Three to five percent. That's the time trial improvement from heat acclimation — a protocol that costs absolutely nothing. For context, a $300 aero helmet buys you less than one percent. Deep-section wheels, maybe two. Ten to fourteen days of riding your turbo trainer with the fans off and the door closed delivers more free speed than most equipment upgrades combined.
Tim Kerrison built purpose-built heat chambers at Team Sky during the Froome era. You already own the equivalent — it's called a spare room with a closed door. Turn off the fans. Put on an extra layer. Ride easy for 45 minutes. The science behind this is some of the most robust in endurance sport, and professional teams have been exploiting it for over a decade.
What Most People Do vs. What Actually Works
Most cyclists approach the indoor trainer as a necessary evil. The fan goes on full blast, the window opens, the garage door stays up. Everything is oriented around staying cool, staying comfortable, staying in a position where you can push watts without feeling like you're about to melt into the carpet.
That makes sense if your only goal is to hit a power target on a Tuesday evening. But if your goal is to get faster — actually, measurably faster — you're leaving one of the most potent adaptations on the table.
Heat acclimation is one of the most effective legal performance interventions in endurance sport. Not one of the most effective supplements. Not one of the most effective pieces of equipment. One of the most effective interventions, full stop. The research shows 3-5% improvements in time trial performance. Let me be really clear about this: 3-5% is enormous. That's 1-2 minutes over a 40km time trial. Your aero helmet gives you maybe 0.5-1%. Your deep-section wheels, another percent if you're lucky. Heat acclimation gives you more than both combined, and it costs you nothing except sweat and stubbornness.
The Physiology — Why Heat Makes You Faster
Once you understand what's happening inside your body during heat acclimation, you'll wonder why you ever rode with a fan on during a base session.
When you ride in hot conditions repeatedly, your body doesn't just tolerate the heat better. It fundamentally remodels how it manages temperature, blood flow, and cardiovascular output. The adaptations are real, measurable, and — here's where it gets really interesting — they benefit your performance in all conditions. Not just hot ones.
The first adaptation is plasma volume expansion. Your body starts producing and retaining more blood plasma. More blood volume means more stroke volume per heartbeat, which means more oxygen delivered per minute to working muscles. This is the same mechanism that altitude training targets, except you don't need to move to a mountain or pay for a hypoxic tent. Your body responds to heat stress by expanding its total blood volume, and that expanded volume sticks around for weeks after the heat block ends.
The second adaptation is improved sweating efficiency. Your sweat glands become more responsive. You start sweating earlier in a session and your sweat composition changes — you lose fewer electrolytes per litre of sweat. Your body gets better at the mechanical business of cooling itself down, which means you can sustain higher workloads before thermal stress starts compromising performance.
The third is a lower resting core temperature. Your baseline body temperature drops slightly after acclimation, which gives you a larger thermal buffer before you reach the critical core temperatures where performance falls off a cliff. You start cooler, which means you can work harder for longer before hitting the red zone.
Then there's reduced heart rate at submaximal intensities. For the same power output, your heart rate drops. You see this within days of starting a heat protocol. Your cardiovascular system becomes more efficient — it can deliver the same output with less effort, which means you fatigue more slowly at race intensity.
Professor Stephen Seiler has been clear on this point in his research on endurance athletes: the plasma volume expansion from heat acclimation improves performance in all conditions, not just hot ones. More blood volume means more oxygen delivery regardless of whether it's 35 degrees or 15 degrees on race day. You race better in the cold because your cardiovascular system is simply more capable.
The Professional Precedent
This isn't some fringe concept from a fitness influencer. This is established practice at the highest level of the sport.
Tim Kerrison, the head of performance at Team Sky during the Froome era, pioneered the systematic use of heat chambers in professional cycling. Before Grand Tour stages expected to hit extreme temperatures — and the Tour de France regularly sees stages above 35 degrees — Froome and his teammates would undergo structured heat acclimation blocks. Post-ride sauna protocols. Heat chamber sessions. Deliberate, progressive exposure designed to trigger every adaptation I just described.
This is documented. This is widely known in the professional peloton. And the results spoke for themselves across four Tour de France victories.
When I had John Wakefield on the podcast, he talked about how Bora-Hansgrohe approach environmental preparation. It's a standard part of Grand Tour preparation now. Dan Lorang built environmental protocols into his planning for Grand Tour stages in hot conditions — the idea that you'd show up to a race in extreme heat without heat acclimation would be considered professionally negligent at WorldTour level.
The good news is that your indoor trainer replicates the same stimulus. You don't need a heat chamber. You don't need a post-ride sauna. You need a room with a door, a trainer, and the willingness to be uncomfortable for 45-90 minutes.
The Protocol — Exactly What To Do
You're not new to the bike. You know what zone 2 feels like. You know what tempo feels like. The protocol here is simple. The hard part is not the sessions — it's the discomfort.
Phase 1: Days 1-5 — Building the stimulus. Ride with your fans off. Close the door. If you have a window open, shut it. Put on a base layer you wouldn't normally wear. The goal is to let your core temperature rise naturally during an easy zone 2 ride. Start with 45-60 minutes. That's it. Don't try to be a hero on day one. The first session will feel surprisingly hard for what should be easy power. Your heart rate will be elevated. You'll sweat more than you've ever sweated on the trainer. This is the point. This is the stimulus.
