F90 Shaft

Good evening everyone, can someone please explain the functioning of an F90 shaft in connection with a condensing boiler in a new building? Specifically exhaust gas routing, fresh air supply, etc. Thank you very much & best regards Roland

Hello Roland,

a gas condensing boiler (Gas-Brennwert-Therme) is connected using an LAS (Luft-Abgas-Schornstein) for supply air and exhaust gases.

Simply put, the LAS provides fresh combustion air on one hand, and on the other hand, the exhaust gases (which are not particularly hot (about 40 degrees) and therefore must be conveyed to the roof by a fan in the gas condensing boiler) are discharged. Of course, in separate circuits. The LAS is usually made of plastic.

The FW90 shaft (FW = fire resistance class) can—depending on the federal state and, I would say, depending on the mood of the BSFM (Bezirks-Schornstein-Feger-Meister)—be required to protect the floor above the heating system in case of fire.

The shaft is a fire-resistant box around the LAS, usually from the ceiling of the ground floor (or basement, depending on where the gas condensing boiler is located) up to under the roof.

You don’t have to understand this. I didn’t understand it either back then and had to work with prefabricated parts for time reasons (see below: money, expensive).

There are many ways to create an FW90 on site, including prefabricated parts, e.g., Skuberne. It is always effort and costs money. An unfavorable exhaust gas routing causes wall offsets; please pay attention to this and, if necessary, have the exhaust gas routing changed. Having such a dismantled box in the middle of the bathroom or bedroom is unsightly.

In your case—as I interpret the question—it is required. Discuss the options on site to choose the most cost-effective way of installation.

Best regards
Thorsten

If it is a single-family house, a low-rise building according to the state building code, then a maximum of F30 is sufficient or, after consultation with the responsible BezSchStFeger, the installation in a GKB drywall construction without combustible components (CW profiles) is also sufficient. The planning, especially the roof or wall penetration, taking into account the fire protection requirements and the [FeuerVO] of the respective federal state, is the responsibility of your architect.

Hello Thorsten, hello wpic
thank you for your detailed and understandable answers.

The functionality now seems more reasonable to me.

Exactly for the mentioned reason with the wall projections, we had this shaft installed in a drywall partition, of course after consulting with our architect.

This shaft is quite large and I look inside and what do I see? Such a small milky mini pipe, diameter maybe about 50, and I wondered is this pipe for the supply air or exhaust air or what function does a single small pipe serve in such a large shaft?

Best regards
Roland

Hello Roland,

the shaft is larger than the pipe. Significantly. That is fine.

Mostly the LAS start with DN60 (I believe that is the outer diameter).

Just ask your architect what he installed.

Best regards
Thorsten

Hello Thorsten, of course I could do that. I just didn’t want to be like the proverbial "ox in front of the mountain" again...

I thought there was a standard for how fresh air and exhaust air are routed, meaning whether the exhaust air or supply air is in the small pipe or whether the exhaust gas or the supply air goes through the shaft without anything, like in a chimney.

Regards
Roland