Switching the Communal Heating Back On: The Silent Trigger Behind October Infestations in the Paris Region

There is one date nobody writes down in their pest-control calendar, and yet it explains a good half of our November call-outs: the day the building's boiler room starts up again. In the Paris region, this usually happens between 15 September and 15 October, depending on the building's rules and the weather. On that day, an entire building changes state. The risers warm up, the service ducts go from 16 to 24 °C, the cellars and crawl spaces emerge from their summer torpor.

For residents, it's a non-event. For German cockroaches, bed bugs and house mice, it's the most powerful "go" signal of the year. We see the result three to five weeks later, when the calls change tone: "we hadn't seen a thing all summer, and now they're everywhere."

Brown rat with greyish-brown fur moving through the green grass of a garden

What switching the heating back on really changes in a building

Temperature, not dirt, sets the biological pace

We often hear that pests turn up "when a place is dirty". In nine buildings out of ten, that's simply wrong. What governs the speed of an infestation is the ambient temperature of hidden areas, because it directly controls how long an insect's development cycle takes.

Take the German cockroach (Blattella germanica), the dominant species in French multi-occupancy housing. Going from egg to adult takes roughly 100 days at 22 °C, but drops to around 50 to 60 days at 30 °C. In other words: a plant room that goes from 18 to 27 °C because a hot-water riser runs through it sees its population potential double for exactly the same reproductive effort.

The heating network is a transport network

A riser doesn't pass through a wall, it passes through every stacked flat. The hole drilled for the pipe is almost always wider than the pipe itself, and the original sealing — compacted mineral wool, cracked plaster, aged expanding foam — degrades over twenty years. The result: a stairwell service duct links the ground floor to the sixth floor vertically without a single serious obstacle.

This explains the pattern we see during surveys: an infestation affecting flats 12, 22, 32 and 42 — in other words the same vertical stack — while sparing next-door neighbours on the same landing. The problem isn't the neighbour, it's the duct.

An adult German cockroach squeezes through a 1.5 mm gap; a house mouse (Mus musculus) gets through a 6 mm hole, roughly the diameter of a pencil. On a building-wide scale, poorly sealed service penetrations are wide-open doors.

Three pests, three ways of exploiting the restart

Cockroaches: summer's invisible population becomes visible

Many buildings believe they have "solved" their problem because the summer was quiet. In reality, cockroach colonies retreat in summer to the most stable areas: the boiler room, domestic hot-water risers, the back of rubbish chutes. They don't disappear, they concentrate. Switching the heating back on reopens the whole building to them, and dispersal towards kitchens and bathrooms begins again.

The early sign to know isn't the insect itself, but the droppings: tiny black specks scattered like pepper, often along the hinges of wall units, behind the dishwasher, or on top of kitchen cupboards. A thorough torch-lit inspection of warm, dark corners tells you more than a week of passive observation — a powerful LED torch is the single most cost-effective tool for a DIY survey.

Bed bugs: the nocturnal heating rebound

Bed bugs (Cimex lectularius) don't migrate through heating ducts, but they are extremely sensitive to temperature. Below 18 °C, their biting activity slows sharply and the interval between blood meals lengthens. Many households report a "lull" at the end of summer, particularly in homes left empty over the holidays.

When the heating comes back and the bedroom settles durably at 20-22 °C with an occupant present every night, the starving bugs resume their normal rhythm. Hence a very common paradox: bites that "come back" in October when nothing new has been brought into the home. It isn't a new infestation, it's the same one, reactivated.

At this stage, objective monitoring beats memory: bed bug mattress encasements fitted to the mattress and base block off harbourages and make any remaining bug visible on the white surface, while interception traps under the bed legs confirm a presence without dismantling the bedroom.

Modern bedroom with a bare mattress and no bedding, purple headboard, beige wardrobe and open bathroom door

Rodents: the end of the comings and goings

As long as outdoor temperatures stay mild, brown rats (Rattus norvegicus) move between sewers, green spaces and the surroundings of buildings. They come in, they go out. From mid-October, two things happen at once: outdoor food sources dwindle, and heated basements become far more attractive than the sewer network.

That's the moment you shift from a passing-through problem to a settling-in problem. A population that settles nests, breeds and damages electrical conduits and pipe insulation. The clues to look for in communal areas: dark greasy smear marks along cellar corridor skirting, droppings shaped like dark grains of rice, gnawed packaging, and above all a scurrying noise in suspended ceilings when the heating is running.

The September calendar: what to do now

The useful window is short. Acting before the heating restarts costs three to five times less than curative building-wide treatment in December.

1. Take stock of the service penetrations (week 1)

Before the heating comes back on, the managing agent and residents' committee can walk the accessible ducts, inspection hatches and plant rooms. Every unsealed pipe penetration should be recorded. Filling is done with materials pests can't get through: mortar, filled foam, or, for small gaps, stainless steel wool mesh packed in before rendering. Plain acrylic sealant is gnawed through within weeks.

