condensing boiler working under pressure?

[rbuzzy]

i was told that a condensing boiler works better under pressure? is this correct?
surley a condensing boiler is a condensing boiler for a reason, and no matter the situation it will still condense?!
im currently looking into fitting a boiler but ive been told to use a 30kw boiler, this house is not small it's got roughly 15 radiators some of wich are of abnormal size, stone walls poorly insulated and arround 60 years old.
Can anyone give any advice??

 

[Billy Bob Bob]

what do you mean by under pressure?

 

[SimonG]

flogging it's puddings out?

 

[GrahamM]

The boiler should be sized using the whole house calculation. It will work out the correct size of boiler based on the house characteristics. It also depends what type of system. ie combi or conventional boiler. Any decent gas fitter can advise.
No point in fitting a boiler that cannot heat the house or too big a boiler that will cost more and never hit its full potential.

 

[GrahamM]

what do you mean by under pressure?

Believe he means pushed to its limit!

 

[Puddle]

So do we have a theory that as ,over sized rads increase the condensing mode,thus makes more efficient ,so it derives that under-sizing the boiler or out put, to a certain extend has the same effect ?

 

[Best]

Would have thought oversizing the radiators will mean the rooms will heat up properly, even in worst of weather, with boiler stat turned low, so help keep boiler condensing. Undersizing the boiler itself would mean risking not heating house properly in mid winter. Modulating gas boilers at least drop their output. However, not good idea to fit too large a fixed output boiler like an oil boiler as it heats up quick but cycles more, unless you lower the output as I would.

 

[WHPES]

Its all about heat transfer. Oversizing the radiators means that the water doesn't have to be as hot to achieve the same rate of heat transfer as there is more surface area available.

Undersizing the boiler would mean that if the peak rate of heat transfer was required, the boiler would be incapable of delivering and so the target temperature would not be able to be attained.

 

[GrahamM]

So do we have a theory that as ,over sized rads increase the condensing mode,thus makes more efficient ,so it derives that under-sizing the boiler or out put, to a certain extend has the same effect ?

Think it is more factual than theory now.
I believe for optimum efficiency an approx. 20°C change in the flow & return temp must be achieved. So slightly oversizing the rads would help achieve this. Within reason! Fitting vastly oversize rads is going to cost, so would offset any potential saving.
The main issue is to ensure the boiler is capable of heating the house first, (no point in oversizing either as again it will cost more for no return) then we can look at getting the heat differential.
It's a 3 step procedure, house, boiler, rads.