In the last few weeks, I've done both the 2 x 20 and Friel tests to determine my functional/anaerobic threshold, and both times came up with 153 bpm. Both tests were done indoors, on a trainer, with a big fan positioned a couple feet away.
On my ride today, I had an 87-minute stretch on the way home in which my average heart rate was 157.4. I felt great and put in a good effort, but wasn't focused on my heart rate at all during the ride... nor did I push the pace as hard as I could that whole period. I also stopped briefly at a few traffic lights, and had to slow for traffic/intersections several times.
Should I be using 157 as my FT number? Or stick with 153 on the trainer, and go with 157 on the road (and/or do a test outside!)
FYI I am 40 years old; highest actual HR I've observed is 184 (though I've only been using a HR monitor for about 3 months/400 outdoor miles).
My opinion: your LT should be around 90% of MHR or around 165. A rider below elite level won't be able to hold LT for an hour. 20 minutes is more usual. The trainer problem, and the problem with the sticky is that it's very, very difficult to really go on a trainer. It's difficult to get the hormones flowing on a trainer, and it's hard to find an uninterrupted 20 minutes outdoors. The easiest thing is to find at least a 1000' steady climb, or 2000' for elite riders. If you live in the mountains, that's not so hard. Having a stronger rider you're trying to keep up with is nice, too.
I have the best luck on a trainer by doing it this way: warm up in zone 2 for 1/2 hour, including 2 1.5 minute intervals at about the 20 and 25 minute points. Then reset your computer and HRM and hit it for 5 miles. Go as hard has you can and still finish the 5 miles. At the end, your should be near MHR. Your average will be a decent approximation of LTHR. I do best at near 100 cadence, but that's individual. You can also do this outdoors if you can find a level 5 mile stretch with no intersections.
However you do it on the trainer, try going harder and at a higher cadence at the beginning and making it hurt a lot more than you may be used to, and just let your HR slowly climb. Once it gets up above 160 it will actually get easier. You're trying to produce a lot of watts and you need a high cadence to do that. You'll have to be well rested to have the mental fortitude to go through with it. Having to quit is not failure. Learn from it and try again next week. It's really gonna hurt. You have to try to get your head around the idea that pain is good.
" your LT should be around 90% of MHR or around 165"
that number is completely arbitrary and its also not realistic to think everyone would have an LT that is 90% of their max HR.
I think you also have to take into account that the duration on the interval will give you different threashold numbers, too.
If I recall, Chris Charmicheal uses a 3 mile outdoor course or an 8 minute indoor interval
Clearly, that will give you a higher number than a 20 minute interval. If you are going to use the number you get to set up training zones, then I would say follow whatever proceedure is outlined for the self test in the book you are using. You will get different numbers depending on the duration of the interval, so if you don't do the same test your training zones won't be equal to the training zones in the text you are using as a guide.
IMO, don't worry about it. As veloGeezer suggests, use the results from the testing protocol that goes with the zones you are using.
HR can be a funny thing. From your graph, it looks like you got your HR up on a hill, and then held it on the long decent. It may be possible your HR stayed high just from the adrenaline of going faster on a downhill section.