For a room where people spend long periods, 15°C is generally too cold as a routine winter target. It is below the UK Health Security Agency's advice to try to heat frequently used rooms to at least 18°C. A brief reading of 15°C does not establish an emergency, but who uses the room, how long it stays cold and whether it can be warmed all matter.
What Temperature Should an Occupied Home Be?
UKHSA's cold-weather guidance for England recommends a comfortable indoor temperature and trying to keep frequently used rooms, including living rooms and bedrooms, at least 18°C. This is public-health advice, not a precise boundary between safe and unsafe for every person.
Some people need more warmth. The guidance identifies higher-risk groups including adults aged 65 and over, young children, people with long-term health conditions and those with low mobility. Do not use someone else's tolerance of a cool room as the standard for the whole household.
Can a healthy adult be comfortable below 18°C?
The Met Office notes that an active adult under 65, wearing appropriate clothing, may be comfortable slightly below 18°C. That does not establish 15°C as a suitable long-term target for everyone. Your activity, time spent in the room and individual needs remain relevant.
Is 15°C suitable for a bedroom overnight?
Do not assume one bedroom setting suits all ages and health needs. Met Office guidance highlights the value of at least 18°C overnight for older people and those with existing health conditions, while recognising different needs for healthy younger adults with appropriate bedding. Seek individual advice if you are unsure about a health condition.
First, Check What the 15°C Reading Represents
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A thermostat may display either the target or the measured temperature, depending on its mode. Check which number you are reading. Use a room thermometer according to its instructions and avoid a position directly beside a radiator, in sunlight or in a strong draught.
Record the room, time, reading and heating settings. A single measurement in a hallway should not be treated as the temperature of every occupied room. Repeated readings help explain whether the issue is timing, uneven warmth or a home that cannot reach the desired temperature.
Does a House at 15°C Automatically Develop Mould?
No. Temperature alone cannot establish whether mould will develop. Government ventilation guidance explains that moisture from activities such as cooking and showering can become trapped and condense on cold surfaces when ventilation is inadequate.
Warming the room may help with cold surfaces, but it does not repair a leak or resolve every ventilation problem. Keep intended ventilation working: the government's home ventilation guide advises leaving trickle vents open and unblocked, including in winter, and maintaining extraction systems according to their instructions.
Persistent condensation, damp patches or mould need their cause assessed. Government damp-and-mould guidance stresses that addressing underlying causes is essential; simply removing visible mould is insufficient. In a rented home, report the problem to the landlord or managing agent and keep photographs and dates.
Avoid treating damp as proof of a heating fault. Moisture entry, building defects and ventilation can also be relevant. Equally, advice to heat or ventilate more should not replace investigation of an underlying defect.
What Can You Check if the House Will Not Warm Up?
Check the programme and target temperature
Confirm the active schedule, clock and any away or holiday setting using the normal controls. Note whether the system responds when heat is requested. The guide to heating that does not respond to the thermostat covers that specific symptom.
Compare rooms and radiators
Record whether the problem affects every room, one floor or a single radiator. If one radiator stays cold while others heat, use the guide to a radiator that is not heating up. If the difference follows the floors, see heating that works downstairs but not upstairs.
Review timing and heat retention
Allow for the time the home needs to warm before occupied periods. The article on daily heating hours explains how to build a schedule. If you are weighing continuous against timed operation, see whether leaving central heating on is cheaper.
If rooms warm but cool quickly, consider the wider building as well as the heating equipment. The guide to improving home energy efficiency covers that separate question. Do not block purpose-provided ventilation to reduce draughts.
When Should You Get Help?
Arrange heating assessment if occupied rooms remain cold despite appropriate settings, the system does not respond, or performance has changed unexpectedly. For complete loss of service, the guide to no heating or hot water addresses the next steps.
For an unclear heating problem in London, our heating service is the starting point. Explain the actual room readings, affected areas and how the system behaves. A low temperature alone is not enough to recommend a new boiler, power flush or other specific treatment.
If cost is preventing adequate warmth, ask your local authority about available energy and financial support. If you are concerned that cold is affecting someone's health, seek healthcare advice rather than waiting for a heating appointment.