Milling is a mechanical intervention in the flue gas path that goes beyond standard brush cleaning. It is used when the chimney cross-section has been narrowed by dried tar deposits, when an old or damaged liner needs to be removed, or when the structural condition of the chimney flue does not match the appliance being connected. Unlike cleaning, this involves work on the chimney structure itself and is not routine maintenance.
When milling comes into consideration
The most common reason is a change of appliance or fuel type, for instance, switching from solid fuels to a gas condensing boiler requires a different diameter and material for the flue than the original chimney had. Another reason is long-term neglected cleaning, where a hard layer of soot and tar forms in the chimney that a standard brush cannot remove within the time limits described in the article. Chimney cleaning: deadlines by fuel type and price will not remove. Milling is also used for chimneys with cracked or damaged inner linings.
A typical example is an older brick chimney at a house where coal was burned for many years and the installation of a wood-burning stove or a condensing gas boiler is now planned. The original flue is often too wide, uneven or blocked, and without intervention the new appliance would not operate safely or efficiently.
How milling is carried out
The technician inserts a milling head into the chimney, which gradually removes deposits, irregularities or remnants of an old liner along the entire length of the flue gas path down to the original masonry. Depending on the condition of the chimney, multiple passes may be required. The aim is to prepare the flue so that it has a smooth and clean surface for subsequent steps: either continued operation or relining.
Approximate price per metre
The cost of flue milling is usually calculated per metre of chimney and depends on the diameter, material, and degree of soiling or damage. As a guide, it ranges from 800 to 1 500 Kč per metre. For taller or harder-to-access chimneys, the price may be higher due to more complex equipment installation. For a detached house with a standard chimney height, the total job including transport of equipment usually comes out slightly higher than a simple sum of the per-metre rate would suggest.
Milling and subsequent insert installation
After milling the flue duct, lining usually follows, inserting a new stainless steel or ceramic liner that ensures the correct diameter and tightness of the flue gas path for the specific appliance. Without lining, a milled chimney would often fail to meet the conditions for safely connecting a new boiler or stove. The choice of liner material depends on the fuel type, with gas condensing boilers, condensate drainage is also addressed; with solid fuels, resistance to higher temperatures is prioritised.
Inspection following chimney work
Milling and subsequent lining are structural modifications to the chimney, after which Decree No. 34/2016 Coll. Requires an inspection of the flue gas path before it is put back into operation. Details about the inspection and its prices are described in the article Chimney inspection: price, obligation and procedureIt is therefore advisable to arrange milling, insert installation and inspection with a single supplier in sequence, so that the appliance can be connected immediately upon completion of the work without unnecessary waiting.
How milling differs from inspection and cleaning
Milling is not part of the standard annual inspection or regular cleaning; it is a one-off construction intervention beyond their scope. An overview of what each regular obligation includes can be found in the article. Chimney inspection, cleaning and certification: the differences.
How to tell if your chimney needs reaming
The first sign is usually a deteriorating draft, smoke is drawn off less effectively and combustion gases are noticeable in the room even when the stove door is properly closed. Another indicator is the report from a regular inspection, where the chimney sweep points out a reduced cross-section or a damaged liner. In older houses where the chimney has long been neglected, milling is often a necessary step simply to allow a new appliance to be safely connected to it.
Who carries out milling
Milling of the flue gas passage is carried out by a chimney sweep company with appropriate equipment. This is a mechanical intervention beyond routine cleaning, not a separate trade. Many companies offer milling, lining and subsequent inspection as one package, saving time and coordination between different suppliers.