The Chimney Check List

If you want to build a new house or renovate your old chimney system, you will have to make many decisions. Our check list takes you step by step to the chimney that is exactly right for you.

"Marry in haste, repent in leisure …“

Although installing a chimney system is a slightly less romantic proposition than this quotation from Friedrich Schiller would indicate, it is equally important to take your time and check thoroughly before you make a decision. The check list below will show you the criteria by which you should judge which chimney system is right for you.

The Check List

Click on the headlines below to learn more.

1. Which type of fireplace would you like to connect?

Three types of fireplaces

Chimney and fireplace experts roughly distinguish between three types of fireplaces:

  1. gas and oil fireplaces
  2. tiled, chimney, or pellet stoves with solid fuel (wood, coal)
  3. biomass boilers (using logs, grains, or wood chips)

For gas and oil fireplaces operating under wet conditions, moisture resistance is an important factor (see entry 3 on the check list).

For tiled, chimney, or pellet stoves operating under dry conditions, soot fire resistance is a decisive factor (see entry 2 on the check list).

Biomass heating in particular offers an enormous potential but its use requires the installation of intelligent chimney systems which must be soot fire resistant and moisture resistant at the same time (see entry 4 on the check list).

2. Soot fire resistance

Using a chimney under dry conditions

When burning solid fuel in the fireplace or the tiled stove, there is always a soot residue which clings to the inside of the chimney and is highly inflammable. All chimney systems used with dry-operation stoves must therefore be soot fire resistant in order to maintain their functionality after a possible soot fire.

These are our soot fire resistant chimney systems:



NISOTT Isolierschornstein

 * also available as the lightweight version OSMOTEC LB

3. Moisture resistance

Using a chimney under wet conditions

When using fireplaces with low flue gas temperatures (e.g., oil and gas heaters, pellet stoves or pellet boilers), it is possible that condensate will accumulate in the interior pipe. This is called operation under wet/humid conditions.

Wet/humid conditions require a moisture-resistant chimney system.

These are our moisture-resistant chimney systems:




4. W3G – classification

W3G quality

A chimney system with a W3G classification is a tested flue gas system that is moisture-resistant, noncorrosive, and soot fire resistant. The name of the classification “W3G” refers to:W3G

  • the condensate resistance class W
  • the corrosion resistance class 3
  • and the soot fire resistance class G.

A chimney system with a W3G classification is therefore suitable for dry and wet conditions with all fireplaces and fuel types.

These are our W3G chimney systems:




5. Independence of ambient air

Fireplace operation dependent on or independent of ambient air

Domestic fireplaces such as tiled or chimney stoves obtain their air supply either via the rooms of the house (they are dependent on ambient air) or via an external air supply outside the building (they are independent of ambient air).

Using a fireplace with a chimney that is independent of ambient air has the advantage that the room where the fireplace is installed does not need to provide a minimum air volume.

These are our chimney systems independent of ambient air:



 * also available in the lightweight version OSMOTEC LB

6. Concentric air supply

Separate or concentric air supplyair/flue gas system

If a fireplace is used independently of ambient air, the air supply can be provided in a separate or concentric manner.

Using the fireplace in conjunction with an air/flue gas system allows a concentric air supply, meaning that air supply and emissions transport are part of a closed system (see diagram). Air/flue gas systems are highly efficient flue systems with an excellent combustion process.

These are our air/flue gas systems:



 * also available in the lightweight version OSMOTEC LB


7. Twin-duct chimney

Twin-duct chimneysMixed use

If you would like to heat using a chimney or pellet stove in addition to your gas or oil heating system, you have to keep in mind that the combination of different fuel types in a single-duct chimney is permitted only in exceptional cases.

As a rule, flue transport must take place in a twin-duct chimney. Our OSMOTEC chimneys can be combined with each other in twin-duct chimneys, in accordance with all demands and requirements.

8. Multi-purpose use

Multi-purpose use permit

If you want to make sure that you will be able to connect additional fireplaces in the future and use them at the same time, your chimney should have a multi-purpose use permit.

With modern air/flue gas chimneys, for example, the air can be obtained from any location or direction.

9. Certified cowl design

Chimney cowl design

The design of the chimney cowl should guarantee that emissions and fresh air are clearly separated from each other to prevent re-circulation, i.e., the return of flues into the air supply. Whether the chimney cowl is designed in such a way should be tested and certified.

The chimney cowl design should also meet fire protection regulations and be resistant to UV light. This especially applies to combined systems with solid fuel and flue gas connections.


Our chimney systems: a quick overview

The table below will tell you which of our chimneys meet your requirements.

soot fire resistant










W3G classified





indep of amb air





concentric air supply





twin-duct version available





multi-purpose use





certified cowl design





* also available in the lightweight version OSMOTEC LB

Zum Kontaktformular - Wir beantworten Ihre FragenProduct inquiries

Do you have any questions about Hillen technology or products?

If so, just give us a call (0049-2631-95596-0) or simply use our contact form and we will reply to you promptly and provide you with all the information you need.