Installing Air Source Heat Pumps in Period Properties

Why Period Homes Need a Different Conversation

Air source heat pumps work beautifully in older houses — but only when the whole system is designed around the building rather than bolted onto it. A heat pump delivers heat at a lower temperature than a gas boiler, typically 40–50°C rather than 70–80°C. That single fact changes everything: how much insulation matters, how large your radiators need to be, and how quickly the house warms up on a cold January morning. In a Victorian terrace or a 1930s semi, the fabric of the building becomes part of the heating design. Get that right and you will have a quiet, efficient system with lower running costs. Rush it and you will have a heat pump that never quite keeps up.

Start With Heat Loss, Not the Box Outside

Before anyone quotes you for a unit, you need a room-by-room heat loss calculation. This tells you how many kilowatts the house actually loses at design temperature — usually around -2°C to -3°C in most of the UK — and it is the only reliable way to size both the heat pump and the emitters. Anything less is guesswork.

As part of that assessment, look hard at the fabric:

• Loft insulation: top up to 270–300mm if you are below that. It is the cheapest heat you will ever save.
• Draughts: original floorboards, sash windows, chimney flues and letterboxes. Draught-proofing is inexpensive and disproportionately effective.
• Solid walls: if you have them, consider internal or external insulation using breathable materials such as wood fibre, and keep lime plaster and render where you can. Trapping moisture in a solid wall causes more problems than it solves.
• Windows: secondary glazing on sash windows is often the best compromise between thermal performance and heritage character.
• Ventilation: once a house is less draughty, it needs deliberate ventilation. Trickle vents, extract fans or a mechanical ventilation system with heat recovery will manage moisture and keep the building healthy.

For a listed building, do this work sensitively and take advice early. There is usually more scope than people assume, particularly with internal insulation and secondary glazing.

Radiators: Bigger, Slower, Better

Your existing radiators were almost certainly sized for a boiler running at 70°C or above. At a 45°C flow temperature, a radiator that produces 1,000 watts on paper might deliver only 400–450 watts. That is not a fault — it is physics. There are three ways to bridge the gap: increase radiator size, add more of them, or improve insulation so the demand drops.

Practically, expect to replace many radiators with double or triple panel versions, often with extra convector fins. In larger rooms, two medium radiators on opposite walls usually beat one enormous one, because they distribute heat more evenly and run at a lower temperature. Where floors are being lifted anyway, wet underfloor heating is worth considering — it pairs superbly with heat pumps and gives a lovely gentle warmth underfoot.

Do not forget the hot water cylinder. Most heat pump systems need a larger cylinder than a combi boiler household is used to, typically 200–250 litres for a family, with a well-insulated coil designed for lower flow temperatures.

Pipework and the Hidden Work

This is where period properties can quietly catch you out. Older systems often use microbore pipework of 8–10mm, which simply cannot carry enough heat at low flow temperatures. In many homes, upgrading the main runs to 22mm and the radiator branches to 15mm is essential. It is disruptive work — floors up, carpets lifted — but it is the difference between a system that performs and one that disappoints.

Other details worth planning for:

• Positioning the outdoor unit: allow space for airflow, avoid bedrooms where possible, and check permitted development rules. Most units are around 40–45 decibels at one metre, so siting matters as much as sound ratings.
• Buffer volumes and controls: a small buffer vessel or volumiser often helps the system run steadily, and weather compensation controls are not optional — they are central to efficiency.
• Electrical supply: check the consumer unit and whether a dedicated circuit is needed.

Choosing the Right Unit and Installer

Once you know the heat loss, the emitter sizes and the pipework reality, the heat pump choice becomes straightforward. Look for a unit that modulates down low, so it can tick over gently in milder weather rather than cycling on and off. Ask for a design that states the expected flow temperature, the seasonal performance estimate, and how domestic hot water will be prioritised.

Use an accredited installer who will do a full heat loss survey rather than a quick glance at your floor area, and who will put the design in writing. Grant funding is available for eligible installations, and the paperwork is the installer's job — a good one will guide you through it without fuss.

Living With It: What to Expect

A heat pump is best run steadily rather than in short bursts. Set a comfortable temperature and leave it there; letting the house cool right down and then blasting it back up wastes energy. Expect longer run times, softer radiator temperatures and a house that feels evenly warm rather than alternately hot and cold. Give it a full heating season to settle, keep an eye on your controls, and don't be afraid to ask your installer to fine-tune the weather compensation curve once winter is underway. Done properly, a heat pump in a period property is not a compromise — it is an upgrade.