Heating: boilers, radiators and furnaces

Heating and hot water take "over half of what you spend in a year on energy bills", on the Energy Saving Trust's figure (a UK body; the page gives no year),¹ and their small faults are the ones most likely to bring out an engineer who wasn't needed. A radiator that's cold at the top, a gauge that has crept below the green, a furnace filter nobody has looked at since the spring: each of these has a check or a fix a householder can do, and each has a version that looks the same and isn't yours at all. This lesson shows you how to tell which system you have, what the pressure gauge is telling you, how to bleed a radiator and top the pressure back up, what the thermostat and the radiator valves actually do, and which parts of a furnace or heat pump are yours to look after. How hot the water from your taps should be is the question for lesson 8, "Hot water: scalds, legionella and the tank".

Many boilers and furnaces burn gas, so the callout's first line matters here more than in the water lessons. Everything this lesson gives a householder happens on the water side or the air side: the radiators, the pressure gauge, the filling controls the maker shows you, the filter slot. Anything behind the boiler's casing, or the furnace's burner, is somebody else's, and lesson 10, "Gas: the law and the engineer", sets out exactly why. For radiators, the callout's "isolate" means heating off and radiators cool before you touch a bleed valve, which is the callout's isolate-first line in a new place. One section below states the law on gas work, and it carries this label in short; here it is in full. This is general information, not legal advice. The rules differ between England, Wales, Scotland and Northern Ireland, and between US states and cities, and they change.

Which system do you have?

You can't use the right advice until you know which system you're standing in front of, and the sources for each country describe different equipment.

In the UK: three kinds of boiler

In the British homes these sources describe, the heating is wet central heating: a boiler heats water, a pump sends it round radiators, and the same boiler often makes the hot water too. The Energy Saving Trust (EST), the UK's government-funded energy advice body, sorts the boilers into three kinds.¹

• Combi. "Combi boilers don’t use a hot water cylinder", EST says; the boiler heats water as you run the tap. EST says they suit smaller homes and are less suited to houses with several bathrooms.
• System. "System boilers have a separate water cylinder to store hot water."
• Regular (heat-only). These work like system boilers, but "have two extra header tanks to store cold water, usually installed in the loft if there is one."¹ (The loft is the attic.)

A condensing boiler, in EST's account, is one built to capture and reuse more of the heat in the gases that would otherwise escape up its flue, and EST adds a quick way to spot one from outside: "The flue is made of plastic. If it’s made of metal, it’s unlikely to be a condensing boiler."¹

A cylinder can be one of two kinds. Vaillant, a boiler and cylinder maker, describes a vented cylinder as fed by a cold water storage tank, "usually in the loft or airing cupboard", and an unvented one as fed "directly from the cold water mains", needing no storage tank.²

Here is the course's best reading of a question lesson 3, "Taps and toilets", left open: why a hot tap might keep running after the stop tap is off. This course read no source that traces the pipes, so this is only its own reading of the two descriptions above. In a home with a vented cylinder, the hot water is fed from a tank inside the house, on the house's side of the stop tap, so turning the stop tap off may not stop it. The course read nothing on how to isolate a vented cylinder, so a hot tap that won't stop remains a plumber's question.

In the US: furnaces, boilers and heat pumps

The US Department of Energy's Energy Saver pages, which were taken down and are cited here as archived copies, put the main difference in two sentences: "Furnaces heat air and distribute the heated air through the house using ducts. Boilers heat water, and provide either hot water or steam for heating."¹¹ A furnace's ducts carry a filter in the air path, which is the part of it that's yours. The third kind is the heat pump, which DOE says can cut electricity use for heating by up to 75% against electric resistance heating, for an air-source model.¹²

Hot water in an American home usually comes from a separate water heater: a storage tank, or a tankless (on-demand) unit. DOE's archived page says tankless heaters typically deliver 2 to 5 gallons a minute (about 7.6 to 19 litres, by this course's conversion; the page's own litre figure for 5 gallons is wrong), and for homes using 41 gallons (about 155 litres) or less a day can be 24% to 34% more efficient than a storage tank.¹⁶ For maintenance it says only: "Read your owner's manual for specific maintenance recommendations."¹⁶ DOE's companion page on tank heaters could not be read, and the tank heater's safety valve and maintenance are lesson 8's.

