Our first impression of the measurements reveals:
- poor thermostatic control, with expected improvement from moving the room thermostat from an external corner with relatively low airflow. The team involved suggested mounting one or more thermostats on the aisle side of pillars where the pew is too blocked to sit on. We’re just thinking about the temperature variation and draughts before suggesting how many to use.
- substantial variation in the side wall surface temperatures depending on whether the measurement was taken directly above a pew or not
- fairly good heat retention at the stained glass windows, all things considered.
- a hefty draught from gaps in the back door that sweeps air right past seats at the back and through the roof vents, making people feel cold – and then out the louvres in the tower.
- relative humidity on the low side, suggesting that the space currently has more ventilation than required.
One result of our investigation should be a better idea of how to set the adjustable louvres in the tower to keep the church dry enough but reduce the heat loss. When they were first installed, they would have been operated with a rope from below as needed – and the clerestory windows would have been opened whenever the building needed a really good airing. The modern approach is controlled ventilation – automatically opening and closing vents to match the current conditions, just like room thermostats start and stop boilers to keep a constant temperature.
We will confirm these perceptions and get results (and pictures!) up on the web as soon as we can manage to do that, but it may take us some time even to process the worksheets and release raw data.
Heat loss through nave ventilation
We’ve also taken our first readings from the hot wire anemometer from the louvres on the ducts coming off the nave, but need to confer before we have a solid estimate of the heat loss. The rough calculation for measurements taken at roughly 11:00 on 17 Feb was as follows:
- Flow rates: 0.144 m3/s (louvre nearest manway) and 0.058 m3/s (other louvre; shutter less open)
- air temp: 19C
- assumption: heat capacity is 1.2kJ/m3/K
- assumption: air ingress to nave is at 10C
Lost heat through nave vents = 1.2 x (0.144 + 0.058) x (19-10) = 2.2 kW. However, the Tinytags show that the external temperature was 5C, so unless there was a reason why the measurers assumed 10C, 3.4 kW might be a better estimate. That’s a bit like running an electric fire or a kettle continuously.