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Central heating pump sizing
- Thread starter Reddwarf4ever
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View the thread, titled "Central heating pump sizing" which is posted in Central Heating Forum on UK Plumbers Forums.
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 🤞
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.
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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.
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....
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 👍
Please do. I have a basic understanding i believe but I'm sure you'll teach me otherwise lol
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.
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.
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.
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.
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
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
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
Turn the boiler temp down from 72 to 65 or even 60 make sure your how water cylinder is 60dc tho
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.
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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 ?
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 ?
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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....
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....
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is the system hot already or cold ?
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.
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.
"What is the theory about having a lower flow Vs inlet temperature ? Is it simply it has more residence time in the rads ? "
More sometimes equals less.
Yes, but you also need to keep the mean radiator temperature as near design temperature as possible, above you have flow/return temps of 68/57, mean of 62.5C which results in the rad emitting 81% of its rated output based on a room temperature of 20C, if you are happy with this but require a return temperature of say 50C but also require the same mean rad temperature of 62.5C then there is only one way of achieving this and that is to raise the boiler/rad flow temperature to 75C and keep reducing the flowrate until the return temp is 50C, this still gives a mean rad temp of 62.5C. and no problems in heating your DHW cylinder.
More sometimes equals less.
Yes, but you also need to keep the mean radiator temperature as near design temperature as possible, above you have flow/return temps of 68/57, mean of 62.5C which results in the rad emitting 81% of its rated output based on a room temperature of 20C, if you are happy with this but require a return temperature of say 50C but also require the same mean rad temperature of 62.5C then there is only one way of achieving this and that is to raise the boiler/rad flow temperature to 75C and keep reducing the flowrate until the return temp is 50C, this still gives a mean rad temp of 62.5C. and no problems in heating your DHW cylinder.
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Thanks, will experiment with your suggestions tomorrow...defo looks like this pump is doing a much better job than the old one......If CP1 doesn’t give a low enough head to achieve a good DT, is there another setting to try ?
1- does anyone know if the heat recovery is a constant, as the inlet temperature falls ? Or as I assume it’s a curve, giving less heat recovery as the temperature falls, think the majority must be achieved at around 50-55oC ?
2-I read it’s a good idea to increase rad sizes when changing to a condensing boiler to assist the low return temperature, so is my idea of changing the 2 rads in my lounge to double fin, a good way to go ?
3- is it normal to heat the hot water the same time as having the heating on ? Because the heat removed from the immersion cylinder will be no where near what the rads remove, and having both would ‘ share’ the flow....
4-.I assume in the summer months with hot water only, condensing mode would not easily be achieved
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I would increase the flow temp to 75C then go to CP2 and write down the boiler deltaT and the pump flow,head & watts.
Then ditto for CP1, PP3 & PP2.
Remember though that if the head is too low then some rads will run colder than others and require balancing, for the moment don't bother with balancing.
I run my boiler at 75C with TRVs on 8 of the 10 rads and I've often seen 40C return temps, I have never balanced the rads, I run on PP3 which, with my system, gives ~ 12LPM @ 3.4M head. I would expect your system to run at [email protected] on CP1, 8.5LPM@3M on CP2, [email protected] on CP3. 9 [email protected] on PP3, & [email protected] on PP2.
Q1. If you mean boiler heat recovery = efficiency then the lower the return temp the greater the efficiency due to enhanced condensing effect.
Q2. I have used a factor of X2 on any replacement rads to allow for weather compensation etc if my present non condensing boiler ever gives up the ghost.
Q3. A lot of boilers now use DHW priority where, when DHW is required, the boiler switches to HW heating and automatically increases the boiler SP temp to 80C which gives very fast heating times but little or no condensing benefit and then switches back to CH after the 30/45 mins or so required.
But if you are not too worried about the length of time required to heat the cylinder then you can install a balancing gate valve on the cylinder return and by throttling it in to say 1/4 turn open you can easily get 50C for a lot of the heating period even with a flow temp of 75C.
Q4. As above.
The effect of return temps on condensing benefit is clearly shown below, not any huge benefits at 50/55C but below 50C, not bad at all.
Happy testing.
Then ditto for CP1, PP3 & PP2.
Remember though that if the head is too low then some rads will run colder than others and require balancing, for the moment don't bother with balancing.
I run my boiler at 75C with TRVs on 8 of the 10 rads and I've often seen 40C return temps, I have never balanced the rads, I run on PP3 which, with my system, gives ~ 12LPM @ 3.4M head. I would expect your system to run at [email protected] on CP1, 8.5LPM@3M on CP2, [email protected] on CP3. 9 [email protected] on PP3, & [email protected] on PP2.
Q1. If you mean boiler heat recovery = efficiency then the lower the return temp the greater the efficiency due to enhanced condensing effect.
Q2. I have used a factor of X2 on any replacement rads to allow for weather compensation etc if my present non condensing boiler ever gives up the ghost.
