How to attach a flue pipe to a furnace


Although gas furnaces are relatively simple pieces of equipment when compared with regular boilers, they do, however, require a gas expulsion system. The way in which gas boilers heat water is based on fuel burning, and this generates gaseous waste that needs to be expelled outside of the room in which the boiler is located. Today, we will discuss how to attach a flue pipe to a furnace. After reading this article, you understand everything you need to know about this subject and several other related details.

What is a furnace? And how does it differ from boiler?

Let’s begin by explaining what a furnace is so that we can have a better understanding of how your gas outlet system works and how to install it correctly. At the beginning of the article, we mentioned that a furnace and a boiler have some basic differences. So, what are these differences?


A boiler is a piece of equipment that burns fuel to provide hot water for heating the home and for the domestic hot water system (DHW). The latter also relates to a combi boiler, which is currently the most common type of boiler.


A furnace, unlike a boiler, only generates domestic hot water, and not heating. It also works by burning fuel (gas) and increases the temperature of the water flowing through it by means of a coil which is heated through the combustion process. An advantage of a furnace is that it can provide an unlimited amount of hot water, as long as it is the correct size for your home’s water consumption needs, taking into account the number of people in the home and the number of water consumption points (showers, sinks, washing machines, etc).


Obviously, units that require a gas expulsion system work by burning fuel (boilers and furnaces as electric water heaters don’t need these). Manufacturers include a basic flue pipe with gas furnaces, which may not be sufficient in many cases, but is fine in others.

Depending on the space available, or the distance between the furnace and the chimney, or even depending on whether or not there is an individual chimney or if the furnace’s output has been incorporated into a communal chimney, you may need to resort to a more basic gas exhaust system.

Types of gas expulsion systems

There are 3 basic gas expulsion systems that are applicable to both boilers and furnaces (and even electric water heaters). These parts are classified as “natural flow”, “forced flow” or “watertight”, based on how they have been manufactured. Let’s take a look at each of these separately.

Natural flow system

This kind of gas furnace is the simplest out of all of them, the most popular and widely used, and the least prone to clogging or breaking down. It doesn’t force the gas in or out, but allows the process to take place naturally.

The process takes place through a pipe with a specific diameter that allows gas expulsion through an opening in the furnace. These pipes should be at least 20cm in length to be able to transport the gas outside.

This kind of system requires adequate space, meaning that the room in which the furnace is located must be large enough and well-ventilated. A good aspect of this system is that it is compatible with all kinds of gas: natural, butane or propane.

Forced flow system

A basic characteristic of the forced flow expulsion system is that it is driven by an electric fan that forces the gas to be expelled outside of the furnace.

One of the disadvantages of this kind of expulsion system is therefore that it requires the furnace to be connected to an electric current. This involves an additional expense and also an indirect increase in your furnace’s polluting capability.

These kinds of furnaces are ideal for confined spaces that don’t have their own ventilation system. They have a smaller design (they are 20% smaller than other boilers) and therefore provide less hot water. They are only recommended for small families.


Watertight system

What is unique about watertight boilers is that both the oxygen inlet and the gas outlet are completely isolated from the room in which the boiler is located. The oxygen for gas combustion enters from outside the home via a pipe or chimney, and gas expulsion is also carried out remotely with a series of pipes connecting the system to an individual or shared chimney.

The new regulations on heating and DHW provide that all gas expulsion systems should have a watertight design. This is not because they are safer, but also because they are more fuel efficient.

Browse boilers and accessories or sale at Suner

Suner offers you a great range of boilers that comply with the most advanced regulations in terms of reducing pollution emissions. These boilers have watertight systems, and are optimally designed and modern with respect to size, elegance and energy efficiency.



It is better to use a professional for installations

It is better to use a qualified professional if you need to install a gas boiler. A professional has all the technical knowledge in order to correctly install the unit, and they have all the necessary tools, they know how to install the unit safely, and they are aware of the legal regulations regarding boiler installations which must comply with the following new regulations:

I. T. E. | ErP | ELD


The 3 kinds of gas outlet designs

There are various ways of expelling gas that are designed not around the unit itself but the space in which it is installed. As is the case with boilers, furnaces can be used with three flue gas outlet designs: coaxial, vertical coaxial, and double pipe.

Choosing one of these three designs will depend on factors such as the space where the boiler is located, the distance between the unit and the outsides, if the final outlet pipe is individual or shared, etc. Based on this, there are three gas outlet designs, and the one you choose will depend on the following:

Coaxial gas outlet

As its name suggests, in the case of a coaxial installation, both pipes (the gas inlet and gas outlet) are situated one inside the other concentrically. This therefore means that one of the pipes must be smaller in diameter than the other one.


Through the larger diameter pipe (or rather through the space between the smaller pipe’s wall and the larger pipe’s inner wall) oxygen is absorbed from the atmosphere that functions as an oxidizer for the gas; while gases are expelled to the outside via the smaller pipe that has been inserted into the larger one.

This kind of installation is sufficient in most cases, and in fact is the most commonly used in conventional boiler installations. It is normally built into the boiler at the time of manufacturing. The maximum distance from the boiler to the final gas outlet should not be longer than 4 to 6 m.

