Central heating pump sizing

[Reddwarf4ever]

6 extra rooms and a 30% increase to the longe will add quite a lot of resistance to the circuit, on top of what was already there.

That’s what I was afraid of....tried this morning with the Hot water on also, at first it looked like like the flow was better ( maybe it should be with such a short circuit, maybe equiv to a CH bypass ) but although initially it needed 10KW and had a DT of 27oC it settled down to the usual 7-8KW DT of 21-22oC with a target temperature of 72oC, so not much difference.....I hope getting a 7m head pump will help improve the flow

 

[John.g]

7.5kw heating demand at a deltaT of 21C = a flowrate of 5.1 LPM which requires a 5.1M head assuming a healthy pump. You require a flowrate of ~ 15 LPM which theoretically requires a head of (15/5.1)^2 X 5.1 or 44M so hope you have a very unhealthy pump just now.

 

[Reddwarf4ever]

7.5kw heating demand at a deltaT of 21C = a flowrate of 5.1 LPM which requires a 5.1M head assuming a healthy pump. You require a flowrate of ~ 15 LPM which theoretically requires a head of (15/5.1)^2 X 5.1 or 44M so hope you have a very unhealthy pump just now.

I read this several times but even though I have worked as a refinery process operator for 20 years & have C&G in process control, I just can’t understand this.....sorry

 

[Reddwarf4ever]

Yes, that's my pump, 6M or whatever always refers to the differential pressure head.

Referring to these numbers.

Wilo 4215516 YONOS PICO 25/1-6-130-(Row), Green​

the 4215516 is probably the Wilo model number.
25 is the diameter of the pump ports in MM.
1-6 is the operating range in meters.
130 is the distance in MM between the pump end connections and is very important as some pumps are "longer" so measure this on your own pump, if not 130mm then you will have to crop and remake one compression fitting which would mean system drain down.
Don't know what ROW means.

Am I correct in assuming that your house with extension has 10 rads?.

Here is a link to a DAB Evosta 3 which is a 7M pump (which you may prefer) which displays power, flow rate and pump head, just make sure before purchase that you are getting this exact model with that display. It also seems exceptional value for money and I would have no hesitation in recommending it.

https://www.stuartplumbing.co.uk/dab-evosta-circulating-central-heating-pump-replacement-grundfos-pump/

Found it cheaper https://plumbingsuppliesdirect.co.uk/dab-evosta-replacement-grundfos-circulating-heating-pump/
just need to confirm it’s a straight swop for my Wilo Gold, will phone them tomorrow

 

[John.g]

I read this several times but even though I have worked as a refinery process operator for 20 years & have C&G in process control, I just can’t understand this.***.sorry

OK, you have determined that you had/have a heating demand of ~ 7.5kw with a deltaT of 21C. By calculation, the flowrate is, 7.5*860/21/60, or 5.1 LPM. Go to the pump curves and you will see that the punp will give this flow at a head of 5.1/5.2M. Now you either have a big restriction somewhere or a very unhealthy pump because if the pump is OK then to increase that flowrate to 15 LPM will need a differential head of 44M, flow is prop to the sqroot of head and head is prop to flow squared. so required head is 5.1X (15/5.1)^2, 44M.

 

[SJB060685]

John do you mind sharing how you calculate flow is proportion to square root of head? I think I'm doing it wrong

 

[Reddwarf4ever]

OK, you have determined that you had/have a heating demand of ~ 7.5kw with a deltaT of 21C. By calculation, the flowrate is, 7.5*860/21/60, or 5.1 LPM. Go to the pump curves and you will see that the punp will give this flow at a head of 5.1/5.2M. Now you either have a big restriction somewhere or a very unhealthy pump because if the pump is OK then to increase that flowrate to 15 LPM will need a differential head of 44M, flow is prop to the sqroot of head and head is prop to flow squared. so required head is 5.1X (15/5.1)^2, 44M.

Thanks, seems cleaner now, obviously it would be impossible to give my pump a 44m head....the DAB pump you linked to looks perfect, found an even cheaper site £62.39 inc and free postage......but is this interchangeable for my Wilo gold.....the first numbers are different, don’t I need a 25mm pump ?
one other thought, just before the expansion pipe was joined to the F&E inlet I was experiencing occasional hot water venting into the F&E tank, if this still continues, but now goes into the inlet, could this be causing the inefficiency ? Maybe I can check where it’s joined and se if it’s hot thanks

 

[John.g]

John do you mind sharing how you calculate flow is proportion to square root of head? I think I'm doing it wrong

I have never calculated it but this chap Bernoulli has and it seems to be reasonably accurate.

 

[SJB060685]

I have read on him yes. Im just running numbers and trying to figure out the resulting flow from the square root of head. Clearly im not doing something right

 

[John.g]

Thanks, seems cleaner now, obviously it would be impossible to give my pump a 44m head....the DAB pump you linked to looks perfect, found an even cheaper site £62.39 inc and free postage...but is this interchangeable for my Wilo gold..the first numbers are different, don’t I need a 25mm pump ?
one other thought, just before the expansion pipe was joined to the F&E inlet I was experiencing occasional hot water venting into the F&E tank, if this still continues, but now goes into the inlet, could this be causing the inefficiency ? Maybe I can check where it’s joined and se if it’s hot thanks

I think the 25 or 15 refers to the pump ports internal diameter so shouldn't affect any of the other dimensions.

No, don't think any problem now as the reason for joining the cold feed and vent was/is to destroy the effect of any imbalances in a U tube.