Free Heat Loss Survey Tool – What Size Heat Pump?

Free Heat Loss Survey Tool

If you have a gas boiler and a smart meter, you can estimate your home’s whole-house heat loss using real energy data from cold weather.

This free heat loss survey tool takes one or more cold 24-hour gas readings, adjusts for boiler efficiency and hot water use, and converts them into a design heat loss figure (kW). That’s the same headline number installers use when deciding what size heat pump a property needs.

It’s not a replacement for a proper full house and room-by-room survey by a skilled engineer, but it’s a solid, reality-based starting point.

Table of Contents

Use the Table of Contents below to jump straight to the section that matters most or scroll through at your own pace.

What this free heat loss survey tool does

This tool:

• uses real gas consumption from cold days

• converts gas into useful space-heating energy

• derives a whole-house heat loss rate (W/°C)

• applies that to a cold design temperature

• outputs a design heat loss in kilowatts

It’s designed for homes that still have gas heating and want a clearer picture before speaking to installers or comparing proposals.

Use more than one day of data if you can

Although the calculator works with a single cold day, it’s strongly recommended to run it across many cold days across the same winter and compare the results.

Daily heat demand is affected by more than just outdoor temperature, including:

• solar gain through windows

• cooking and oven use

• people, pets, and general occupancy

• appliances and background heat from electrical loads

• windy conditions, which can increase heat loss through infiltration

Looking at multiple days helps smooth out these effects and gives a clearer picture of the home’s underlying heat loss.

The input data (what each field means)

Gas used on the coldest day (kWh)

The total gas used over a full 24-hour period on a cold day.

For best results:

• the heating should be on most or all of the day
• the house should already be up to temperature
• outside temperatures should be around 0 °C or below

Smart meters and supplier apps usually show daily gas totals.

Boiler efficiency (%)

Gas boilers don’t convert all the used gas into useful heat.

If you don’t know your actual efficiency:

• 80–85% is a sensible default for many modern boilers
• older systems may be lower

This converts gas input into useful heat delivered to the house.

Hot water energy that day (kWh)

This removes domestic hot water from the total so the calculation reflects space heating only.

As a rough guide, the following amount of gas is required to take water from 10 °C to 50 °C (assuming 85% efficiency).

• 100L (5.5 kWh)
• 150L (8.0 kWh)
• 200L (10.7 kWh)
• 250L (13.3 kWh)
• 300L (16.0 kWh)

Average indoor temperature (°C)

The actual temperature inside the house on the day the gas usage was recorded.

For the most reliable results, try to keep the indoor temperature steady for the full 24 hours. This makes the gas usage reflect ongoing heat loss rather than energy used to warm the building back up.

If the house drops to, say, 15 °C overnight and is then boosted to 20 °C during the day, the simple average might be 17.5 °C. However, that day’s gas use will also include extra energy needed to reheat the building fabric, which can skew the result.

Also, the tools works best when all rooms in the house are heated to the same temperature.  If you have rooms that are ‘switched off’ and cold, then this will affect the outcome.

Average outside temperature (°C)

The average outdoor temperature for the same 24-hour period.

Weather apps and websites like Weather Underground are good sources for daily averages.

Cold, overcast days where the temperature stays low throughout the day tend to give the most reliable results. Days that swing from 0 °C up to 6 °C, especially with sunshine, can distort the average due to solar gain.

Target indoor temperature (°C)

The indoor temperature you want the house to hold on a cold design day.

The MCS calculations use 21 °C.

Choosing a design outside temperature

The design outside temperature (DOT) represents a cold, worst-case condition, not a typical winter day. It’s used to estimate how much heat your home needs during the coldest periods.

Colder inland and northern areas use lower values, while milder coastal areas use higher ones.

Typical UK examples (illustrative):

• Scotland (inland): −4 °C to −6 °C

• Newcastle: around −4 °C

• Manchester: around −3 °C

• Birmingham: around −2 °C

• Oxford: around −2 °C

• Penzance: 0 °C to +1 °C

Local conditions matter too. Homes on higher ground or in exposed locations can be colder than nearby sheltered areas. If that applies to you, it’s reasonable to test a slightly lower value.

If you’re unsure, the calculator defaults to −3 °C, which is a sensible middle ground for much of England.

An experienced heat pump installer should be able to choose the correct DOT for your home and location.

Heat loss calculator

Tip: For best results, run this using several cold days and compare the outputs.

How the calculation works (with example)

The calculator follows the same steps every time. Here’s the working using the default values.

Step 1: Convert gas into useful heat

Gas used: 118 kWh
Boiler efficiency: 85%

118 × 0.85 ≈ 100 kWh of useful heat

Subtract hot water:

100 − 8 = 92 kWh for space heating

Step 2: Convert daily heat into average power

92 kWh ÷ 24 hours = 3.83 kW

This is the average heat output required to hold the house at temperature on that day.

Step 3: Derive the heat loss rate (W/°C)

Indoor temperature: 21 °C
Outdoor temperature: 0 °C

Temperature difference:

21 − 0 = 21 °C

Heat loss rate:

3,830 W ÷ 21 = 182 W per °C

This means the house loses about 182 watts for every degree difference between inside and outside.

Step 4: Apply design temperatures

Target indoor temperature: 21 °C
Target outside temperature: −3 °C

Design temperature difference:

21 − (−3) = 24 °C

Design heat loss:

182 × 24 = 4,368 W

Final result: ~4.4 kW whole-house heat loss

What size heat pump do I need?

This is usually the first question people ask, and it’s the one this tool is designed to help with.

Heat pump sizing is driven by heat loss, not floor area or the size of your existing boiler. The design heat loss figure calculated above represents how much heat your home needs on a cold day.

For example, if your design heat loss is around 4.5 kW, then a heat pump capable of delivering “at least” that much heat at design conditions should be sufficient for space heating.

Further Reading (how much electricity does a heat pump use?)

Once you know your whole-house heat loss, you can estimate how much electricity a heat pump would use across different outdoor temperatures.

This is covered in more detail in How Much Electricity Does a Heat Pump Use?, which shows why electricity use isn’t linear with temperature and how heat loss feeds directly into those calculations.

How Much Electricity Does a Heat Pump Use?

And for more renewable and heat pump articles, please head over to the blog section of the website where you will find lots of extra material.

Renewables & Heat Pump Guides | Energy Stats UK Blog

Important Limitations

This free heat loss survey tool provides a whole-house estimate, not a room-by-room calculation.

It’s best used to:

• get ballpark heat loss figures for your home
• sense-check installer figures
• understand whether all numbers are in the right ballpark
• build confidence before moving to detailed design

A full whole house and room-by-room heat loss calculation is still required for heat pump system design and installation.

Frequently asked questions

Is this a real heat loss survey?

It’s a whole-house estimate based on real gas usage, not a room-by-room survey. It’s intended as an early planning and sense-check tool.

Can this tell me what size heat pump I need?

It gives you the key input: whole-house design heat loss. That’s the number used to size heat pumps, alongside hot water and system details.

Do I need a heat pump already to use this?

No. This tool is designed for homes with gas boilers that want to estimate heat loss before switching.

How accurate is the result?

Accuracy improves when you run it across several cold days and compare the outputs. One day gives a useful estimate; multiple days give a clearer picture.

Does this replace an MCS heat loss calculation?

No. This tool is a practical cross-check using real-world data but it does not replace and proper full house and room by room heat loss by an experienced installer.