Correctly sizing a combination boiler

[nysk2008]

Hi all,

I am wondering if there is a rule of thumb that some of you use when sizing up for the install of a combi boiler. I fully appreciate U values, Flow rates, water pressure etc etc but is there anything that I can/need to do to get an overall feel for what the customers should be buying when they put you on the spot?

i.e Each rad 3kw + 10 (thats only a make-up example by the way)

Cheers

 

[diamondgas]

mysk2008,
Having been called out to soooooooooooooooooo many 24kW combi's over the winter period year after year, I'd say stay away! only fit one if the cold water flow rate is pants! Otherwise I ballance cold water supply & custards desires!

 

[nysk2008]

Thanks mate,

But this combi isn't for me. So many people are sold on the idea of a combi and refuse to listen to other options so if I have to give the customer what they want I just wanted to know how to size a combi there and then based on the bare minimum facts.

Cheers for your reply mate

 

[diamondgas]

Well 24kW will heat the vast majority of houses. I think it's about 80,000btu's old money. So a 28/30kW, 100,000btu's boiler will heat a 4/5 bed house no probs! I honestly question the custard on how many taps they use coz I'm prety much certain a 28kw will more than be enough to heat the house, and if the house needed more than 28kW then it's more than likely gonna need something other than a combi.... IMHO

Vast majority of complaints I've had to deal with when changing from stored hot water to a combi is water temp related!

 

[AWheating]

you could take a rough estimate at 2.5kw for a double rad and 1.5kw for a single rad, which is over the top but gives you an idea if your on the spot. There will not be many situations where you will increase the size of a combi to increase heating performance, not many houses will have a large amount of rooms without an increase in hot water demand.

 

[kirkgas]

when sizing a combi i never even consider the heating side, only the hot water requirement, the flow rate causes all the heartache, so get that right and the heating looks after itself as it is fully modulating, a 24kw will only supply about 10 ltr/min which is mince but will supply about 8 rads of various sizes, minimum these days is 28kw to get anywhere near decent flow which will then supply 12+ rads easy. your rule of thumb of 2kw per rad +? isnt required as you can only have hot water OR heating, not like a conventional boiler which requires heat and hot water at the same time, as a very quick rule of thumb, for the type of houses i work in i use 2.5 kw per rad, as the big ones balance out the wee ones, so it is simply how many rads x 2.5 + 3kw if hot water required

 

[nysk2008]

thanks for all your replies, v.helpful. appreciate it