15M pump required, one large Vs 2 small in series?

[Michael Groves]

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?

 

[ShaunCorbs]

no you cant sorry to say

 

[Michael Groves]

Why not, Grundfos confirmed it would work?

 

[ShaunCorbs]

Why not, Grundfos confirmed it would work?

will put undue load on them

 

[Michael Groves]

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.

 

[ShaunCorbs]

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

 

[snowhead]

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.

 

[Michael Groves]

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 ?

 

[Michael Groves]

have a look at dab pumps

Seem good value, about half the price

 

[Chuck]

I'm specifying a new system. It has a 15M head,

This seems like a no brainer to me, what am I missing?

Why does the system require a 15 m head pump? It's an unusually high pressure.

 

[Michael Groves]

5 Storey Victorian house, Boiler in the basement. It's actually 13.5M, (I measured it exactly) so 15M being the closest.

 

[Chuck]

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.

 

[Michael Groves]

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?

 

[ShaunCorbs]

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?

 

[Michael Groves]

unvented cylinder?

Yes

 

[ShaunCorbs]

Yes

i would say your shy on your heat loss calcs

unvented cylinders require 15-20kw on there own

 

[Michael Groves]

i would say your shy on your heat loss calcs

unvented cylinders require 15-20kw on there own

Cylinder is 20kW, Radiators are 17kW.

 

[Michael Groves]

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.

 

[ShaunCorbs]

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?

 

[Chuck]

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!)