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Vaillant Gas Boiler and Windhager Pellet Boiler

Condensing Gas Boilers

Vaillant Gas Boiler installed with weather compensation in Colchester

This section is titled "Condensing Gas Boilers" but what exactly is a Condensing Boiler?

In practice it is a generic term for a modern heating appliance which is very efficient, the products of combustion are so cool, the water vapour "Condenses" and results in the characteristic pluming encountered from the flue terminal with these appliances.

These appliances are also "Room Sealed" and draw the air required for combustion from outside the property, rather than using the air within the property.

The room sealed technology has two major benefits regarding efficiency, firstly the boiler is not using the air from the room which we are paying to heat and secondly, a large air vent is not generally required in an outside wall for ventilation, allowing cold air to be drawn in to the room from outside the property.

If your own non-condensing boiler is due for replacement, it is very likely that being replaced by a Condensing Boiler will reduce your energy consumption and with further strategies applied, additional savings can be made.

Importantly, room sealed condensing appliances are also inherently safe being sealed from the room and reducing the likelyhood of carbon monoxide entering your property

If you wish for your boiler to be repaired, replaced or serviced, simply call us on 01206 432 999.

However, are you Interested in Heat Lab Ltd improving the efficiency of your gas boiler?

We are glad you are still reading and are interested in energy efficiency, our passion. Modern Condensing appliances are rated around 100% efficient but are typically only 90%* efficient.

The efficiency can be improved by ensuring the heating water in the heat exchanger is as cool as possible and this can be acheived in a number of ways;

All of these products are available from ourselves and have a thorough description in their relevant section of this website or just click on the description above.

It should also be noted that manufacturers use the "Net" calorific value of the gas used to calculate efficiency but at Heat Lab, we use the "Gross" calorific value and arrive at real world efficiency ratings.

In the forthcoming weeks, we will be installing a heat meter onto a modern natural gas condensing appliance, measuring the heat generated and distributed through the property. This will cover the heating and hot water requirements of a renovated 1960's property.

* According to SEDBUK ratings

The Technical Side of Condensing Gas Boilers

Types of Condensing Gas Boilers

There are predominently two types, a Regular Boiler or a Combination Boiler. Each is available from a multitude of manufacturers, costs vary, various kw ratings are available and personal preference or previous experience often dictates buying of the appliance.

This section will explain the key differences between each type of Condensing Boiler and will also show schematics for their conventional installations.

Regular Gas Boiler Schematic

Regular Boiler

The regular boiler is probably the simplest and most adaptable available, irrelevant of the manufacturer. Depending upon the exact model, they can be installed open vented or on a sealed (pressurised) system, on a "S", "Y" or "W" plan, with or without a Domestic Hot Water Cylinder or Thermal Store.

As can be seen by the plumbing schematic to the left, there are a greater number of external components and as such, the space required for the installation is greatly increased. The installation is much more flexible in design, with inclusion of additional boilers, wood burning stoves and solar thermal not only possible, it is recommended. Installing Combination Boilers on multi-fuelled installations, while possible, is more complicated depending upon the application.

The instantaneous Domestic Hot Water performance is not governed by the performance of the regular boiler, as the water is stored ready for use and is mearly restricted by either the cold main entering the property, the static head pressure, the booster pumps used or the storage volume.

Regular Boilers are often used with open vented type not water cylinders, the system relys on the height difference between the cold water storage tank and the hot water outlet. This type of installation is simple and effective, although pressure and/or water flow can be limited.

Also available are Unvented Cylinders or Thermal Stores, which can provide much better hot water performance if correctly installed and again, we would recommend referring to the links above for further information.

A possible scenario could be a modern 2 bedroom bungalow with a low heat loss of 7kw and the bathroom has luxury drench shower requiring 30 litres per minute. Fortunately, the Regular Boiler would be capable of heating the property and it would be capable of providing enough domestic hot water for the shower, given the correct hot water storage/boosting equipment. This type of installation is generally very successful.

Combi Gas Boiler Schematic

Combination Boiler - Combi

Externally Combination boilers look very similar to regular boilers, they are generally slightly larger and more pipework emits from the unit itself, to the left is a typical system schematic for a combination boiler.

Internally Combination boilers feature atleast another heat exchanger, a diverter valve, a pressure relief valve and circulation pump, this adds to the purchase price of the actual boiler. As many of the components required for our heating system are internal, the complexity of the system is reduced and possibly less space is required for the installation. Generally, Combination Boilers are also installed onto sealed heating systems and require additional components to ensure the water pressure remains within safe margins.

The Combination Boiler has two downsides, Domestic Hot Water is not stored and the available hot water flow would be limited 10-15 litres per minute, depending on the boiler.

The Cold Main water pressure also needs to be suitable for the Combination Boiler, as the water pressure forces the water through the boiler and out of the hot tap/s. If the water pressure and/or flow is below the level required, please refer to our products and solutions for Cold Water Boosting in the above links.

The selection of the heat output of the Combination Boiler is dictated by the required domestic hot water requirements and not the space heating requirements.

In our previous scenario of a modern 2 bedroom bungalow with a low heat loss of 7kw and a bathroom with a luxury drench shower requiring 30 litres per minute. Unfortunately, although the Combination Boiler would be capable of heating the property, it would not be capable of providing enough domestic hot water for the shower and as such, a Combination Boiler would not be installed.

