No offense taken, you have made a very valid point, I appreciate your comments. I have some reading to do........
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- Thread starter Michael Groves
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No offense taken, you have made a very valid point, I appreciate your comments. I have some reading to do........
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.
Used the Plum Nation Heat Calc, Heating Calculator - BTU Calculator
comes out at 17kW, so 40kW boiler gives me 5kW to play with
- May 23, 2015
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i would say try a different one 😉 just worked it out old school and its 300w out on a single rad
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.
- May 23, 2015
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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
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.
They need that information to estimate the total pipe run to the index rad and thus the resistance of the circuit.Michael Groves said:
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.
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.
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 ?
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.
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?
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!
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!
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?
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.
I think 30kW would be too small, I also have 500L water cylinder with 20kW heat exchanger.
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?
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?
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