Fellrnr Heat Chamber

A heat chamber is a heated enclosure used for Heat Acclimation Training, typically with an indoor bike trainer. It gives far more control than a Fellrnr Heat Suit, and is much more comfortable for the long sessions that heat training requires, at the cost of space, money and building work.

This page describes the chamber I built in my garage in the UK, including what I would do differently.

A heat chamber is a sealed, heated space that you are inside. If you get into trouble you are inside the heat source, and if you collapse nobody will know unless you arranged for it. Make sure the door opens from the inside, keep it unobstructed, keep a phone within reach, and make sure somebody knows you are in there and when to check. Read the safety and first aid sections of Heat Acclimation Training before using one.

Do not use a heat chamber for weight loss or weight cutting. The weight lost is water rather than fat and returns within a day or two, while the dehydration makes everything more dangerous. Sweat-based weight cutting in hot rooms and saunas is the practice that has killed athletes, and it led the NCAA to ban hot rooms above 26c/79f for weight management.

How I got here

I started heat training in the UK simply by overdressing in the garage on the bike trainer. I then added a fan heater, then surrounded the bike area with plastic sheeting, and finally built a proper chamber from insulation board with the help of my eldest son. If you are not sure heat training is for you, that progression is worth repeating: each step is cheap and tells you whether the next is worth it.

The structure

• Roughly 1.2m wide, 2.4m high and 2.4m long, which is just big enough for the bike and some extra gear.
• Built from 25mm insulation boards.
• A simple flap cut into the rear wall lets me open it to dump heat and CO₂. I leave it closed while pre-warming, which takes 15 to 45 minutes depending on the ambient temperature.
• I removed the bike's front wheel to save space and replaced it with a simple wooden stand.

The structure is fragile, because I did not know whether the whole approach would work. If I built it again I would make a timber frame to attach the insulation to, use thicker insulation, and insulate the floor for better thermal efficiency. But good enough is good enough: it is wabi-sabi, nothing is permanent, nothing is finished, nothing is perfect.

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Heating

• Two fan heaters, a 3kW and a 1.8kW, which is as much as my garage supply will take.
• The fans draw air partly from outside through a hole in the insulation. This matters: without it they cut out at around 30c. With it, the chamber reaches 40–50c.

Monitoring the environment

• A four-channel K-type thermocouple device, with sensors in various positions around the room and one in front of the main heater output. Several sensors are worth having, because a single reading tells you very little about a space with a fan heater in it.
• A CO₂ sensor. I aim to keep CO₂ below 2,000 ppm, and open the flap when it climbs. A sealed, insulated box containing a hard-breathing human accumulates CO₂ faster than you would guess.

Note that environmental monitoring tells you about the chamber, not about you. See the core temperature section of Heat Acclimation Training for why measuring your own temperature is harder than it sounds, and why I have to leave the chamber to take a rectal reading at all.

Humidity

The heat and low humidity dry my throat badly if I push the effort, so I added an ultrasonic humidifier. It helps, though I do worry about mould building up in the tubing and would clean it more diligently than I do.

Low humidity also means a lot of sweat evaporates, which is good for cooling and therefore means you can tolerate more heat. A humid chamber is a much harsher environment at the same air temperature.

Protecting electronics

The heat causes real problems for electronics, which are not designed for 45c ambient. I use freezer ice packs:

• Two under the laptop and two either side of the touchpad.
• The phone rests on an ice pack.
• Another ice pack behind the iPad I use for entertainment. This one needs replacing every 30 to 45 minutes; the iPad struggles the most of any device.

This applies to thermometers too. A digital thermometer left in the chamber becomes very hot, and I have had thermometers terminate a reading before insertion and report chamber air temperature instead. See Heat Acclimation Training for the detail.

Sweat

Managing the volume of sweat is a genuine problem. Even at low humidity, where much of it evaporates, a lot still ends up on the floor. I put various trays under the bike to catch what I can. If I were building again I would plan drainage properly rather than improvising trays.

Equipment

• A bike trainer, in my case a Tacx Neo 2, with a custom build of GoldenCheetah to control it. I have also modified GoldenCheetah to display and graph the CORE sensor metrics.
• Typical sessions are 60 to 120 minutes, drinking 2 to 3 litres of electrolyte solution, roughly a quarter teaspoon of salt per litre.
• A good saddle and bib shorts matter more than you would expect at these durations. I use an ISM PR 3.0 saddle, which is heavy for outdoor riding but comfortable, and Castelli Espresso bib shorts. Bib shorts rather than waisted shorts, to avoid waist compression and the GI problems that follow.

Blood and iron

I have blood tests through heat training. As expected, my red blood cell concentration dropped as blood volume increased, then returned to normal. That transient effect has since been documented, with haemoglobin mass falling from day 4 of heat training before returning to baseline.

Because building extra red cells needs iron, and iron is hard to absorb from plant sources and supplements, I eat black pudding regularly, aiming to get 50 to 100% of my RDA from it. See Heat Acclimation Training for the caveats on iron.

Is it worth it?

A chamber and a heat suit produced similar increases in haemoglobin mass in elite cyclists, around 2.4% and 2.6% respectively. So the chamber is not buying you a bigger adaptation. What it buys is comfort, control and repeatability over long sessions, which in practice is what determines whether you actually do the training.

If you are doing a two-week block before a hot race, overdressing or a Fellrnr Heat Suit will do the job. A chamber makes sense if heat training is going to be a regular part of your year.

See Also

• Heat Acclimation Training
• Fellrnr Heat Suit
• Running in the Heat