Plumbing Reference Material

Plumbing reference material

Please find below a selection of good plumbing reference material, broken down and explained in a way that is easy to understand for the benefit of you, our customers, and the general public. If there is anything else you think we should show here, we would welcome your added contribution in order to help keep the information accurate and up-to-date.

OFTEC registered technician

Whether you are selecting a technician to give your oil boiler, cooker or stove its annual service, or planning a much bigger project – a home extension or conversion of your heating to run on oil, you will want to ensure that you employ the best person for the job. You need an experienced technician who has been trained on modern, high efficiency appliances, who can give you sound advice when necessary, and whom you can trust to carry out the work properly. In short, you need an OFTEC Registered Technician.

There is a simple way to avoid the risk of falling victim to a ‘cowboy’ operator. Before you ask a technician to call, enquire if he or she is OFTEC Registered (and if not, why not?).

  • They are individually trained at an OFTEC Approved Training Centre, and their skill independently assessed before they can apply for OFTEC Registration 
  • Have their work inspected from time to time by an OFTEC Inspector 
  • They are re-assessed every five years to maintain their Registration 
  • Carry an identity card confirming the type of oil heating work for which they are registered 
  • Have to hold appropriate insurance to work in your home 
  • They are able (in England and Wales) to self-certify work for which they are Registered. This saves you time and money, as non-Registered technicians must, by law, notify Local Authority Building Control, or submit plans when installing oil-fired appliances 
  • They will leave with you a written statement of the work they have done on your appliance, replacement parts fitted and recommendations for work needed in the future

You can find details of OFTEC Registered Technicians in your area on or by phoning OFTEC on 0845 65 85 080 .

Condensing boilers

A condensing boiler is a high efficiency modern boiler that incorporates an extra heat exchanger so that the hot exhaust gases lose much of their energy to pre-heat the water in the boiler system. When working at peak efficiency, the water vapour produced in the combustion process condenses back into liquid form releasing the latent heat of vaporisation.

A side effect is that this water, known as condensate, which is slightly acidic, has to be piped away to a drain or soak away.

The photo (below) shows a cutaway combination condensing boiler. It is mounted on a wall and the exhaust gases will rise through the plastic flue in the top left corner. Hot water is provided by a small storage tank on the right: the tank (which is covered by insulating foam) has been cut open to show the tightly wound quick refresh coil inside it. At the bottom of the photo are a number of pipes going into the boiler. One carries the gas for the burner and there are two (in and out) for the central heating system. The plastic pipe on the right carries the condensed water vapour produced by burning the gas. This water contains dissolved oxides of sulphur and nitrogen, making it slightly acidic.

Ian Byrne, the Foundation’s Deputy Director writes: “YES! Although not many homes in Britain have condensing boilers, over half the new boilers in the Netherlands have been of this type for a number of years. The National Energy Foundation carried out extensive monitoring work on new homes built in Milton Keynes in the late 1980s, and it was found that operating efficiencies of 90% or better could be obtained in normal use. I have had a condensing boiler in my own home for 15 years; it works very well and my heating bills are lower than my neighbours in similar homes with old-fashioned boilers (although most have installed condensing boilers themselves since this page was first written in 1996).”

Most existing boilers are measured by their output in British Thermal Units per hour (Btu/h), but all current boilers are sold in the metric equivalent of kilowatts (kW). The calculator below will give the approximate metric value, but you should bear in mind that in the past, central heating engineers often installed boilers that had a higher output than strictly necessary. Although this meant that there was no possibility of the boiler failing to meet the demand for heat, even in the most arctic of conditions, it also meant that they were mainly operating at a part load, and so running below their maximum efficiency. If you have installed additional loft or cavity wall insulation since the last boiler was fitted, it is highly likely that you will need a smaller boiler than before.

We recommend seeking professional advice from an OFTEC registered installer before choosing an appropriate replacement boiler.

All OFTEC registered oil installers in the UK should be able to fit a condensing boiler.

Good heating controls require a minimum of four things:

  • An electronic timer or programmer that allows separate switching of heating and hot water 
  • A room thermostat 
  • Thermostatic radiator control valves (TRVs) 
  • Separate thermostatic control on the hot water system 


Additional controls that may be worth considering to get the very best from a central heating system include intelligent heating controls, a weather compensator, a boiler energy manager and full zone control.

