Swapping a Keston

[jonnyswamp]

Priced and won a job a while back to change a Keston 130 for a vaillant equivalent
My head's been in the shed lately and I assumed it was a heat only open vent, but been back today to put some cleaner in the system and found it's pressurised, not open vent
Anyway, I sized the existing rads and they come to 17kw (these are all adequate according to customer) it has a megaflow 300l unvented, what kw do these require?
Do I really need to size the boiler to the max of the cylinder's requirements plus rads? (only 2 occupants at mo)
Was going down the route of a 628, rather than a 637 to match the Keston
Cost isn't an issue as I priced it for a 437 (rediculously overpriced BTW)
And why the 35mm primaries, do these megaflows neeed them
Pump in a/cupboard is a 25-80 about 4m from boiler
Am I getting uptight over nothing or just overcomplicating
What would the big boys do

 

[chris watkins]

As long as you are sure the rads are indeed only 17kW then add 3kW (no more) for the hot water.
Why don't you do a whole house heat loss just to double check !

 

[chris watkins]

http://www.google.co.uk/url?sa=t&rc...hw25iK3pMjcITOEqjgkp63A&bvm=bv.64507335,d.bGQ

 

[mfgs]

You need more than 3kw of heat to heat a 300L unvented in a reasonable time. The coil can take about 20kw of heat or something silly.

 

[vern]

You need more than 3kw of heat to heat a 300L unvented in a reasonable time. The coil can take about 20kw of heat or something silly.

24kw pal on these...... 637 all the way, just done exactly the same job, but used a WB 40 CDi......worked like a dream

 

[Ermintrude]

Yep the regular cdi is where my money at too. You can't shift 40kw through 28mm that's why it's 35! Just 28mm the gas and check pumps working properly, didn't the keston have an internal pump?