Question 1

Which is better, a combi or a conventional boiler?

Accepted Answer

Neither is better in every home. A combi is better for smaller homes with one bathroom and good mains water, while a conventional or system boiler with a cylinder is better where several people need hot water at the same time.

Question 2

What is the most common type of boiler in the UK?

Accepted Answer

Combi boilers are widely regarded as the most common type in UK homes, because they are compact, need no tanks and suit the many homes with one bathroom.

Question 3

Can I replace a regular boiler with a combi?

Accepted Answer

Usually, if your mains water pressure and flow are good enough and your pipework and showers can cope with mains pressure. An engineer should check both at the survey before you decide.

Question 4

What is the difference between a system boiler and a regular boiler?

Accepted Answer

Both heat water into a cylinder. A system boiler has the pump and expansion vessel built in and needs no loft tanks, while a regular boiler relies on a cold water tank and a feed and expansion tank in the loft.

Question 5

Do system boilers need a hot water cylinder?

Accepted Answer

Yes. A system boiler heats water that is stored in a cylinder, usually an unvented cylinder fed directly from the mains, which then supplies your taps and showers.

Question 6

Inhibitor, cleaner, filter or power flush?

Accepted Answer

These do different jobs and work together. Product or service What it does When it is used Inhibitor Protects metal surfaces and slows corrosion and scale Always, as the ongoing protection System cleaner Loosens existing sludge and debris Before a new boiler, or when a system is dirty Magnetic filter Catches magnetite and debris as water circulates Fitted on the return pipe to the boiler and cleaned at each service Power flush Pumps water and cleaner through at high flow to remove heavy sludge When a system is badly contaminated BS 7593:2019 recommends an in-line filter as well as chemical treatment. Every new boiler package we fit includes a magnetic system filter. If your system is already badly sludged, inhibitor on its own will not fix it. It needs cleaning first. Our guide to power flushing and the signs your radiators need it helps you decide.

Question 7

Do I need to put inhibitor in my central heating?

Accepted Answer

Yes. Without it, the metals in your system corrode, sludge builds up and parts such as the pump and heat exchanger wear out sooner. BS 7593 and boiler manufacturers both expect the system to be treated.

Question 8

How often should central heating inhibitor be changed?

Accepted Answer

BS 7593:2019 calls for the level to be checked every year and the system re-dosed after five years unless a test shows it is still at the right concentration. It also needs replacing whenever the system is drained and refilled.

Question 9

Can you put too much inhibitor in a central heating system?

Accepted Answer

Follow the product’s dosing instructions for your system size. The recommended dose is what the product is designed to work at, so adding extra brings no benefit, and an engineer can test the concentration if you are unsure.

Question 10

How do I know if my central heating has inhibitor in it?

Accepted Answer

You cannot tell by looking. Clear water is a good sign but not proof. An engineer can test a sample at your annual service and tell you whether the level is correct.

Question 11

Does a magnetic filter replace inhibitor?

Accepted Answer

No. A filter removes debris that has already formed, while inhibitor slows down the corrosion that creates it. BS 7593:2019 recommends both.

Question 12

Which central heating controls are best?

Accepted Answer

For most homes, a programmable room thermostat or smart thermostat combined with TRVs on the radiators gives the best mix of comfort and efficiency. Homes with a cylinder also need a working cylinder thermostat. If your boiler supports it, weather or load compensation can add a further saving.

Question 13

Is it cheaper to leave the heating on low all day?

Accepted Answer

In most homes with a gas boiler, heating only when you need it, using a timer and thermostat, uses less energy than keeping it on low all day, because a warm house loses heat faster than a cool one. Heat pumps are different, as they are designed to run steadily for longer.

Question 14

Where should a room thermostat be placed?

Accepted Answer

On an inside wall in a room you use regularly, at a comfortable height, away from radiators, direct sunlight, draughts and appliances that give off heat. A poorly placed thermostat can make the heating run too long or switch off too early.

Question 15

Why does my heating not respond to the thermostat?

Accepted Answer

Check the thermostat is set above the current room temperature, the programmer is calling for heat and the batteries are not flat. For wireless models, check the receiver near the boiler still has a connection. If all of that is fine, the boiler, a zone valve or the wiring may need an engineer.

Question 16

Do I need a new boiler to have a smart thermostat?

Accepted Answer

No. Most smart thermostats work with existing boilers by switching them on and off. To use advanced features that control the boiler’s output, the boiler needs to support the right communication protocol.

Question 17

How likely is a boiler to leak carbon monoxide?

Accepted Answer

A modern, correctly installed and regularly serviced room-sealed boiler is very unlikely to leak carbon monoxide. The risk rises with poor installation, missed servicing, damaged flues and older open-flued appliances.

Question 18

Can a boiler leak carbon monoxide when it is switched off?