Phase 2: Days 6-10 — Extending the exposure. Push the duration to 60-90 minutes. Stay in zone 2. You'll notice something interesting by day 6 or 7: it starts feeling slightly less awful. Your heart rate at the same power begins to come down. Your perceived effort drops. These are the early signs that acclimation is working — your plasma volume is expanding, your thermoregulatory system is adapting. Don't be tempted to add intensity yet. The adaptation is driven by sustained moderate heat stress, not by hammering yourself into a puddle.
Phase 3: Days 11-14 — Adding intensity. Now you can begin including moderate-intensity work within the heat sessions. Tempo efforts within a 75-90 minute ride. Not VO2max intervals. Not threshold work. Steady, controlled tempo. By this point, your body has made the major acclimation adaptations and can handle higher workloads in the heat without the same cardiovascular strain you felt in week one.
Maintenance: Ongoing. Once you've completed the initial 10-14 day acclimation block, 1-2 heat sessions per week maintains the adaptations. You don't need to keep doing it every day. The plasma volume expansion, the improved sweating efficiency, the cardiovascular gains — they persist as long as you maintain occasional heat exposure.
Full acclimation takes 10-14 days of consistent heat exposure. You start seeing measurable adaptations within 5-7 days, with plasma volume expansion happening earliest. The whole process from start to fully acclimated fits inside a standard training block.
Safety — The Stuff You Cannot Skip
Let me be really clear about this: heat training is safe when you approach it with basic common sense. It is not safe when you approach it with ego.
Hydration is critical. You will sweat more during these sessions than you've ever sweated on the trainer. Drink significantly more than usual. Pre-hydrate before the session. Have a bottle within arm's reach throughout. And replenish electrolytes — not just water. When you're losing that much sweat, plain water alone isn't enough.
Watch for warning signs. Dizziness. Nausea. Confusion. A sudden inability to think clearly or a feeling that something is wrong. If any of these appear, stop immediately. Not "finish the interval and then stop." Stop. Get off the bike. Cool down. These are signs of heat exhaustion, and the line between heat exhaustion and heat stroke is one you do not want to find.
Never do high-intensity intervals in heat until you are fully acclimated. Days 1-10 are not the time for VO2max work in a closed room with no fan. The combination of high metabolic heat production and high ambient temperature without acclimation creates a dangerous thermal load. Easy riding first. Moderate intensity later. High intensity only when your body has fully adapted.
Avoid heat training sessions when you're already fatigued or dehydrated. If you did a hard group ride in the morning, don't come home and do a heat session in the evening. The cumulative stress is not worth the marginal adaptation. Rest, recover, and do the heat session when you're fresh.
Build the exposure gradually. Ten to fourteen days is the timeline for a reason. Your body needs time to produce the additional plasma volume, to retrain the sweat glands, to recalibrate the thermoregulatory set points. Rushing this process doesn't make it faster. It makes it dangerous.
When To Use Heat Training
There are two ways to think about timing.
The first is event-specific preparation. If you have a target event in hot conditions — a summer sportive, a Gran Fondo in southern Europe, a triathlon in July — begin your heat acclimation protocol 2-3 weeks before the event. This gives you the full 10-14 days for acclimation plus a few days to taper and recover before race day. You'll arrive at the start line with a body that's already adapted to the conditions your competitors will be fighting against.
The second is as a general performance intervention during base phase. Because the plasma volume expansion and cardiovascular improvements benefit racing in all conditions, heat training works as a winter or spring training tool. You're sitting on the indoor trainer anyway during base phase. You might as well close the door and get the free adaptation while you're at it. Professor Stephen Seiler's work is clear: the gains transfer. You don't need to be racing in heat to benefit from heat acclimation.
Why Most Cyclists Miss This
The key insight about free performance gains: they're almost always uncomfortable. There's no product launch. No marketing campaign. No review on a cycling website. Nobody's filming an unboxing video of "turning off your fan and closing the door."
Heat training is uncomfortable. Properly uncomfortable. The kind of uncomfortable where you question your choices and watch the clock and wonder if something is actually wrong or if you're just hot. That's the point. The discomfort is the stimulus. Your body adapts to the discomfort by becoming fundamentally more capable, and that capability transfers to every ride you do afterward.
It's not sexy. It's not new. There's no gadget to buy. But the physiological adaptations are among the most powerful legal performance interventions available in endurance sport. Three to five percent in time trial performance. Expanded blood volume. Lower heart rate. Better cardiovascular stability. For zero cost.
The fixable problem for most cyclists isn't their equipment, their nutrition plan, or their interval structure. It's that they've never deliberately made themselves uncomfortable on the trainer in a way that triggers real adaptation. The indoor trainer isn't just a place to do structured workouts when the weather's bad. It's a heat chamber you already own.
Close the door. Turn off the fan. Start with 45 minutes of easy riding. See what happens over two weeks.
If you want the structure, the coaching, and a community of riders who are serious about getting faster without the nonsense, the Not Done Yet community on Skool is where we put all of this into practice. No hard sell. Just riders who want to be better than they were last season.