The pressure gauge on a sealed system

Worcester Bosch's figures are for what it calls a sealed system: the water in the radiators is held under pressure, and a gauge on or under the boiler shows it. Ideal's guide is for combis, and it notes that system and heat-only boilers may have a filling loop elsewhere. Worcester Bosch's filling guide, for its own boilers, says: "A sealed system relies on an operating pressure of between 1 and 1.5 bar to deliver heating and hot water at a consistently comfortable and reliable level."⁴ (Lesson 2 used the same unit for the water supply, with WaterSafe's conversion of 1 bar to about 14 psi.)

Here is where the course's question, law or advice, whose, and where?, earns its keep. That figure is a maker's, for its own boilers, and makers differ:

Three makers give 1 to 1.5 bar cold; Baxi gives 1.0 to 2.0 as its normal operating range and doesn't say cold or hot. None of them is a standard, and none speaks for anyone else's boiler. So the number for your boiler is the one in your boiler's manual. EST's regular boiler takes its water from tanks in the loft, and no maker read here says whether its gauge figures apply to that arrangement; your manual does.

The gauge moves for reasons the makers explain. Worcester Bosch says bleeding radiators loses pressure: "Boiler pressure can be lost when bleeding radiators."⁷ Ideal lists that among the small, normal losses, with air let out through automatic vents and tiny amounts escaping at valves and joints, and says the gauge "measures the pressure within the whole central-heating circuit, not just the boiler."⁵ Pressure rising a little while the heating runs is normal, Ideal says.⁵ Worcester Bosch draws two lines: "If pressure rises by more than 1 bar when the heating is up to temperature then the expansion vessel may require repressurising. This will require a service engineer." The expansion vessel is a part of the sealed system; this course read no maker's account of how it works, only Worcester Bosch's line that repressurising it is a service engineer's job. Its second line: "The pressure in the system will usually require topping up once or twice a year." Much more often than that, it says, contact your installer.³ Ideal says the same about once or twice a year, and adds that a fault is more likely "when the pressure drops repeatedly over a period of days or weeks, falls to zero, or requires frequent topping up."⁵

Topping the pressure back up

Putting water back into a sealed system is a job some makers write up for householders. It is Yours, on conditions: your boiler's own user instructions give you a filling method, you follow them to your boiler's own figure, you can clearly identify the filling controls and they are under or built into the boiler, the boiler is off and cold, and it's your home to repair or your landlord agrees. If you rent, ask your landlord first. This lesson gives two makers' accounts to show what those instructions look like, not to replace yours.

Worcester Bosch's System Filling Guide, for its own boilers, says there are three kinds of filling link, depending on what the installer fitted: an external looped hose beside or beneath the boiler, a keyed link under the boiler, and a keyless one worked by a lever.⁴ For the two built-in links, the guide has you pull a catch down and take off the boiler's bottom panel to reach them; that panel is as far as it goes. For each, the method is the same in outline: open the link, "Watch the gauge closely as the system fills. It may fill quite quickly so be ready to shut off the link when the needle is between 1 and 1.5 bar", and close it. If you overshoot, it says to bleed a radiator until the gauge is back in range.⁴

Ideal's guide, for its own boilers, is narrower and more cautious. It is written for combis only, those "where the filling loop or filling controls are clearly identifiable underneath or built into the boiler." For a system or heat-only boiler with a filling loop somewhere else, it says to contact your installer or a heating engineer. Its other conditions are ones this course would copy whatever make you have. Its steps for a combi, in short:⁵

1. Switch the boiler off and let the system cool, because the pressure "should be checked and adjusted while the boiler is cold."
2. Find the filling controls. "Only continue if you can clearly identify the correct filling controls. Do not operate unidentified valves underneath the boiler."
3. Open the filling valve or valves slowly, watching the gauge, "as the pressure may rise quickly."
4. At about 1.0 to 1.5 bar, return the valves to where they were, and make sure they are fully closed, because a valve left partly open may let the pressure keep rising.
5. Bleed the radiators, heating still off and cool (the steps are in the bleeding section below), then check the pressure again.
6. Switch the boiler back on and check it works normally.

