How to Size a Heat Pump for a Welsh Stone House
Quick Summary
How to size a heat pump for a Welsh stone house starts with a room-by-room heat loss survey, not the stone itself. Calculate how much heat each room loses on the coldest day, include draughts and ventilation, then match the heat pump output and radiator sizes to that total.
Stone construction alone does not decide heat pump size; total heat loss does.
Measure every external wall, window and door, and note each wall's material and thickness.
Count draughts around sashes, chimneys and floorboards, as leaving them out causes under-heating.
Radiators give less heat at heat pump flow temperatures, so check each room's output at the planned temperature.
Include domestic hot water demand in sizing, and avoid oversizing, which costs more and can run less efficiently.
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Table of Contents
Last Updated: November 11, 2026
Sizing is the step that decides whether an air source heat pump works well in a Welsh stone house. A heat pump sized correctly keeps the home warm on the coldest days without wasting electricity. One sized wrongly either struggles through winter or cycles on and off, costing more than it should. When you need to size a heat pump for a Welsh stone house, you start with how much heat the building loses, not with the label on the wall.
Can a Heat Pump Work in a Welsh Stone House?
Yes, a heat pump can work well in a stone house, provided the system is designed around the building. Older stone properties often have thick walls, high heat loss and radiators that were never meant for low flow temperatures. Each of these affects the design, but none rules a heat pump out.
Many homeowners assume the stone itself is the problem. In practice, the design choices matter more than the material.
Thick walls hold heat well but warm up slowly.
High heat loss means a larger system output is needed.
Existing radiators may need upgrading to run at lower temperatures.
With careful sizing, a stone house can be heated efficiently and comfortably.
Why Stone Construction Alone Does Not Decide Heat Pump Size
When you learn how to size a heat pump for a Welsh stone house, remember that the wall material on its own does not set the size of the heat pump. What counts is how much heat the whole building loses on a cold day.
Two stone houses of the same age can need very different systems. One may have good loft insulation, sealed windows and draught-proofing. The other may have none of these. The second house will need a larger output, even though the walls look identical.
So the right question is not "is this a stone house?" but "how much heat does this house lose, room by room?" That question leads to a sizing calculation you can trust.
Carrying Out a Heat Pump Room-by-Room Heat Loss Survey
A room-by-room heat loss survey is the foundation of correct sizing. It measures how much heat each room loses at the design outside temperature, which is the temperature the system must still heat the house at.
A survey typically records:
Room dimensions, including ceiling height
Wall type, thickness and insulation level
Window and door sizes, types and condition
Roof or ceiling insulation
Floor construction
Draught levels and ventilation
Each room's heat loss is added up to give the total for the building. That total is the heat output the heat pump needs to deliver on the coldest day.
If customers contact us directly we will carry out the survey for free, saving them £250.
Heat Loss Calculation for an Old House Explained
Heat loss is calculated from the area of each surface, its insulation value and the temperature difference between inside and outside. In an old house, the walls and floors are often the biggest losses.
Older houses also tend to have gaps and draughts. These add to the heat loss and must be counted, or the system will be undersized.
Measure every external wall, window and door.
Note the material and thickness of each wall.
Account for air leakage around windows and doors.
Check the state of the roof and floor insulation.
A calculation that skips these details will give a misleading result.
Working Through a Worked Sizing Example for a Welsh Stone House
A worked example makes the sizing logic visible. The figures below are illustrative, not a substitute for a design calculation, but they show the sequence a competent designer follows and the points where a shortcut can mislead.
Consider a two-storey solid-stone cottage on a hillside in mid Wales. The walls are roughly 600 mm of rubble stone with lime mortar, the roof is slate with 100 mm of loft insulation, and the windows are original single-glazed timber sashes. The house has a kitchen, a living room, a front room that is rarely used, three bedrooms and a bathroom.
Step 1: Establish the design outside temperature. The design temperature is not the average winter temperature. It is the temperature that is exceeded only for a small fraction of hours in a typical year, and it varies with altitude, exposure and distance from the coast.
Step 2: Calculate fabric heat loss for each element. For each room, the designer multiplies the area of each element (external wall, window, door, roof or ceiling, floor) by its U-value, which describes how readily that element conducts heat, and by the temperature difference between inside and the design outside temperature.
Step 3: Add ventilation and infiltration loss. Older houses leak air. Draughts around sashes, chimneys and floorboards are counted separately as air-change losses, calculated from the room volume and an allowance for the air change rate. Leaving this out is one of the most common reasons an old house ends up under-heated.
Step 4: Sum the room totals. The room totals are added to give the whole-building heat loss at the design temperature. This figure, and not the floor area, sets the required output of the system. A rule of thumb based on square metres can be a useful sense-check but should never replace the room-by-room calculation for a stone building.
Step 5: Match the output and check the flow temperature. The heat pump is chosen so its output at the planned flow temperature covers the building total. The radiators are then checked against the same room losses.