Energy myths

The Government pushed this one heavily in the mid-1990s; but in practice the answer is a lot more complex. The easiest way to test this out is to put the plug in the bath next time you have a shower, and see how much it fills up. If, at the end of the shower, there is less water than you would usually have in a bath, then you will probably save money by taking a shower. But it’s not quite that simple…

The simple answer assumes that you are using the same source to heat water for the bath and the shower. This will be the case if you are using your central heating boiler or an immersion heater. But a lot of British homes have a “Power Shower” that uses electricity to provide a powerful and instant shower. Not only do they use water more quickly than a shower fed from a hot water system, they also use on-peak electricity that costs around 7p a unit, compared to 2p for gas. At this point it gets complicated, as to give a firm answer you would also need to know the efficiency of your heating boiler, the amount of insulation on the hot water tank and the losses on the pipework between the tank and your shower.

In fact, it may not make any difference either way, if you are using the central heating boiler to warm the water. For in most cases, especially in summer, we leave the boiler on for a fixed period of time, allowing it to heat up the tank of water. If the bath or shower is drawn off after the end of a water heating period, it will not affect the heating cost on the day it was used (for that has already happened) but on the next heating cycle; and if the tank cools down then it will not be significantly affected by how much hot water was used after the previous cycle.

This one is easy. It is always best to place the water heating on a timer, as the energy lost from a hot water tank depends on the temperature difference between the surface of the tank and its surroundings. It’s a common myth that it somehow takes more energy to keep heating up a tank than to maintain it at a high temperature.

Of course, as with all the urban myths on this page, there are a few “ifs” and “buts”. If the tank is highly insulated (so standing losses are very low) and there is an effective thermostat on the tank, then the losses through leaving it on can be much reduced. And some people have a need for large quantities of hot water all day long, in which case they may have no alternative to leaving the immersion heater on. But in general, it is much better to install a timer – a heavy duty one, suitable for immersion heaters, should cost less than £20 and if you can fit it yourself safely, could pay for itself in a few months: an excellent energy efficiency investment.

This idea became quite widespread about 20 years ago, when it was being supported by an official government institution. However, it is not true.

Thermostatic Radiator Valves (TRVs) will only switch the flow to a single radiator on or off. They do not stop the boiler from firing (and so using energy). They are useful, but tend to be a rather crude control of temperature in a room, as they are affected by siting (please don’t put the sofa directly in front of them!) and are often not set at the right temperature.

In contrast a room thermostat is accurate to within a degree or so. What’s more, if it has been wired up correctly (in what may be called an “interlock”) it should send a signal back to the boiler to switch itself off if there is no demand for heating for either the heating or hot water circuits. This stops the boiler firing when the internal water temperature has slipped – ie. it stops so-called “dry cycling”, and definitely saves energy.

There are other advantages to modern room thermostats. They can contain an optimum start temperature sensor, which delays the boiler firing on relatively mild days, again saving energy. And if combined into full zone control, they can set different temperatures for different floors of a home – meaning that living rooms can be snug, without over-heating bedrooms.

One final warning: don’t put a TRV on a radiator close to the room thermostat, as the TRV’s operation will mislead the room stat.

Maybe from your boiler, but not from mine! The answer to this one depends very much on how efficient your boiler is, especially when it is operating at part load, as it does in summer when there is no central heating demand. Broadly, if you have a modern high efficiency boiler (say rated ‘D’ or better on the SEDBUK scale) linked to a hot water tank thermostat, then it will be better to use the boiler to heat water, all year round. But, providing you have a timer and well-insulated tank, it may be cheaper to use the electric immersion heater in summer if your boiler is not a high efficiency model.