2. Deal with the bin store before, not after (weeks 1-2)

This is the nerve centre of any building. Closed bins, a washable floor, no stored cardboard, a floor drain trap kept filled with water. A bin store backing onto a boiler room and left dirty is a nursery that feeds the whole building for six months.

3. Inform residents intelligently (week 2)

A clear notice beats a meeting. It must say three things: what to report, to whom, and within what timeframe. Under-reporting is the number one cause of failure — one occupant in two waits until they're certain before speaking up, which means waiting until the population is established.

4. Set up monitoring markers (week 3)

A few non-toxic detection traps, placed at identified hotspots — behind the boiler room, the bike store, the first-floor duct — give an objective picture of the situation before winter. Sticky detection traps for crawling insects cost a few euros and turn a suspicion into hard data.

5. Redo seals and thresholds inside the flats (week 4)

On the private side, the useful measures are modest but real: seal the pipe penetrations under the sink and behind the toilet, fit a brush draught excluder to front doors and cellar doors, check the ventilation grilles. None of this eliminates an existing infestation, but all of it slows colonisation down.

Modern bedroom with a double bed and bare mattress, grey headboard, light wood wall and beige curtains

What you absolutely must not do when the heating goes back on

Don't spray aerosol insecticide into the service ducts. It's the single most expensive reflex there is. A repellent aerosol applied in a duct kills only a fraction of the individuals and scatters the rest into neighbouring flats. Every autumn we pick up cases where an infestation confined to two flats ended up affecting eight after exactly this kind of initiative.

Don't throw out furniture at the first suspicion of bed bugs. Carrying a mattress down the stairwell without wrapping it means sowing the infestation along the entire route. If disposal is decided, the item must be wrapped on the spot in a sealed plastic cover and rendered unusable.

Don't confuse treatment with cleaning. An immaculate building with open ducts and a boiler room at 28 °C will stay infested. A modest building with properly sealed penetrations holds up very well.

Don't treat flat by flat. This is the hardest lesson to get across to residents' associations. On a shared heating stack, an isolated treatment is like emptying a bath while the tap is still running.

The legal framework: who pays for what

Responsibility follows a simple logic, set out in the Act of 6 July 1989 and the ELAN Act for the decent-housing aspects:

Since the Act of 27 July 2023, known as the "anti-squat" law, Article 6 of the 1989 Act specifies that the landlord must hand over a home free from infestation by pest and parasitic species. In practice, in the case of a building-wide infestation, the only effective route remains a coordinated intervention decided by the managing agent, with access to private areas arranged within grouped time slots.

What a proper building survey looks like

A genuine survey doesn't mean spending ten minutes in a flat. It includes:

• a review of the technical areas (boiler room, ducts, crawl space, bin store, risers);
• identification of evidence by vertical stack, not by landing;
• a map of unsealed penetrations;
• the installation of detection devices with readings at 7 and 21 days;
• a phased treatment plan with a set follow-up date.

For cockroaches, the reference method remains gel bait applied at multiple points along movement routes, combined with an insect growth regulator and successive readings. For bed bugs, heat treatment or a steam + residual insecticide combination, always with at least a second visit three weeks later to cover egg hatching. For rodents, sealing is the absolute priority, chemical control being only a tightly regulated add-on — ANSES regularly restates the restrictions on anticoagulant use and the risks of secondary poisoning of wildlife.

For flat owners who want to understand the levers available to them before appointing a professional, a practical guide to running a residents' association saves a lot of deadlock at the general meeting: most treatment delays stem from disagreement over cost allocation, not from disagreement over the diagnosis.

Unmade bed with a duvet and crumpled white sheets, seen from above

Key takeaways

• Switching communal heating back on, between mid-September and mid-October, mechanically accelerates insect life cycles and encourages rodents to settle.
• Infestations spread vertically, along stacks and ducts, not horizontally from landing to landing.
• September is the only window in which you can act cheaply: sealing, bin store, detection.
• Aerosols in ducts and isolated treatments make the situation worse instead of fixing it.
• A building-wide plan coordinated by the managing agent remains the only strategy that holds up over time.

If your building has had even the slightest episode of cockroaches, bed bugs or rodents in the past two years, the next three weeks are worth more than the three months that follow. A survey before the heating restarts allows you to treat a population still concentrated in a few square metres of plant room, rather than one dispersed across thirty heated flats.

Sources and references: ANSES (fact sheets on cockroaches, bed bugs, and anticoagulant rodenticide use), Ameli (fact sheet "Punaises de lit : que faire ?"), the ministry responsible for Housing (Act no. 89-462 of 6 July 1989 as amended, the ELAN Act, the Act of 27 July 2023), Chambre syndicale 3D for professional best practice.