Boiler sizing - simple question

[Gilmore Lewes]

Hopefully simple answer - I want to check the size of my system boiler. Do I do this against the output value of all my radiators or against the heat requirement of my rooms?

I know I have to add some for the HW tank.

Surprisingly I can't find the answer on the web.
Thanks

 

[flameport]

The heat requirement of the rooms and the building as a whole, which will also give you the size of the radiators required.
Do NOT use any existing radiator sizes, as you have no way of knowing if they were correct when installed, and even if they were it's likely that the building requirements have changed over time due to adding insulation, double glazed windows and so on - plus any new system will designed for much lower flow temperatures than systems from years or decades ago.

I know I have to add some for the HW tank.

Only if you have a time machine and are going back to the 1970s.

 

[MNW67]

Hopefully simple answer - I want to check the size of my system boiler.

A boiler you already own?

 

[Gilmore Lewes]

Thank you Flameport, I thought that would be the way to go as the radiators vary from 30 years old to new and add up to a lot of kWs. I note your comment about HW - is the modern assumption that it will never have to heat the HW at the same time as the radiators and therefore will have plenty of power?
MNW67 - It is a fairly new 40kW Bosch boiler but the system has grown up over the years and is fairly random. I am currently balancing it and wondering if it is all sized correctly.

 

[flameport]

it will never have to heat the HW at the same time as the radiators and therefore will have plenty of power?

Partially.

New systems can and usually should be set up so that HW takes priority, with all of the boiler power being used to heat the hot water cylinder, so it heats in the shortest time.

However even if heating and HW are used simultaneously, it still works for most of the year - the system would be designed for a very cold external temperature, and on most days it won't be anywhere near that, so the reduced power to the heating won't make any difference except on the coldest of days.

In both cases, the interior temperature should not change much, as with even modest levels of insulation, the heat should be retained for for far longer than the time it takes the HW cylinder to heat up.

It is a fairly new 40kW Bosch boiler

Unless your house is vast and poorly insulated, that boiler is grossly oversized.
However some of the radiators may well be undersized if they were intended for an older system with much higher flow temperatures.

 

[muggles]

A 40kW boiler should do around 30-40 radiators

 

[MNW67]

It is a fairly new 40kW Bosch boiler but the system has grown up over the years and is fairly random. I am currently balancing it and wondering if it is all sized correctly.

Are you just wanting to work out whether your boiler is powerful enough? Or are you aiming to turn down the boiler's maximum output?

 

[Gilmore Lewes]

Thanks for all the responses which are helpful and reassuring.
1) I didn't know about the priority for HW but I think our system is not smart enough for that. Only the controller knows what circuits are on and the boiler just shoves out hot water.
2) 30-40 radiators sounds good, we have 24 plus 4 towel rails on the HW circuit.
3) I am just checking it is powerful enough, I wasn't going to interfere with the settings as I think it automatically reduces its output.

Thanks again for taking the time to respond.