A flat that stays cold despite the heating being on is not usually a heating failure. It is usually one of four specific problems, and they are diagnosable in an evening with a hand and a candle.
The order below is by cost, cheapest first, because the cheap fixes solve most cases.
Diagnosis: the four causes
1. Draughts (cold air entering). Test: hold a lit candle or a wet finger near the edges of windows, doors, skirting boards, and any pipe where it enters a wall. A flickering flame or a cold spot indicates a draught.
2. Single glazing (heat leaving). Test: the inside of the window glass is cold to the touch, and condensation forms on it. Single-glazed windows lose heat roughly twice as fast as double.
3. Radiator problems (heat not distributed). Test: with the heating on, feel each radiator top and bottom. Cold at the top = air in the system. Cold at the bottom = sludge. Cold throughout = valve or boiler issue.
4. Damp (moisture absorbing heat). Test: a musty smell, condensation that does not clear, or patches on external walls. Wet walls conduct heat away far faster than dry ones — a damp wall can feel 5 °C colder than a dry one at the same air temperature.
The order matters: fix damp first if present, because it defeats everything else. Draughts second (cheapest). Radiators third. Glazing last (most expensive).
Fix 1: draughts — under $30
Draughts account for a surprising proportion of heat loss in older flats, and the fixes are trivial.
| Location | Fix | Cost |
|---|---|---|
| Bottom of doors | Draught excluder (brush or foam) | $8–15 |
| Letterbox | Letterbox flap or brush seal | $8–12 |
| Keyhole | Escutcheon cover | $5 |
| Window frames | Self-adhesive foam strip | $8–15 |
| Chimney (unused) | Chimney balloon | $25–45 |
| Pipe entry points | Silicone or foam filler | $8 |
Total for all six: about $60, and in many flats this alone makes a noticeable difference.
The chimney balloon deserves a mention. An open flue in an unused fireplace can lose as much heat as an open window. A balloon costs $25–45 and is the single highest-impact draught fix in a period flat.
The caution: do not block ventilation in a room with a gas appliance or an open fire that is used. Chimney balloons are for unused flues only, and blocking ventilation in a room with a boiler can be dangerous.
Fix 2: radiators — under $20
Bleeding a radiator (cold at the top) is a five-minute job:
- Turn the heating off and let radiators cool
- Hold a cloth under the bleed valve at the top corner
- Turn the valve key anticlockwise a quarter turn — air hisses out
- Close it as soon as water appears
- Check the boiler pressure afterwards; you may need to top it up
Cost: a radiator key is $3–5.
Why it matters: a radiator that is half full of air is a radiator that is half working. This is the most common cause of "the heating is on but the room is cold".
Radiator reflector panels (foil behind the radiator, on external walls) cost $10–20 per radiator and reflect heat back into the room instead of into the wall. Effectiveness is debated — measured savings are in the 5–10% range, and only on external walls.
The sludge case (cold at the bottom): this needs a power flush, which is a plumber's job at $200–400. Before that, a chemical cleaner added to the system ($15–25) sometimes resolves mild cases.
Fix 3: single glazing — $20 to $500
The cheap option: secondary glazing film. A clear plastic film taped to the frame, shrunk tight with a hairdryer. Cost: $10–20 per window.
Effectiveness: it creates an air gap, which is what double glazing does. Measured effect is a real reduction in heat loss, though it does not insulate as well as actual secondary glazing.
The drawback: windows cannot be opened without damaging the film, so it is a winter-only solution. And it looks like plastic film, because it is.
The middle option: heavy curtains. Thermal-lined curtains, hung close to the window and overlapping the frame, cost $40–120 and reduce heat loss meaningfully — especially with a pelmet or a blind above them.
The detail that matters: curtains must not cover the radiator. A curtain in front of a radiator traps heat between the fabric and the window, which loses more than it saves. Short curtains or a blind are better when the radiator is under the window.
The expensive option: secondary glazing panels or window replacement. $200–500 per window. Effective, but rarely possible in a rental.
Fix 4: damp — variable cost, highest priority
Damp defeats heating, because wet walls conduct heat away. Fix the damp and the flat feels warmer at the same thermostat setting.
Diagnose the type first:
| Sign | Type | Cause |
|---|---|---|
| Patches on external walls, worse in winter | Condensation | Warm moist air meeting cold surface |
| Tide marks rising from the floor | Rising damp | Failed damp course |
| Localised patches, often near a pipe or gutter | Penetrating damp | Water entering from outside |
| Black mould in corners, behind furniture | Condensation | Poor ventilation |
Condensation is the most common in flats, and it is behavioural rather than structural:
- Ventilate — open windows briefly but fully (10 minutes with a wide opening beats 2 hours with a crack)
- Dry laundry with a window open, or in a ventilated bathroom
- Keep the flat heated to at least 16 °C (cold air holds less moisture before condensing)
- Wipe condensation from windows daily in cold weather
- Keep furniture 5 cm off external walls so air can circulate
The counter-intuitive point: heating a flat more often reduces condensation, because warmer air holds more moisture before it condenses. A cold flat with normal cooking and washing produces more mould than a warm one.
Rising and penetrating damp are landlord issues. Report them in writing; they are structural.
The heating cost comparison
Approximate running costs for a 40 m² flat, per winter month (varies hugely by tariff and climate):
| Measure | One-off cost | Effect |
|---|---|---|
| Draught sealing (all) | $60 | Meaningful, felt immediately |
| Chimney balloon | $30 | Large in period flats |
| Bleeding radiators | $5 | Restores lost output |
| Thermal curtains | $80 | Moderate |
| Secondary glazing film | $15/window | Moderate |
| Radiator reflectors | $15 each | Small |
| Keep flat at 16 °C minimum | Ongoing | Prevents damp |
The combination that costs under $100: draught sealing, radiator bleeding, and a chimney balloon if there is a flue. In most flats that produces a noticeable improvement, and it is cheaper than a month of extra heating.
The thermostat strategy
Running heating at a constant low temperature is generally cheaper and more comfortable than intermittent blasting, for two reasons: it keeps the walls dry, and it avoids the reheating cost of a cold building.
| Strategy | Cost | Comfort | Damp risk |
|---|---|---|---|
| 16 °C constant | Moderate | Good | Low |
| 18 °C morning and evening | Moderate | Good | Low |
| Off during the day, 21 °C in the evening | Higher | Poor (cold surfaces) | Moderate |
| Off most of the time | Lowest heating | Bad | High |
The last option is false economy. A flat kept cold grows mould, and mould remediation costs hundreds.
The honest summary
A cold flat usually has draughts, air in the radiators, single glazing, or damp — and the first two cost under $30 to fix.
The single most common cause of "the heating is on but the room is cold" is an unbled radiator. Five minutes with a $4 key.
The single most expensive mistake is keeping the flat cold to save money: the mould that results costs far more than the heating saved.