Everything you need to know about condensing boiler drains


Condensing boiler drains are well-known for their excellent fuel efficiency. With condensing boilers, drainage however is an important issue because the recycled heat always ends up as condensation and converts into water. In this article, we will be discussing in detail everything you need to know about your condensing boiler’s drain: how it works, how to install it, and the most recent regulations around it.

What is a condensing boiler?

Generally speaking, there are three types of boilers and this is based on the way in which they work:

  • Watertight boilers: This type of boiler functions in a way that is very safe for the user, but is not that efficient in terms of fuel use. Its combustion chamber is completely hermetic to the outside elements, and the fuel has no outlet connecting it to the room in which the boiler is located. Its only connection with the outside is the air pipe that enters from the turbine and the gas expulsion system, also known as the chimney.
  • Atmospheric boilers: This system is not as safe as a watertight one as the combustion chamber is open to the outside (hence the name of the boiler). The air or oxidizer used to burn the fuel comes from the surrounding atmosphere, meaning the room in which the boiler is located. Installing this kind of boiler has not been permitted in Spain for a number of years.
  • Condensing boilers: This boiler has the most technologically advanced, efficient, and modern design. Its design is based on the sealed boiler design but has been improved in terms of fuel efficiency. The combustion chamber is also closed to the outside elements, and it is therefore considered very safe. However, what is unique about this type of boiler is the fact that before the heat is expelled through the chimneys, it is reused to preheat the water that returns cold in temperature after heating the heating system. This means that when the boiler starts again, it won’t need as much fuel to reheat the water that serves the heating system.

Condensing boilers are one of the most innovative units in their field. Traditional boilers typically expel a large portion of their latent heat into the atmosphere via the smoke expulsion system. With conventional boilers the thermal energy is discarded, but with condensing boilers, this energy is recycled, and then reused for the heating and domestic hot water system.

A condensing boiler is truly exceptional when it comes to fuel efficiency. It can easily exceed 100% fuel efficiency (and can often even reach 110% fuel efficiency). This is more than the 90% fuel efficiency that conventional boilers (seal and atmospheric) have. This positive feature of the condensing boiler is precisely due to its ability to extract additional heat from the waste gases and condense the water vapor so that it can make the most of the latent heat present in the vaporization process.

How a condensing boiler’s drainage works

The initial installation of a condensing boiler is slightly more expensive than that of a sealed-only boiler. However, you will soon recover this investment by significantly saving on fuel costs as time goes on, in addition to helping to reduce the harmful, polluting emissions from your home.

There is an important detail to note about condensing boilers. A specific kind of drain is needed to remove the condensation generated by this type of boiler. This should be studied in detail as it could affect the correct functioning of your boiler.

When a condensing boiler has finished using the heat from the fumes inside, it cools it down, condenses it, and then turns it into small water droplets. This is why boilers with this type of design need a tray inside to collect the heat that has been condensed into water. This tray should be emptied on a regular basis so that it doesn’t overflow.

Some experts believe that a condensing boiler can be installed without a drain. This is, however, nothing more than a myth. It is not convenient to install a condensing boiler without also installing a drain pipe at the same time, or at least attaching a larger tank than the one that came with the unit so that it can collect the water produced by the heat recycling process. The only way that this can be avoided is by emptying the small tank that came with the boiler several times a day.

If you want to change your condensing boiler’s fuel efficiency and low polluting emissions rate from low to practical, it is important that the equipment is correctly installed with a drain that doesn’t have any blockages. For the system to function optimally and correctly, the condensation residue needs to be expelled properly.


How much water does a condensing boiler produce?

As already mentioned, condensing boilers normally come with a small drain tray from the manufacturer. However, this can quickly fill up and spill over easily if you do not empty it regularly. It is not a very good idea to use this small tray to collect the waste from your condensing boiler.

You might want to consider installing a larger drain for your condensing boiler to make life easier for you. Otherwise, you will end up with a lot of unnecessary work by having to empty the small tray more often than you have to.

The amount of water that a condensing boiler produces varies from model to model. It normally depends on the level of use, the season, the setpoint temperature indicated on the thermostat, the geographical area in which you live, and even the manufacturer’s boiler design. Whatever the case, generally speaking, these types of boilers (when working at full capacity) should generate 1 to 2 liters of water per day.

If you use a large enough container with the drain, or you install pipe directly to your home’s general use drain, you will have the convenience of not having to empty out the water manually. However, if you choose to use the container that came with your boiler, you will have to empty it about 3 times per day.

Installing a drain in a condensing boiler

It is far better to fit a drain when your condensing boiler is being installed. The smartest option is for the condensing boiler drain to expel water via your home’s general drain. To do this, you won’t need a special pipe, but you will need one that has been adapted for this purpose and one with the same outlet diameter as your boiler’s.

