Heating terminology



Traditionally a dial type wall mounted control.  They now come digital, wireless and smart home connected.  You can still get the standard hard wired dial though, don't worry.


A timer is the term for a single zone timed control, usually just heating or just hot water.  One or the other.


A programmer has more than one zone, heating and hot water is most likely.  You would usually still have a separate thermostat.


Either in the airing cupboard, under the boiler or in the boiler.  You will have at least one pump on any modern system to circulate heated water through the radiators, underfloor heating or cylinder.

Cylinder thermostat or sensor

We need to know when to start and stop heating a hot water store.  Older systems used a simple on/off mains thermostat strapped to or integrated with the cylinder.  Nowadays we can use resistance lead sensors wired to either a 3rd party control or the boiler itself.

Thermostatic radiator valves - TRV

These are thermostatic heads which open and close depending on the ambient air temperature in the room.  There are several manufacturers who now make smart wireless battery operated TRV's as well as standard stand-alone heads.

Motorised valves

For systems with more than one circuit (e.g heating and hot water or two heating zones), motorised valves can be used to close and open whenever the demand requires.  2-port and 3-port valves are very common and a likely culprit in a breakdown situation.

Boiler thermostat / potentiometer

The temperature dial on the boiler itself.  This will govern the temperature the boilers set point.  Depending on the boiler it could be a cut out switch, modulation guide point or part of a weather compensation system.


Boiler Types

Condensing boiler
All gas boilers sold in the U.K are condensing boilers*, this simply means they are capable of condensing which is explained further in another post.  A common misconception is "condensing" and "combination" being the same thing.

*Note - There are some exceptions to the rule on condensing boilers but for clarity I haven't gone into it.

Open vent or heat only

These are "traditional"  boilers which simply make heat.  The system will still need an external pump, motorised valves, a water store and some way of pressure accommodation, either a loft tank or an expansion vessel. 

These are still popular due to cost and ease of install although don't necessarily give the optimum upgrade.

System boiler

A system boiler incorporates a pump and pressure vessel, it is also configured for and has the ability to be attached to an external hot water store.

These are great for integration and more intelligent control of heating and hot water.

Combination boiler

Probably the most commonly known boiler type, especially in cities.  A combination boiler combines heating and hot water production within one box, it doesn't need tanks or a cylinder for hot water.

These are great for smaller properties, they are not so great for high demand hot water.  They aren't as efficient and don't cope with high flow rates.

Storage combination boiler

A bit of a niche product, a storage combination has a small internal tank and the boiler feeds it to provide hot water.  Not as efficient as a separate cylinder generally but better for running costs and flow rates than a standard combination boiler.

These are great for a medium sized property with multiple showers but restricted space for the hot water store.

Hybrid gas boiler and heat pump

As the name suggests they have a gas boiler but integrate an air source heat pump.

These are great for more clean homes as they don't rely on fossil fuels as much and were part government grants (which hopefully will come back one day).  Of course if you have solar PV and can use the generated power to run the heat pump then this would be a massive benefit.


Condensate/ Condense

As previously mentioned most boilers fitted in the U.K now are condensing.  This means literally that they are capable of passing condensation down through the heat exchanger to increase the amount of heat taken from products of combustion.

Instead of expelling all the heat in the form of vapour into the atmosphere a condensing boiler aims to trap the fluid to increase efficiency.  This is then terminated into a drain safely after going through a siphon.

The condensate itself is clear and odourless but quite acidic, it must be terminated via plastic into a suitable drain, not iron or cement.  If this is not possible then a netraliser or pump can be used.


Heating systems can be dirty, they are often old and may have been previously open vented and neglected.

It is mandatory when fitting a new boiler, cylinder or other major appliance to flush the system.  This ensures that the contaminants that were in the old system don't damage or reduce the life-span of the new appliances.  The method of flush is usually chosen by the engineer on site but should be selected to ensure the cleanliness and longevity of the new components.

Hot flush

One of the most simple.  The engineer puts a hose on the system somewhere, turns the boiler on and maintains a constant pressure in the system while maintaining a constant flow from the hose to the drain.  Care must be taken to avoid short cutting, it is easy to miss large parts of the system with this method.

Magnetic flush

This involves using a large magnetic filter, placed ideally just before the heat source and filling loop.  The filter grabs any of the dirt in the system as it is magnetic, chemicals and agitators are used and heat will speed up the process.

