New central heating

[ghost]

Hi all!

I am in the process of updating the central heating in my home. I have been looking into a few options and after doing some investigating research I am considering the following based on CH requirement of 29.94kw

Boiler: Viessmann 100/200 30kw (to buy)
Faral Rads (bought)
200L water cylinder (bought)

I am just looking for some advice into boiler size as my calculations state I require just under 30kw, therefore will a 30kw sized boiler work efficient??

Also some sites state that you should allow 3kw for the water cylinder, but as I believe it the HW will take priority from the CH and will therefore not require any extra KW.

 

[crashndie]

personally i would look at 35 kw boiler build some redundancy in to the system that way if you upgrade any rads or there's some more demand you'll have sum back up. the modern boiler can be range rated an will modulate to what the demand is on the boiler.

 

[AfterburN003]

Id be inclined to go for the 35kw boiler. If your calculations are correct then only allowing 0.5kw for heat loss in pipework etc is cutting it a bit fine.

 

[ghost]

Thanks for the advice guys

I had just read that it was better to have a higher KW demand on the CH than the actual boiler KW rating, so it would work more efficiently. This has been mentioned with regards to oversizing the rads.

Also I was going to replace my combi boiler with a new condenser combi, but have also been advised to go for a system boiler instead due to now fitting the 200L water cylinder. Any thoughts?

 

[D_Hailsham]

Boiler: Viessmann 100/200 30kw (to buy)
Faral Rads (bought)
200L water cylinder (bought)

Do you have a tame Gassafe registered engineer who will install your boiler?

Do you realize that the installer will not have any liability if the boiler or any of the parts you have bought are faulty go wrong?

as my calculations state I require just under 30kw

Have you compared your calculations with the results given by Sedbuk Boiler Calculator or EST online boiler calculator

Don't forget that boiler sizing assumes an outside temperature of -1C. In view of the recent weather, you might think it advisable to allow for a lower temperature. It's proportional. So if you decide to opt for -6C and the calculator says 30kW for -1C and room temp of 21C, you will need (assuming 2kW for water):

(30-2) x (21-(-6))/(21-(-1))kW = 28 x (27/22)kW = 34.5kW for heating

Adding back the 2kW for water gives you 36.5kW, say a 37kW boiler.

Also some sites state that you should allow 3kw for the water cylinder, but as I believe it the HW will take priority from the CH and will therefore not require any extra KW.

That all depends on how the hot water is connected. If you have a S Plan or Y plan, you have to allow for water heating in the boiler size as both can be running at the same time. If you opt for a W plan, this is not necessary as only one can be on at a time. Hot water takes priority.

I had just read that it was better to have a higher KW demand on the CH than the actual boiler KW rating, so it would work more efficiently. This has been mentioned with regards to oversizing the rads.

I think you are referring to the need to oversize rads because their output is lower when run at a 20C differential. The demand is not any higher.

Radiators are not like electric fires which give of a constant amount of heat, their output is temperature dependent. This feature is used by weather compensators.

Rad output specs are quoted based on flow temps of 75C and return of 65C. If the temps are 75C and 55C the output is some 15% lower, so a "1000W" radiator will only produce 855W. You therefore have to install a radiator with a nominal output of 1170W to actually produce 1000W. It is possible to size the radiators so they produce the required heat with virtually any water temperatures.