Vertical coaxial gaas outlet

Generally speaking, a vertical coaxial design doesn’t differ at all from a normal coaxial design. The only difference is in the position of th flue gas pipe which is vertical and has an upward direction. This option should be chosen if the boiler is located in a position or in a room that does not allow a horizontal outlet.

With this design, the pipes remain concentric or coaxial with one another. Ther vertical position doesn’t affect anything technical to do with the flue gas outlet pipe when it is installed in a watertight boiler. However, if it is a natural flow installation, the vertical position may have an advantage in that it facilitates the exit of gases and prevents the fumes from moving backwards.


Double pipe gas outlet

The non-coaxial double pipe design is the perfect solution for when a final gas outlet is very far from the boiler (it can even be 20 m away).

In this kind of design, the two pipes (the one for the air intake for oxidation and the other one for the gas expulsion) are separated. Each pipe should have a diameter of about 8cm.

This double pipe design is far less common than the previous two designs. However, it may be the best solution, or even the only solution in certain circumstances.

(See more details in our article “Regulations for diesel boiler chimneys: for clean air”).

Frequently asked questions about gas expulsion in boilers

1 – Can I install a forced flow system to a natural flow boiler?

Yes, you can install a forced flow system to a natural flow boiler, and it is not difficult to do so. In fact, it is recommended as it is better for the boiler to comply with the current regulations around ecological design for hot water and heating appliances.

There are many advantages of adding a forced flow system to a natural flow boiler: it is a safer gas expulsion system, you will no longer have birds nesting in the chimney, the fumes will no longer return into the room in which the boiler is located, and it is quite possible that you will see a reduction on your energy bill.

Forced flow kits are available for sale and include a fan and temperature sensor. Natural flow boilers don’t have electricity, so the forced flow will need to be connected to the power supply in some way.

The flow should be located in the vertical tube, and in the direction in which the blades move the air, in addition to the sensor needing to be placed next to the factory-fitted one that came with the boiler. It is not recommended that you tamper with or diable the factory-fitted sensor.


2 – When it rains, water enters my boiler’s chimney

This is a common occurrence that can happen often, especially in boilers that are located outdoors, especially if they have a vertical gas outlet. In these cases, when it rains heavily, water can enter through the chimney’s mouth and obviously the boiler might stop working.

Below are some points that you can resolve. Firstly, you would need to make sure that the tube coming out of the cabinet (using a cabinet with outdoor hot water appliances is stipulated in the regulations) has been sealed well around the corners. You can also place an elbow joint after the vertical part of the beginning of the pipe, and you can add in a horizontal joint at the end. Of course, it is essential to add a small, well-attached metal roof at the end of the chimney.


3 – My furnace is natural flow and smoke returns through the outlet pipe

Although manufacturing and selling boilers that aren’t watertight is not prohibited, using another type of system in a boiler that has been installed before the regulations came into effect is not allowed.

In certain kinds of older installations, the natural flow gas boiler’s flue gas outlet may have been horizontal. And because this kind of design expels the smoke directly from the boiler (without even using a fan to force the smoke out), sometimes the smoke can spontaneously reverse and go inside, contaminating the space in which the boiler is located.

The solution to this problem may be to place an elbow joint vertically at the boiler’s outlet pipe (it cannot be a PVC pipe as this melts, so it should be metal), and place the first section of the gas outlet pipe vertically. This ensures that the smoke is expelled upwards and then outwards so that it doesn’t return into the boiler or the room.

4 – Can a watertight boiler be installed with a shared shunt with gas boilers?

Technically you cannot connect a watertight boiler to a shared shunt with gas type boilers. The shunt for gas boilers is different to that of watertight appliances.

5 – Can I use a brick chimney as a vent for a boiler?

Not really. A boiler’s flue pipe can be inserted via a brick chimney, which is used as a path to a typical outdoor boiler chimney. However, the brick chimney itself can’t be used as the only gas outlet for the boiler.


6 – Distance / pipe diameter ratio

The relationship between the distance of the smoke outlet and the diameter of the pipes to be used might depend, to some extent, on the model and brand of the boiler. In coaxial pipes, the measurements can be 60-100 or 80-125.

The 60/100 measurement is quite common and works well in most cases. The 80/125 measurement is for appliances where the pipe must travel a longer way to reach the outside.

Either way, ensure that this installation is carried out by a qualified professional. With the use of the user manual and their technical experience, the professional will know which diameter pipe you need.

7 – What parts should I use to install a smoke vent?

With watertight boilers (which are the only ones that can currently be manufactured legally), the maximum distance from the gas outlet pipe is determined by the individual manufacturer and according to the appliance’s characteristics. This, in turn, is what determines how many and the kind of parts to use.

In the boiler industry, it is common for manufacturers and sellers to sell these at varying prices to the user, which is based on the parts needed for each appliance.

The boiler’s price will vary slightly depending on whether the user needs a short or long term installation, smaller or larger diameter pipes, a vertical or horizontal design, coaxial or 2 pipes, for individual or shared chimneys, etc.

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