What Our Putney Architect Discovered About Our Boiler That Saved the Extension From Being Cold Six Months a Year

Nobody thinks about the boiler when planning an extension. You think about the kitchen layout. The worktop material. The bifold doors. The rooflight. The connection to the garden. The boiler is in a cupboard in the hallway doing its job quietly and you assume it will keep doing its job after the extension is built.

Our architects putney practice checked the boiler during the first visit. Not because we asked them to. Because they check it on every project. What they found explained why so many Putney extensions feel cold in winter even with underfloor heating installed.

Our boiler was too small for the extended house. And nobody would have noticed until the first cold snap in November.

What Too Small Means

Our boiler was a 24kW combi. Adequate for our three bedroom Victorian terrace in its original configuration. Three radiators downstairs. Four upstairs. One bathroom. The boiler coped fine. Hot water on demand. Radiators warm within twenty minutes. No complaints.

Add a thirty square metre kitchen extension and the equation changes. The new space needs heating. Whether thats radiators or underfloor heating, the boiler has to supply the hot water to warm it. Thirty square metres of additional floor area with a large glass wall losing heat to the garden requires significant heating output.

Our architect calculated the additional heating demand. The new extension needed roughly 4kW of additional output. Our 24kW boiler was already running at about 20kW during peak demand on cold mornings. Adding 4kW would push it to 24kW. Maximum capacity. No headroom for particularly cold days. No spare capacity for running hot water simultaneously.

In practice this means the extension would be warm most of the time. But on the coldest mornings when you most need the heating to work properly, the boiler would struggle. Radiators in the existing house would heat slower because the system is sharing capacity with the new extension. Hot water might drop in temperature while the heating is running.

Not a catastrophe. But a daily frustration from November to March that would get worse every year as the boiler ages and loses efficiency.

What Most Extensions Get Wrong About Heating

Most builders include underfloor heating in the extension as standard. Homeowners see it on the quote and assume the heating is sorted. Job done. Underfloor heating installed. The extension will be warm.

But underfloor heating is just pipework in the floor. It still needs hot water from the boiler. If the boiler cant supply enough hot water at the right temperature, the underfloor heating runs lukewarm instead of warm. The floor feels tepid rather than cosy. The room never quite reaches temperature on cold mornings.

Nobody tests this until winter. The extension is built in summer. The heating is commissioned during mild weather. Everything seems fine. Then December arrives and the kitchen that was meant to be the warm heart of the house feels noticeably cooler than the rest of it.

Our architect said she sees this on roughly one in three extension projects she is called to assess after completion. The homeowner complains the extension is cold. The builder says the underfloor heating is working correctly. Both are right. The heating is working. The boiler just cant feed it properly.

The Two Thousand Pound Fix

Our architect recommended upgrading the boiler from 24kW to 32kW as part of the extension project. A new combi boiler with enough capacity for the existing house plus the new extension plus simultaneous hot water demand.

Cost. About two thousand installed. Including removing the old boiler, fitting the new one, and adjusting the system controls.

Two thousand during construction when the plumber is already on site and the system is being modified anyway. Compared to about three thousand if done as a standalone job after the extension is finished. Because a standalone boiler replacement requires a separate plumber visit, separate drain down of the system, and separate commissioning.

We added the boiler upgrade to the builders scope. It appeared as a line item in the quote. Priced alongside the underfloor heating, the radiator modifications, and the hot water connections. Seamless. No separate appointment. No additional disruption.

What November Felt Like

Warm. Everywhere. The extension. The existing rooms. The hot water. Everything working at the capacity it was designed for.

Our neighbours extension was completed around the same time. Same builder. Same underfloor heating specification. They kept their original 24kW boiler. By mid December they were complaining that the kitchen floor never felt properly warm in the mornings. The radiators in the hallway took forty minutes to heat up instead of twenty.

Their builder said the underfloor heating was installed correctly. He was right. It was. The system was fine. The boiler was the bottleneck. A 24kW boiler trying to heat a house that now needed 28kW.

They replaced their boiler in January. Three thousand for an emergency replacement during the busiest time of year for heating engineers. A thousand more than it would have cost during their extension build six months earlier.

The Check Every Extension Project Needs

Ask your architect to calculate the total heating demand of the extended house. Not just the new extension. The whole house including the existing rooms.

Compare that demand to your current boiler output. If the boiler is running at or near capacity before the extension, it will be over capacity afterwards. An upgrade is not optional. Its essential.

This calculation takes about thirty minutes. Your architect or a heating engineer can do it during the design stage. The cost of the calculation is zero if included in the standard design service.

The cost of not doing it is a cold extension every winter and an emergency boiler replacement at premium rates when you finally figure out why.

Six to eight months from first conversation to completion. Two thousand for a boiler upgrade during construction. Warm kitchen every November. Because someone checked the cupboard in the hallway before designing the room at the back.