Recently, a number of people have picked up on this, believing it to be a problem that only affects condensing boilers. They point out that in summer, when the boiler is only heating water, it operates at part load and does not normally enter condensing mode. “Aha!” they say, “in that case it must be wrong to use a condensing boiler only for water heating.” Actually, that’s not true. They are right to point out that the boiler may not be in condensing mode, but even so the larger (or second) heat exchanger unit in a condensing boiler still means that it will operate at a higher efficiency than a conventional boiler (unless it has a very high internal thermal mass). So a condensing boiler is still likely to be cheaper to run than a conventional one in summer. The Government’s A-G boiler energy rating (SEDBUK) is based on average seasonal efficiencies, and takes into account the lesser performance of boilers in summer, so an “A” rated boiler will use less fuel over the year than a “B” rated one, and so on…

Ten years ago, there were a lot of high-pressure salesmen selling “Boiler Energy Managers” with this claim. (I know, I bought one.) But you could make the same savings by switching off your boiler for 10% of the time, and that is what most of these domestic boiler energy managers did. If they are fitted by the manufacturer, then they will save money, but retrofitted the same effect can often be achieved by turning down the thermostat by a degree or so. Both delay the firing of the central heating system. Modern boilers and with good controls should not need an additional strap-on energy manager.

The salesmen have moved on to another product – the magnetic ring that aligns the gas molecules and achieves amazing improvements in boiler efficiency. Well, I would be amazed if these worked as well as they are claimed to, as in some cases they would end up giving boilers more than 100% efficiency. Preliminary investigations by Government scientists suggest the claims are just hot air – I expect that by the time we know for certain, the salesmen will be selling us something else again…

On the other hand, there is evidence that strap-on devices designed to stop scale forming in hot water pipes may be effective in hard water areas. These apparently work by precipitating out limescale into microscopic particles that stay suspended in the hot water, rather than coalescing as scale on the walls of the pipe. It’s not clear how much energy they can save (and it’s unlikely to be anywhere near 10%), but hopefully there will be independent test results available in the near future.

Technical jargon explained

A flue system designed by the boiler manufacture, which permits flexibility in terminal location i.e. low level, vertically or horizontally.

An energy source derived from renewable material (such as wood, wheat or oilseeds) that can be burned to generate heat, power or motion.

A liquid bio-fuel.

An enclosure inside which an oil storage tank can be located which is capable of holding the contents of the oil storage tank plus 10% so as to prevent oil damaging the environment in the event of a leakage or spillage. A Bund used to be known as catchpit.

Construction Industry Research and Information Association.

Communities and Local Government.

A boiler that provides domestic hot water on demand. Also provides space heating.

A high efficiency boiler that recovers the heat from the flue gases which would otherwise be lost to atmosphere.

A switching device fitted to the domestic hot water cylinder which controls the temperature of stored water. This should not be higher than 60 degrees centigrade.


A device fitted in the oil line close to the heating appliance to help the draw of oil up to a level higher than the level of oil in the tank. It does this by removing the air and vapour created when the pump in the oil boiler applies suction to the high level oil line.

Department for Environment, Food and Rural Affairs.

A storage vessel normally made of copper in which domestic hot water is stored. Inside the cylinder is a coil of pipe through which water from the boiler passes to heat the water, which becomes available through the hot water taps.

An electronic gauge includes a transmitter, probe and receiver. The transmitter sends a signal indicating the level of oil in the oil storage tank to a remotely located receiver and display unit. The probe is a sensor located within the oil storage tank that detects the level of oil in the oil storage tank, this is connected to a transmitter. The receiver picks up the signal from that transmitter and displays the level of oil in the oil storage tank.

European Performance of Buildings Directive.

Of a higher viscosity than kerosene, a clean burning oil commonly used for commercial oil firing installations. Also known as 35 second, Class D to BS 286.

Boiler and Room heater.

Home Information Pack (Sellers or estate agents have to produce a Home Information Pack before marketing any property for sale). Requirement of the Homes Bill.

Institute of Domestic Heating Environmental Engineers.

An oil storage tank that is constructed at the factory to include a bund facility plus a lid to keep the bund clear of rain water.

This is a light, low sulphur and very clean burning heating oil suitable for use with most types of oil and cooking appliance. Also known as 28 second, Class C2 to BS 2869.

An electrically powered valve for the control of water flow through pipes, it can be operated by a time switch or thermostat.

National Association of Chimney Sweeps.

A chimney taking flue gases from an appliance which terminates in the open air, usually above roof level. Masonry chimneys must always be fitted with a flexible flue liner to prevent the gases coming into contact with the chimney itself. The liner needs to be replaced each time a boiler is fitted. An appliance connected to an open flue must be provided with a combustion air supply directly from the outside.

