What should I ask before buying a new boiler?

Ask what the calculated heat loss of the property is, what the flow rate and pressure at the cold main measured, whether the gas pipe from the meter needs upgrading, where the flue and condensate will run, and what is being done about the old pipework and system water. Then ask for the exclusions in writing. Those questions test whether a survey happened rather than testing the brand.

How do I know if the boiler is the right size for my house?

The output should come from a heat loss calculation based on room measurements, construction, glazing and insulation, not from counting radiators. On a combi, the hot water demand usually sets the size and the heating demand is smaller, so ask which of the two drove the decision. Ask for the heat loss figure to be written on the quote. An oversized boiler cycles more, which wears the ignition components and reduces condensing time.

Why does the surveyor need to measure my water flow rate?

A combi heats water as it flows through, so the hot water performance you get is limited by the flow the incoming main delivers, not by the boiler output. Without a measured figure in litres per minute and a static pressure in bar, the hot water performance of the new system is a guess. This is the usual reason a new combi feels weaker than the system it replaced, and it is predictable at survey stage.

Should the gas pipe be upgraded when I get a new boiler?

Often yes, particularly with a larger combi in an older property where the existing run is narrow or long. The pipe has to deliver enough gas while other appliances run, which is confirmed by a working pressure test at the appliance during commissioning. Ask whether the run has been traced from the meter and whether any upgrade is inside the quoted price, because it is one of the most common late additions.

Does the system need cleaning before a new boiler goes in?

Yes, and it is not an upsell. Steel radiators shed magnetite, a black iron sludge, and modern high efficiency heat exchangers have narrower passages than the boilers they replace, so debris an old appliance tolerated now causes noise, cold spots and early failure. Ask which cleaning method is used, whether a magnetic filter is included, and whether inhibitor is added and recorded at the end of the job.

What should be excluded from a boiler quote?

Get the exclusions listed in writing. The usual ones are gas supply upgrades, non standard or vertical flue kits, moving the boiler position, a condensate pump, system cleaning, filter and controls, making good and decoration, removal of an old cylinder and tanks, access equipment, and anything found once the old appliance comes off the wall. Agree in advance that variations are priced and approved before the work is carried out.

Which boiler brand should I choose?

Judge brands on how they fail and how they are repaired rather than on marketing. The practical questions are whether common parts are stocked locally, whether the appliance can be worked on without stripping it, and whether the manufacturer still supports the model in ten years. Vaillant and Worcester Bosch are strong on parts and access. Ideal and Baxi sit lower on price with widely stocked parts. Viessmann builds well but has a thinner parts network.

Do I need a new boiler or can mine be repaired?

Ask for the fault in components, not adjectives. Old and inefficient is not a diagnosis. Genuine replacement reasons are parts no longer made for the model, a failed heat exchanger where the part approaches the cost of the appliance, a change of system type, or repeated failures across different components indicating end of service life. If none of those applies, ask what a repair would involve before agreeing to a replacement.

Is a same day discount on a boiler quote worth taking?

Treat it as a technique rather than an offer. Nothing about the cost of your installation changes at midnight, so a price that expires today exists to stop you comparing it with anything else. The same applies to finance discussed before scope. A firm confident in its survey will leave a written, itemised quote with the model, output and exclusions on it and expect you to think about it.

Does a sponsored listing mean a boiler company is better?

No. Paid placement in search results and on maps is bought per click, and that advertising spend is a real cost that has to be recovered from the work customers pay for. It is a legitimate way to find customers, but it measures a marketing budget rather than competence. A top position tells you nothing about whether anyone measured your cold main or sized the gas run. Judge the survey instead.

What to Ask Before Agreeing to a New Boiler

Ask five things before you sign anything: whether the heat loss was calculated or guessed, what the flow rate at the cold main measured, whether the gas supply is big enough for the boiler proposed, where the flue and condensate are going, and what is happening to the old pipework and the water inside it. A survey that answers those five produces a quote you can check line by line. A visit that skips them produces a number, and a number is not a survey. Almost every new boiler that disappoints its owner was specified wrongly on the day of the survey, not built wrongly in a factory.

What questions should I ask before agreeing to a new boiler?

Ask what was measured, what was assumed, and what is excluded from the price. Those three cover almost everything that goes wrong later. A surveyor who measured gives the numbers without hesitating, because they wrote them down an hour ago. A salesperson who assumed steers towards the brand, the finance and the paperwork, because that is where the prepared material is.

