24kw or 30kw boiler for a flat

[Sirodyam]

Hello, I'm not sure whether to get a 24kW or 30kW combi boiler for a flat (only £10 difference)

The 30kW boiler would only just be OK for the size of gas pipe that can fit in the building - if I've worked it out correctly.

Please can someone advise on getting a 24kW boiler in a small 2 bed flat, or 30kW for a good flow rate on thermostatic showers?




If anyone has time to check heres my calculation for the 30kW boiler:


30kWh boiler stated 3.12 m3/h max flow rate = 0.0008666667m3/s

From CIBSE tables: http://www.cibse.org/getmedia/b7008...025a/Guide-C-2001-Natural-gas-tables.pdf.aspx

22mm 5.5 Pa pressure drop per meter
28mm 1.5 Pa pressure drop per meter

From meter - 28mm for 11.5m, 6no. Elbows - 3m, Total length: 14.5m x 1.5 = 21.75 Pa pressure drop (first part)

Reduce to 22mm for 10m, 5no. Elbows - 2.5m, Total length: 12.5m x 5.5 = 68.75 Pa pressure drop (second part)

Total pressure drop = 90.5 Pa = 0.905 mbar

The 28mm pipe can't be any longer as it won't fit in the building neatly.


Is a theoretical drop of 0.905mbar too close to the limit? Should I just get the 24kw?



Need to make a decision on the boiler soon. However building won't be ready till March so it seems too early to get quotes and ask these questions.



Thank you for reading this far!

 

[Dan Robinson]

Both boilers are insanely over sized for heating.

So assuming we are dealing with combination boilers, what is the water flow rate?

 

[Sirodyam]

Hello, Thank you for the reply.

Yes they are combination boilers and the flow rates are 9.8 l/min for 24kW and 11.8 l/min for 30kW @Delta 35c.

The new water supply for the flat has not been installed yet so I cannot tell you flow rate/pressure of the supply, but it will all be new pipework from the main in the road.


A concern of the smaller boiler is that there have been posts with complaints that the shower experience of a 24kW combi boiler is considerably worse than a 30kW.


Thanks

 

[wannocks]

The important figure is the flow rate from the tap. That will decide which boiler you need.

 

[shambolic]

Get a 35 or higher. Most still provide 24 to heating which is far far too much for your hoose.
Gas supply Meh no one bothers

 

[Sirodyam]

Hello, I got a neighbour (in another building) to allow me to test his flow rate/pressure.

Worked out 15.6 l/min @ 0.5 Bar at my boiler height (took off 0.5bar for an 5m increase in height). Static pressure 3.7 Bar.

His is an old 15mm copper supply pipe though so I'm not sure if the new 25mm MDPE supply will have better pressures/flow rate.



Is this helpful or will I have to wait for the new supply to be installed to get a definite answer?

Thanks

 

[muggles]

Your 25mm MDPE should be much better. Do you know what the required heating load for your flat is?

 

[Sirodyam]

Hello, Sorry I'm unsure how to work out the heating load - unless its the total BTU of the radiators?

However the walls are insulated, as well as the floor and the roof of the flats adjacent.

From an online calculator - BTU of all the radiators (3 + towel rail) is about 15,000 BTU. Seems like any boiler would be fine CH wise?

Thanks

 

[Dan Robinson]

Is there a reason why your installer isn't working all this out?

 

[muggles]

From an online calculator - BTU of all the radiators (3 + towel rail) is about 15,000 BTU. Seems like any boiler would be fine CH wise?

Thanks

Not really - 15k BTU = approx 4.5kW. You therefore really need a boiler that can modulate down to that kind of figure and lower or it'll be constantly cycling on and off, making it VERY expensive to run. Vokera's Unica i 28 is probably your best bet, or maybe the 32

 

[Sirodyam]

Thank you for your replies.

I did get one quote but think the price was increased due to the building not being ready - therefore I was waiting before getting further quotes and asking these questions.

When I asked about the gas pipe size/boiler he said some of it will have to be 28mm reducing to 22mm for a 24kW - without accepting the quote I couldn't get it worked out exactly.



The reason I attempted to work out the gas pipe pressure drop was to find out if a 30kW combi boiler would be able to run off the length of 28mm pipe that can easily fit in the building.

I am worried that a 30kW would use too much gas for the proposed pipe size/length but also that a 24kW might not be enough for a good shower or modulate at a good level - as mentioned above.


I worked out the BTU and put in the radiators and flow/return to save money - so it will effectively be a boiler replacement and new gas pipe than a full install.


Any advice is much appreciated.

 

[Dan Robinson]

With 4kw Heatloss, any boiler is going to be shagged anyway make sure whichever one you choose can be range rated in heating mode.

Personally I'd be looking at the Vokera Mynute for this with its 10:1 ratio and OpenTherm controls.

 

[shambolic]

You do know that you won't get anymore than 18kw down a 22mm flow and return! Even if the boiler is 24kw to heating. (Told this on Thursday by one of our boffins, don't know how true it is? @Dan_Robinson )

 

[Dan Robinson]

Depends on delta T....

Would have to work it out, but at 20 degrees you'd be OK....

Also remember, Combi's are spec'd on hot water. Heat output usually lower.

 

[Dan Robinson]

30kw @ 20 degrees.
15kw @ 11 degrees.

@shambolic..... Now I know you're just testing