Rad sizing

[mutley racers]

When sizing rads using the method ft3 times 5=BTU's does this include the extra 20% for condensing?

Also, i am quoting for a whole house install and am thinking of putting in a system boiler. Does anyone have any makes they like? I am looking at viesmann or Vailiant. Probably vailiant 618.

So all floorboards up and down stairs. rip all old out and replace with new system. Boiler, 10 rads and hot water cylinder. How long should this take on my own do you think?

And any rough ideas of pricing? I like the add 20% to final bill method as I always seem to come up short in the end from something i missed

[petercj]

I've got a Valliant ecomax pro which has developed problems due to restriction in the small bore pipe used in production of the heat exchanger - having sought opinions on here about boilers with larger bore heat exchangers, the Viessmann and the Atag came well recommended on this site, you might want to check out opinions on this thread:-

http://www.ukplumbersforums.co.uk/ce...78-boiler.html (Which Boiler?)

In every other way the Valliant has run without fault, but I wouldn't buy another one with such a small bore heat exchanger fitted.

[fuzzy]

When sizing rads using the method ft3 times 5=BTU's does this include the extra 20% for condensing?

Also, i am quoting for a whole house install and am thinking of putting in a system boiler. Does anyone have any makes they like? I am looking at viesmann or Vailiant. Probably vailiant 618.

So all floorboards up and down stairs. rip all old out and replace with new system. Boiler, 10 rads and hot water cylinder. How long should this take on my own do you think?

And any rough ideas of pricing? I like the add 20% to final bill method as I always seem to come up short in the end from something i missed

extra 20% for condensing? rads are designed with low flow temps so if sized correctly as per rad schedule and condensing boiler is set up properly then the boiler will condense. i find its just clouding the issue to consider it at this stage, it shouldnt come into it

[chris watkins]

R U having a laugh mutley is this what you call heatloss calculation to size rad's ??? This (ft3 x 5 = BTU's x 20%) is no better than guessing !! you might as well stick to the does it fit under the window. Why don't you get your self a heatloss program & do a professorial job Myson, Heatline or Stelrad all do them on either CD or to down load for FREE & it does not take to long to master them. (& you don't have to use there rad's either).

[mutley racers]

oh is that right fuzzy? these days we don't need to add 20% then for the boiler to condense?

Chris, i have looked on here for ways to calculate as a rule of thumb and that method has come up a couple of times

[fuzzy]

the boiler must have a return of around 55 degrees to condense. If the flow is set at around 68 then with properly sized rads it should return at a temp which will ensure it condenses. in reality who sets up the flow so low? most people whack it up anyway and it barely condenses

[petercj]

On the sizing issue, have a look at Valliant's range before scratching your head too much - the choice in the eco max pro range fives years, or so, ago was 18.5kw, or 28kw. According to pencil and paper calculations, the 18.5 was too small, and the 28 was too big for my property.

Because of the condensing issues I opted for the 18.5. The undersizing has hardly ever been a problem, i.e. if everything was on at once the boiler wouldn't keep up, but with TRVs it's not a problem in practice. But if you are sizing for a customer, I guess oversizing is a safer bet for you than undersizing - although maybe Valliant have more choices in their latest range.

If they've got kids, allow a couple of extra KW's - they never shut doors!

[SimonG]

Nowt wrong with cubic feet x multiplier (dependant on room) to give an estimate of rad size.

[petercj]

Allow 1kw per child on top.

[mutley racers]

wow so many different ideas and methods people use. Thanks for the input guys. So simon, would you or have you ever used the cubic ft way? I also remember being on the honeywell course and they said you should add 20% extra for the rads. But only calculate boiler size for the original calculation without the 20%. It is all a bit confusing.

[chris watkins]

Nowt wrong with cubic feet x multiplier (dependant on room) to give an estimate of rad size.

Because as a professorial you would not surely want to estimate the size of something as important as a heat emitter.
Ever noticed the difference in size between a rad in a new house & one in an older one yet they are the same size rooms ??

[SimonG]

Because as a professorial you would not surely want to estimate the size of something as important as a heat emitter.
Ever noticed the difference in size between a rad in a new house & one in an older one yet they are the same size rooms ??

Must be unprofessional then.

I know what works and what doesn't and I'd rather use the cubic foot method and take two minutes to find the rad size than take longer to get exactly the same result.