Central heating pump sizing

[Reddwarf4ever]

Your old pump isolating valves are probably 1/4 turn ball valves that you shut with a screwdriver (slot) or a allen key, nothing sticks out, but unfortunately they almost always leak after operation, you can easily buy new ones with gate valves.

It would seem that you can get a Dab Evosta3 with 130mm centres. it might be a 6.5/7M pump but you should ask your supplier if they have one, this will also be adequate IMO, the pump curves for all their models are in the link.

https://evosta.dabpumps.com/wp-content/uploads/2019/03/Evosta-2-3-Installation-and-Operation-Manual_UK.pdf

have ordered the DAB 3 21412 model, not sure which curve is applicable to the pump I ordered.... some websites don’t give the numbering, just the performance data, the site I bought mine from had some contradictory info, confirmed it was 130mm and 6.9m head by phone and has a 5 year guarantee not 3 as shown, hopefully they will change the info on their site...ordered it from https://plumbingsuppliesdirect.co.uk/dab-evosta-replacement-grundfos-circulating-heating-pump/ but the invoice is from STUART PLUMBING & HEATING SUPPLIES, but on their website it’s more expensive and have to pay postage 🤯 so assume PSD is owned by Stuart plumbing.....must be offering competition to their own company.....marketing strategy 😎 the pump seems to have numerous control options, what’s my best setting to start with ? And best setting long term....

P.S. the pump iso valves are gate valves, not quarter turn, with a single square 1/4” approx post, no thread that I can see to hold a handle, can’t understand why they are sold like this, to shut off in an emergency, requires hunting around for an adjustable ( or normal ) spanner.....so much easier with a handle......maybe will replace them when I change the pump 🤔

 

[SJB060685]

John what would happen to the watts display if there was a pump problem or restriction in the system?

 

[John.g]

have ordered the DAB 3 21412 model, not sure which curve is applicable to the pump I ordered.* some websites don’t give the numbering, just the performance data, the site I bought mine from had some contradictory info, confirmed it was 130mm and 6.9m head by phone and has a 5 year guarantee not 3 as shown, hopefully they will change the info on their site...ordered it from https://plumbingsuppliesdirect.co.uk/dab-evosta-replacement-grundfos-circulating-heating-pump/ but the invoice is from STUART PLUMBING & HEATING SUPPLIES, but on their website it’s more expensive and have to pay postage 🤯 so assume PSD is owned by Stuart plumbing.*.must be offering competition to their own company..marketing strategy 😎 the pump seems to have numerous control options, what’s my best setting to start with ? And best setting long term.

P.S. the pump iso valves are gate valves, not quarter turn, with a single square 1/4” approx post, no thread that I can see to hold a handle, can’t understand why they are sold like this, to shut off in an emergency, requires hunting around for an adjustable ( or normal ) spanner..so much easier with a handle...maybe will replace them when I change the pump 🤔

You have the correct type isol valves as the gland nut can be tightened up if any leak, so no need for renewal.

Would suggest constant pressure, either CP2 or CP3 see below, answer to SJB.

 

[John.g]

John what would happen to the watts display if there was a pump problem or restriction in the system?

Look at the pump curve on pge 25 in attachment which is for a 7M Evosta2, can,t find one for a 7M evosta3.
you will see that on constant pressure CP3 that the pump will maintain 4.8M from 0 flow, if the system demand is 1 m3/hr 16.7 lpm then the pump power is 35w, if a restriction in the system and the flow fell to zero then the power is 16.5w and so on in between. more later

 

[SJB060685]

Look at the pump curve on pge 25 in attachment which is for a 7M Evosta2, can,t find one for a 7M evosta3.

Which chart am I looking at?
Through common sense I imagine if there was a restriction on PP then the pump would modulate down and display a lower wattage?

 

[SJB060685]

Thanks John. As I said the other day, the ins and outs of pumps is new to me, so still trying to figure the charts. I understand what you said above but feel free to share more in case I missed something 👍

 

[Reddwarf4ever]

Thanks John. As I said the other day, the ins and outs of pumps is new to me, so still trying to figure the charts. I understand what you said above but feel free to share more in case I missed something 👍

I am in the same boat as you, things are always so complicated until you understand them, then you wonder why it was so difficult......it’s not rocket science.....so we‘ll get there 🙏 keep asking questions, it’s the only way to learn.....also when people understand something so well it’s sometimes easy to downplay an answer.....I will persevere, I want to understand this.....trouble is the pump I ordered has lots of options so need to understand that also, just hope the others are patient with me 🤞

 

[John.g]

Thanks John. As I said the other day, the ins and outs of pumps is new to me, so still trying to figure the charts. I understand what you said above but feel free to share more in case I missed something 👍

Page 25 shows two graphs the top one is the normal pump curves and I am just assuming that you have choosen the pump to run on setting CP3 and just suppose that your system will circulate 16.67 LPM at this setting of 4.8m. if you look vertically downwards you will see another graph of power vs flow for all the different pump settings, you have choosen CP3 and because the system is circulating at 16.67 LPM then the pump power required is 35W, if the system, say boiler Hx starts getting dirty/fouled then the pump speed will reduce to maintain this constant head of 4.8M and if it got completely blocked the pump will still maintain 4.8m but with no flow and still require 16.5W. If you had a smaller system in that even when spotelessely clean only circulates 8.3 LPM (0.5m3/hr) then you can see that the pump only requires ~ 25.5W at the constant pressure of 4.5M.

