Oil boiler - how big should I go?

ellives

Hi

Using a few of the calculators suggests I need a boiler capable of approx. 65000 BTU or approx. 19Kw

My question is....does it matter if I get one with a considerably larger output capability?

I ask as I may add an extension at some point but it may be years off, plus will buy a more powerful one anyway if it is worthwhile.

I'm wondering if it's like owning a 1.6 litre car and replacing it with a 3 litre - still gets from A-B, albeit faster and smoother, but costs more to run?

Any thoughts?

macman

A higher output just heats the property faster, it takes the same amount of heat to get from temp x to temp y, whether it's fast or slow.

ellives

Does it take the same amount of effort....read fuel to get from x t y?

macman

Yes, fuel converts to heat as a constant-assuming that the boiler has the same efficiency at both level of output.

ellives

I appreciate this is like pulling teeth......so are you saying getting a more powerful boiler than is strictly necessary has no downside apart from being more expensive to buy?

Plus, has the upside of being faster?

Cardew

In these days of condensing boilers it seems that thinking has changed about over-sizing a boiler.

http://www.nef.org.uk/energysaving/boilers.htm

What size of Boiler do I need?


Old boilers may have their output measured in British Thermal Units per hour
(Btu/h), but all current boilers are sold in the metric equivalent of kilowatts
(kW). The calculator below will give the approximate metric value, but you
should bear in mind that in the past central heating engineers often installed
boilers that had a higher output than strictly necessary. Although this meant
that there was no possibility of the boiler failing to meet the demand for heat,
even in the most arctic of conditions, it also meant that they were mainly
operating at a part load, and so running below their maximum efficiency. If you
have installed additional loft or cavity wall insulation since the last boiler
was fitted, it is highly likely that you will need a smaller boiler than
before.

If you 'Google' 'sizing a central heating boiler' it appears most are in agreement with the above.

Don't forget that on a Combi the higher rating, say 24kW, is for the Hot Water output.

macman

OP is seeking an oil boiler, not sure how efficiency varies under load compared to a gas boiler?
As ever, this is best resolved by a proper site survey by the proposed installer.

ellives

macman wrote: »

OP is seeking an oil boiler, not sure how efficiency varies under load compared to a gas boiler?
As ever, this is best resolved by a proper site survey by the proposed installer.

Good point....it is for an oil boiler not gas.....

Cardew

I appreciate the OP uses oil, but I would have thought the same principle applies. The aim is to get it into condensing mode as much as possible.

Or put it this way, I have seen nothing to suggest that it is wrong to oversize gas, but doesn't matter with oil!

Ben84

The car analogy isn't so good in this case, cars with bigger engines also tend to be much heavier (hence needing a bigger engine to perform as expected) so they have to burn more fuel to move more mass everywhere they go. A lot of the difference you see in car fuel costs is down to the weight of the car, not the engine size, with the catch that bigger engines sizes pretty much always mean heavier cars. It's one of those true, but perhaps not for the reasons you might think situations. Anyway, boilers are radically different to car engines, aside from both consuming fuel the similarities are limited.

Gas boilers can sometimes have a device fitted to limit the gas supply so output matches the house better (can be removed later if needed) - is this also possible with an oil boiler?