Node.js Web Socket H15 Idle Connection timeout on Heroku
disclosure: I'm the Node.js platform owner at Heroku.
Are you seeing this on the client side? Or just the error-reporting dashboard?
I'd recommend focusing on reproducing the errors on at least one client, to see if there's actually any impact. Otherwise, you may be wasting time debugging false positives.
If you do decide to continue debugging, you may want to install a free papertrail / nodetime / new relic / strongloop addon to figure out exactly which requests are triggering the H15s.