Siting a Replacement Boiler: Flue Terminal and Condensate

Two measurements decide which wall it ends up on

A twenty-year-old open-flued boiler is sitting on the chimney breast, still firing, still heating the house, and the householder wants the new one in exactly the same place because that is where the pipes are. It is a reasonable request and it is the wrong starting point. The appliance going up in its place needs two things the one coming down never needed: a terminal position that can be measured against every opening, corner, gutter, boundary and neighbouring terminal near it, and somewhere for a continuous trickle of acidic water to run downhill to without ever being asked to run uphill.

Neither of those is settled by where the pipework happens to arrive. They are settled outside the building and under the floor, with a tape, a spirit level and the appliance's own installation instructions, while the old boiler is still on the wall and the plaster is still intact. The failure this exists to prevent is the ordinary one: a straight-swap price on Monday, the old case off at half past eight on Tuesday, and by eleven the discovery that the only position clearing the neighbour's window has no waste under it and no gas leg near it.

The two constraints frequently disagree. A terminal that clears everything sits on the gable; the only drain is on the opposite elevation. A cupboard with a soil stack behind it puts the terminal under a bedroom window. The position that gets built is the intersection of the two, and everything else — gas, electrical supply, water connections, servicing access — follows that intersection rather than choosing it.

The terminal is measured from things, not from the wall

There is no single distance to memorise, which is precisely why this gets done badly. Three documents apply at once and the strictest wins. In England the flue outlet position is governed by Approved Document J, Combustion appliances and fuel storage systems, with the detail in BS 5440-1; in the United States the terminal clearances sit in NFPA 54 / ANSI Z223.1, the National Fuel Gas Code, and in the International Fuel Gas Code. Over all of them sits the appliance's own installation instructions, frequently more onerous and the document an inspector will hold you to. A table of millimetres here would be worse than useless, because the figure differs by document, by edition, by appliance and by whether the flue is fanned or natural draught.

What does transfer between all of them is the list of things you measure from, and that list is longer than most surveys allow for. Openable windows, doors and rooflights. Air bricks, trickle vents and any other ventilation opening, including a neighbour's. Below eaves, below and beside gutters, soil pipes and rainwater downpipes. Above ground level, above a balcony, above a flat roof somebody stands on. Internal corners and external corners, which carry different figures. Boundaries, and any surface facing the terminal across a narrow gap. Other terminals, horizontally and vertically, including the one the neighbour fitted last year. Take each measurement, write it on the survey sheet against the obstruction it belongs to, and do it standing outside rather than from a photograph.

One rule is absolute in every document and is broken constantly: a room-sealed flue terminates outdoors. A carport with a roof over it, a covered side passage between two houses, an enclosed light well, a lean-to that started as a canopy and acquired sides — as far as the flue is concerned these are inside the building. Structures like these appear after a boiler was installed far more often than before it, which is why the elevation you are surveying may already carry a fault that has nothing to do with your work.

Then there is what the terminal has to survive. Within reach of a path, a drive, a garden used by children, or anywhere a ladder is likely to be leaned, it needs a guard, fitted at the clearance the maker states rather than pushed up against the cowl. This rarely changes the position on its own, but it belongs on the quotation as an item, because a guard argued about afterwards is a guard nobody gets paid for.

The hole that is already there proves nothing

The existing penetration is evidence of what was once acceptable, not of what is acceptable now. The terminal position rules have moved, editions have been amended, and — much more commonly — the elevation has changed around it. A single-storey extension went up next door. A window was added to the return wall. A gutter was replaced with a deeper profile. A fence became a wall. The previous installation may have been entirely correct on the day it was commissioned and fail the same table today, and nothing about a new appliance inherits the old one's position by right.

The flue itself rarely transfers either. An open-flued boiler venting into a lined chimney is a different arrangement altogether from a room-sealed condensing appliance, which needs a sealed path for both combustion air and products of combustion. Where a chimney is genuinely reusable it is reusable as a route — a shaft to pass a flue system up — and only with the system the appliance maker lists for it. A liner sized for the old appliance, or a proprietary system from another manufacturer that happens to share a diameter, is not an alternative. That is settled by reading the instructions and looking up the chimney with a torch, not by discovering on the day that the kit on the van does not go where the old flue came out.

Plume is a constraint nobody quotes for

A condensing appliance discharges flue gas at low temperature and close to saturation, so on any cold morning it produces a visible plume. Nobody who has not had one next door expects it. The complaint arrives about the sight of it; the damage arrives from the water in it, landing on render, on painted timber, on a fence panel, on a path that then ices over at the same spot every winter.

Where the plume goes is a separate question from whether the terminal clears. A terminal can satisfy every distance on the table and still discharge across a shared footpath, into a passage the neighbour uses daily, or at a boundary they are about to build up to. Stand where the discharge will be and look at what is in front of it for the next few metres. That is in no table, and it is the thing that generates the phone call.