Ideal sends you to your installer or a Gas Safe engineer if you can't find or can't confidently identify the filling controls, if the hose has been removed, or if water is leaking from radiators, valves, joints or pipes.⁵

Notice what these tasks share. Every step is on the water side, it is in the maker's own guidance to householders, and none of it goes further into the boiler than a panel the maker's user guide tells you to remove. That's the boundary.

Gas and the boiler, in brief

This is general information, not legal advice.

In Great Britain the law requires anyone doing gas work to be competent (regulation 3(1) of the Gas Safety (Installation and Use) Regulations 1998) and anyone doing it as a business to be on the Gas Safe Register (regulation 3(3)); HSE's code of practice sets competence at an industry-recognised course followed by an assessment.¹⁷¹⁸ Northern Ireland has its own 2004 Regulations, and their regulation 3 mirrors this one.²¹

The official advice points one way. HSE "strongly advises that all gas appliances, flues and pipework should be installed, regularly maintained and serviced at least annually by a Gas Safe registered engineer."¹⁹ What a householder may do is what the appliance's user instructions give the user to do.²⁰ So a boiler repair, a fault code the manual doesn't tell you to clear, or anything inside the casing beyond that panel is Call someone, and lesson 10 has the rest.

The cold radiator: top or bottom?

This is the diagnosis the whole of this lesson's hands-on part rests on, and it takes a hand on the radiator.

The mechanism, as the makers give it: air gets into a heating system over time, and Vaillant says "Air will naturally rise as it is lighter than water", so it gathers at the top of a radiator, where hot water can't reach.⁸ Sludge is the other end of the same system. Vaillant says the oxygen in trapped air "causes corrosion which can result in leaking and debris in your radiators".⁸ Neither maker says more about what the sludge is made of, and this course won't either. What matters is where it sits, and that a bleed key can't reach it.

A third pattern: the whole radiator is cold, with no sign of trapped air. Vaillant's advice is to check its valve first, since a thermostatic valve may simply be set too low, and if the valve is right and the radiator still won't heat, the valve may be blocked by sludge or limescale, which it sends to a heating engineer.⁹

Bleeding a radiator

Two UK makers publish the method for householders, Worcester Bosch and Vaillant, and they agree on the steps. It is Yours, on both makers' advice, for a radiator on a wet central heating system with a bleed valve at the top, if it's your home to repair or your landlord agrees.⁷⁸

1. Find the cold ones. Worcester Bosch says run the heating first and feel which radiators are colder at the top. Vaillant adds: use gloves, because they'll be hot.⁷⁸
2. Heating off, radiators cool. Worcester Bosch: "Turn off your heating system and wait for the radiators to cool." Vaillant says about an hour, or two to be cautious, so that hot water or steam can't burn you, and that the radiator valves should be fully open while you bleed.⁷⁸
3. Key in, cloth ready. The bleed valve is at the top of the radiator, usually at one end; Vaillant describes it as a small round hole with a little square inside. Hold a cloth under it (Worcester Bosch) or a bucket beneath and a cloth against the wall (Vaillant).⁷⁸
4. Open slowly, anticlockwise. Vaillant says a quarter to half a turn is enough, and not to keep opening once it hisses. The hiss is the air leaving.⁸
5. Close when water runs. Worcester Bosch: keep it open "until water starts to flow steadily", then turn the key clockwise. Vaillant says let a little water run first, and "Don't close the valve too tight, otherwise you might damage it." Vaillant also notes the water may be discoloured, and that this isn't a cause for alarm.⁷⁸
6. Check the gauge before the heating goes back on. Vaillant says to check the pressure before switching back on, and Worcester Bosch says to top it up afterwards, because bleeding loses pressure.⁷⁸

