Heating design..

[josh]

What is the rule of thumb for sizing a low loss header, 2 pipe sizes above the main flow/returns?

did have the book somewhere but can't find it, what maximum KW can 15, 22, 28, 35, 42mm copper tube carry?

doing a fair size job at the mo and have done a first fix of each floor and know how we'r doing it but just need to decide the best way to go about the plant. The total heating load is 60kw serving 32 radiators and a fair bit of ufh.

Have done similar jobs in the past but have kept the boilers separate serving different parts of the building but thinking of marrying them up on this one.

how would some of you chaps do it?

 

[Ermintrude]

1500mm long 76mm vertical header. 54 reducing tees off for the boilers and then 35mm off to the relevant bits?

 

[doitmyself]

What is the rule of thumb for sizing a low loss header, 2 pipe sizes above the main flow/returns?

The diameter of the LLH should be 3 times that of the connecting pipes.

what maximum KW can 15, 22, 28, 35, 42mm copper tube carry?

It all depends on the maximum acceptable velocity (m/sec).

Assuming a 60kW boiler and a 20C differential, the flow rate will be 0.72 litres/sec.

The velocities will be as follows:

28mm = 1.34 m/sec
35mm = 0.86 m/sec
42mm = 0.58 m/sec
54mm = 0.34 m/sec

If the velocity is above 1.5 m/sec, the water might be heard flowing in the pipes. Below 0.3 m/s can give rise to sludge settling. A flow between 0.6 and 1 m/sec is normally acceptable.

So 60kW will require a 42 mm pipe (39.6 mm id), the id of the header should be 118.8 mm, i.e 120 mm

How many boilers will be providing the 60kW?

 

[josh]

Thanks for the replies, much appreciated.

There will be two boilers, will be frowned upon here but i do like the main heat only's. Each zone comes back to the boiler room so shouldn't need to use 42mm, the biggest zone comes into the 35mm mark.

Was thinking of using the header to take each boiler to it, wanted to know the load each size of pipe can carry to see if i could get away with 28mm from each boiler to the header if that makes sense, the distance will be minimal so suppose i could find that if i put 30kw into a flow rate and see what velocity i would get through 28mm, is that right or not doitmyself?

 

[doitmyself]

WB publish a useful Technical Bulletin on cascading boilers.

It appears that you are thinking of connecting the boilers as in diagram 1; I was thinking of diagram 2.

The header still needs to be sized to carry all 60kW.

You also may find this from the CIBSE Journal helpful.

Have you designed for a 20C differential, i.e 'oversized' the radiators to take account of the reduced output compared to a 10C differential?

30kW through a 28mm pipe will give a velocity of 0.67 m/sec with a 20C differential.

 

[Ermintrude]

Wb recommend NRV on returns and one boss on to LLH. I use reverse flow on logics as this splits load perfectly

 

[josh]

Thats a good point doitmyself, just used the mears calculator which is still 10c differential i believe, its an older calculator. It normally is oversized on it anyway so i'm sure allowing a 20 would be fine, we'r yet to get a complaint of undersized rads.

so 28mm on the primary circuit would be fine, what does that translate into flow rate and i can size the shunt pumps to that, you would set the pump as to provide a constant flow rate for the full 30kwwouldn't you so there was no risk of overheating, right or wrong?


Cheers Ermi will remember that.

 

[Ermintrude]

Pump to constant as no variable head. 28mm from each boiker to Leg to headder

 

[Ermintrude]

http://www.worcester-bosch.co.uk/cache/file/31/cascading-greenstar-regular-gas-boilers.pdf

 

[Ermintrude]

Second schematic

 

[doitmyself]

CIBSE Journal Feb 2014 has a useful article on LLH design on page 58.

 

[josh]

Thanks for all the help guys.

 

[josh]

Have done the heating side of the job, just cylinder to fit and bathrooms to 2nd fix next year.
got the header made, went with 67mm in the end. Here are the pics, will get better ones when it's all finished.

 

[Ermintrude]

Looks good but those pipes in slab look to be un insulated! Nice work but what the sweet Jesus are those boilers doing attached to that nice pipework? Come on solos? Really

 

[josh]

Not insulated because it's not finished! Nothing wrong with solos, works really well.