What is the calculated heat loss, and how was it arrived at?
What did the flow rate and pressure at the cold main measure?
What size is the gas pipe, and does it need upgrading?
Where is the flue terminating, and does that position work?
Where does the condensate run, and is any of it outside?
What is happening to the existing pipework and the system water?
What is in the price, and what will come back as an extra?
Why this boiler, and what is servicing it like in five years?

Did anyone calculate the heat loss, or just count the radiators?

A real survey measures rooms, notes construction, glazing and insulation, and produces a heat loss figure in kilowatts that the boiler is chosen to match. Counting radiators and adding a margin is not that. If the answer is a round number that happens to be the size the firm usually fits, the calculation did not happen.

Oversizing is the most common outcome, and it is sold as headroom. What it does is make the boiler cycle. A unit far larger than the load satisfies the demand quickly, shuts down, cools and fires again, and that repeated ignition cycle is hardest on the components that fail first, the fan, the ignition electrode and the gas valve. It also spends less time in condensing mode, which is the efficiency you paid for.

On a combi the output is split between heating and hot water, and hot water demand normally sets the number, so a large combi in a small flat is not automatically wrong. What is wrong is not knowing which drove the decision. Ask the surveyor to say it out loud: this size because the hot water needs it, or because the heat loss needs it.

Ask for the heat loss figure in writing. It takes one line on the quote. A firm that calculated it adds the line without comment. A firm that did not will explain why the line is unnecessary.

What did the flow rate at the cold main actually measure?

If a combi or an unvented cylinder is being proposed, someone should have put a gauge and a weir cup on an outlet and written down the static pressure in bar and the flow in litres per minute. Without those two numbers the hot water performance is a guess, and it is the commonest reason people say the new boiler is worse than the old one.

The arithmetic is unforgiving. A combi heats water as it passes through, so the flow it receives is the flow you get, and if the incoming main delivers modestly a bigger boiler changes nothing. Two showers at once on a single combi is a known disappointment in London flats, and it is predictable at survey stage.

Static pressure in bar, taken at an outlet, not quoted from the water company.
Flow in litres per minute with the outlet fully open.
Whether the main is old lead or narrow bore, and whether it was traced.
What the household uses at once, which decides whether a combi suits at all.

Is the gas supply big enough for the boiler being quoted?

The gas pipe from the meter has to deliver what the appliance needs while every other gas appliance runs, and on a larger combi in an older property it often will not without an upgrade. Ask the size of the existing pipe, the length of the run, and whether replacing it is in the quote. A surveyor who followed the pipe from the meter knows. One who did not will say it should be fine.

This is where quotes diverge most sharply and least visibly. A new gas run through a Victorian conversion, up a stairwell and into a kitchen is real work and real making good. The firm that priced it is not the expensive one, it is the one that told you. The proof the check was done is the working gas pressure test at commissioning, so ask whether that will be recorded.

Where is the flue going, and where does the condensate end up?

Ask to be shown the proposed flue terminal position and the condensate route before the price is discussed, because both can move the boiler to a different wall. A flue must terminate clear of openings, boundaries and anywhere the plume blows back into a window or across a neighbour path, and in dense terraces and light wells the workable positions are limited.

Condensate is the one people regret. A modern boiler produces acidic condensate continuously and it has to drain somewhere, ideally internally into a soil stack or waste pipe. An external run in narrow plastic pipe is the classic freeze point in a cold snap, and you lose heating on the coldest night of the year for a reason designed in during the survey. If external is unavoidable, ask the diameter, whether it is insulated, and whether the fall is continuous.

Flats add a layer. If the flue runs through a void or communal area, ask who approves the work and what the building manager wants to see. That conversation is slow, so start it before committing to a date.

What happens to the old pipework and the water inside it?

A new boiler on a dirty system is a new boiler with a shortened life. Ask what cleaning method is proposed, whether a magnetic filter is being fitted, and whether inhibitor goes in at the end. The answer should be specific about method and sequence, not the word flush used as a noun.

The mechanism is mostly iron. Steel radiators corrode internally and shed magnetite, a black sludge that settles in radiators and circulates through the pump. In a modern boiler the water passes through a narrow heat exchanger, so debris an old cast iron boiler tolerated for twenty years now blocks passages and produces noise, cold spots and early failure. Cleaning is not an upsell, it is what protects the purchase.

Ask about the old pipe runs too. Microbore feeds in a nineteen eighties flat, sludged and partly blocked, will not suddenly perform because the boiler changed. A surveyor who flags the existing pipework as a limitation is telling you something a sales visit never will.

Which cleaning method, in what order, and for how long.
Whether a magnetic filter is included and where it will be mounted.
Whether inhibitor is added and recorded at commissioning.
Whether any pipework is being left in place that the surveyor has doubts about.

Which boiler brand are you fitting, and why that one?