Vaillant adds one order for a house with more than one floor: bleed the ground floor first, then upstairs.⁸ Vaillant adds a word about repetition: "If you have to repeatedly bleed radiators, then discuss this with your installer when the boiler is serviced as there may be an underlying problem."⁸

Two radiators, two households

A combi flat on Tyneside

This is the simple case. In a first-floor flat on Tyneside, the bedroom radiator is cold across the top and warm below. The boiler is a combi, and its manual gives 1 to 1.5 bar cold. The gauge reads 1.2 bar with the heating off.

The owner runs the heating to check the other radiators (all warm), then turns it off and waits an hour. Key in, cloth under the valve, a quarter turn anticlockwise: a hiss, then after a while a steady trickle of water, and the key goes back clockwise, not too tight. The gauge now reads 0.8 bar.

A system boiler in Northamptonshire, with a gap for you

This is the harder case. A 1990s detached house in Northamptonshire has a system boiler and a cylinder in the airing cupboard. The back bedroom radiator is hot along the top and cold along the bottom. The owner bled it anyway; nothing hissed, and water came straight out. And since October the pressure has needed topping up about every fortnight to stay in the maker's range.

Two facts, and the temptation is to treat them as one problem. Before you open the answer, decide what each fact means on its own, and whose job each is.

Thermostats and radiator valves

EST describes the room thermostat as a switch with a target. It "turns the heating on until the room reaches the temperature you’ve set. Once it reaches the set temperature, it turns the heating off until the temperature drops again."¹⁰ So the setting decides when the heating stops, not how hard it works while it is on. Set it to 25°C and the house heats at the same rate, then keeps heating past 20°C. That's the course's reading of EST's description. EST's advice is to set the lowest temperature you're comfortable at: "For most people, this is between 18°C and 21°C."¹⁰ EST adds that a thermostat needs free air around it, so keep it clear of curtains, furniture and radiators, or it reads the wrong temperature.¹⁰

A thermostatic radiator valve (TRV) works on one radiator. EST: TRVs "work by sensing the air temperature around them. If the room is warmer than the setting on the TRV, the valve will close a little." EST puts what that does in terms of flow: closing cuts "the volume of hot water flowing into the radiator". The scale usually runs from 0, off, to 6, fully open, and EST suggests 3 to 4 for the rooms you use most and 2 to 3 for bedrooms.¹⁰

EST also describes the boiler's own dial, which "sets the temperature of the water that’s pumped from the boiler through the radiators", the flow temperature.¹⁰ This course read EST's explanation and no maker's instructions for changing it, so it gives no setting. If you change it, do it by your boiler's user instructions.

In a US home with a heat pump, one DOE rule changes the habit of turning the heat right down at night: "Do not set back the heat pump's thermostat if it causes the backup heating to come on."¹² DOE gives the reason on the same page: "Backup heating systems, such as electric resistance or electric strip heaters, are usually more expensive to operate because they are less efficient."¹²

Furnaces, filters and heat pumps

What an efficiency rating leaves out

A furnace or boiler in the US carries an AFUE rating. DOE's archived page defines it: "An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home". It says older systems ran at 56% to 70% and modern ones reach up to 98.5%.¹¹ Then comes the catch. AFUE excludes the heat lost from the ducts, "which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic, garage, or other partially conditioned or unconditioned space."¹¹ So a 95% furnace with its ducts in a cold attic could, at worst, deliver about 62% of the fuel's energy to the rooms: 95% times 0.65, by this course's arithmetic. The rating describes the furnace, not the house.

Who services what

DOE says a furnace's or boiler's maintenance "should be provided by a heating system professional", and gives the heat exchanger as an important safety reason: "Furnace heat exchangers mix combustion gases with house air when they leak", which can cause carbon monoxide poisoning.¹¹ That's lesson 11, "Carbon monoxide and the alarms that warn you". For heat pumps, DOE says: "You should have a professional technician service your heat pump at least once a year."¹² ENERGY STAR, the US government's efficiency programme, recommends a contractor's check-up each year, the cooling system in spring and the heating in autumn, and lists what the contractor checks, including "all gas (or oil) connections, gas pressure, burner combustion and heat exchanger."¹⁴ All of that is Call someone.

