When I am on my turbo trainer I use a Garmin chest HR strap which is connected to Zwift and the HR recorded by my Forerunner 55 has generally been pretty close to what Zwift is recording.
Since the weather has turned colder it's been off by 40 - 60 bpm lower eventually catching up after about 30 minutes. Today when early on it was below zero where I train it averaged 40 - 60 bpm lower for the whole hour session. Yet when I did a walk at lunchtime the HR recorded by the watch was what I would have expected.
Has anyone else experienced this during cold weather or could the watch be faulty?