Ventilation & heat recovery — Chislehurst BR7
Big houses, long duct runs, and the room at the end of them
Chislehurst has some of the largest housing we work on, and it brings a problem the smaller stock never has: distance. On a long run, the number on the side of the fan and the number at the far grille are not the same number — and the room that suffers is always the one furthest from the plant. That is a design problem, not an installation one, and it is solved before anything is ordered.
Every metre of duct, every bend, every grille and every insect screen takes something off what a fan can actually deliver. On a short run out through a wall that hardly matters. On a large detached house, where the plant is in the loft and the furthest room is most of the length of the building away, it decides whether the system works.
It is not the same problem scaled up. Resistance does not rise politely in step with length, and it is bends and restrictions that do most of the damage rather than straight duct. The consequence is always the same and always predictable: the room at the end of the longest run is the one that never quite gets there. Usually a top-floor bedroom or an en-suite over a garage.
Which is why a large house is designed rather than fitted. The duct sizes, the route, the number of bends and the fan are chosen together against the actual plan, and then every terminal is measured and adjusted when it is in.
What we do differently on a big plan
Size the duct for the run, not the fitting
The temptation on a long run is to keep the duct the size of the spigot on the terminal. Going up a size on the main runs costs a little more in material and a little more space in the void, and it buys back a great deal of the resistance a long route imposes — because moving the same air more slowly through a bigger pipe is far cheaper in pressure than forcing it through a small one.
Spend the effort on the bends
A tight bend costs far more than the metre of straight duct it replaces. On a long run the route is planned to sweep rather than turn, and to turn as few times as the structure allows. This is most of what separates a system that hits its rates from one that is permanently disappointing.
Balance it, terminal by terminal
A branched system does not distribute itself fairly. Left alone, the nearest rooms take more than their share and the far ones take what is left. Balancing means adjusting every terminal until each room gets its design rate, then measuring and recording all of them. On a big house this is a real part of the day rather than a five-minute check.
Insulate every run across the cold loft
Large houses mean large roof spaces, and both outdoor runs cross them. Warm wet air inside a cold duct condenses on the inside of it and runs back down. Every outdoor run is lagged as standard, and the condensate drain is set to fall continuously to somewhere sensible — a pipe that dips holds water, and water in a cold loft freezes.
Look for the cold bridges at the joists
On these houses the mould we are called to very often appears in straight lines across a ceiling or in a band at the wall head. That is the structure showing through — timber conducting heat differently from the insulation between it, or insulation that stops short. It is worth naming because it changes what fixes it: ventilation deals with the moisture, but the pattern is telling you where the building is cold.
Plan for the filters being changed
A unit in a large loft is only serviceable if somebody can reach it. Boarding, a light and a clear route to the unit are worth insisting on at installation, because the alternative is a system whose airflow quietly falls away for years. It is the cheapest thing on this list and the one most often left out.
Older, larger and often listed or in a conservation area
A good deal of the stock here is old enough and distinctive enough to carry constraints on what may appear on an elevation. Where that applies, the same rules we use in Dulwich apply here: favour the rear elevation, use the soffit where the overhang allows it, work back to an opening that already exists, and say up front where anything will be visible from. We are not planning consultants and will not tell you what will be approved — we will give you the options in writing with the airflow consequence of each.
The other thing worth saying plainly: a big house does not automatically need a big system. Plenty of the properties we survey here need a properly specified extract in two or three wet rooms and background ventilation that works, and nothing else. Whole-house heat recovery earns its money where the building is genuinely airtight, where bedrooms are showing damp as well as bathrooms, or where the house is being renovated anyway and the ducts can go in before the ceilings do. The survey is what tells those apart.
Questions we get asked in Chislehurst
Almost always because it is at the end of the longest duct run, and the system has never been balanced. A branched system does not distribute itself fairly on its own: the rooms nearest the plant take more than their share and the far ones take what is left. On a big house that difference is large enough to be obvious to whoever sleeps in the far room. It is usually correctable without new ductwork — balancing means adjusting each terminal until every room gets its design rate, then measuring and recording all of them. If it cannot be balanced, that tells you something real about the duct sizing, and it is better to know.
Yes, and it is not the length that does most of the damage — it is the bends and restrictions. Every metre of duct, every turn, every grille and every insect screen takes something off what the fan can deliver, and resistance does not rise politely in step with distance. The practical consequence is that the figure on the side of the fan and the figure at the far grille are not the same figure, sometimes not close. That is why a large house is designed rather than fitted: duct sizes, route, number of bends and fan chosen together against the actual plan, and then every terminal measured when it is in.
You are looking at the structure through the mould. Timber joists conduct heat differently from the insulation between them, so the plasterboard directly under a joist can run colder than the plasterboard between joists — and mould grows where the surface is coldest and stays damp longest. A band along the top of a wall is usually the same thing at the wall head, or insulation that stops short of it. It is worth naming because it changes the conversation: ventilation deals with the moisture in the air, which is the half that is reliably fixable, but the pattern is telling you where the building is losing heat. We will say which of the two we think is dominant rather than selling you a fan for a cold bridge.
For a four-bedroom house, a full heat recovery system supplied, installed, commissioned and measured runs £9,000 to £13,000 including VAT. Five bedrooms or a genuinely complicated retrofit is £13,000 to £18,000. What moves it within a band is how the ducts can be routed, where the unit can go and how far it is from the outside, which unit you choose, and how much making good is needed. If the house is being renovated anyway and the ducts can go in before the ceilings do, it is roughly 25 to 30 per cent less — which on a house this size is a large enough number to be worth planning around.
Quite possibly not, and a big house does not automatically need a big system. Plenty of the properties we survey here need a properly specified extract in two or three wet rooms and background ventilation that works, and nothing else — and if that is what yours needs, that is what the report will say. Whole-house heat recovery earns its money where the building is genuinely airtight, where bedrooms are showing damp as well as bathrooms, or where you are renovating anyway. Extract fans do nothing at all for a bedroom, so if the damp is in the bedrooms the answer is different from if it is in the shower rooms.
The filters, yes, provided we put the unit somewhere you can actually get to — which is why boarding, a light and a clear route to it are worth insisting on at installation rather than afterwards. It is the cheapest thing on the whole job and the most often left out, and a filter nobody can reach is a filter that never gets changed. What is worth having done properly once a year is the rest: the heat exchanger cleaned, the condensate trap checked, and every valve measured with the figure recorded against last year's. One reading tells you it works today; a reading every year tells you whether it is drifting, and why.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, both sat in August 2026. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.