The filter, which is yours

One job on that list is the householder's. ENERGY STAR: "Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump. Your contractor can show you how to do this."¹⁴ DOE's archived pages give other intervals: every 3 months "or as recommended by the manufacturer or installer" for heat pumps, and "every month or two during the cooling season" for air conditioners.¹²¹³ This course reads them together as: check monthly, change it when it is dirty, and at least as often as the maker says.

For the method, this course read one maker's. Carrier, a US furnace maker, gives these steps on its website for a furnace filter:¹⁵

1. Buy a replacement that "matches the size required by your unit".
2. Find the filter, usually in the furnace's blower compartment (the blower is the fan that moves the air) or in the duct leading into it.
3. Turn the system off, at the thermostat or at the furnace's own power switch on the outside of the cabinet.
4. Open the access panel: Carrier names the blower cabinet door or a panel on the duct. Before taking the old filter out, note the airflow arrow printed on its side. Carrier: "Air should flow from the ductwork into the furnace."
5. Slide the old one out and put the new one in with its arrows pointing the same way, towards the blower.
6. Close the panel and turn the system back on.

It is Yours, on conditions: the size your unit takes, and the filter slot or panel your furnace's manual shows for it. Nothing else on a gas furnace comes off for this job.

What people get wrong

"Turning the thermostat up heats the house faster." EST: it "won’t make your home heat any faster!" It sets when the heating stops.¹⁰

"A radiator that's cold at the bottom needs bleeding." Cold at the top is air; cold at the bottom is likely sludge, which bleeding won't clear (Worcester Bosch).⁷

"1 to 1.5 bar is the right pressure for every boiler." It's what Worcester Bosch, Vaillant and Ideal give for their own boilers cold. Baxi gives 1.0 to 2.0 as its own operating range. Check your boiler's manual, which is also the only place read here that can tell you whether a gauge figure applies to a regular boiler with loft tanks.³⁵⁶⁸

"Topping up the pressure every few weeks is just maintenance." Worcester Bosch and Ideal both put normal at once or twice a year. Much more often, Worcester Bosch says, contact your installer; Ideal says a fault becomes more likely, and to contact it.³⁵

"A 95% furnace puts 95% of the fuel's energy into my rooms." AFUE excludes duct losses of up to 35% of the furnace's output (DOE, as archived).¹¹

Practice

For your plan

This lesson adds to the calendar: the boiler, furnace or heat pump serviced once a year by a professional (HSE for gas appliances, and in a let home in Great Britain the landlord's yearly check, lesson 10; DOE for heat pumps; ENERGY STAR's autumn heating check-up), the filter checked monthly and changed at least as often as its maker says, the boiler pressure read cold in autumn before the heating season. It adds to the map: which system you have, where the gauge and filling controls or the filter are, and the pressure figure from your boiler's manual. It adds to the stop list: anything inside the boiler or furnace casing, a pressure that needs topping up more than twice a year, a rise of more than 1 bar when hot, and any fault code the manual doesn't tell you to clear.

Connections

Lesson 1, "How a house works, and where it switches off", set out the job labels, and the callout's isolate-first line applied here as heating off and radiators cool. Lesson 2, "Water in: supply, leaks and frozen pipes", used bar for the cold supply; this lesson used it for the heating circuit, which is a different system with its own gauge. Lesson 3, "Taps and toilets", asked why a hot tap might not stop, and this lesson's cylinders gave the course's best answer. Lesson 8, "Hot water: scalds, legionella and the tank", takes the same cylinders and water heaters and asks how hot their water should be. Then lessons 9 and 10, "Gas: the smell and the valve" and "Gas: the law and the engineer", take the boiler's fuel side, and lesson 11, "Carbon monoxide and the alarms that warn you", takes up what a leaking heat exchanger lets out.