The most common diameter is normally 2 cm, but depending on your boiler’s brand, the diameter could be anything up to 3.5 cm. To find out what the diameter is, you should consult the user manual that came with your boiler. The total length of piping required will depend on the distance between the boiler and that drain you intend connecting it to. If you are not able to have a separate outlet for your boiler, bear in mind that you could consider using a drain outlet that the boiler shares with another unit. You could also even share the drain with other household appliances (for example, a washing machine).


The material from which the pipes and other accessories is made is very important. The water that your boiler gets rid of is not regular water. It has acidic and corrosive properties. PVC is therefore the best material for this kind of pipe. This material ensures that the pipe will be highly resistant to extreme water temperatures, as well as wear and tear. It is also advisable that your pipe fittings are made from PVC. Copper or lead pipes are not recommended, even though, traditionally, they have been the most widely used materials for piping.

You are already at an advantage if your boiler is located above the ground or on an upper floor. The water moves from the boiler to the drain through the pipe and via the force of gravity. It therefore doesn’t need any other components to help it move (e.g. a pump). However, if your boiler is not higher than ground level or on an upper floor, you will need to install a special pump that can move the water to the drain. This would be a special pump for condensing boilers.

Drain accessories are available for purchase at Suner

You can find original brand name drain accessories to install in your condensing boiler in the Suner store. Click on the following link to find the most suitable drain accessories for you:

Buying from Suner is simple and the logical choice. We only sell original brand name accessories and components, which are produced by second or third parties. You can therefore rely on Suner if you need to install a condensing boiler. With us, you will be able to select spare parts and accessories from our catalogue of more than 30,000 part references from the most prestigious brands on the market. In addition, we offer excellent customer and after-sales service.


Other tips for condensing boilers

  • If you want to install a new boiler, always consider a condensing boiler. The new regulations mean that these boilers are really the only ones that comply, especially in terms of environmental regulations. These new regulations do not mean that other kinds of boilers (that are already installed) are not allowed, but condensing boilers will soon become the most common boilers, and it will be much easier to find spare parts and maintenance for them.
  • Condensing boilers have been shown to be very reliable in many instances. Thanks to their excellent heat recycling system, they normally exceed 100% fuel efficiency. There are low NOx boilers available that are not condensing boilers, but condensing boilers are naturally low on NOx anyway.
  • Condensing boilers are, strictly speaking, the only ones that comply with low pollution emissions regulations in the atmosphere (CO2 and NOx). They also offer great performance and low fuel consumption, making them attractive options for manufacturers.
  • Repair and maintenance of condensing boilers is much easier. Practically all of its parts can be accessed from the front of the unit, including the expansion vessel (which is difficult to access on other boilers).
  • If you use a condensing boiler in conjunction with low-temperature radiators, fan coils, and underfloor heating, your heating bill could be reduced by up to 30%. Even if you only switch to conventional radiators and other traditional parts, your energy bill could drop to up to 15%.
  • Another useful device to use with a condensing boiler is an external probe. When the boiler knows what the outside temperature is, it can take steps to ensure that the boiler operates at the lowest possible power to maintain the temperature set. It therefore operates at maximum fuel efficiency without compromising on comfort.

Drainage regulations for condensing boilers

The regulations very clearly state that not only must each condensing boiler have a drain, but there are strict guidelines on how it should operate. These guidelines have been established in the “Regulations for thermal installations in buildings”, also known by the acronym: R.T.I.B.). The following are some of the clauses that refer to the drainage of boilers, according to R.T.I.B.:

  • The room should have an efficient drainage system, either by means of gravity or, if necessary, with a pump.
  • The connection between the drain valve and the drain itself should be created in such a way that the water passage is visible. The valves must be protected against accidental movement.
  • When the unit is fed with gases that are denser than air, there should be no connection with the lower level (i.e. drains, sumps, ventilation ducts at ground level, etc). This refers to the area around the unit (a 1m radius).

There are other points in these regulations which relate to this article. For instance, although it is not explicitly stated, in practice the rules are that only condensing boilers may be installed. This is due to the fuel efficiency guidelines for heating systems that require them to adhere new objectives regarding the reduction of polluting emissions into the environment.


Boiler manufacturers are required to have a fuel efficiency of more than 86% in their heating equipment. Practically the only boiler type that is able to meet these ecological standards is the condensing boiler. In addition to the Spanish heating regulations, mandatory directives from the European Community regarding polluting emissions (known as ErP [Energy related Products] and ELD [Energy labeling Directive]) also apply.

Click on the following links to explore the European and Spanish regulations on heating installations:

The new European regulations in favor of energy efficiency have been in force since September 2015. These regulations are part of the package of measures approved by the European Union to commit to reducing carbon dioxide emissions by 2020.

These regulations had already been approved since 2009, and the commitment included that after a 6-year transition period, from 2015 the manufacturing or use of heating or sanitary equipment that do not comply with these regulations is prohibited.

The two actionable parts of the directive are the ErP (which deals with regulating energy efficiency and reducing polluting emissions) and the ELD (which requires relevant products to have the correct energy efficient labels, and manufacturers to sell equipment with the relevant documentation along with reports stating how the equipment complies with energy consumption, and noise pollution regulations).

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