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Air heat meter

Hi,

This is my first post, apologies if this is in the wrong category. The following describes the motivation and a partial implemented of an air heat meter. All feedback appreciated, if there is a better solution or if there is a pitfall I’ve not spotted please let me know.

I want to measure the heat flow from a variable flow rate fan coil (air con) unit. The unit provides heating or cooling and is linked to an external heat pump via a refrigerant loop. I’m looking for a solution to do short term studies of energy balances in PassivHaus buildings. I would like the data gathered to slot into OEM.

I’ve looked for commercial sensors to measure flow rate in the duct but I’ve only found components that are engineered for process control. Therefore, I have started to look at a DIY solution. I’ve looked at various DIY Pitot tube projects and they don’t match the flow rates I need (< 3m second) and they would need to be calibrated.

Most low-cost, commercial MVHR commissioning equipment that can measure the right flow rates are not designed for easy integration. Testo have recently released a set of sensors that link to smart phones for logging and display. The Testo 405i is hot wire anemometer that measures velocity and temperature.

My son and I analysed the Bluetooth comms between a 405i and smart phone and he has reverse engineered a Python application running on a RPi to configure the device, put it into monitoring mode and then post the data to OEM. He uses the Bluez Gatttool running inside Python using Pexpect to parse the output. In OEM I have used a series of processes to calculate the temp difference between the duct contents and the room, the volume based on the duct size and velocity and finally the Wh from volume and delta t.

There are a number of things still to do:

  • the 405i is battery powered with a 15h battery life - we are planning to use the PRi USB power source to substitute for the 3 AA batteries
  • we need a mount to hold the device at the right position in the duct
  • the 405i switches off after 3+ hours of sending data, we need to figure a work around to prevent this, possibly sending update Bluetooth commands via the Python prog or power cycling the device and then reconfiguring it from Python
  • the flow rate calc uses a single measurement in the middle of the duct, this needs to be calibrated against duct traversals at different flow rates

Many thanks for feedback,

Rod