Q3. A lot of boilers now use DHW priority where, when DHW is required, the boiler switches to HW heating and automatically increases the boiler SP temp to 80C which gives very fast heating times but little or no condensing benefit and then switches back to CH after the 30/45 mins or so required.
But if you are not too worried about the length of time required to heat the cylinder then you can install a balancing gate valve on the cylinder return and by throttling it in to say 1/4 turn open you can easily get 50C for a lot of the heating period even with a flow temp of 75C.
Q4. As above.
The effect of return temps on condensing benefit is clearly shown below, not any huge benefits at 50/55C but below 50C, not bad at all.
Happy testing.
Thanks that graph is exactly what I was looking for, even asked in a scientific forum, but got no where 🤯
trying various options with the boiler
CP1 Boiler target 68oC 8KW limit inlet 46oC
8.6-10W
1.8-2.0m
0.4l/h
CP2 Boiler target 68oC 8KW limit inlet 46oC
13.2-17W
2.6-3.0m
0.5-0.7l/h
not sure why the fluctuation, found if impress the Mode button once quickly, it cycles through the values, the manual says pressing it changes the mode, but only when it’s displaying the mode ( multipurpose buttons not helpful ) I previously held the button in until it displayed a rotating ‘arrow’ then pressed once, but although it display the readings, it had gone through a degas so values had changed, easier now I know.
One problem IMO apart from the pump, is the valiant boiler Eco tec plus 418, it does not modulate very well, often continuing to hard fire as it reaches the target temperature and thus overshoots, made worse by the fact it cuts out when only 3oC over temperature. Takes a long while going through the tripping restarting before it settles down again. Sometimes I drop the KW limit to 5KW and ‘ nurse ‘ it back up by slowly increasing the KW.....IMO I should t have to do this, & if I can do this then the boiler should. I wonder if the boiler is actually oversized, which again antagonises the problem. if it were sized as say 12 KW instead of 18 KW, wouldn’t it be able to cope with lower flows ? If that is the case then it will always be a struggle.....
I have 7 radiators which could be made double fin.....wonder how much effect that would have on the delta T ???
trying various options with the boiler
CP1 Boiler target 68oC 8KW limit inlet 46oC
8.6-10W
1.8-2.0m
0.4l/h
CP2 Boiler target 68oC 8KW limit inlet 46oC
13.2-17W
2.6-3.0m
0.5-0.7l/h
not sure why the fluctuation, found if impress the Mode button once quickly, it cycles through the values, the manual says pressing it changes the mode, but only when it’s displaying the mode ( multipurpose buttons not helpful ) I previously held the button in until it displayed a rotating ‘arrow’ then pressed once, but although it display the readings, it had gone through a degas so values had changed, easier now I know.
One problem IMO apart from the pump, is the valiant boiler Eco tec plus 418, it does not modulate very well, often continuing to hard fire as it reaches the target temperature and thus overshoots, made worse by the fact it cuts out when only 3oC over temperature. Takes a long while going through the tripping restarting before it settles down again. Sometimes I drop the KW limit to 5KW and ‘ nurse ‘ it back up by slowly increasing the KW.....IMO I should t have to do this, & if I can do this then the boiler should. I wonder if the boiler is actually oversized, which again antagonises the problem. if it were sized as say 12 KW instead of 18 KW, wouldn’t it be able to cope with lower flows ? If that is the case then it will always be a struggle.....
I have 7 radiators which could be made double fin.....wonder how much effect that would have on the delta T ???
Ok on CP1... pump 9W 1.8m 0.5l/h fairly stable
boiler 68oC target & actual, inlet 55oC
So am just on the condensing temperature & it’s slowly going up.
i have 12 rads total
All 100x50cm
downstairs
2 double ( one of which only gets really hot when on CP3 in extension room)
2 single
1 towel
upstairs
2 towel
5 single
Detached brick house built 1987, 3 bed, game room, store room, lounge, kitchen, cloakroom, bathroom and shower room....usually at 23oC, wonder what size boiler I would need for this, need to compare with the 18KW I have.
wonder if, as per my previous post, the boiler is over sized, assuming a smaller one can handle a lower flow, which would help the DT.....all I can think of doing at this point is increasing rad sizes.....or living with an inefficient heating 🤯
boiler 68oC target & actual, inlet 55oC
So am just on the condensing temperature & it’s slowly going up.
i have 12 rads total
All 100x50cm
downstairs
2 double ( one of which only gets really hot when on CP3 in extension room)
2 single
1 towel
upstairs
2 towel
5 single
Detached brick house built 1987, 3 bed, game room, store room, lounge, kitchen, cloakroom, bathroom and shower room....usually at 23oC, wonder what size boiler I would need for this, need to compare with the 18KW I have.
wonder if, as per my previous post, the boiler is over sized, assuming a smaller one can handle a lower flow, which would help the DT.....all I can think of doing at this point is increasing rad sizes.....or living with an inefficient heating 🤯
As you can adjust the output of the boiler work out your rad kw and add them all together that’s what you set your boiler out put to
I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.
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