If you take my system as a example which circulates ~ 15 LPM (0.9m3/hr) @ 3.5M.......I could set the pump to CP2 which is constant pressure at 3.8M, my system would then circulate 15.7 LPM (0.94 m3/hr) @ 3.8M.
with a power demand of 26W.

I woud suggest CP2 (3.8M)as a reasonable starting point, it can very quickly be changed especially with all the info that will be displayed on this pump.

 

[SJB060685]

I am in the same boat as you, things are always so complicated until you understand them, then you wonder why it was so difficult...it’s not rocket science..so we‘ll get there 🙏 keep asking questions, it’s the only way to learn..also when people understand something so well it’s sometimes easy to downplay an answer..I will persevere, I want to understand this.***.trouble is the pump I ordered has lots of options so need to understand that also, just hope the others are patient with me 🤞

My background is oil fired appliances, thats all we were, burner technicians. The system side of things is not something we did much of, so this last year I've been studying and learning a lot of new things to better my understanding. I have a passion for learning and a desire to at least have a good understanding in all areas of heating systems. John here has taught me a few things that I didn't know before joining this forum and to be honest, when it comes to pumps he's probably one of the best on here.

 

[Reddwarf4ever]

John what would happen to the watts display if there was a pump problem or restriction in the system?

Hi
i can only relate to my experience in the oil industry ( Mobil Coryton refinery ) when the pump discharge was shut back ( or there was another restriction to flow ) on a centrifugal pump the Amps ( power ) would drop off, as the pump would just be stirring the liquid and not doing any real work......not sure if that’s applicable, but IMO should be....

 

[John.g]

That would be pretty correct, all very large centrifugal pumps are started with the discharge valve shut to avoid the motor tripping on overload, the discharge vale is then opened gradually.

 

[SJB060685]

Page 25 shows two graphs the top one is the normal pump curves and I am just assuming that you have choosen the pump to run on setting CP3 and just suppose that your system will circulate 16.67 LPM at this setting of 4.8m. if you look vertically downwards you will see another graph of power vs flow for all the different pump settings, you have choosen CP3 and because the system is circulating at 16.67 LPM then the pump power required is 35W, if the system, say boiler Hx starts getting dirty/fouled then the pump speed will reduce to maintain this constant head of 4.8M and if it got completely blocked the pump will still maintain 4.8m but with no flow and still require 16.5W. If you had a smaller system in that even when spotelessely clean only circulates 8.3 LPM (0.5m3/hr) then you can see that the pump only requires ~ 25.5W at the constant pressure of 4.5M.

If you take my system as a example which circulates ~ 15 LPM (0.9m3/hr) @ 3.5M.***...I could set the pump to CP2 which is constant pressure at 3.8M, my system would then circulate 15.7 LPM (0.94 m3/hr) @ 3.8M.
with a power demand of 26W.

I woud suggest CP2 (3.8M)as a reasonable starting point, it can very quickly be changed especially with all the info that will be displayed on this pump.

Thanks again John. I've checked everything against the charts and did the maths for your bottom paragraph and it all adds up. Thanks for taking the time 👍

 

[John.g]

I'll go through another two settings, constant curve (CC) and the most interesting one of all, proportional pressure (PP) control, based on my own system, tomorrow.

 

[SJB060685]

I'll go through another two settings, constant curve (CC) and the most interesting one of all, proportional pressure (PP) control, based on my own system, tomorrow.

Please do. I have a basic understanding i believe but I'm sure you'll teach me otherwise lol

 

[John.g]

Ok, at tremendous expense here are a few numbers/attachments based on my requirements of 15 LPM @ 3.5M head and also based on the TRVs closing down with flow demand as low as 5 LPM.

speed2 will give 16 LPM@4M and 29.5W
speed2 will give 5 [email protected] and 27.5W

CP2 will give 15.9 [email protected] and 26W
CP2 will give 5 [email protected] and 17W

PP3 will give 16.8 [email protected] and 30W
PP3 will give 4.5 [email protected] and 12W

PP2 would almost do the job, [email protected] and (only) 12.2W.

 

[John.g]

Don't seem to be able to attach other file above so attached it here, but can't.
Its zipped excel file so don't know why not.

 

[SJB060685]

Thanks John. Could you give me a quick explanation from your view of CC, CP and PP please.

 

[John.g]

CC or constant curve or fixed speed, the pump runs at a constant speed, the faster the speed the greater the head, all pumps have 3 CCs to cater for different size systems, the problem is the power consumption is almost constant because as the heating (flow) demand goes down due to zone vale closing or TRVs throttling in the pump head increases and the pump efficiency falls off as you can see by the tiny differences in the power consumptions between 15 LPM and 5 LPM.
CP or constant pressure is one of the best controls, the head stays constant due to the pump speed changing up/down to maintain the same head for different flowrates and gives big energy savings like above. 17W vs 26W.
PP or proportional pressure control can give the biggest savings of all as the pump (speed) and head decreases as the flow demand decreases, unfortunately because of the limited number of settings one sometimes has to select a higher setting than necessary like above, where PP2 falls just short of the required flowrate but only consumes 12W, the reason I choose the Wilo is that the PP settings can be incrementally changed in 0.1M stages to give any flowrate required.
Also PP control reduces the (speed) head as the flow decreases, and vica versa unlike the CC settings.

 

[SJB060685]

Thanks John for a clear explanation. One last stupid question. I take it long before the introduction of erp products and modulating pumps all circulating pumps were just CC?

 

[John.g]

Correct,

 

[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.