Go deeper

• Worcester Bosch, System Filling Guide, 2016, free. Read here in full. Eight pages, for Worcester Bosch's own boilers, with drawings of the three kinds of filling link. Even with another make, it shows you what to look for under the boiler.
• Energy Saving Trust, "The most economical ways to use your central heating", free. Read here in full from an Internet Archive capture. What a room thermostat, a TRV and the boiler's flow temperature each do. Its savings figures in pounds are tied to a price period, which is why this lesson left them out.
• ENERGY STAR, "Maintenance Checklist", free. Read here in full. One page listing what a US contractor's annual check-up should cover and the one job it leaves to you.

Sources

1. Energy Saving Trust, "Boilers", read from Internet Archive capture 20260919232017 (the live site refuses automated readers). Read: full text. The three kinds of boiler, the plastic flue, and heating and hot water as over half of energy bills.
2. Vaillant, "Hot water tank temperature". Read: full text, 25 September 2026. A maker's page; used only for its description of vented and unvented cylinders.
3. Worcester Bosch, "How often should I need to top up the pressure in the heating system?". Read: full text; re-read on the live page 25 September 2026. A maker, about its own sealed systems.
4. Worcester Bosch, System Filling Guide, dated 03/05/2016 in the file. Read: full text, 25 September 2026. A maker's guide for its own boilers.
5. Ideal Heating, "Topping up the pressure on your Ideal Boiler". Read: full text, 25 September 2026. A maker, about its own boilers; written for combis.
6. Baxi, "What should my boiler pressure be set at?". Read: full text, 25 September 2026. A maker, about its own boilers.
7. Worcester Bosch, "How to bleed a radiator". Read: full text; re-read on the live page 25 September 2026.
8. Vaillant, "How to bleed a radiator". Read: full text; re-read on the live page 25 September 2026.
9. Vaillant, "Why is my radiator not working?". Read: full text, 25 September 2026. Used only for the whole-radiator-cold check.
10. Energy Saving Trust, "The most economical ways to use your central heating", read from Internet Archive capture 20260417064839. Read: full text. The savings figures on the page are tied to a price period and are not used.
11. US Department of Energy, Energy Saver, "Furnaces and Boilers", as archived: Internet Archive capture 20250102062644 (the page no longer exists on energy.gov). Read: full text.
12. US Department of Energy, Energy Saver, "Operating and Maintaining Your Heat Pump" (capture 20250102062920) and "Heat Pump Systems" (capture 20241231180018), as archived. Read: full text of both.
13. US Department of Energy, Energy Saver, "Maintaining Your Air Conditioner", as archived: capture 20241215045250. Read: full text.
14. ENERGY STAR, "Maintenance Checklist". Read: full text; re-read on the live page 25 September 2026.
15. Carrier, "Changing Furnace Filters". Read: full text, 25 September 2026. A US maker's page, which also promotes its own products; used only for its filter steps.
16. US Department of Energy, Energy Saver, "Tankless or Demand-Type Water Heaters", as archived: capture 20250102084025. Read: full text.
17. The Gas Safety (Installation and Use) Regulations 1998 (SI 1998/2451), regulation 3. Read: in full; re-read on the live page 25 September 2026. Extends to England, Wales and Scotland.
18. Health and Safety Executive, Safety in the installation and use of gas systems and appliances, L56, fifth edition, 2018. Read: guidance paragraphs 52 to 55, ACOP paragraph 81, guidance paragraphs 82 to 87, and the glossary entry "gas engineer".
19. Health and Safety Executive, "Gas safety: home owners". Read: full text.
20. Gas Safe Register, "Don't DIY when it comes to gas appliances", Internet Archive capture 20251023004454 (the live site blocks automated readers). Read: full page text of the capture.
21. The Gas Safety (Installation and Use) Regulations (Northern Ireland) 2004 (SR 2004/63), regulation 3. Read: full text of regulation 3. Northern Ireland's own instrument.