Accepted Answer

A boiler only produces carbon monoxide while it is burning gas, so a boiler that is not firing is not producing it. If your alarm sounds with the boiler off, treat it as real and follow the emergency steps, because another appliance could be the source.

Question 19

Can you smell carbon monoxide?

Accepted Answer

No. It has no colour, smell or taste. The smell of gas comes from an odour added to the gas supply, which is why you should never rely on your nose to detect CO.

Question 20

Where should a carbon monoxide alarm go if my boiler is in a cupboard?

Accepted Answer

Fit it in the room where the cupboard is, on the ceiling or high on a wall, between 1m and 3m from the boiler. Follow the alarm maker’s instructions as well.

Question 21

Do I need a carbon monoxide alarm with a combi boiler?

Accepted Answer

An alarm is strongly recommended with any fuel-burning appliance, whatever type of boiler you have. It is expected when a new boiler is fitted, and landlords in England must provide one in rooms with a fixed combustion appliance.

Question 22

Can a plumber fit it and a Gas Safe engineer connect the gas?

Accepted Answer

This question comes up often. In theory, a plumber could do the water pipework and a registered engineer could connect the gas and commission it. In practice most Gas Safe engineers will not do this, because by connecting the gas and commissioning the boiler they take responsibility for the safety of the whole installation, including the flue, the condensate pipe and the parts they did not fit. They would need to check everything, which often takes as long as doing it themselves.

Question 23

Is it illegal to install your own gas boiler?

Accepted Answer

The law requires anyone who does gas work to be competent, and HSE guidance makes clear this includes DIY work on your own home. Anyone doing it as a business must be Gas Safe registered. For a homeowner without gas training, fitting a boiler would not meet that standard.

Question 24

Can I replace an electric or oil boiler myself?

Accepted Answer

Other boilers bring their own rules. Electrical work in homes falls under the Building Regulations and some of it is notifiable, and oil boilers have their own installation standards and certification. Use a qualified, registered installer for either.

Question 25

Can I fit the radiators and pipework and get an engineer to do the rest?

Accepted Answer

You can discuss it, but most engineers will not commission a system they did not install because they become responsible for its safety. Any saving is often lost in the time needed to check your work.

Question 26

What paperwork should I get with a new boiler?

Accepted Answer

You should receive a completed Benchmark checklist, a Building Regulations compliance certificate, confirmation that the manufacturer’s warranty has been registered and the user manual. Keep them together for servicing, warranty claims and selling the house.

Question 27

Can I service my own boiler?

Accepted Answer

No. Servicing means opening the case and working on gas components, which is gas work. Manufacturers also require the annual service to be done by a Gas Safe registered engineer to keep the warranty valid.

Question 28

How efficient is it?

Accepted Answer

A plug-in fan heater or oil-filled radiator turns one unit of electricity into one unit of heat. A heat pump moves more heat than the electricity it uses. The Energy Saving Trust gives three to four units of heat per unit of electricity as a typical figure for heat pumps, and air-to-air units work on the same principle. In practice the figure depends on the outside temperature, the model and how the system is used. It is highest on mild days and lower on cold ones. Each model has a seasonal efficiency rating on its energy label, which gives a fairer picture across a whole heating season than the best-case figure. For a more detailed look at what affects the electricity it uses, see our page on air conditioning running costs. If you are thinking about a system that heats and cools, our air conditioning installation page explains how we size and fit one.

Question 29

Does it work in winter?

Accepted Answer

Yes, with some limits worth knowing about. For the UK climate, a correctly sized unit copes well through most of the winter. On the coldest days an undersized unit may struggle, which is why sizing matters.

Question 30

Is it cheaper to heat with air conditioning or gas?

Accepted Answer

It depends on your tariffs and how efficiently each system runs. A heat pump air conditioner uses far less electricity than an electric heater, but gas is cheaper per unit than electricity, so against a modern gas boiler the difference can be small either way.

Question 31

Can air conditioning replace central heating?

Accepted Answer

In a small, well insulated home or flat it can be the main heating, with a unit in each main room. It does not heat water, so you would still need another way to get hot water, and in a larger home the number of indoor units needed often makes radiators the more practical choice.

Question 32

Why is my air conditioner blowing cold air in heat mode?

Accepted Answer

Check it is set to heat mode and that the set temperature is above the room temperature. Short spells of cool air are normal at start-up and during a defrost cycle. If it never produces warm air, the Daikin manual notes a refrigerant leak can be the cause, which needs an F-Gas engineer.

Question 33

Do all air conditioners have a heating mode?

Accepted Answer

Most wall-mounted split systems sold for homes are heat pump models that heat and cool, but some are cooling only, and many portable units only cool. Check the specification for a heating capacity figure before you buy.

