What size boiler do I need?
Bigger is not better. An oversized boiler costs more to buy, more to run, and wears out sooner.
Boiler output is measured in kilowatts, and the right figure comes from your property rather than a rule of thumb.
What the size depends on
- The number of radiators and their sizes
- The number of bathrooms and how often they are used at once
- The insulation, glazing and age of the building
- Your incoming water pressure and flow rate
A three-bedroom house with one bathroom has very different demand from a three-bedroom house with two bathrooms and a rainfall shower.
Why oversizing is the common mistake
An oversized boiler short-cycles: it fires, heats quickly, shuts off, then repeats. That constant cycling is inefficient and it wears components faster. You pay more upfront, more monthly, and replace it sooner.
It happens because oversizing is the safe option for an installer — nobody complains that their house is warm. You are the one carrying the cost.
What to ask
Ask whoever is quoting how they arrived at the output figure. A proper answer refers to your radiators, your bathrooms and your water flow rate. "That's what we usually fit" is not sizing.
If you have low incoming water pressure, that also affects whether a combi is right for you at all — worth establishing before anyone orders a boiler.
Want this priced for your property?
Send BPS Plumbing & Heating a few photos and the rough measurements and you'll get a written quote that sets out exactly what's included — so you can compare it properly against anything else you've been given.