Very interesting podcast on heat training (member only I believe): Ice buckets and paint suits: How heat and cooling both aid performance - Escape Collective
One of the interesting discussion points was how you can do heat training on recovery days because you’re not introducing extra muscle fatigue and so should still be able to recover 
Hard hard no. Heat training is crazy stressful, talk about an easy way to dig a hole
Yeah just try this once and you’ll see how utterly exhausted you are sweating 8lbs under and hour on the trainer.
I have done it. Didn’t notice any significant additional stress that impacted hard training that week. Not sure I sweated 8 lbs.
Fair points. There is more to fatigue than just muscular fatigue 
The title says it all. Why do you want to do any sort of “training on recovery days”?
Agree - and I do sweat that much. I think it may take acclimation prior to using it regularly, but once acclimated it’s no problem
If you arent sweating much, you arent heat training. If heat training doesnt affect your week, you probably didnt need a rest day to start with
Heat training can be a great tool, but the timing is key. While heat adaptations can give you a performance bump, those adaptations can be gained quickly and they are also go away pretty quick. If you are killing yourself in the heat 4 months out from your event, I think you are doing more harm than good. If it’s just an easy recovery day in the heat (ie - easy spin, not a struggle), there probably isn’t much downside stress and it’s basically no different than just getting in a sauna when not riding. Early season, I think there are better things to work on during recovery rides. It’s a good time to work on holding aero/aggressive positioning on the bike, maybe work on off-road bike skills, go for a fun spin with friend/family, etc.
My perspective might be a little skewed from living in central TX. I know plenty of heat training is coming prior to my target events whether I want the heat training or not. Very little desire or benefit to doing heat training early in the year. For folks who don’t live in hot climates and have to “create” the environment, I wouldn’t recommend starting heat training until ~4-6 weeks out from your target events.
First, I would recommend that everyone actually listen to the podcast before chiming in…
Second, I am going to defer to a noted exercise physiologist, who has worked with WT teams and lead the heat adaptation protocols for multiple Hour Record attempts before almost anyone here. Not a slag against forum members, but a recognition of his expertise.
Is what he saying counter-intuitive (at least to most of us)? 100%. But he has the education, training and experience to assess the effectiveness of the recommendation.
You and WT teams are not the same
As far as I can see, it’s a paid site, so probably not a reasonable expectation.
I honestly recommend looking into signing up if you are into cycling journalism and the cost isn’t too much of a burden. Their content is super top notch, no ads, and interesting.
There is a 22 minute version of it available on the normal Escape Collective podcast feed…free to all.
So his expertise does not apply to “you”?
Put another way, he works with WT teams because of his expertise in the field…and his advice in the pod is very much geared towards athletes in general, not WT athletes.
To be perfectly clear and hopefully helpful - here is what I did for heat training. It was similar to what was outlined in the podcast. It was on my trainer in a 71 degree basement. I wore a baselayer, thermal jersey, cycling cap, but just regular shorts. No fan and spun in Z1 for 60-ish minutes. I was sweating plenty. Was it uncomfortable - yes. Did it feel like it did any muscular damage to my legs - no. I made sure to hydrate appropriately the rest of the day and was ready for my workout the following day, just like any other recovery day.
I have no precise measurement to quantify the benefits. In fact it was done in a lead up to a mtb race and so I don’t even have a power file. But I can say it was one of my best ever results on the bike, so I believe (maybe falsely) that it had at least some marginal impact on my performance.
Out of curiosity, are you just doing this by “feel” / “leave the fans off” or do you back it up by a core body temperature measurement? I use the Core Body Sensor since a few weeks and found that getting into the “Heat adaption zone” became quit difficult after some heat training. I usually wear normal, short bibs, a long sleeve baselayer, jersey and a long sleeve jacket and leave the fans off and it takes me up to 1h to even reach the heat training zone.
I was using the feel-o-meter, so can’t objectively measure my protocol or results. I will say usually started up at higher Z1 low Z2 just to build up some heat and usually backed down a bit once I got going. So maybe it was placebo, but it was good enough that I have repeated and will do so again. YMMV
Thats a good way of doing it. I also try to quickly bump the core body temperature by going into SweetSpot areas and by now I would think I can feel quite accurately when I hit the heat training zone…its basically when you are dripping of sweat. In my first workouts I would sometimes exceed the zone and overheat too much, but somehow my body got trained to cool itself better now and I dont really have this issue anymore. It just got more difficult to hit the targeted core body temperature.
TLDR: The Core Body Sensor is nice to have, ideally you use it for a few weeks to get a feel for it, but I strongly believe now that you can get the same gains without it.