Power flush

A very commonly  known term, this is the more expensive but comprehensive option.  A small machine with a powerful pump is set up to circulate the system water, it can be reversed to help move the dirt around and into the machine. 

Power flushing is best done with heat, chemicals and a magnetic filter.

Water testing

It isn't good enough to simply look at the water to know when the flush is finished.  Although the water could be quite clear to the eye there may still be plenty of dissolved solids and an incorrect ph level.

Flushing a dirty system will increase the efficiency, reduce fuel use, increase life expectancy of all components and reduce the chance of break downs.

Following a flush it is highly recommended that a primary filter is fitted to give long term protection.


Historically in the U.K we as heating installers and plumbers have simply added inhibitor to the system to protect it without looking at the ramifications of overdosing or any understanding of the contents of the chemicals. 

We like to assess each situation individually, there is no panacea.  We may use inhibitor in some situations but we would prefer not to.

Waste handling and disposal

Any tradesperson who works in your home or out of it has a requirement by law to hold a current waste carriers license.

If they do not have one then they should not transport any waste as part of their job, there are fines in place which can be issued by the police.

Any waste generated as part of an installation will ordinarily be removed by the installation team and disposed of legally and environmentally.

Waste metals are recycled where practical and profit may be made from their disposal.

Gas Safe Register

It is absolutely crucial that any individual working on gas appliances or pipe-work must hold current and relevant Gas Safe qualifications.

Some people still mistakenly believe Corgi to be the regulatory body, they lost the contract in the U.K in April 2009.  Since then the name "Gas Safe Register" has been created with the HSE so although the scheme operator may change, the name will always be the same.

You should always ask to see the photo card of the engineer when they start work in the house if they are going to be working on gas at any point.

You can check all registered businesses and individuals on https://www.gassaferegister.co.uk/

System filters/ Magnaclean

Magnetic filters are units fitted on the return to a boiler that grab the sludge before it can into and damage the appliance.

There are now a huge variety of filters made by many different companies:

Magnaclean is the most famous and one of the first widely available filters, a plastic cylinder design with a powerful magnet in the centre.  The dirt sticks to the magnet and can be removed and cleaned during maintenance.

Spirotech make the MB3, a brass body filter with a small magnet and internal water filters to drive the dirt to a low point ready to be flushed out during maintenance.  They also make an RV2 de-airator for removing air particles from the water, air is a huge cause of water quality issues as it is essentially aiding rust.

Fernox TF1 is very similar to the Magnaclean in construction although has a valve underneath for draining and flushing.

Fernox Omega is a nickel plated brass body filter and incorporates the best aspect of the other filters.  A central magnet, internal baffles for trapping dirt, all metal construction and a flushing point underneath.  Unlike some of the other filters it will not restrict flow if it does fill up with dirt.

Carbon Monoxide

Carbon monoxide is produced by any fossil fuel when it's being burned.

You can't smell it or see it or hear it but it can kill an adult in a very short space of time.

The first to be effected are children and small pets.

It is rare that a well serviced appliance will get into a state that could allow carbon monoxide into a living space but an appliance like an open flued gas fire or boiler could very easily become dangerous if one of several crucial safety devices fail or the ventilation is blocked

Modern boilers are less likely to allow carbon monoxide into the living space due to their sealed design and concentric flue but installation error of neglect could still allow them to be dangerous.  A simple case seal or flue jointing issue could create a dangerous situation.

These reasons are a large part of the legal requirement for landlords to have annual safety inspections at their properties.  Many insurance companies for shared accommodation are also now stipulating this.

More than fifty people a year die from carbon monoxide poisoning and many more are affected.

Fortunately there are steps you can take to reduce the risk significantly:

Firstly having a registered gas company or individual conduct an annual service on all gas appliances in the home.

Secondly install and regularly check carbon monoxide detectors in all rooms that burn fuel, including wood fires.

Thirdly educate yourself on what to look for in carbon monoxide poisoning.

How does carbon monoxide poisoning affect a person?

In very low concentration carbon monoxide may give headaches, dizziness and blurred vision.

In medium concentration carbon monoxide will give the above with disorientation, muscle cramps and breathing difficulty.

In high concentration carbon monoxide will cause paralysis, induce a coma and eventually death.