Watch out: if the calculation is done at a warmer design temperature than the site justifies, the system will look adequate on paper and then fall short on the first severe cold snap.
Recording each step, with the values and assumptions used, lets the design be reviewed before it is installed.
The worked example follows five steps, each with the key point to get right.
Step | What to do | Watch for |
1. Design outside temperature | Use the temperature exceeded only a small fraction of hours | Exposed upland sites usually need a lower figure |
2. Fabric heat loss | Multiply each element's area by its U-value and temperature difference | Thick solid stone often causes the largest share of loss |
3. Ventilation and infiltration | Count draughts around sashes, chimneys and floorboards | Leaving this out is a common cause of under-heating |
4. Sum room totals | Add room losses to get the whole-building figure | Floor area alone is only a sense-check |
5. Match output and flow temperature | Choose a heat pump covering the total and check each radiator | One undersized radiator leaves a room cold |
Checking Radiator Sizing for a Heat Pump
Radiator output depends on the temperature difference between the water in the radiator and the room air, not on the boiler that heats the water. Heat pumps usually run at lower flow temperatures than boilers, so each radiator gives off less heat than its rated output at boiler temperatures.
This is why a radiator that was adequate on the old boiler can be too small on a heat pump, even if the room has not changed.
Read the radiator schedule at the design flow temperature. Manufacturers publish outputs at a standard temperature difference, usually written as delta T50, meaning a 50 °C difference between water and room air. The designer converts that rating to the planned flow temperature and room temperature, then compares the result with the room's calculated heat loss. Most installers will record this as a radiator schedule, with one line per room showing the existing output, the required output and any change.
Check the rooms that are hardest to heat. Rooms with the highest heat loss per square metre, such as those with large single-glazed windows or external stone walls on two sides, are usually the first to fall short. Bathrooms often need extra output to reach a comfortable temperature at a low flow temperature.
Consider the options when a radiator is too small. The usual choices are a larger radiator, a longer or double panel, an additional emitter, or lowering the room's heat loss through draught-proofing or insulation. Underfloor heating can suit ground-floor stone rooms but requires floor build-up and is not always practical in an older house.
Watch out: a radiator that is correct for the room but sized for a higher flow temperature will still fall short if the system runs cooler than planned. Confirm the design flow temperature with the installer and check that the controls will hold it.
When the schedule is complete, every room has a documented output that meets its heat loss at the planned flow temperature, which makes the design straightforward to verify before installation.
Domestic Hot Water, Oversizing and Verifying Your Design
Sizing must also cover domestic hot water, not only space heating. Hot water demand depends on how many people live in the home and how much hot water they use. This demand is added to the heating load when choosing the system.
Oversizing is a common mistake. A larger unit costs more to buy and can run less efficiently.
To verify a design, check that:
The calculated heat loss matches the surveyed room data.
The heat pump output covers the building total at the design temperature.
The radiators can deliver the heat at the planned flow temperature.
The hot water demand is included in the sizing.
Good designs are documented, so the reasoning can be reviewed before installation.
Using a Heat Pump Survey Checklist Before You Book Your Installation
Before booking an installation, use this checklist to confirm the survey covers what it needs to:
Room-by-room heat loss calculated for every room
Wall, roof, floor and window details recorded
Draught and ventilation levels noted
Design outside temperature confirmed
Heat pump output matched to total heat loss
Radiator sizes checked at the planned flow temperature
Hot water demand included in the sizing
Design written down and available for review
If you want help working through this for your own home, the Heatgeek Wales team can talk you through the survey and sizing process. Get your instant heat pump estimate today.
Frequently Asked Questions
How do you calculate the heat loss of a stone house?
Heat loss is calculated room by room. The surveyor measures each room's dimensions, identifies the wall, roof, floor, window and door areas, and records the construction and insulation level of each element. Each element's area is multiplied by its U-value and the design temperature difference, and the results are added together with an allowance for ventilation and draughts. For a solid-walled stone house, the wall U-value is the key input, so an accurate assessment of wall construction and any existing insulation matters more than the stone itself.
What size heat pump does an older stone house need?
The size depends on the calculated design heat loss of the whole property, not on its age or the stone it is built from. Two houses of the same floor area can need very different outputs if one is insulated and the other is not. A proper room-by-room calculation gives the heat pump output needed on the coldest design day, which is then matched to a suitable model. Oversizing a heat pump can reduce efficiency through short cycling, so the goal is a match, not the largest unit available.
Can existing radiators work with a heat pump in a stone house?
Sometimes. Heat pumps usually run at lower flow temperatures than oil or LPG boilers, so each radiator gives off less heat. Radiators are checked against the room heat loss at the lower flow temperature. Where a radiator is undersized for this, it may need replacing with a larger one, or the room may need additional emitter output. Checking radiator output is part of a complete heat pump design rather than an afterthought.
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