Combustion - Natural Gas

Using heat for comfort is not a recent innovation, humans have been using fire for thousands of years but we now have a very thorough understanding of the science involved. The chemistry does depend upon the fuel used but all domestic or commercial boilers rely on the combustion triangle.

Combustion Triangle

The Combustion Triangle comprises of the three conditions to create fire, or combustion. If you take any of the three conditions away, the chemical process can not be supported and the combustion stops.

CH4 + 2O2 = CO2 + 2H2O

The above chemical equation is representing what is happening with combustion using CH4, Methane (Natural Gas) as a fuel.

Therefore the water vapour that is visible from a Condensing Gas Boiler Flue Terminal is present in all boilers, it is just at a lower temperature and critically, is below the dewpoint of the atmospheric conditions - hence it can be seen.

We must therefore be aware that although pluming at a flue terminal is an indication of low flue temperatures, which with Condensing appliances is desirable, it is still an indication that heat is being wasted. The change of state that is taking place forming condensation is using energy and this is happening outside of the boiler/flue system.

This change of state is using latent heat from our products of combustion and it is energy not being used to heat our property or domestic hot water, which is obviously not good for our boiler efficiency.


As we have discovered in the Combustion Triangle above, we require Oxygen to support Combustion and tradiationally with open flued appliances, the air within the heated room was used and an air vent in the outside wall allowed fresh (possibly cold) air to enter from outside.

This ventilation, although critical for combustion, is actually cooling the room we are trying to heat - not good.

Flat Roof Flue Terminal

This is another major advantage of "Room Sealed" appliances, they are sealed to the room and require no air vents to be added to external walls, unless the appliance manufacturer requires additional ventilation for cooling the appliance.

To the left is a picture of a Flat Roof Flue Terminal, the red arrow is showing the route of products of combustion and the blue arrows are showing the inlet for the combustion air to enter the Condensing Boiler. This type of flue is called a balanced flue.

If we are using an existing open flued appliance, we do not have this arrangement for the combustion air to enter the boiler and an additional air vent must be installed. The size of this air vent is calculated according to the Net Heat Output of the appliance in kilowatts and below is an example of such a ventilation calculation.

20 (Net Heat Output of Open Flued Boiler in kw) - 7 (adventitous air) * 5 (cm2 per kw) = 65cm2 Vent or Larger.

It is clear that a vent to the outside air of this size has a potential to cause draughts and fortunately, is mainly only required on open flued appliances.

Inadequate ventilation is potentially a very serious situation.

Firstly a lack of ventilation will not allow complete combustion to occur and carbon monoxide will be formed, potentially entering the room the appliance is located. Secondly, a lack of ventilation will also deteriorate the performance of a open flued chimney system and the lack of draught will likely not clear the products of combustion (which are also likely to be very high in Carbon Monoxide).

Carbon Monoxide is a killer. Carbon Monoxide is formed whenever there is incomplete combustion and is present in the flue system of even the best performing boiler.

Routine maintenance is required to ensure that only minimal levels of Carbon Monoxide are formed, check all seals/gaskets in the boiler are in good condition and that the flue system is operating correctly to ensure that these products of combustion do not enter the premises.

The situation of products of combustion entering the room is known as "spilling". If encountered, an appliance should not be used, the area well ventilated and an engineer called prior to entering the property.


The commissioning of a Condensing Boiler is critical if it is to perform efficiently, effectively and most importantly, safely.

Listed below are some key factors which Heat Lab Ltd verify before the Boiler is commissioned;

Condensing Boiler Zero Governor

It is important to select a heat output of the boiler that is suitable for the property in which it is installed, with modern Condensing Boilers, this is simply adjusted from the control panel on the appliance and this must be done after the appliance has been commissioned on "high fire".

As the gas valves are "zero governors" (pictured left) in these modulating appliances, the burner pressure and therefore heat output can not be adjusted, unlike conventional gas control valves.

It is important that the boiler remains on high fire throught the commissioning process, the boiler has the ability to reduce the heat output of the appliance automatically depending on the demand of the system.

If the boiler is becoming close to the temperature setpoint, the boiler will modulate down by decreasing the speed of the combustion fan and this will effect the pressure applied to the "zero governor", reducing the quantity of gas entering the appliance for combustion.

Generally Condensing Boilers are more efficient at the lowest heat output available on the appliance.

The zero governer is a valve contained within the boiler and it controls the flow of gas into the burner depending upon the speed of the combustion fan, which is dictated by the demand on the system. These valves should only be adjusted by the appliance manufacturer, unless specified in the manufacturers instructions.

Once the commissioning process is complete and all factors have been passed, the installation is now ready for use.

A little about Heat Emitters

As mentioned previously, there are various strategies available to optimise this condensing condition and primarily, they all rely on reducing the temperature of the water flowing through the plate heat exchanger.

It is commonly misunderstood that it is only the boiler return temperature that must be below 50 Deg C, this is incorrect.

What is important is the COMPLETE heat exchanger temperature, the heating water returning must be as cool as possible but the water flowing out of the boiler must also be as cool as possible. Further information is included in the "Oversizing Radiators" section in the technical articles above.

After reading the section on "Oversized Radiators" in the above links, it is clear that there is no definitive return temperature that a Condensing Boiler suddenly starts to condense and becomes more efficient. It is the average temperature across the heat exchanger that matters and the golden rule is, the cooler the better.

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