The honest answer names the brand, gives a reason based on parts and serviceability, and admits the tradeoff. The weak answer is that the brand is simply the best, which usually means the firm has a supply arrangement with one manufacturer and fits nothing else. That is not scandalous, but you should know it is the reason.

Our view, formed from repairs rather than brochures, is that the question is not which brand fails least but which can be put right quickly when it does. Every boiler has a bad day. What separates them a decade later is whether common parts are stocked by merchants across London, whether the exchanger and fan can be reached without stripping the appliance, and whether the manufacturer still supports the model. Vaillant and Worcester Bosch score well on parts and on being sensibly laid out. Ideal and Baxi sit lower on price with widely stocked parts, which matters more than people think. Viessmann builds well but the parts network is thinner, so a simple fault can wait longer.

Ask one follow up above all: is this a boiler the firm repairs, or only installs. Those are different relationships with a product, and an engineer who has stood in front of the ten year old version with a multimeter has an opinion worth more than the specification sheet.

The honest answer to which boiler is best is that it depends on the property, the mains flow rate and who is going to service the thing, so an installer who names the same brand for every job is telling you about their buying arrangement rather than about your house.

It is equally fair to ask which boiler brands to avoid in a property like yours and why, because the reasoning behind an exclusion tells you more about how much the installer knows than any recommendation does.

Asking which is the most reliable boiler brand invites a brochure answer, so ask instead what they replace most often on the range they are proposing and where those parts come from.

Beware the substitution. If the quote names a model, check the appliance fitted is that model, with the same output and from the same range. Ranges within a brand differ, and the entry range from a good manufacturer is not the flagship.

What is in the price, and what will come back as an extra?

Ask for the exclusions in writing, because that is the list deciding whether the cheapest quote stays the cheapest. Inclusions are easy to say. Exclusions are where a fixed price becomes a moving one, and a firm that surveyed properly can name them on the spot.

The usual candidates are below. Ask too what happens if something is found once the old appliance comes off the wall. That should be an agreed process, with variations priced and approved before the work is done, rather than a decision made while your heating is in pieces.

Gas supply upgrade, flue kit variations and boiler relocation.
System cleaning, filter, inhibitor and controls.
Making good, decoration and removal of redundant tanks and cylinder.
Access equipment, and what happens if asbestos is found.
How a variation found on the day gets priced and approved.

Do I actually need a new boiler, or is this repairable?

Ask what is wrong with the existing appliance, in components rather than adjectives. Old, inefficient and on its way out are not diagnoses. A failed fan, a leaking heat exchanger, an obsolete gas valve with no supply route: those are diagnoses, and each carries a repair cost and a parts availability answer.

The genuine reasons to replace are worth stating. Parts no longer manufactured for the model. A failed heat exchanger where the part approaches the price of the appliance. A system changing type anyway, for example a stored system becoming a combi. Repeated failures across different components, which means end of service life rather than one bad part. If the person in your house can name which applies, the recommendation has a basis.

The uncomfortable part is the incentive. A business that only installs makes nothing from telling you the boiler is repairable, and one that only repairs has the mirror image problem. So do not ask whether they think you need a new boiler. Ask what the fault is and what repairing it would involve. Facts are harder to shade than opinions.

A boiler diagnosis from somebody who is not also quoting for the replacement is the most useful second opinion available.

Nobody should agree to a boiler replacement on the strength of a fault code read from the doorway.

This is also the point to settle combi or system boiler with a cylinder, because changing the type midway through a quote changes the pipework, the flue route and the scope of the job.

What London housing stock changes about the survey

The building decides more than the brand does, so the survey should be visibly about your property rather than a generic script. London water is hard across the supply area, and that alone changes what you ask about hot water components and their service life.

In a Victorian conversion the constraints are the gas run, the flue position in a light well or on a party boundary, and pipework buried under floors nobody can inspect. Ask where the stopcock is and whether it works, because a seized one turns installation day into a longer day. In a purpose built block the constraint is usually the existing system type, and flats built around a thermal store such as a Pulsacoil are not simply converted to a combi. In a new build with underfloor heating in liquid screed, the circuit runs cool and is not accessible, so appliance and controls have to suit the emitter.

Scale deserves its own question. Hard water deposits inside the hot water side of a combi and on any stored system, and it is the largest single influence on how these components age here. Ask what protection is proposed on the incoming main, and what the surveyor saw on the old installation. The state of the old parts is free evidence about what the new ones face.

What a sales visit looks like next to a survey

A survey spends most of its time looking at the house and little talking about the product. A sales visit inverts that. If the person looked at the boiler, the meter, the main, the flue position, the radiators and the loft, you had a survey. If they looked at the boiler, opened a tablet and started showing you packages, you had a sales visit, and the price on the screen was decided before they arrived.

