Device for mixing a combustible gas and combustion air for supplying to a burner, comprising: - an air line for the supply of combustion air; - a gas line for the supply of a combustible gas which is provided with a control valve ; - a first measuring line with a first outer end (a) which is connected to the air line and a second outer end (b) which is connected to the gas line ; - a second measuring line with a first outer end (c) which is connected to the first measuring line at a point between the first (a) and second outer end (b) of the first measuring line , thus forming a three-way intersection, and with a second outer end (d) which is connected to the gas line and/or the air line ; - a thermal mass flow sensor , comprising: ∘ a first temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the air line during use; and ∘ a second temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the gas line during use; - a controller connected to the thermal mass flow sensor and the control valve and configured to control the control valve on the basis of a difference, measured by the thermal mass flow sensor , between the mass flow rate of the gas flow between the three-way intersection and the air line and the mass flow rate of the gas flow between the three-way intersection and the gas line . - an air line for the supply of combustion air; - a gas line for the supply of a combustible gas which is provided with a control valve ; - a first measuring line with a first outer end (a) which is connected to the air line and a second outer end (b) which is connected to the gas line ; - a second measuring line with a first outer end (c) which is connected to the first measuring line at a point between the first (a) and second outer end (b) of the first measuring line , thus forming a three-way intersection, and with a second outer end (d) which is connected to the gas line and/or the air line ; - a thermal mass flow sensor , comprising: ∘ a first temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the air line during use; and ∘ a second temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the gas line during use; ∘ a first temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the air line during use; and ∘ a second temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the gas line during use; - a controller connected to the thermal mass flow sensor and the control valve and configured to control the control valve on the basis of a difference, measured by the thermal mass flow sensor , between the mass flow rate of the gas flow between the three-way intersection and the air line and the mass flow rate of the gas flow between the three-way intersection and the gas line . <EOS>