A masonry heater is run on one burn a day. Between two fires, one question remains: what becomes of the stored heat, and where does it escape? That is the job of the dampers.
The combustion air damper
It controls the amount of air admitted into the firebox. At the start of the burn, it opens wide to launch a brisk combustion; as the fire becomes established, it gradually closes to avoid burning too fast and sending unused heat up the flue.
Driven by an electronic control — on our Brunner appliances, the EAS combustion management system —, it closes the heater without any action by the user once combustion is over. This is a real comfort: nobody has to get up in the evening to close the damper.
The end-of-circuit damper
The second damper sits at the outlet of the flue path, just before the flue. Its function is different: once the fire is out, it cuts the residual draught.
Without it, a hot masonry heater keeps drawing air from the room, which passes through the still-hot masonry and escapes through the chimney stack. This is an invisible loss, which lasts for hours and carries off part of the very heat you have just stored.
According to the manufacturer, adding this damper brings a gain in heat release of around 10 to 15%. We fitted it for the first time in 2017, on a project in Saône-et-Loire, and have fitted it on later appliances since.
On which appliances
These two dampers are offered as options on heaters built around a Brunner firebox combined with an accumulator, a configuration found on a good share of our projects. They require a power supply near the appliance — something to plan for during the structural work, not afterwards.
The projects fitted with them: Côte Chalonnaise, in Viré (71) and Gironde Strait, in Saint-Thomas-de-Conac (17).
In a new-build house
Regulations for new-build houses require the main heating to be switchable off by hand and controlled automatically according to the indoor temperature. A masonry heater meets this in two ways, to be decided with the project’s thermal engineering firm: either it is the main heating, fitted with automatic control of the combustion air and additional convection distribution driven by a thermostat; or it remains the back-up heating of a house that has another system. In both cases, its combustion air comes from outside through a dedicated duct, and the appliance is sealed: it does not interfere with the house’s airtightness test.