Ventilation & heat recovery — Petts Wood, Orpington & Chislehurst BR5, BR6
The housing round here is about the best in London for heat recovery
Most of what we survey is a compromise. Petts Wood and the 1930s stock around Orpington is the exception: generous lofts, sensible roof pitches, wet rooms stacked over each other and room to run ducts where they should go rather than where they will fit. If you have ever been told a whole-house system is not realistic in a normal semi, this is the housing that proves otherwise — with one detail that has to be right.
Petts Wood was laid out on the garden suburb principle from the late 1920s by the developer Basil Scruby, with the architect Leonard Culliford supervising and Noel Rees as master builder. The rules Scruby imposed — generous plots, controlled building lines, controlled roof heights, good materials — are why the area still looks coherent. They are also, entirely by accident, why it is such good ventilation stock.
Four things follow from it, and together they are the difference between a straightforward installation and a fight:
- The lofts are big and reachable. A decent pitch over a full-width plan gives real standing room near the ridge, which is where a heat recovery unit wants to hang. You can get to it to change a filter, which decides whether it is still working in five years.
- The plan is regular. Bathroom over kitchen, or bathroom over hall, stacked the way these houses were laid out — so a riser has a partition to drop down that agrees on both floors instead of crossing a room.
- There is somewhere to put the terminals. Deep eaves overhangs take a soffit terminal that is close to invisible, and the gable end on a semi gives a clean run out at high level.
- Ceilings have not usually been meddled with. No downlights punched through a sealed ceiling every 800mm, which is both a duct route and an air leakage path into the roof.
Petts Wood West, which began in 1933, is generally plainer and more standardised than the earlier east side — two builders rather than the forty-odd who worked on the first phase. For our purposes that is good news: standardised plans mean fewer surprises above the ceiling.
The one detail that has to be right
Get this wrong and the loft is where the condensation ends up — which is a worse problem than the one you started with.
Both outdoor runs must be insulated
A heat recovery unit in a loft has two ducts to the outside world: the intake bringing cold air in, and the exhaust taking warm wet air out. Both cross a cold, ventilated roof space. Warm moist air inside a cold duct condenses on the inside of it, runs back down and drips — onto the insulation, the ceiling, or into the unit. Every outdoor run we install here is lagged as standard rather than as an extra.
The condensate has to fall the whole way
A counter-flow exchanger cools the outgoing house air below dew point, so it produces water. That drain needs a trap and a continuous fall to somewhere sensible — usually out through the soffit into the gutter. A pipe that dips anywhere along its length holds water, and a pipe holding water in a cold loft is a pipe that freezes.
The loft floor stays usable
"Do I lose my loft?" is the first question anybody asks, and the answer here should be no. The runs ride up under the rafters and down the slope to the eaves rather than lying across the insulation where you want to board out. It is a slightly longer route with a couple more bends, which the fan is sized for.
And the eaves stay clear
The roof void depends on air moving through it at the eaves. Insulation pushed out into them — which is exactly what happens every time somebody tops it up — blocks that path, and then the roof itself starts to sweat. We check it on every survey here whether or not you are buying ventilation from us.
Two things about the area worth knowing
Petts Wood is an Area of Special Residential Character. That is a Bromley planning designation rather than a conservation area, and the two are not the same thing — but the practical effect is similar enough to plan around: the coherence of the street scene is protected, so a terminal on a front elevation is worth avoiding on principle. In this stock it very rarely has to be there anyway. The eaves and the rear elevation deal with almost every case, which is one more reason this housing is easy to work with.
Not every house here needs a whole system, and we will say so. A 1930s semi with a working extract in the bathroom and the kitchen, and trickle vents that are not painted shut, may need nothing more than putting those right. Heat recovery earns its money where the house has been properly insulated and sealed, where the bedrooms are showing damp as well as the wet rooms, or where somebody wants the warmth back out of the air they are throwing away. The survey tells the two apart. If yours is the first one, that is what the report will say.
Questions we get asked in Petts Wood and Orpington
Yes, and this is the housing type where it is genuinely straightforward rather than a fight. What makes it work is the loft: a decent pitch over a full-width plan gives real standing room near the ridge for the unit, and room to run ducts properly. The plan helps too — these houses stack their wet rooms sensibly, so a riser can drop inside a partition that agrees on both floors instead of crossing a room. The awkward retrofits are flats with no loft at all, and houses where the ceilings are full of downlights and the roof space is full of a conversion. A standard Petts Wood or Orpington semi is neither.
For a two to three bedroom house, retrofit, expect £6,000 to £8,500 including VAT, supplied, installed, commissioned and measured with the figures for every terminal in writing. A larger four-bedroom house runs £9,000 to £13,000. 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 — there is a real spread in efficiency, running noise and what the filters cost each year — and how much making good is needed. If you are doing the work anyway, before the boards go on, it is roughly 25 to 30 per cent less, which is the best argument there is for ringing early.
You should not, and it is the first thing worth designing around. Ducts can be run up under the rafters and down the slope to the eaves rather than laid across the insulation where you want to board out. It is a slightly longer route with a couple more bends, so the fan is sized for the resistance it actually faces — but it keeps the floor of the roof space clear and it keeps the runs out of the way of anything you store up there. The unit itself hangs off the structure near the ridge where the headroom is, and it needs to stay reachable, because a filter you cannot get to is a filter that never gets changed.
Because two of the four runs carry outdoor conditions across a cold space. The intake brings cold air in and the exhaust takes warm, wet house air out, and both cross a ventilated roof void that is close to outside temperature all winter. Warm moist air inside a cold duct condenses on the inside of it, collects, and runs back down — onto the insulation, through the ceiling, or into the unit. It is the single most common reason a loft-mounted system causes a problem instead of solving one, and it is entirely preventable. Both outdoor runs are lagged as standard on everything we install here, not offered as an upgrade.
It is worth planning around rather than worrying about, and it is worth being precise: an Area of Special Residential Character is a Bromley planning designation, not a conservation area, and they are not the same thing. What it means in practice is that the coherence of the street scene is taken seriously, so a visible terminal on a front elevation is something to avoid on principle. In this housing that is rarely a constraint at all — the deep eaves overhangs take a soffit terminal that is close to invisible from the pavement, and a semi's gable end gives a clean high-level run out at the side. We design for the discreet route by default and tell you where anything will be seen from before you agree to it.
Quite possibly not, and we would rather say so than sell you something. If the bathroom and kitchen extracts genuinely move what they should, the trickle vents are not painted shut and nothing is going mouldy, a whole-house system is a comfort and efficiency purchase rather than a fix — and it should be sold to you on those terms or not at all. Where it does earn its money is where the house has been properly insulated and sealed and now feels stuffy, where bedrooms are showing damp in the corners as well as the wet rooms, or where you are about to do building work anyway and it would cost a third less to do it now. An hour on site tells us which of those you are.
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.