Central heating pump sizing

[Reddwarf4ever]

My maths was done quickly and using mass flow rates equation. Similar to Ohms law it uses a magic triangle and if you know two factors you can work out the third. Output in KW at the top triangle, with flow rates in litres per second bottom left and DT times specific heat capacity on the bottom right, specific heat capacity usually taken as 4.2.
So 5kw with a DT across the boiler of 30°c means the flow through the boiler will be about 2.4 litres per minute as you say but 5KW output needs 3.6 litres per minute.

Does the calculation on this website make sense then ? How do I use the f,ow rate with the pump curve chart ? Looks right at the bottom of the bottom axis

one other question, the Grundfoss ‘standard’ pump has quite a higher specified Power rating than my Wilo pump, does that imply it may have, ‘ potentially have a higher fow rate ?

 

[John.g]

If that circ pump is not A rated which i don't think it is and is like the old traditional 3 speed ones then even if working perfectly the power at any given speed will vary very very little between no flow and max flow, all the meter will show IMO is that it is running at its requested speed/head which at speed 3 is 65W.

The other calculations are very easy to do from first principles......ie
(Flow in LPM) X 60)) X deltaT (deg C) / 860 = Heat demand in KW
ie LPMX60XdeltaT = KW
Example. boiler output is "known" to be 5kw, deltaT is 20c, find flowrate in LPM
LPM = (5*860)/(20*60) = 3.58 (LPM) and so on.

 

[SJB060685]

Johns method and mine seems to differ slightly but still arrive at the same answer and your method you mentioned seems to add up as well.
Pumps will supply a set flow rate at a given head generated, that is correct yes John?

 

[Reddwarf4ever]

If that circ pump is not A rated which i don't think it is and is like the old traditional 3 speed ones then even if working perfectly the power at any given speed will vary very very little between no flow and max flow, all the meter will show IMO is that it is running at its requested speed/head which at speed 3 is 65W.

The other calculations are very easy to do from first principles.***..ie
(Flow in LPM) X 60)) X deltaT (deg C) / 860 = Heat demand in KW
ie LPMX60XdeltaT = KW
Example. boiler output is "known" to be 5kw, deltaT is 20c, find flowrate in LPM
LPM = (5*860)/(20*60) = 3.58 (LPM) and so on.

Hello
so as this problem has been present since fitting my new valiant 8-9 years ago, with the new pump, I don’t think it’s pump sludge. My old boiler a potterton netaheat, which I believe was 60% efficient, which never had issues, most likely as the tech was very low, may have had a lower pressure drop....

So all I need to research is an A graded pump ? Or is there anything better ? Thanks

just saw mention on a website that a CH getting too hot may need changing, obviously the pump contents are between 55-75oC so will be hot anyway, but if I touch the area around where the flow control knob is, it’s too hot to touch, is this normal, was told not to insulate the front ( obviously not the electrical connection box )

thanks

 

[John.g]

yes, correct in principle, if one takes my example above of 3.58 LPM or 0.21 m3/hr then thee above pump if running in speed3 provide this flow rate at a head of 5

Johns method and mine seems to differ slightly but still arrive at the same answer and your method you mentioned seems to add up as well.
Pumps will supply a set flow rate at a given head generated, that is correct yes John?

yes, correct in principle, if one takes my example above of 3.58 LPM or 0.21 m3/hr then the above pump if running in speed3 will provide this flow rate at a head of 5.3M which means that IF the pump is performing normally then there has to be a big restriction somewhere as there is no way that any domestic system should require this sort of head with such a low flow rate.

 

[John.g]

Hello
so as this problem has been present since fitting my new valiant 8-9 years ago, with the new pump, I don’t think it’s pump sludge. My old boiler a potterton netaheat, which I believe was 60% efficient, which never had issues, most likely as the tech was very low, may have had a lower pressure drop.***

So all I need to research is an A graded pump ? Or is there anything better ? Thanks

just saw mention on a website that a CH getting too hot may need changing, obviously the pump contents are between 55-75oC so will be hot anyway, but if I touch the area around where the flow control knob is, it’s too hot to touch, is this normal, was told not to insulate the front ( obviously not the electrical connection box )

thanks

If the new Valiant was installed without a system clean/power wash then it would quite likely foul the Hx very early on as this acts as a perfect filter with a dirty system so, hopefully not, the boiler may require a new Hx and magnetic filter plus system power wash.
That boiler Hx (clean) requires a head of 1.2m at the required flowrate of 13.33 LPM at 18kw output so a 6M pump should be quite adequate for your 10 rad system.
Just ensure that whatever pump you choose that it displays the power in watts.
I have a 10 rad system (OF boiler) and my 6M Wilo yonos Pico provides a flow rate of 11.5 LPM @ ~ 3.5M head to give me a deltaT of 25C through my 20kw boiler

 

[Reddwarf4ever]

Any thoughts on the heat I mentioned of the pump front ? If I paid for a power flush, would a differential pressure figure be given before and after ? To see if cleaning helped, would. A power flush clean the HX ? and or if there is a problem.....how do you determine the amount of head required ? Is it from the boiler or pump curves not familiar how to do this.

if I pay to hire a power meter, will that be of any benefit ? Thanks

Also, could piping configuration, particularly changes made during the extension build be responsible for this restriction or could it only be solids build up ?

