I have both a Garmin and a Wahoo HRM. I use both with my 2 Garmins and the TR App on my Windows tablet. Both HRMs, on all machines, read very low for the first 10 -45 minutes (approx) before getting over themselves and catching up with what my heart is actually doing. Just for fun, like today, I got a reading of 225 bpm for the entire 40 kms of my ride. I never saw that when I was young and fit. Can anybody explain what might be happening?
Wet the strap and your skin if you haven’t already been doing this.
You need a good connection, if you don’t wet the strap and your skin you have to wait until you sweat enough to get reliable readings.
Other causes of bad readings are:-
- Batteries low or the battery not seated in the battery housing correctly leading to intermittent contact.
- Baggy clothes flapping and the HR monitor picking that up as ‘fake’ heart beats.
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The straps degrade because of salty sweat and corrosion. If the HRM value seems stuck (i.e. reads the same value for a long time), or reads too low or too high, it’s often the strap that’s degraded too much. Buy a new strap for your HRM.
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Thanks for the comments. One of the straps is brand new and I always wet them first. Batteries changed and my ‘fuller’ figure prevents clothes flapping. I must confront the elephant in the room, it’s me 
You could try to clean the insides of the HRM with isopropyl alcohol. I had issues with a Garmin HRM having low readings and read online that the connection between the battery and connecting could be corroded/filled with dust. Tried to scrub it with cotton buds and some isopropyl alcohol and I stopped having low readings. basically free and worth a try.
Do let it dry before you reinsert the battery.
Sometimes the simple fixes are the best.
Hope it helps.
Many thanks, I’ll try that.
I found that water doesn’t actually work all that well for HRM straps, but licking them works a treat and mine is always accurate now that I do it. It might sound weird, even a little gross, but give both sides a slobbery lick before putting it on and see if that works any better.