What does a boiler survey actually check?

Most people picture a boiler survey as somebody standing in the kitchen looking at the old boiler and nodding. That is not what it is.

A survey is a set of measurements taken around the boiler rather than at it, because the appliance is the one part of the job that is already known. What is not known is whether your house can take the boiler that is being proposed, and what has to be changed before it can. Every item below is a real check, and almost every one of them can move a price.

The gas side: does the supply reach?

This is the check that surprises people most, because it has nothing to do with the boiler and everything to do with the pipe feeding it.

Gas installation pipework has to be designed so that the pressure lost between the meter outlet and the appliance inlet does not exceed 1 millibar on natural gas. That is a hard limit in the manufacturer's own installation instructions, not a preference. A long run in an undersized pipe fails it, and the answer is a bigger pipe from the meter to the boiler, which is a real morning's work and sometimes a floor coming up.

The engineer works this out from the pipe size, the length of the run and the number of bends, then confirms it on the day with a working pressure test at maximum output. If the meter itself is the constraint rather than the pipe, that is a separate job with a separate owner, because the meter is not the installer's to change.

Source: Worcester Bosch, Greenstar 8000 Life installation and maintenance instructions

The water side: how much comes in, and what is in it

If a combi is being proposed, the incoming mains is the ceiling on everything it can do. A combi heats water as it passes through, so it cannot deliver more water than the street gives you. The measurement is flow rate in litres per minute, taken at a tap with everything else off, and it takes about a minute to do.

Static pressure gets checked at the same time, and so does the condition and size of the incoming supply pipe, because an old narrow supply is a common reason a powerful combi disappoints the person who paid for it.

Water hardness belongs in this section too. In a hard water area the manufacturer's instructions call for scale protection on the cold feed, which is a component and a bit of pipework that should appear on the quote rather than on the invoice.

The flue: where can it come out

A flue has to terminate a set distance from windows, doors, air bricks, boundaries, corners and anything above or below it, and those distances come from the manufacturer's instructions and the building regulations together.

The survey establishes the route the flue would take, how far it has to travel, whether it needs to go up through a roof rather than out through a wall, and whether scaffolding or a plume management kit is involved. A boiler that has to move even a short distance can change the flue from the simplest part of the job to the most expensive part of it.

The condensate: where does the water go

Every modern boiler produces acidic condensate that has to be piped away. The guidance is to run it internally wherever possible to reduce the risk of it freezing, with a minimum fall along its whole length so it drains properly. Internal runs through unheated lofts, basements and garages are treated as external runs, and a rainwater downpipe can only be used where that pipe goes to a combined drain.

On a straight swap this is usually already solved. Move the boiler, or put one into a room that has never had one, and finding a legitimate drain becomes one of the jobs that quietly decides where the boiler can go.

Source: Worcester Bosch, Greenstar 8000 Life installation and maintenance instructions

The system the new boiler is joining

A new boiler inherits whatever is already in the pipework. The survey should look at the radiators, the age and material of the pipes, whether there is a filter, whether the water coming out of a radiator is black, and whether anything obviously leaks.

This matters because cleaning the existing system to the relevant code of practice is a requirement of the compliance guide that sits under the building regulations, not an upsell, and the manufacturers reference it directly in their instructions. It is also one of the most common reasons two quotes differ: one installer has priced cleaning the system properly and the other has not mentioned it.

Source: Worcester Bosch, Greenstar 8000 Life installation and maintenance instructions

The house, not the appliance

The output figure a boiler needs is a property of your house, not of the boiler you happen to like. A survey should count radiators, ask how many bathrooms get used at once, ask about insulation, loft conversions and any extension you are planning, and take the room sizes seriously enough to do a heat loss calculation rather than a guess.

The reason this matters more than it used to is that the sizing question has changed shape. It is no longer only about whether the boiler is powerful enough. It is about whether it can turn down far enough to match how little heat the house needs on a mild day.

The unglamorous checks that decide the day

The rest of the list is short and boring and it is where jobs go wrong when it gets skipped.

• Position and clearances. The boiler needs space around it to be worked on, not just to fit. A cupboard that takes the appliance but leaves no room to service it is a problem you inherit for the next decade.
• The electrical supply and the isolation point, and whether the existing controls and wiring can carry over.
• Access. Stairs, lofts, a hatch too small for the boiler, parking, and where the old appliance leaves from.
• What is behind the boiler. Older properties in Northampton and Milton Keynes still turn up back boilers behind fireplaces, and removing one is a much bigger job than a wall hung swap.
• Making good. Who fills the holes, what happens to the tiles behind the old unit, and whether that is in the price or in a sentence at the bottom of it.

How to tell whether you had a survey or a look

You do not need to check the work. You only need to check that the measurements exist, because a quote built on assumptions is a quote that can change on the day.

Ask what the mains flow rate was, in litres per minute. Ask what the gas pipe run is and whether it needs upsizing. Ask where the flue and the condensate are going. Ask what the heat loss came to. Someone who surveyed will answer all four without hesitating. Someone who looked will tell you it will be fine.

There is nothing wrong with a quote given without a survey, as long as everybody understands what it is: an estimate that assumes the easy version of every one of the checks above.

What to do with this

Print the four questions if it helps. The point of them is not to catch anybody out, it is that once two installers have both answered them, the difference between their prices usually explains itself.

If you want quotes from installers who survey before they commit to a number, we can put you in front of Gas Safe registered firms who work in your area.