None of the tells is proof of bad intent on its own, but pressure works. A discount that expires today is a technique, not an offer, because nothing about the cost of your installation changes at midnight. Finance presented before scope is the same move in another form. A total with no breakdown leaves nothing to compare later.

The advertising angle is worth understanding rather than resenting. Paid placement in search and on maps costs money per click, and that is a genuine business expense recovered from the work customers pay for. It does not make an advertised firm worse. It does mean position is bought rather than earned, so a top listing tells you about a marketing budget and nothing about whether anyone measured your cold main.

A price appears before the flue, gas run and main have been looked at.
A discount that expires today, or a price that drops when you hesitate.
Finance discussed before the scope of work is agreed.
A total with no breakdown, and no written list of exclusions.
No named model and output on the quote.
Reluctance to put the heat loss or measured flow rate in writing.

Questions we get asked

What should I ask before buying a new boiler?: Ask what the calculated heat loss of the property is, what the flow rate and pressure at the cold main measured, whether the gas pipe from the meter needs upgrading, where the flue and condensate will run, and what is being done about the old pipework and system water. Then ask for the exclusions in writing. Those questions test whether a survey happened rather than testing the brand.
How do I know if the boiler is the right size for my house?: The output should come from a heat loss calculation based on room measurements, construction, glazing and insulation, not from counting radiators. On a combi, the hot water demand usually sets the size and the heating demand is smaller, so ask which of the two drove the decision. Ask for the heat loss figure to be written on the quote. An oversized boiler cycles more, which wears the ignition components and reduces condensing time.
Why does the surveyor need to measure my water flow rate?: A combi heats water as it flows through, so the hot water performance you get is limited by the flow the incoming main delivers, not by the boiler output. Without a measured figure in litres per minute and a static pressure in bar, the hot water performance of the new system is a guess. This is the usual reason a new combi feels weaker than the system it replaced, and it is predictable at survey stage.
Should the gas pipe be upgraded when I get a new boiler?: Often yes, particularly with a larger combi in an older property where the existing run is narrow or long. The pipe has to deliver enough gas while other appliances run, which is confirmed by a working pressure test at the appliance during commissioning. Ask whether the run has been traced from the meter and whether any upgrade is inside the quoted price, because it is one of the most common late additions.
Does the system need cleaning before a new boiler goes in?: Yes, and it is not an upsell. Steel radiators shed magnetite, a black iron sludge, and modern high efficiency heat exchangers have narrower passages than the boilers they replace, so debris an old appliance tolerated now causes noise, cold spots and early failure. Ask which cleaning method is used, whether a magnetic filter is included, and whether inhibitor is added and recorded at the end of the job.
What should be excluded from a boiler quote?: Get the exclusions listed in writing. The usual ones are gas supply upgrades, non standard or vertical flue kits, moving the boiler position, a condensate pump, system cleaning, filter and controls, making good and decoration, removal of an old cylinder and tanks, access equipment, and anything found once the old appliance comes off the wall. Agree in advance that variations are priced and approved before the work is carried out.
Which boiler brand should I choose?: Judge brands on how they fail and how they are repaired rather than on marketing. The practical questions are whether common parts are stocked locally, whether the appliance can be worked on without stripping it, and whether the manufacturer still supports the model in ten years. Vaillant and Worcester Bosch are strong on parts and access. Ideal and Baxi sit lower on price with widely stocked parts. Viessmann builds well but has a thinner parts network.
Do I need a new boiler or can mine be repaired?: Ask for the fault in components, not adjectives. Old and inefficient is not a diagnosis. Genuine replacement reasons are parts no longer made for the model, a failed heat exchanger where the part approaches the cost of the appliance, a change of system type, or repeated failures across different components indicating end of service life. If none of those applies, ask what a repair would involve before agreeing to a replacement.
Is a same day discount on a boiler quote worth taking?: Treat it as a technique rather than an offer. Nothing about the cost of your installation changes at midnight, so a price that expires today exists to stop you comparing it with anything else. The same applies to finance discussed before scope. A firm confident in its survey will leave a written, itemised quote with the model, output and exclusions on it and expect you to think about it.
Does a sponsored listing mean a boiler company is better?: No. Paid placement in search results and on maps is bought per click, and that advertising spend is a real cost that has to be recovered from the work customers pay for. It is a legitimate way to find customers, but it measures a marketing budget rather than competence. A top position tells you nothing about whether anyone measured your cold main or sized the gas run. Judge the survey instead.