Regular Boiler - Size Matters - Why?

Lokolo

Can someone explain what a higher output on a regular boiler would actually do.

For example, if I have a hot water tank and 10 radiators.

If I had a 25kW boiler, and a 30kW boiler. How would the 30kW boiler impact the system? It would refill the hot water tank faster? For radiators does it mean they heat up faster, and/or does it impact the number of radiators that can be heated (so can a 10kW boiler only heat up 2 radiators, or can it still heat up 10, but requires more time to do so)?

alleycat`

You're really in the wrong section (this is for alternative fuels).


The Higher output boilers are based on instant hotwater demand for things like showers running straight off the boiler (so Combis).



Your hot water, like mine, is coming out of the tank so you don't need that level of instant "kick".


So unless you have an enormous house, or it's ancient and badly insulated, a system boiler would not need to be anywhere near that size.


Over sizing the boiler would, potentially, actually be worse as the boiler would operate outside of "condensing" mode.

Lokolo

alleycat` wrote: »

You're really in the wrong section (this is for alternative fuels).


The Higher output boilers are based on instant hotwater demand for things like showers running straight off the boiler (so Combis).



Your hot water, like mine, is coming out of the tank so you don't need that level of instant "kick".


So unless you have an enormous house, or it's ancient and badly insulated, a system boiler would not need to be anywhere near that size.


Over sizing the boiler would, potentially, actually be worse as the boiler would operate outside of "condensing" mode.

I didn't even think about just posting in the Energy subforum. For some reason completely bypassed it!

However thanks for the explanation. Exactly what my thinking was.

G_M

alleycat` wrote: »

Over sizing the boiler would, potentially, actually be worse as the boiler would operate outside of "condensing" mode.

Indeed. I'm no expert but when I was replacing my boiler it was explained to me that an over-sized boiler would be constantly switching itself on and off as it quickly got up to the required heat, and this is inefficient.

But equally an under-sized boiler would run non-stop as it struggled to produce enough hot water to circulate round the 10 rads (or whatever number).

So the boiler should be sized by a competant installer who assesses the system as a whole.

Happy to be corrected though if that was just sales talk....!

Cardew

I certainly agree that with a HW tank a high boiler output is of little relevance.

The output for radiator heating should be determined(sized) by the requirement to heat the whole house to, say, 20C when the temperature outside is, say, 0C. That will be determined by the size of the rooms and standard of insulation.

Indeed. I'm no expert but when I was replacing my boiler it was explained to me that an over-sized boiler would be constantly switching itself on and off as it quickly got up to the required heat, and this is inefficient.

Most(all?) modern boilers are modulating so the boiler output is reduced(modulated). On my large boiler down to 8kW.

In addition I have weather compensation which reduces/increases the water temperature as the outside ambient temperature varies. When the boiler came on in the last couple of weeks, the water temperature was between 30C and 40C.

The effect of these controls is to maximise the time the boiler is in condensing mode.

Lokolo

Cardew wrote: »

The output for radiator heating should be determined(sized) by the requirement to heat the whole house to, say, 20C when the temperature outside is, say, 0C. That will be determined by the size of the rooms and standard of insulation.

Thanks Cardew. To me that sounds like the boiler can heat up certain amount of radiators to a certain temperature, the bigger the boiler, the more and faster it can heat up radiators.

Uxb1

That's basically it.
I recall a rule of thumb that for those places which are infrequently heated - churches being the common example you need to size the boiler for 25% more than for a normal property.
This is because the place/pipes/rad's etc will be cold and you want the whole lot warmed up as quickly as possible.
So you are in this case sizing for warming the building up in a fixed time case rather than maintaining a temperature.

Legendino

I understand the way to 'size' a regular boiler i.e stored domestic HW is 1.5kw per radiator + 3kw for HW.


So a 10 rad house + HW = a 18kw boiler. I would always had a 'little extra' so that when heating and hw water is 'called for' the boiler is not working at max. So in this instance maybe 20 or 21kw ?