Fireplace glossary

Fireplace Professional Jargon - words and phrases from the world of fireplaces

Below you will find explanations for the most commonly used professional terms in fireplace construction.

BY-PASS VALVE / DIVERSION DUCT
In many cases, cold chimneys do not have sufficient draught performance. Therefore, the flue gas exiting the firebox must reach the chimney via the shortest possible route. The by-pass valve is essentially a diverter valve that allows this. It switches the flue gas path from direct to indirect. Using them improves efficiency and prevents smoke spillage.

COMBUSTION AIR
The amount of air required to burn the fuel (wood). We distinguish between primary, secondary, and (less commonly) tertiary combustion air. Primary air enters through the grate, while secondary air serves to burn off wood gases via air introduced later into the combustion chamber.

COMBUSTION PRODUCT REMOVAL EQUIPMENT
Also known as a chimney. It ducts flue gases generated in the fireplace insert to the outdoors. There are internal chimneys and those mounted on external walls; the latter must be insulated to achieve proper draught.

COMBUSTION CHAMBER
The central part of the fireplace insert where the gases released from the wood are burned. In both air-heating and water-jacket models, it is typically made of cast iron, or occasionally steel plate with a firebrick (chamotte) lining.

FLUE CONNECTION ELEMENTS
Components (made of stainless steel or steel) that connect the firebox to the chimney to channel flue gases outdoors.

FLUE OUTLET
The connection point on top of the firebox “dome” used to vent combustion products. Water-jacket inserts usually require a larger diameter due to higher total power output.

ROOF / SMOKE HOOD
The bell-shaped “dome” of the fireplace that gathers combustion products before sending them into the chimney. In water-jacket models, this is often replaced by a water tank heat exchanger.

IGNITION TEMPERATURE
The lowest temperature at which wood gases mix with air and ignite without an external flame. For firewood, this is approximately 270°C.

ASH DRAWER
It is located at the bottom of the fireplace firebox, directly beneath the fire grate, and is used to collect ash. It is an essential component of solid-fuel fireplace inserts.

HEAT SHIELD
Located on the inner back wall of the firebox. It protects the main casting from thermal shock by absorbing the brunt of the heat and distributing it evenly, extending the unit’s lifespan.

DRAFT REGULATOR BUTTERFLY VALVE
A structural unit used to regulate the amount of flue gas exiting the insert. It is a standard accessory located in the flue outlet (smoke socket) and is manually operated from the front.

FIREPLACE INSERT
The main structural element where fuel is burned. It is placed inside a built structure (surround) and connects to the chimney via a flue pipe. Categories include air-heating and water-jacket (boiler) inserts.

CONVECTION SPACE
The air gap between the fireplace insert and its external cladding. It ensures air circulation; the accelerated air cools the firebox (increasing its lifespan) and heats the room. Proper design is essential for performance and safety.

SOOT CHAMBER
The section of the flue gas exhaust system—that is, the chimney—located below the lowest connection point of the vertical flue.

EXTERNAL AIR OPTION
Instead of drawing oxygen from the living area, the fireplace draws combustion air from outdoors via a dedicated duct. This requires a connection socket at the bottom of the insert to feed air directly into the combustion chamber.

BASE LEGS
The legs at the bottom of the insert, often adjustable, used for leveling. Benkel models come with built-in adjustable legs as standard.

AIR-HEATING FIREPLACE INSERT
A type of fireplace insert capable of heating the room where the firebox is installed. By channeling warm air through a duct, it can heat adjacent rooms and even the entire house. It is made of graphite-gray cast iron.

AIR SLUICE
A manually operated valve for supplying and regulating combustion air. Used to supply external and internal combustion air to stoves. Inlet cross-sectional area: 150 cm². Hermetically sealed when closed.

OPEN-AIR FIREPLACE FIREBOX
A fireplace insert that draws combustion air from the room in which it is installed and vents the combustion products to the outdoors through a flue system.

OPEN FIREPLACE
A heating appliance that is not separated from the room’s interior space.

OPEN SYSTEM CENTRAL HEATING
A heating system using an open expansion tank to balance pressure. While possible with water-jacket fireplaces, closed systems are now more common for modern efficiency.

