The underlying issue is normally that static electricity from your running top is finding an alternate pathway through the belt, and masking the genuine electrical signals from your heart. There are a number of ways of dealing with this:
* wet the contact pads on the strap before your run to ensure better electrical contact between strap and skin - especially in cool/low humidity conditions. The issue normally goes away a few minutes into your run once you start sweating (salty sweat is an excellent conductor). I usually just run wet fingers over the contact pads, but some people use electrode gel.
* wash the strap regularly - over time salts from sweat can accumulate in the fabric of the belt, making alternate pathways for static easier. Best practice is to give them a quick rinse under the tap after each run, and wash them once a week or so.
* replace the strap when washing no longer resolves the problem - the fabric strap should be regarded as a consumable to be replaced every 12-18 months (swap the sensor unit). Unfortunately Garmin's HRM-RUN and HRM-TRI are not removable.
* consider what running shirt you are using - the technical fabrics in some tops can be a static nightmare. I have a couple that I avoid on cool/low humidity days (I'm too cheap to throw out a perfectly functional running top). I have also used anti-static spray on these shirts which is effective, but a hassle.
* wet the contact pads on the strap before your run to ensure better electrical contact between strap and skin - especially in cool/low humidity conditions. The issue normally goes away a few minutes into your run once you start sweating (salty sweat is an excellent conductor). I usually just run wet fingers over the contact pads, but some people use electrode gel.
* wash the strap regularly - over time salts from sweat can accumulate in the fabric of the belt, making alternate pathways for static easier. Best practice is to give them a quick rinse under the tap after each run, and wash them once a week or so.
* replace the strap when washing no longer resolves the problem - the fabric strap should be regarded as a consumable to be replaced every 12-18 months (swap the sensor unit). Unfortunately Garmin's HRM-RUN and HRM-TRI are not removable.
* consider what running shirt you are using - the technical fabrics in some tops can be a static nightmare. I have a couple that I avoid on cool/low humidity days (I'm too cheap to throw out a perfectly functional running top). I have also used anti-static spray on these shirts which is effective, but a hassle.