The weather is getting warmer and today is the first time I spotted evidence of the automatic by-pass on the Mechanical-Ventilation-with-Heat-Recovery (MVHR) unit being activated.

In winter, a key benefit of the MVHR unit is that it recovers most of the heat from the outgoing air and uses that to heat up the incoming air, with the result that you get fresh air coming in at pretty much the same temperature as the stale air going out – providing good ventilation without wasting (much) heat. Similarly, in the middle of summer, it does the reverse and cools down the incoming air using the outgoing air.

In between these two extremes, it makes sense to counteract the natural tendency of the building to heat up (due to passive solar gain and appliance usage within the building) by bringing the air straight in rather than attempting to heat it up. This is known as “by-pass mode” on the MVHR unit (the incoming air by-passes the heat exchanger) and is activated automatically based on temperature limits set on the MVHR unit’s controller. It’s completely automatic so pretty much the only evidence is from the temperature monitoring on the MVHR system – see the graph below (click once to open the image and click again to enlarge).

MVHR Automatic By-Pass Activated

Slightly annoyingly, there seems to be no way to access the data being collected by the PAUL MVHR unit controller so this data is monitored via a separate set of sensors – see here for more details. The image above is a screen-shot from Grafana displaying data stored in an InfluxDB database.