Choosing the right boiler - somewhere between 18kw and 42kw depending on where I look.....

[Last Plumber]

They do a system version I believe;

Greenstar i System (27kW and 30kW) | Worcester, Bosch Group



Perhaps not quite finger in the wind, it seems a fairly detailed estimate using this calculator;

Calculate heat requirements

I would recommend you get a Heating Engineer to look at it. Either to price you for the work or to give a report and Heat loss calculations.
They will probably charge for the latter but it would be worth it in my opinion.
I just took a quick look at the calculator you used and it did not give the answers I expected to see.

 

[Raydeeator]

I would recommend you get a Heating Engineer to look at it. Either to price you for the work or to give a report and Heat loss calculations.
They will probably charge for the latter but it would be worth it in my opinion.
I just took a quick look at the calculator you used and it did not give the answers I expected to see.

Do my numbers look on the low side? The boiler sticks out 30kw CH against my estimation of 19kw, so I figure that even if I'm out by a huge margin then I'm still on the safe side. And when I did my calcs I was pessimistic about certain things (e.g. wall insulation - fairly sure they're all cavity insulated by chose non-insulated in the calculator).

That said, I will do as you suggest and get in the pro's......

As a matter of interest, what tool do the pros use?

 

[Last Plumber]

Do my numbers look on the low side? The boiler sticks out 30kw CH against my estimation of 19kw, so I figure that even if I'm out by a huge margin then I'm still on the safe side. And when I did my calcs I was pessimistic about certain things (e.g. wall insulation - fairly sure they're all cavity insulated by chose non-insulated in the calculator).

That said, I will do as you suggest and get in the pro's....

As a matter of interest, what tool do the pros use?

Personally I use a Mear Calculator, as well as knowledge and experience.
I have looked at a few online tools/calculators but I can't trust them the same.
I would size the Boiler for whichever has the largest requirement, DHW or CH, in your case.
Others will look at that different.

 

[Riley]

Mears all the way unless I just need a quick advisory calc

 

[dan_the_plumber]

Evening all,

Our boiler hasn't been doing very well recently. Just doesn't get the radiators hot at all. It was a relic of a smaller house, which was then extended. They didn't replace the boiler.

We've decided to replace the boiler and sort out some pipework, install new valves etc, but I can't settle on the boiler size required. All radiators are already installed etc, so it's a question of finding the right sized boiler to heat it all appropriately;

- 4 bed detached house
- 1 bath, 2 showers
- 2200 sq/ft
- 15 rads, 2 towel rails

We are going for a system boiler. My calculations are as follows

- Current radiator output (Watts - ΔT = 50°) = 22620 Watts (includes towel rails and two rads in a glass conservatory)

- House heat loss = 19000 Watts (17000 if remove conservatory)

The boiler which has been suggested is the Worcester-Bosch Greenstar 30i. As far as I can tell it has 24kW CH output, and 11ish LPM flow, with a max BTU of 100,000.

If I trawl the internet I find all sorts of stuff about 3kw per radiator, or any 4 bed houses needing 42kW boilers etc. I appreciate that in the past it's been common to massively oversize boilers, but it just makes me nervous that I'm missing a trick and the boiler won't be capable. With the numbers above would the 30i suffice and are there any apparent issues? I have the numbers but don't really know how to interpret them...

My alternative is to have the 35CDI or something, at a marginally greater cost.

Many many thanks.

Well done on your maths work mate. TBH you’ve done a better job than most plumbers. Speaking from experience you should be fine with Worcester 30i system boiler, or Worcester 30 CDI is Avery good boiler for a bit more money.
If you wanted full peace of mind you could upgrade to the 35kw boiler, but yo give some idea I’m maybe looking at the 35CDI to heat a block of four flats whilst providing all the hot water too. Dan

 

[kris]

I would say that's for a standard copper cylinder not an unvented, As if you allow let's say 2kw

And the unvented cylinder can do a reheat time of 30 mins with 20kw, that will give you a re heat time of 5 hours from cold

So if you use all your hot water it will take 5 hours for it to be fully hot[/QUOTE

I know its different as electricity but Even an immersion heater say 2kw of electric power would heat the tank in short period