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

[Michael Groves]

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........

 

[Chuck]

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.

 

[Michael Groves]

how many rads? seams low only 8 2kw rads

you done a room to room heat loss? and not just measured the existing rads?

Used the Plum Nation Heat Calc, Heating Calculator - BTU Calculator

comes out at 17kW, so 40kW boiler gives me 5kW to play with

 

[ShaunCorbs]

Used the Plum Nation Heat Calc, Heating Calculator - BTU Calculator

comes out at 17kW, so 40kW boiler gives me 5kW to play with

i would say try a different one just worked it out old school and its 300w out on a single rad

 

[Chuck]

comes out at 17kW, so 40kW boiler gives me 5kW to play with

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.

 

[ShaunCorbs]

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

 

[doitmyself]

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.

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.

 

[doitmyself]

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

You will find these helpful:

 

[Michael Groves]

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.

 

[Last Plumber]

It sounds nearer to what I would be expecting !