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?

 

[Last Plumber]

I think it sounds a lot to be honest with you.
I still think you would be wise to get a Heating Engineer to spec this job for you. Boiler size, Pump size and Domestic Hot water storage, any Cold water storage that may be necessary. You would benefit from the design and the advice they will be able to give. It will cost you a few hundred pounds but I think you will save a lot more. Plus you will have the peace of mind that it will be correct, the knowledge that the system is a as efficient as possible and someone to blame if it goes wrong.

 

[doitmyself]

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.

By "default figure" do you mean 11 deg C as on page 2 (46 in the formula for flow rate on same page)? If so, that is incorrect for your boiler which is designed for a 20 deg C differential. 46 then becomes 84. If you redo your flow rates and resistance using 84, you will find that the head is about 30% of what you originally calculated. That's because resistance varies with flow rate squared.

What was "tiny"? Don't understand your reference to 3.5M and 8.3M.

You also need to take the 20C differential and lower operating temperatures (needed to ensure condensing) when selecting your rads, as rad output will be reduced.

It would have been helpful if you had mentioned HMO in your opening post.

Have you thought about having two boilers, a 17kW for the heating and a 20kW for the cylinder? A smaller heating boiler will modulate much lower and will be condensing more often - if the system is correctly designed. You could also include weather compensation.

The HW boiler needs to run with a flow temp of 70-75C and a 10 deg differential, otherwise the cylinder will not heat up to 60C quickly enough. This means the boiler will not condense.

 

[Michael Groves]

You make some very valid points, I think I need to wind this back to make sure I'm getting correct results.
Calculating head is complicated, I'm now thinking about a single boiler using W-Plan. If I also have a 400L cylinder the heat exchanger is 17kW. This way I can use a single 18kW boiler to run both.
Worst scenario if the boiler can't cope I could buy a second 18kW boiler further down the road and have a dedicated boiler for each.
I recall being told by Worchester a while back you want your boiler running at 100% for best efficiency?

Thoughts?

 

[ShaunCorbs]

Think it will be more than 17kw as normal 300l ones have between 20-25kw coils

Also you just got the one cylinder??

 

[Michael Groves]

Seems to be various heat exchangers, Gledhill 300L has 30kW heat exchanger !!!, I'm looking at Grahams Therma.

I'll just go for a single cylinder.