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

[Michael Groves]

I'm thinking about getting a Magna3 which has auto adapt. If I'm using 28mm pipe would I get a 25-nn or a 32-nn ?

 

[Chuck]

...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.

 

[Michael Groves]

Are you aware that you don't just add the flow resistances of radiators together to get the total.

Yeah got that, added the ***ulative figures of all shared pipework.

 

[Michael Groves]

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?

 

[Vee]

Don't forget the flow in your equation. Its flow against head. Its a curve.

 

[Last Plumber]

Some of the Schools I work in only have 4 meter head pumps installed.
There is more to this than I think you realise.
The Magna 3 is way oversized for your needs in my opinion.

My advice would be to ask a Heating Engineer to calculate this for you. You will need to pay for this spec obviously as it takes time, effort, knowledge and skill but it will work out cheaper than buying a commercial pump.
Don't make the mistake of thinking that oversized is best. It does not work like that and you will end up regretting your decision, an oversized pump will cause issues and is a waste of your hard earned cash.

I hope this helps!

 

[doitmyself]

used figure in literature for boiler, again very small.

Which literature were you looking at? There's nothing about heat exchanger resistance in the 40CDi Regular Installation Manual (I assume you are talking about the open vent version, not the system one). Did you take it from Technical Bulletin 48? If so the resistance of a 40kW CDi hex is about 3.5 metres, which isn't exactly small.

The 30CDi (which should be more than enough, including HW) has a resistance of approx 1.75m at full output.

What temperature differential did you assume when calculating the system resistance?

 

[Michael Groves]

I used the default figure in the sizing resistance in copper tube. It was tiny. 3.5M is much bigger. Yes looking at 40CDi regular, but with Robo kit. I thought 35CDi system was just too tight. In that case I would need 8.3M head.

I think 30kW would be too small, I also have 500L water cylinder with 20kW heat exchanger.

 

[Last Plumber]

I used the default figure in the sizing resistance in copper tube. It was tiny. 3.5M is much bigger. Yes looking at 40CDi regular, but with Robo kit. I thought 35CDi system was just too tight. In that case I would need 8.3M head.

I think 30kW would be too small, I also have 500L water cylinder with 20kW heat exchanger.

500 litres?
Why?
What do you have in the way of Hot water?

 

[Michael Groves]

It's a HMO, 11 tenants.
Currently only 3 showers, but every room has WHB. There is one communal kitchen. I plan to install maybe another 5 private showers in the larger rooms. Ultimately they'll still be 11 tenants max, but you could get a larger simultaneous draw off.
I must admit I've struggled with the capacity for this, how does it sound now with a little more background info?