 yet, waiting for the milder weather, wonder if anyone knows of a way I can measure the boiler inlet temperature? It’s displayed in the boiler, but accessed via keypad, would like permanently displayed so can check its blow 55oC.....need a sensor wire with a gauge thanks

 

[Reddwarf4ever]

i installed the new DAB 3 pump today.....have been in touch with DAB for guidance, but terminology seems to fluctuate between website, instructions and received emails....reference to constant speed & pressure seem get mixed up.....I have been advised to set at constant curve setting @ speed 3 is maximum as a starting point.

think this is the TOP symbol....getting 33-35W 5.4-5.6m & 0.7m3/Hr flow....sometimes it slows down ??? Got no one to help me and watch the boiler, so can onot relate what the pump is doing......I tried Heating + Hot water and Hot water only and readings were the same. Leaving it here for now and monitoring the boiler DT....in the past few days it got worse, outlet temp of 72oC was only achieving 55oC return temperature...so was struggling to keep the boiler in condensing mode..... o idea why the DT had moved away from 20oC.....

I really hope this pump will resolve my problems, not sure if the top option is the best for

 

[ShaunCorbs]

Turn the boiler temp down from 72 to 65 or even 60 make sure your how water cylinder is 60dc tho

 

[John.g]

i installed the new DAB 3 pump today..have been in touch with DAB for guidance, but terminology seems to fluctuate between website, instructions and received emails....reference to constant speed & pressure seem get mixed up..I have been advised to set at constant curve setting @ speed 3 is maximum as a starting point.

think this is the TOP symbol....getting 33-35W 5.4-5.6m & 0.7m3/Hr flow....sometimes it slows down ??? Got no one to help me and watch the boiler, so can onot relate what the pump is doing...I tried Heating + Hot water and Hot water only and readings were the same. Leaving it here for now and monitoring the boiler DT....in the past few days it got worse, outlet temp of 72oC was only achieving 55oC return temperature...so was struggling to keep the boiler in condensing mode.. o idea why the DT had moved away from 20oC.***.

I really hope this pump will resolve my problems, not sure if the top option is the best for

There seems to be a fair bit of resistance to flow, however, letting that aside for the moment then from your numbers above, Flowrate 0.70 M3/hr (11.7LPM), deltaT 17C? (72-55) Boiler output = 11.7*60*17/860, 13.88 kw, so as long as the min output of the boiler is > this then there should be no problems. However if you wish to get the return temp down then do as suggested.... reduce the boiler SP. Personally,I don't like running circ pumps (especially A rated ones) flat out, its up to yourself but if you were to consider changing the pump mode to constant pressure , CP3 = 4.5M then you should get a flow rate of 0.63 M3/hr (10.6 LPM) (& 27W) and a slight increase in the boiler deltaT, you can then reduce the boiler SP to suit yourself but remember this will also reduce the rad output/boiler demand and you don't want ro reduce it below the boiler minimum output, can't remember if you have that min output for that boiler?.

If you wish to calculate the boiler output yourself while testing its.... KW = (M3/hr)X60X16.66XdeltaT/860.

 

[Reddwarf4ever]

Looks like there is a 60 minute time out to do editing, which is a pity.

UPDATE
1- I have set the boiler to 18KW, first time ever....its modulating ok, but have had to reduce the target temperature to 68oC to keep the inlet below 55oC.....so quite a low DT
2-occasionally the pump declares ‘LO FLOW’ then shortly after shuts off, restarts with the same message, then after a few minutes reverts to the previous readings I.e. 0.7m3/Hr etc....set it to heating only for now to establish that works ok.

not sure if the higher flow; which it looks like I have, has dislodged some debris and causing the pump problems.....or it’s something else, all rads now very hot, even one in a room which previously struggled to get warm. Ideally want to heat the hot water in the day, together with the heating, as on hot water only, I don’t think I will get a low enough inlet temperature for condensing.... going to try again tomorrow....also if I try to heat the hot water on its own, not sure if the DT between the immersion cylinder and boiler outlet is enough

thanks for the replies......very confused about the pump setting options., is CP3 the middle icon of the three ..?.....happy to have a lower flow if it helps the DT....will look at the charts again...

still worried about the LOW FLOW message, although hasn’t happened since a few hours ago....

also was considering increasing two lounge rads from single to double, won’t increase the flow obviously bit will emit extra heat and help the boiler DT, any thoughts ?

 

[John.g]

The pump shouldn't be displaying that message except its some form of protection, suggest changing to CP3 for now, you can see the different mode symbols on top and the 3 vertical bars are the settings for each mode.

 

[Reddwarf4ever]

Ok I set the pump to D3, the boiler is now at 11KW ( although it’s modulating there is no indicator of actual KW ( Gas ) being burnt.....dropped outlet temperature to 68oC and return @57oC just checked outside and it’s Not condensing.....can’t drop,outlet any more, don’t think it will manage Hot Water at such a low temperature.....should I try D1 ? What is the theory about having a lower flow Vs inlet temperature ? Is it simply it has more residence time in the rads ? Any thoughts on upgrading 2 rads 100cm x 50cm to double fin ? Can’t see it will hurt

been told power flush devices can be hired, will have to drain down in the summer if I fit new rads, could do it then......don’t want to do it if not needed though

P.S. I forgot to say I accidentally left hot water & heating on together, although this should be acceptable to do, have now left it on heating only....

 

[ShaunCorbs]

is the system hot already or cold ?

 

[John.g]

What do you mean by D3 on the pump, is it constant pressure as shown in the diagram, and can you read off the flowrate, the head and the watts, can then calculate the boiler output.

And yes your old pump was kaput or full of sludge as it should have been circulating 0.66 M3/hr (11 LPM) at a head of 4.9M based on your new pump readings above.