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If you’re choosing a gas fire, you’ll often see the terms Class 1 and Class 2. They describe the type and size of chimney or flue that a gas fire is designed to work with. Knowing which one you have helps narrow down the fires that may be suitable, but it isn’t the only thing that matters.
The condition of the chimney, its dimensions and the requirements of the individual fire all need checking before installation.
A Class 1 chimney is usually the traditional type of chimney we see in older properties. If you have a brick-built chimney breast with a chimney pot on the roof, there’s a good chance you’re starting with this type of setup. Traditionally, Class 1 includes larger brick or stone chimneys and suitable lined flues. You’ll often see gas fire manufacturers refer to a 175mm or 7-inch flue when describing Class 1 suitability.
A Class 1 chimney can generally accommodate a wider choice of gas fires, including many full-depth models, because there is normally more space available behind the fire. That doesn’t mean every gas fire will automatically fit.
You still need to check:
If you’re fitting a gas appliance, your Gas Safe registered engineer should check the chimney and flue as part of the installation.
Class 2 normally refers to a smaller flue system. A common example is a prefabricated metal flue, sometimes identified by a metal terminal on the roof rather than a traditional chimney pot. You’ll often see manufacturers refer to a 125mm or 5-inch flue when describing Class 2 gas fire suitability. Because these systems can offer less depth than a traditional chimney, you may need a slimmer gas fire.
This is why you’ll see some inset gas fires described specifically as slimline Class 2 fires, while other models are suitable for both Class 1 and Class 2 installations.
You can see examples in our range of inset gas fires.
Pre-cast flues are another type we regularly come across, particularly in newer properties. Rather than having a large traditional chimney void, the flue is formed using blocks built into the wall. You can often spot one from the outside by the type of terminal on the roof and the lack of a large traditional chimney breast, although we wouldn’t rely on that alone.
The biggest issue with a pre-cast flue is usually space. The fireplace opening and available depth can be much shallower, so a full-depth gas fire may not fit. There are gas fires designed for suitable pre-cast flues, but you need to check the exact appliance requirements. This is a good example of why we’d choose the fire after checking the opening and flue rather than assuming a standard inset fire will fit.
You’ll still see these measurements used regularly:
They’re useful when comparing gas fires, and manufacturers still use them in their specifications. But we wouldn’t treat those figures as the only test of whether a fire is suitable. Different appliances have different requirements, and the chimney or flue also needs to be in suitable condition and work correctly with the appliance.
Always check the installation instructions for the exact gas fire you’re considering.
Sometimes it’s obvious. Other times it isn’t. A traditional chimney breast and chimney pot may point towards a Class 1 chimney, while a metal flue terminal may indicate a prefabricated system.
But if you’re unsure, don’t guess.
A Gas Safe registered engineer can inspect the existing fireplace and flue and tell you what you’re working with.
If you’re asking us which fire may suit your fireplace, it’s useful to send:
That gives us a much better starting point.
Not having a traditional chimney doesn’t mean you can’t have a fireplace. Electric fires and bioethanol fireplaces don’t need a conventional chimney or flue, while some gas fires use a balanced flue system through an outside wall. f you’re considering gas, check the requirements of the exact appliance before you buy
Phone
01773 718718
Email
sales@castfireplaces.co.uk