 

[SJB060685]

What changes were made?
Any idea of pipe sizes etc?

 

[John.g]

You can't determine if a pump is operating correctly by its temperature.
Don't think you will be provided with any differential pressures before/after a power wash.
Others will inform you but IMO a power wash will not clean a blocked Hx.
Don't know if you can actually hire/get a clamp on power measurement meter that gives you watts , it may only give the current in amps which is only of limited value.

Because you didn't or still don't really know if you have a pump or a restriction problem you were able to determine reasonably well IMO from the boiler output and the deltaT across it that the flow rate (by calculation), appears to be between 3 and 4 LPM, say 3.5 LPM, this information, on its own does not tell you that you may have a restriction or not as the flow depends on the pump head which in turn can be read off the pump curves.

Flow rate is 3.5 LPM or 0.21 ms/hr (3.5/16.666)
To read off the pump head, go to 0.21 on the horizontal (X) axis and "draw a line" vertically upwards until it meets the various speed curves and the corresponding head is on the vertical (Y) axis. You can see that IF the pump is running on speed1 the head is 2.9M, if running in speed2, 4.6M and running in speed3, 5.5M.
You havn't definitely confirmed that the pump is running in speed3 (you might do so) but even if it is only running in speed1 you will still only get 0.29 ms/hr, 4.8 LPM @ ~ 5.4M head on speed3 because flow is proportional to the sq.root of head so again this would point to a restriction somewhere IF the pump is operating correctly.

We can talk about it until the cows come home, if it were my system I would just go away and install a 6M A rated pump that displays the power in watts and you will know almost instantly where the problem is and while it will not fix it if the original pump was OK, at least you now know where the problem may be.

Any thoughts on the heat I mentioned of the pump front ? If I paid for a power flush, would a differential pressure figure be given before and after ? To see if cleaning helped, would. A power flush clean the HX ? and or if there is a problem.***.how do you determine the amount of head required ? Is it from the boiler or pump curves not familiar how to do this.

if I pay to hire a power meter, will that be of any benefit ? Thanks

Also, could piping configuration, particularly changes made during the extension build be responsible for this restriction or could it only be solids build up ?

 

[Reddwarf4ever]

You can't determine if a pump is operating correctly by its temperature.
Don't think you will be provided with any differential pressures before/after a power wash.
Others will inform you but IMO a power wash will not clean a blocked Hx.
Don't know if you can actually hire/get a clamp on power measurement meter that gives you watts , it may only give the current in amps which is only of limited value.

Because you didn't or still don't really know if you have a pump or a restriction problem you were able to determine reasonably well IMO from the boiler output and the deltaT across it that the flow rate (by calculation), appears to be between 3 and 4 LPM, say 3.5 LPM, this information, on its own does not tell you that you may have a restriction or not as the flow depends on the pump head which in turn can be read off the pump curves.

Flow rate is 3.5 LPM or 0.21 ms/hr (3.5/16.666)
To read off the pump head, go to 0.21 on the horizontal (X) axis and "draw a line" vertically upwards until it meets the various speed curves and the corresponding head is on the vertical (Y) axis. You can see that IF the pump is running on speed1 the head is 2.9M, if running in speed2, 4.6M and running in speed3, 5.5M.
You havn't definitely confirmed that the pump is running in speed3 (you might do so) but even if it is only running in speed1 you will still only get 0.29 ms/hr, 4.8 LPM @ ~ 5.4M head on speed3 because flow is proportional to the sq.root of head so again this would point to a restriction somewhere IF the pump is operating correctly.

We can talk about it until the cows come home, if it were my system I would just go away and install a 6M A rated pump that displays the power in watts and you will know almost instantly where the problem is and while it will not fix it if the original pump was OK, at least you now know where the problem may be.

Ok, as the pump is rather old, it will not be a bad idea to replace it, of for nothing else than as an insurance against possible early failure....you said “6M A rated pump that displays the power in watts” can you suggest 1 or 2 examples to get me started ?
in your opinion, would you suggest a power flush in he first instance as a good starting point ? ( I have flushed through myself bi annualy and treated with cleaner & then inhibitor but never had a power flush since new in 1987 ) then fit a new pump as suggested and take it from there. P.S. it is on speed 3....

some of this is a little bewildering, I guess everything is until you understand it.....when you refer to “5.4M head on speed3” does pump head mean how high the pump will pump ? Or what head pressure it needs ? may sound a daft question to someone whom such a thing is so obvious, but then hopefully I will have a better understanding, thanks for explaining the pump curve chart

thanks so far for everyone’s input and advice, has really helped a lot.

P.PS. S there an easy way to find out what all the numbers mean on CH pumps ?