Who's right on sizing?

[texforever]

I recently asked 2 engineers to quote on installing a new NG system boiler and unvented cylinder to replace the combi in my large Victorian semi.
The house has approx 225m of living space over 3 floors and there is CH on 2 floors and I intend to have UFH on the top floor and 2nd floor bathroom which is 4.5m x 2.7m x 3m.

Having done a heat loss calc, one engineer taking into account the HW side is recommending a boiler around the 40-43kw mark. He has suggested a Keston C50 due to the limited fluing options that are available to me.

Engineer 2 is suggesting a Valliant Ecotec 630 30kw.
How can 2 engineers be so far apart on the sizing of the boiler? I’ve looked online at boiler size calculators and to me the first chap appears to be right, however wouldn't too large a boiler be inefficent and expensive to run or am I missing something?

Any thoughts would be appreciated.

 

[Agile]

I would guess that a 24 kW boiler would be quite adequate!

The advantage of the Vaillant is that its likely to be still working in three years time, the Keston perhaps not!

We only instructed to add 2 kW for water heating!

Tony

 

[bengasman]

Rule of thumb: never believe anyone recommending a Keston.
Heatloss calculation is irrelevant for replacing a boiler; all you need is to add up the capacity of existing radiators.
As your house needs zoning, it could be argued that you could go 1 or 2 sizes below the total rad capacity and no hot water allowance is needed as the cylinder can be heated during heating down times.
Tony's advice is a lot closer to the truth.

 

[Agile]

I would always size the boiler on the full heat needs to cater for the times the whole house is timed to be on.

But I do follow the instruction to only allow 2 kW for hot water.

I calculated the heat loss quite simply from the information the OP gave to us.

Tony

 

[Onetap]

Rule of thumb: never believe anyone recommending a Keston.
Heatloss calculation is irrelevant for replacing a boiler; all you need is to add up the capacity of existing radiators.

That assumes that the existing radiators are sized correctly.

 

[namsag]

are you sure about your living space what width and length is each floor

 

[matt1e]

I would guess that a 24 kW boiler would be quite adequate!

I would agree
I live in a quite large Georgian terrace house and I run four (zoned) floors comfortably of a 28kw boiler

matt

 

[kevindgas]

Keston = sounds like aircraft taking off also bed partner with Ideal
need we say more!

 

[bengasman]

Rule of thumb: never believe anyone recommending a Keston.
Heatloss calculation is irrelevant for replacing a boiler; all you need is to add up the capacity of existing radiators.

That assumes that the existing radiators are sized correctly.

No it doesn't; it takes it for a fact that as the rads have been there for aeons, the client is gonna keep 'em as they are, otherwise he would have changed them already regardless of what the calculator says.

 

[Kev F]

I would guess that a 24 kW boiler would be quite adequate!

The advantage of the Vaillant is that its likely to be still working in three years time, the Keston perhaps not!

We only instructed to add 2 kW for water heating!

Tony

Ive always been told to allow 3 for water tony, not saying im right but a few lads i know all say that, also allow 2kw heat loss

 

[Agile]

Ive always been told to allow 3 for water tony, not saying im right but a few lads i know all say that, also allow 2kw heat loss

2 kW is the figure that I was taught on the energy efficiency course. I would have expected that you had been on the same course as well as its a required qualification to fit boilers since 2005.

It can be explained because the heating is not needed on full power all the time and the hot water is heated up at different times to the CH.

Tony

 

[texforever]

Hi

Thanks for the posts chaps, it gives me something to think about.

Opinions on Keston duly noted!

Agile: Possible stupid question here ! does 2 kw rule for HW apply regardless of cylinder sizes? Mine is 200ltr

Bengasman: Rads calc approx 114k btu's based on room sizes= 33kw, take on board your point about zoning.

Namsag: Each floor is just short of 9m x 9m

Matt1e: How many bathrooms do you run on your system?

Cheers
Pete

 

[gas4you]

I was also taught 3Kw on the energy efficiency course.

Haven't a clue which one you took Tony. Was it the original and proper C&G or the new cut down one that was made easier to pass

 

[heatingman]

Refer to the cylinder tech/installation specs, it will advise you of the re-heat times and coil requirements to achieve this re-heat time.
If you are going to want to have the heating on full pelt, especially when cold and also re-heat the water at same time due to heavy demand (guests staying over) You need to take it in consideration. Or just go with the whole house calc and add 2/3kw for the hot water. The choice is yours. Personally I always calculate for the worst case and best performance from supplied equipment.

 

[doitall]

An allowance of 2Kw is what it says in my book, personally;- I don't think it's enough for a family house with high pressure (unvented cylinders) systems.

For a 200 Ltr cylinder I would allow up to 5Kw

http://www.boilers.org.uk/