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Commercial Fireplace projects | Architects | Interior Designers | Construction companies

Bio Fires Online, we have a history of working closely with Architects, Interior Designers and Construction companies. Our customer service is unrivalled, as we have a focus on ensuring our customer’s expectations are met at all times. We achieve this through strong planning, project management and clear communication.

Our founder, Mark Edom believes that too many companies are there to just sell you something. Whereas we are here to build a lasting and mutual relationship with our clients. It is, therefore, a must that we impart a certain amount of our knowledge to our clients, in order for them to design fires that can be achieved.

Fires are an unusual element of a building project. They look great, and most people would like one, but there are some complexities when it comes to adding on to your design. Often the design of a fire is driven by aesthetics, rather than function, but it is our experience that you need to combine both. There are a number of key points to understand, and we will start by covering off conventional flue fires first.

The equation that you need to consider when designing a GAS fire into a property is as follows. Quite often we come across companies designing into their plans a large flame, in a large opening because that is what creates the biggest impact. However, it isn’t as simple as that, and here is why.

  • The larger the flame, the more carbon monoxide (CO) is being produced because of the combustion process. CO is colourless and odourless and toxic to humans and animals.
  • The chamber in which the fire sits is basically a box for with the front missing (and in some cases other sides missing too). Therefore the CO being produced by the flame can spill out into the room more easily, the larger the opening.
  • The top of the chamber has a ‘Gather’. This is where the typically rectangular form of the chamber narrows down into a square flue. A bit like and inverted funnel. For new installations, these typically transition into a circular twin wall flue. The angle of this narrowing determines how easy or hard it is for the exhaust gases to rise with the convection and ‘gather’ into the flue. If the gather is too shallow, then that makes it harder for the CO to rise, and therefore is more likely to fall forwards out of the chamber and into the room.
  • The diameter/width of the flue impacts the volume of exhaust gases that can easily exit the chamber. The smaller the volume of the flue, the less Co that can make it out of the chamber, again making it easier for the gases to spill into the room.
  • The length and number of bends in the flue also have an impact. The material in the walls of the flue heat up and retain that heat. This aides with the convection process which is what is removing the gases from the chamber. If the small is too short, then there is not enough material to heat, and therefore the draw is less. Conversely, have a flue that is too long, and the hot gases will have cooled before they exit the flue, which basically means that you are now trying to force cold air up and out of the top of the flue before the CO can escape. This is why gas engineers heat the flue prior to carrying out a spillage test on a fireplace. Otherwise the cold flue is not helping with the draw. It is important to note that chimneys that sit on the outside wall of a property, will require more time to heat up, as the cold air within the flue will have chilled the flue walls. Each time there is a bend in a flue, this restricts the flow of air upwards. That is why there are restrictions on the number of bends that you can have within a flue. There also restrictions on the angles which can be used also. Not all flues rise vertically up and out of the building.

 There are different types of gas fire which can help eliminate the issues above, like glass-fronted balanced flue fires. These use a concentric flue, where there is a flue within a flue so to speak. The combustion air is typically drawn in through the outer section, rather than from the room, with the exhaust gases being expelled through the inner flue section. This means that the fire is completely sealed from the room behind glass. It is more and more common now for the glass to be non-reflective, so you would hardly know that it is there. These fires have the added benefit of being more efficient at heating the room, as 80% of the heat makes it into the room. Whereas with an open gas fire, 80% of the heat is lost directly into the flue and out of the chimney. The consideration when looking at a balanced flue fire is the flue route out of the property. In most cases, they have the flexibility of being able to follow a route that can flow down, as well as up. However, you must abide by the manufacturer’s instructions in terms of the number of bends allowed. This can prove restrictive, and it is our recommendation that flues are one of the first services designed into your building. Other services are not governed by regulation in the same way that flues are. Get this wrong, and you probably won’t find out until the fires are tested, which only happens when the property is finished, with doors windows, and any air movement systems in and functioning. If your open gas fire is found to be spilling at this point, you won’t be able to change the flue because it is embedded within the fabric of your building. So the only other options will be to reduce the size of the burner down to the point where it no longer spills. Or you can reduce the opening, but at this stage, the fireplace is installed, so normally the only way to do that is with stone or glass slips. It is possible but unlikely that you would try and install a balanced flue fire by running another flue inside the existing flue and connecting this to a new balanced flue fire. This would require building works on the fireplace itself. The other option would be to block up the flue and remove the gas supply to the chamber and install a bioethanol fire instead.


One possible solution when the fire is spilling is to install a chimney fan. This creates a mechanical draw through the flue, but it must also be connected to the gas fire. As the fire can not be operated if the fan is not working for any reason. Therefore the fan is connected to a gas solenoid to ensure safe use. Retrofitting this wiring can be a challenge, as can retro fitting a chimney fan without planning permission. If you run calculations at the design stage, then a fan can be planned for, and can even be embedded down into the chimney stack to hide it from sight. This is not a simple silver bullet solution so to speak. There are a couple of considerations, like the cost of a fan. They aren’t cheap. There is the access for future maintenance which we have seen be a major problem for some installations. You must also consider the potential noise from a chimney fan. If the fan is too close to the fireplace, the flue provides a conduit down which the mechanical noise can travel. It may also prove to be noisy when a distance away from the fireplace. Enough air needs to be extracted from the fireplace to ensure that all of the CO disappears up the flue. If the flue is too narrow, then the speed at which the air travels must increase. This can produce a considerable noise if the flue is just too small to allow the volume of air required through it slowly.

Another consideration is ‘free air’ or ‘combustion air’ that must be provided into a room for most gas fires. Typically a modern gas fire over 6.9kW requires 100cm2 of air supply into the room. This must be open to the outside world at all times. It can not be closed in any way, and can not have any form of mesh over it, as this will clog with dust over time and cease to be effective. The holes must be over 5mm in size, and less that 10mm. A gas engineer will check these during the servicing of an open gas fire, and should they not be in order, the fire will be decommissioned until the fault has been rectified.

We have experienced other peoples projects in the past where they did not consider these points prior to installation, and have seen instances where glass header slips over 300mm deep had to be retro fitted. We have seen noisy installations, where the flue was too small for the volume of exhaust gases. There have been multiple flues installed on restorations where they wanted to use many of the fires, but when it came to installing the fires, the flues all proved to be less than half the size of what was required. That meant that they couldn’t have their planned open gas fires.

We have many years of experience of helping people with these types of issues, and our advice has always been the same. Talk to us as early in your project as you can. We can provide you with the advice that allows you to be confident in what you actually achieve when the project is finished. A slight change at the design stage can make a huge impact in the end result. Get it wrong, and that slight change is probably not a viable option once the project has gone too far.

We are here to help you with any designs and planning on your projects, so please feel free to give us a call on 0207 788 4777.


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