Discuss 15M pump required, one large Vs 2 small in series? in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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I'm specifying a new system. It has a 15M head, I called Grundfos and they recommended Magna3 40-150 rrp £1,640. (using rrp for reference only)
A friend recommended 2 no. Grundfos Magna3 32-80 in series, rrp £628 (Total £1,256).
Running costs of 2no. 32-80 is a third of 40-150.
Grundfos technical advised they do not recommend running in series, nor do they recommend you do not run in series, they are mute on the subject.

This seems like a no brainer to me, what am I missing?
 
no you cant sorry to say
 
Grundfos advised 1st pump should be configured with autoAdapt, with second pump fixed rate. This way AutoAdapt will adjust accordingly.
But Grundfos said 2nd pump would always be running at 100% which would reduce life and ties in with your comment.

I've got some more realistic prices, difference is about £200

I'm still leaning towards 2 in series.
 
Grundfos advised 1st pump should be configured with autoAdapt, with second pump fixed rate. This way AutoAdapt will adjust accordingly.
But Grundfos said 2nd pump would always be running at 100% which would reduce life and ties in with your comment.

I've got some more realistic prices, difference is about £200

I'm still leaning towards 2 in series.

have a look at dab pumps
 
If running 2 pumps in series was so much more efficient then all manufacturers would offer twin inline pump sets.
Can't say I've seen any.

The final cost of the installed pumps will be about the same after allowing for mounting and connecting up 2 pumps against just mounting 1 at a slightly higher purchase price.
 
If running 2 pumps in series was so much more efficient then all manufacturers would offer twin inline pump sets.
Can't say I've seen any.

The final cost of the installed pumps will be about the same after allowing for mounting and connecting up 2 pumps against just mounting 1 at a slightly higher purchase price.

Does not make commercial sense to sell pumps in series, when they can make more money selling one.

You have a valid point on extra time and vales, but if I allow £10 for valves and an extra hour work at £20, I'm still £170 better of running in series.

So technically I still see no reason not to run in series ?
 
5 Storey Victorian house, Boiler in the basement. It's actually 13.5M, (I measured it exactly) so 15M being the closest.

If you are talking about a circulation pump, it doesn't need to have a head of 13.5m, which is what you seem to be thinking. This is because the inlet pressure is 13.5m and the outlet pressure is the sum of the pump-inlet plus pump-working pressures. So you only need enough head to overcome the impedance of the pipework, which should be sized to work with a standard 6m pump.
 
If you are talking about a circulation pump, it doesn't need to have a head of 13.5m, which is what you seem to be thinking. This is because the inlet pressure is 13.5m and the outlet pressure is the sum of the pump-inlet plus pump-working pressures. So you only need enough head to overcome the impedance of the pipework, which should be sized to work with a standard 6m pump.

Yes it's a circulation pump, in a sealed system. Will have Worchester Bosch 40CDi Classic with a Robo kit. I get your point if it were open vent, or am I missing the point?
 
Yes it's a circulation pump, in a sealed system. Will have Worchester Bosch 40CDi Classic with a Robo kit. I get your point if it were open vent, or am I missing the point?

unvented cylinder?
 
If you are talking about a circulation pump, it doesn't need to have a head of 13.5m, which is what you seem to be thinking. This is because the inlet pressure is 13.5m and the outlet pressure is the sum of the pump-inlet plus pump-working pressures. So you only need enough head to overcome the impedance of the pipework, which should be sized to work with a standard 6m pump.

I thought it was building height, clearly I am VERY wrong, Grundfos Technical support also confirmed building height.

Here's a link I found, good explanation. I need to reconsider
How to size a Circulator Pump. Taco pump curves.
 
Cylinder is 20kW, Radiators are 17kW.

how many rads? seams low only 8 2kw rads

you done a room to room heat loss? and not just measured the existing rads?
 
Yes it's a circulation pump, in a sealed system. Will have Worchester Bosch 40CDi Classic with a Robo kit. I get your point if it were open vent, or am I missing the point?

