The air flow pattern and magnitude of air velocities in churches and other historic buildings are of interest since they influence the deposition rate of airborne particles on surfaces, and hence affect soiling of valuable artifacts of different kinds. Increased air movements might also cause enhanced sooting from candles and it has an influence on the thermal comfort of people. The type of installed indoor heating units is likely to be important here since these usually induce substantial air movements through natural or forced convection. In an experimental field study, two different heating systems were compared regarding their effect on room air velocities in a medieval stone church: air-to-air heat pumps with indoor fan convectors vs. a combination of bench heaters and radiators. Hot-sphere anemometers were used to record air velocities in the near-zone of the heat pumps and their surroundings, and 3-D sonic anemometers were used to measure downdraught air velocities at the surfaces of a wall and a window. Smoke was used to visualize air flow patterns.

It was found that the heat pumps caused strong buoyant air jets that rose to the ceiling, but that the air velocities were rather low outside of these jets. The bench heaters caused buoyant plumes, which also seemed to attain rather high air velocities and reach the ceiling. As regards downdraught along wall and window, no significant difference between the two heating systems could be seen, although there was a tendency towards slightly higher air velocities at these surfaces when the heat pumps were in use. Since the air flow pattern at the surfaces appeared similar, also the particle deposition mechanisms and soiling rate can be expected to be similar.