Question 34

Is it bad to run air conditioning on heat all winter?

Accepted Answer

No. Heat pump air conditioners are designed to heat through the winter, and defrost cycles are part of normal operation. Keep the filters clean and have the system serviced regularly.

Question 35

Do all new boilers come with a 10 year warranty?

Accepted Answer

No. Warranty length depends on the brand and model, and entry-level boilers often come with shorter cover. Longer terms are often only available when an accredited installer fits the boiler and it is registered on time.

Question 36

Does a boiler warranty cover parts and labour?

Accepted Answer

A manufacturer’s warranty normally covers both parts and the engineer’s labour for a fault caused by the boiler itself. It does not cover faults elsewhere in the heating system or damage from dirty system water.

Question 37

Is the annual service free under warranty?

Accepted Answer

No. The warranty requires an annual service but does not pay for it. Vaillant’s terms state this directly, and other manufacturers work the same way.

Question 38

Does a boiler warranty transfer if I sell my house?

Accepted Answer

Often yes, as the warranty stays with the boiler at the property. Some manufacturers ask the new owner to update the registration and keep up the annual service, so pass on all the paperwork when you sell.

Question 39

What happens when my boiler warranty runs out?

Accepted Answer

Nothing changes with the boiler itself. Repairs are simply paid for by you from then on, so regular servicing and clean system water become even more important for keeping the boiler reliable.

Question 40

Which is cheaper to run?

Accepted Answer

This is the question most people care about, and the honest answer is that it depends on your home and your tariff. A heat pump delivers several units of heat for each unit of electricity, while a boiler delivers slightly less than one unit of heat for each unit of gas. Against that, a unit of electricity costs several times more than a unit of gas. Whether the heat pump comes out cheaper depends on how efficiently it actually runs in your home over a whole year (its seasonal performance) and on the gap between your electricity and gas prices. The Energy Saving Trust puts it plainly: heat pumps should be cheaper to run than most fuel types, but if you already have a newer gas or oil boiler, a heat pump will not necessarily be cheaper. A time-of-use electricity tariff can improve the sums. A heat pump in a draughty home with small radiators will run at a poor efficiency and can cost more

Question 41

Is your home ready for a heat pump?

Accepted Answer

Work through this checklist before you get quotes. If you answer yes to most of these, a heat pump is worth a serious look. If not, you can still get there by improving insulation first.

Question 42

What about air conditioning?

Accepted Answer

Most modern split air conditioning systems are air-to-air heat pumps. They can heat a room as well as cool it, using the same principle as an air source heat pump, but they blow warm air rather than heating water, so they do not heat your radiators or your hot water. They can be a good way to heat and cool a conservatory, extension or home office alongside a boiler. See our air conditioning installation page for how that works.

Question 43

Should I replace my gas boiler with a heat pump?

Accepted Answer

If your home is well insulated, has room for a cylinder and an outdoor unit, and you are happy for some radiators to be changed, a heat pump can cut your carbon and may cut your bills. If your boiler has just failed, or the home needs a lot of work to suit a heat pump, a new gas boiler is often the more practical step for now.

Question 44

What is the downside of a heat pump?

Accepted Answer

The main downsides are the higher upfront cost, the need for a cylinder and an outdoor unit, and the fact that radiators often need upsizing. Running costs depend heavily on how well the system is designed and on your electricity tariff.

Question 45

Do heat pumps work in cold weather?

Accepted Answer

Yes. They keep working in cold weather, but their output and efficiency fall as the outside temperature drops, and the outdoor unit runs short defrost cycles in frosty conditions. A correctly sized system is designed for the coldest days you are likely to see.

Question 46

Are gas boilers being banned?

Accepted Answer

At the time of writing you can still have a gas boiler replaced with a new one in an existing home. Government policy is moving new homes and grant funding towards low carbon heating, so it is worth checking the latest position on GOV.UK if you are planning years ahead.

Question 47

Can I have a heat pump and keep my boiler?

Accepted Answer

Yes, as a hybrid system, where the heat pump does most of the work and the boiler helps on the coldest days. Hybrids are not eligible for the Boiler Upgrade Scheme grant, so you would pay the full cost.

Question 48

What is a good efficiency rating for a boiler?

Accepted Answer

Any new gas boiler fitted in an existing home in England must be at least 92 per cent ErP efficient, which is class A. Most modern condensing boilers are rated a little above that, so the differences between good boilers are small.

Question 49

Does an A-rated boiler save money?

Accepted Answer

Compared with an old non-condensing boiler, yes, because more of the gas you buy ends up as heat. Compared with another A-rated boiler, the savings depend more on how it is set up and controlled than on the rating.

Question 50

How do I make my boiler more efficient?