Makes no difference to the conclusion whether the system is vented or unvented. You would only need a 15m head if you were trying to pump water from, say, a borehole in the basement to an open tank in the roof.

If this is not obvious to you then you should subcontract the design of the system to someone who knows what they are doing. (No offense intended!)
 
Makes no difference to the conclusion whether the system is vented or unvented. You would only need a 15m head if you were trying to pump water from, say, a borehole in the basement to an open tank in the roof.

If this is not obvious to you then you should subcontract the design of the system to someone who knows what they are doing. (No offense intended!)

No offense taken, you have made a very valid point, I appreciate your comments. I have some reading to do........
 
I thought it was building height, clearly I am VERY wrong, Grundfos Technical support also confirmed building height.

The building height is relevant only to the extent that there will be a limit to the maximum pressure difference between the inside and outside of the pump before the seals fail. This is usually a *lot* more than the inlet-outlet pressure difference we are discussing here.
 
And when sizing system boilers, you don't normally just add the HW peak demand to the CH demand. It'll give you an oversized boiler that short cycles most of the time.

yes and no

if there happy with staggering the times then aslong as heatings over the hot water requirement your fine

but if they dont want to stagger the times you need to take the full demand into account
 
if they dont want to stagger the times you need to take the full demand into account
The only time you need full demand is if the house is at -3C and the cylinder is full of cold water. In normal circumstances the house may drop a few degrees overnight and the cylinder may be as low as 40C; so, even first thing in the morning the boiler will not be running flat out. Modern cylinders are so well insulated that they can be heated up at midnight and the water will still be hot enough for a bath in the morning.

Running the system with hot water priority, using a diverter valve not a mid-position, ensures that the cylinder gets heated quickly during the day, so the reduction in room temperature is minimised.

Michael Groves said:
Grundfos Technical support also confirmed building height.
They need that information to estimate the total pipe run to the index rad and thus the resistance of the circuit.
 
Thanks for literature, I had to sit down quietly for a while to understand.
As per literature I've calculated flow resistance of every radiator supplied with 15mm pipe. I then calculated flow resistance of 22mm pipe supplying 15mm pipe, and finally 28mm pipe supplying 22mm pipe. I included every TRV, elbow and tee. I'm sure I missed a few but I can add a little for that. I included cylinder which adds very little and used figure in literature for boiler, again very small.
So to recap I have 5 story house with 22 radiators. My calculation comes at at 4.5M head. This just sounds way too low. What do you think?
There's a lot of 28mm pipe and 22mm pipe which obviously reduces resistance a lot, but still 4.5M sounds very low.
 
...but still 4.5M sounds very low.

It's not possible to tell whether you've done the calculation correctly from your description. But that's a bit higher than I would expect. Are you aware that you don't just add the flow resistances of radiators together to get the total but you have to take into consideration that they are in series with some pipework but are basically fed in parallel by the pump?

The next question is what displacement rate do you require, which is a figure in litres/minute or cubic-metres/hour? You calculate it from the total amount of power you want the circulator to deliver. For the sake of an example, let's assume 40kW. For a 20°C temperature drop that would need a circulation rate of 1.7 cubic-metres per hour.

You then need to look at the pump curve to make sure that the combination of head and displacement rate lie within its operating curves.
 
The next question is what displacement rate do you require, which is a figure in litres/minute or cubic-metres/hour? You calculate it from the total amount of power you want the circulator to deliver. For the sake of an example, let's assume 40kW. For a 20°C temperature drop that would need a circulation rate of 1.7 cubic-metres per hour.

You then need to look at the pump curve to make sure that the combination of head and displacement rate lie within its operating curves.

Yeah managed to calculate that, I’ve not looked at curves yet as still debating head calculation. But I assume Magna3 autoAdapt takes care of a lot of that.

What gets me if a 5 storey house with 22 rads only needs a 15-50, than most average homes are massively over pumped as this is fitted everywhere?
 
Don't forget the flow in your equation. Its flow against head. Its a curve.
 
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