The type of burner commonly used on all modern boilers and a number of cookers. Oil is injected, via a nozzle, into an airstream created by a fan. The oil is then vaporised and ignited by a spark.

A time activated switching device used to regulate the times when space and water heating services will be operated by the boiler. A single channel programmer uses one set of times to operate both services. For space and water heating control it is better to use a two channel programmer, which enables services to be operated independently to each other.

A boiler that heats water. This heated water is then pumped around the system to provide space or water heating.

A valve fitted in the oil line external to the building which incorporates the sensor that is located inside the boiler casing so as to shut off the oil supply to the building externally in case of a fire or overheat at the appliance.

A device which switches heating control equipment on and off according to the temperature which has been set by the user.

Standard Assessment Procedure (measures the energy in buildings).

Scottish Building Standards Agency.

Seasonal Efficiency of Domestic Boilers in the UK.

A transparent tube fitted in the outer pipe close to the tank and run vertically up to just above the highest part of the tank. Where the tube joins the draw off pipe there is a push bottom valve which when depressed enables oil to rise up the tube to the same level as the oil in the tank.

Providing heat for rooms.

Like a regular boiler, but often includes a circulating pump, expansion vessel and safety pressure relief valve within its casing.

Non electric valves which regulate water flow through a radiator in accordance with the amount of heat required to maintain a pre-set temperature in a room.

A type of burner often used in cookers and room heaters, which burns the oil by heating it so that vapour is given off to maintain a flame. Silent in operation.

Providing heat for domestic hot water.

Providing separate space heating control for different parts of a building.

Landlord Certificates

Correct oil fired equipment installation and maintenance is essential for performance and safety. The following has been produced to clarify requirements surrounding rented accommodation:

  • There is no legal requirement in the United Kingdom to obtain a landlord safety certificate for oil fired equipment installed within a let property. 
  • However, BS 5410 Part 1 requires oil fired appliances to be serviced periodically in accordance with the manufactures instructions. 
  • Oil tanks and oil feed pipe work should be checked for general condition and any leaks repaired. This is to reduce pollution incidents which may result from inadequately stored oil. 
  • To promote safety of tenants and instill peace of mind to Tenants, Letting Agents, Landlords and to satisfy some property insurers, it is recommended that an OFTEC Registered Technician services and inspects an oil installation at least annually. 
  • As well as peace of mind, planned maintenance can keep an appliance operating at peak performance, reducing fuel bills, save on costly call out fees, if an appliance fails and reduces the risk of carbon monoxide poisoning.

There have been no recorded deaths related to oil fired appliances and carbon monoxide, but if an oil fired appliance is incorrectly installed and/or maintained this can, although not common, produce carbon monoxide. Carbon monoxide is a highly toxic poisonous gas which is given off by appliances burning all fossil fuels. It is odourless, colourless and tasteless and is therefore difficult to detect. Some symptoms of carbon monoxide poisoning are as follows:

  • Tiredness 
  • Headaches 
  • Dizziness 
  • Nausea 
  • Vomiting 


For extra protection fit an audible carbon monoxide detector manufactured to BS EN 50291 and BS 7860. Carbon monoxide detectors are warning devices only and must never be substituted for regular service and maintenance.

OFTEC Registered Technicians have been trained to work competently, safely and applying recognised industry practices.

OFTEC Registered Service and Commissioning Technicians are the only persons that can obtain industry recognised OFTEC CD/11 Service and Commissioning Forms. The form provides written evidence of service/maintenance confirming the condition of the appliance and installation, any non-conformities and any remedial work required to bring the installation up to current regulations standards. The Landlord/Letting Agent should retain a copy of the CD/11 form so they can demonstrate that they have met their legal duty of care to the tenant by having the installed oil fired equipment properly maintained and tested.

Where tenancy agreements put the responsibility of the maintenance on the tenant, it is highly recommended for Landlords and Letting Agents peace of mind that the tenant is required to use an OFTEC Registered Technician, so they can provide a copy of a CD/11 form to demonstrate that they have met the terms of their tenancy agreement.

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