Accepted Answer

Keep it serviced every year, keep the system clean and protected by a filter, bleed radiators when needed, use good controls and set the flow temperature to suit your radiators. A well-set-up boiler condenses more of the time.

Question 51

Why is my boiler efficiency figure over 100 per cent?

Accepted Answer

That figure has been calculated on a net calorific basis, which leaves out the heat in the water vapour. UK ErP ratings use a gross basis, so a condensing gas boiler will show just over 90 per cent.

Question 52

Does boiler efficiency drop with age?

Accepted Answer

It can. Sludge, scale, worn parts and missed services all reduce how efficiently a boiler runs. Yearly servicing and a clean system help it stay close to its rated efficiency for longer.

Question 53

Is there a deadline because of a gas boiler ban?

Accepted Answer

No. There is no ban on gas boilers in existing homes in England, and you can still replace a broken gas boiler with a new one. The Future Homes Standard, which comes into force on 24 March 2027, applies to new-build homes. Any new gas boiler fitted in an existing home must meet the Boiler Plus rules on efficiency and controls.

Question 54

Is it cheaper to replace a boiler in summer?

Accepted Answer

Boiler prices do not follow a strict seasonal pattern, but replacing in summer lets you plan, compare quotes and choose a date without the pressure of a cold house. That usually leads to a better decision than a rushed winter replacement.

Question 55

How old should a boiler be before you replace it?

Accepted Answer

There is no fixed age. Many modern boilers run well for 12 to 15 years or more with yearly servicing. Age matters most when it comes with frequent breakdowns, hard-to-find parts or poor efficiency.

Question 56

Should I replace my boiler before it breaks?

Accepted Answer

If it is old, unreliable or inefficient, yes. Planning ahead means you choose the boiler and the date, instead of accepting whatever is available during a breakdown in winter.

Question 57

Can a boiler be replaced in winter?

Accepted Answer

Yes. Boilers are replaced all year round. A like-for-like combi swap is often completed in a day, but more complex jobs can take longer, so plan temporary heating if the weather is cold.

Question 58

Is it worth replacing a boiler that still works?

Accepted Answer

If it is a non-condensing boiler or very old, replacing it will cut your gas use and remove the risk of a winter breakdown. If it is a reliable condensing boiler under about ten years old, servicing and repairing it is usually the better value.

Question 59

Combi, system or regular: which suits a three bedroom house?

Accepted Answer

Three bedroom homes vary a lot. A 1930s semi with one bathroom, a new-build with an en suite and a family bathroom, and an extended house with a loft conversion all need different answers. Start with how your household uses hot water. Your home Usually the best boiler type Why One bathroom, shower over the bath or separate shower Combi Hot water on demand, no tank, frees up the airing cupboard One bathroom plus a small en suite rarely used at the same time Combi, sized carefully Works well as long as two showers are not run together Two bathrooms often used at the same time System boiler with unvented cylinder Stored hot water at mains pressure to more than one outlet Weak mains water flow or low pressure System boiler with cylinder, or fix the supply first A combi can only heat the water that comes in Existing tanks in the loft, older pipework you want to keep Regular (heat only) boiler

Question 60

What size boiler does a three bedroom house need?

Accepted Answer

Boiler size is measured in kilowatts (kW) of output. There are two separate jobs to size for: heating the house and heating water.

Question 61

Is a combi boiler good for a 3 bedroom house?

Accepted Answer

Yes, for most three bedroom houses with one bathroom, or one bathroom and a lightly used en suite. It gives hot water on demand without a tank. If two showers are regularly used at once, a system boiler with a cylinder is usually better.

Question 62

What size combi boiler do I need for a 3 bedroom house?

Accepted Answer

Most one-bathroom three bedroom homes suit a combi in the high 20s to mid 30s kW. The right size depends on your mains water flow and how many outlets you use at once, so a flow test at the survey matters more than the number of bedrooms.

Question 63

Is a 24 kW combi big enough for a 3 bedroom house?

Accepted Answer

For heating, often yes, because most three bedroom homes lose less heat than that. For hot water it can feel slow, particularly in winter when the incoming water is colder. Most households with a family bathroom are happier with a slightly larger combi if the mains supply can support it.

Question 64

Should I get a system boiler instead of a combi?

Accepted Answer

A system boiler with a cylinder is better when you have two or more bathrooms used together, a larger family, or a weak mains flow that limits a combi. It needs space for a cylinder, usually in the airing cupboard.

Home Owner's Guide
Plumbing, Heating & More

Honest advice from qualified engineers. No jargon.

Trusted & Accredited Professionals

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Boiler Warranties Explained: What Is Covered and How to Check Yours

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Home Owner's Guide Plumbing, Heating & More

Honest advice from qualified engineers. No jargon.

Trusted & Accredited Professionals

Home Owner's Guide
Plumbing, Heating & More