The manufacturer's answer is a plume management kit — a duct that carries the discharge away from the flue terminal to a new outlet, usually higher or around a corner. Two consequences follow and both get missed. The clearances are re-measured at the new discharge point, because that is now where the products leave; and the kit consumes part of the appliance's equivalent flue length allowance, which may be the allowance you were already spending on bends. A plume kit added as an afterthought to a flue run already at its limit is not an addition, it is a redesign.

Flue length, bends, and the void it would cross

Every appliance has a maximum equivalent flue length in its instructions, and every bend is deducted from it at a stated rate. An external wall spends almost none of that allowance. An internal wall chosen because it suits the kitchen units can spend the whole of it crossing a ceiling to reach daylight. Count the bends on the survey and do the subtraction there — the answer occasionally rules out a position outright, and that is much cheaper to learn with the wall shut.

Concealment is the second cost of an internal position, and it is a joinery cost rather than a heating one. A flue inside a ceiling void, a boxed chase or a bulkhead has to remain inspectable along its length, which means access hatches where the guidance requires them — at joints and changes of direction — rather than wherever the boxing is easiest to cut. Approved Document J and the appliance instructions between them set what is needed, and nobody enjoys explaining after the fact that a new bulkhead needs three hatches in it.

The flue also has a fall, and it runs the opposite way to instinct. On a condensing appliance a horizontal flue is set to fall back towards the boiler so that condensate forming inside it drains into the appliance's own trap rather than dripping out of the terminal onto the wall below. A vertical flue cannot do that, so it gets a condensate collector at its base — and that collector is a second condensate connection to find a route for, on top of the one coming out of the boiler. A loft or vertical-flue position therefore carries two drains, not one, which is the detail that most often turns a tidy loft installation into an untidy one.

Candidate positions, and what each one drags in

By the end of a survey two or three positions are usually still standing, and the choice between them is rarely about the boiler. It is about what each one obliges you to build. The right-hand column below is the part that belongs on the quotation, because it is the part that turns into a variation when it is discovered on the day instead.

Read it as a prompt rather than a ruling. A loft is a poor position in a bungalow with no permanent access and a reasonable one in a house that already has a fixed stair and a boarded floor. What does not change is that every row costs something, and the row nobody priced is the row that loses money.

Settle it with a tape and a torch, before the case comes off

None of this needs an instrument that is not already in the van, and all of it belongs on the visit where the price is agreed rather than the visit where the work is done. The sequence below is the survey in the order that saves a walk: outside first, because the terminal rules out more positions than anything else; then the flue route; then the space; then the two services; then the drain.

Do it with the appliance's own instructions open. On a swap the customer's expectation is set by the thing already on the wall, and the moment to reset it is while you are both standing in the room with a tape in your hand, not after the plaster is off.

1. Get the proposed appliance's installation instructions before quoting — terminal table, equivalent flue length, compartment clearances and condensate requirements all live in them, and they can be stricter than the code.
2. Mark the terminal on the outside face and measure from it to every opening, vent, corner, gutter, soil pipe, boundary and neighbouring terminal, recording each figure against the obstruction it belongs to.
3. Stand where the discharge will be and note what the plume lands on, who sees it, and whether it crosses a boundary or a path.
4. Trace the flue route back inside, counting bends and noting every void or boxing it would need a hatch in.
5. List every other fuel-burning appliance sharing the space and mark which are open-flued, before deciding what happens to the existing ventilation openings.
6. Follow the gas leg back to the meter, noting each change of bore, and record developed length rather than straight-line distance.
7. Hold a level on the intended condensate route from spigot to connection and confirm continuous fall over all of it — a loose length of pipe held in place answers this in two minutes.
8. Photograph the elevation, the marked terminal, the flue route, the drain connection and the meter, and attach them to the quotation.

What the quotation has to say out loud

Write the position down. Name the wall, name the terminal location, name the flue route and its bends, name the condensate termination and whether it is internal or external. A quotation that says supply and fit one combination boiler is a quotation that has agreed to whatever the customer pictured, and what the customer pictured is the old boiler's position. Anything conditional — a plume kit if the neighbour's window turns out to be openable, a new gas run if the working pressure test fails, hatches if the flue is boxed — goes on as a stated provisional item with a figure against it, before the work starts.

Then keep the record. Commissioning documentation such as the HHIC Benchmark Commissioning Checklist, or the equivalent required by the adopted regime, is what the next engineer reads, and it is worth more with photographs of the concealed flue run and the condensate connection attached. The gas work itself is notifiable and, in Great Britain, may only be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 — worth restating, because the siting decisions above are exactly the ones a householder assumes they can settle alone.