BUTTERFLY VALVE
A shut-off damper installed in the flue outlet of fireplace inserts, which allows you to manually regulate the volume of flue gas flowing out of the firebox from the front of the fireplace. For safety reasons, it must not and cannot be closed completely; otherwise, the combustion products cannot escape to the outside through the chimney.

GRATE
Located at the bottom of the firebox where the wood rests. Its main task is to deliver combustion air into the fire. Made of cast iron, it can consist of center and side grates (which help cool the side walls).

WOOD, DRY
The only acceptable fuel. This refers to firewood seasoned for 2 years in a dry, ventilated place. Wet wood wastes energy through evaporation and causes soot/creosote buildup.

SECONDARY COMBUSTION AIR
Secondary combustion air is introduced into the combustion chamber of the fireplace insert through air inlet openings located on the back wall of the firebox. It optimizes combustion in the upper third of the firebox by igniting the previously unburned gases. The air wash can also be considered secondary combustion air, as it not only keeps the glass clean but also contributes to the combustion of these gases. On the back wall of Benkel fireplace inserts, there are 7 holes, each 15 mm in diameter, provided for delivering the secondary air into the firebox.

VENTILATION GRILL
An element used for the outlet (and sometimes inlet) of the circulated warm air. Usually placed near the ceiling on the upper part of the fireplace surround.

FITTINGS
The optional accessories for the fireplace insert are: the handle, ash drawer knobs, draft regulator valve knob, and hinge knobs. For our fireplace inserts, you can choose from 4 types; they are available with black fittings at the base price.

THERMAL DISCHARGE VALVE
The thermal safety valve is responsible for the safety of water-jacketed fireplaces. It controls the cooling circuit of the water-jacketed firebox using a sensor probe with an immersion sleeve. At a factory-calibrated temperature (~95 °C), the valve mechanically opens the mains cold water, allowing it to flow through the separate cooling coil and into the drain until the water temperature inside the fireplace insert cools back down to the factory-calibrated approx. 85 °C. This provides complete safety for the central heating fireplace.

SEALING ROPE & PASTE
High-heat resistant materials used to assemble the insert. Heat-resistant paste (up to 1200°C) ensures the structure is airtight.

REAR FUELING DOOR
This is the rear door of a special type of air-heating fireplace insert, where a Bodok firebox is mounted as an extension onto the back wall of a Pilis type firebox. If you prefer not to manage the firing from the front of the fireplace insert, the rear firing door with lower heat emission is available.

LOG GUARD / FIRE DOG
A cast-iron “rake” or fence attached to the grate that protects the glass from logs rolling against or hitting it.

WATER TANK
The main structural element of the water-jacketed fireplace insert, which is generally located above the firebox. If we wish to increase the water-side output—at the expense of air heating—the cast iron back plate of the fireplace insert can also be replaced with a water tank. The water tank of the Pilis water-jacketed fireplace inserts can feature either a cross-water-tube or a longitudinal-fire-tube design.

WATER-JACKETED FIREPLACE INSERT
Fireboxes that can be connected to a closed or open central heating system are called water-jacketed fireplace inserts. The material of the firebox is cast iron, while the water heat exchanger is made of steel plate. Its advantage over air-heating fireplace inserts is that it is capable of heating not only the air in the given room but the entire house. The system can be combined; by incorporating a heat exchanger or a buffer tank, even underfloor heating can be connected to it.

CLOSED COMBUSTION FIREPLACE INSERT
A fireplace insert designed so that its entire combustion air supply, firebox, and flue gas extraction are airtight and isolated from the room where it is installed. It draws its combustion air from the outdoors and discharges the flue gases to the outdoors through the exhaust system, operating entirely independently of the pressure conditions in the installation room.

CLOSED SYSTEM CENTRAL HEATING
Connecting a water-jacketed fireplace to a closed central heating system is more reliable than an open-system operation. Its main component is the closed expansion tank, which serves to equalize excess pressure. If, for any reason, the circulation pump of the central heating fireplace fails to operate, the overpressure is released by the thermal safety valve and a pressure relief valve located anywhere within the system.

CLOSED FIREBOX FIREPLACE
A heating appliance separated from the room’s airspace by a fireplace door, which can draw the air required for combustion from either an internal or an external air source.