Installation (And Why It Costs You Later)

Here’s something most boiler installation guides don’t mention: the brand of boiler you get matters

a lot less than the size. Not physical size — output size. The kilowatts. And getting that wrong is far more common than the industry likes to admit.

Why Boiler Sizing Gets Ignored

Most companies, when they replace your boiler, will ask one question: what have you got now?

Then they order the same thing. It’s quicker. It avoids the awkward conversation about whether your current setup was ever right in the first place. But a like-for-like swap on a badly sized original means you’ve just installed an expensive version of the same problem.

An oversized boiler ‘short cycles’ fires up, reaches temperature too quickly, and shuts off before the heating system has properly circulated. This on-off pattern puts wear on the heat exchanger, burns through ignition components faster, and — this surprises people — actually makes the house less comfortable. 

The boiler never finds its rhythm. Undersized is the other extreme. The boiler runs almost constantly trying to keep up, bills creep up, and the hot water never feels quite right on a cold morning. If you’ve ever had a house where the radiators seem warm but the rooms don’t, this is often why.

What Proper Sizing Actually Involves

A proper heat loss calculation looks at the floor area of your home, the number of external walls, window size and type, ceiling height, insulation levels, and how many bathrooms you have. It takes roughly 20 minutes to do properly. 

Most engineers can do it from memory for common property types, but for anything unusual — older terraces with solid walls, large detached properties, homes that have had extensions — it should be done on paper.

For most three-bedroom semi-detached homes in Hertfordshire, you’re looking at somewhere between 24kW and 30kW for a combi boiler. A four-bedroom detached with two bathrooms might need 35kW to 40kW. 

Hard Water Makes It Worse

Parts of Hertfordshire sit in a hardwater area. Limescale builds up inside heat exchangers, reducing their efficiency and, over time, their capacity. An already marginal boiler — one that was borderline undersized to begin with — will start struggling within three to four years in a hardwater area where the water hasn’t been treated. 

A correctly sized boiler with a scale reducer fitted during  installation has a much better long-term outlook. This is something we check during every survey visit. If your existing pipework has scale build-up, we’ll flag it before the installation rather than after. See our boiler installation page for what’s covered in a full survey.

The Smart Thermostat Factor

Since the Boiler Plus regulations came in, new boiler installations in England are required to include a time and temperature control — which in practice means a room thermostat or, increasingly, a smart thermostat. A smart thermostat like a Hive or Nest paired with a correctly sized boiler can improve efficiency by 8–12% compared to a basic programmer. 

That’s a real figure — not a marketing claim — based on how the boiler modulates at lower demand instead of cycling on and off.

The combination of correct sizing, a system filter, scale protection where needed, and a smart thermostat is the setup that actually performs. Most blogs talk about brand. This is the stuff that matters more.

What to Ask Your Engineer Before They Order the Boiler

Before any installer orders your replacement unit, ask them: have you done a heat loss calculation, or are you matching what’s there? What kW output are you recommending, and why? Does that account for the number of bathrooms and the property’s insulation? If they can’t answer that clearly, it’s worth pausing before you commit.

At JTS, every installation starts with a proper assessment — not just a look at what’s already there. book a survey and we’ll give you a written recommendation with the reasoning behind it. If you want to understand the full process first, our boiler installation page covers it in detail. We also handle boiler repair and central heating across Hertfordshire, Cheshunt, Waltham Cross and Enfield.

Getting boiler sizing right from the start saves money every year. Book a survey — free, no

obligation, carried out by Gas Safe engineers across Hertfordshire.