Fellrnr Heat Suit

The Fellrnr Heat Suit allows Heat Acclimation Training in ordinary or even cold conditions, with no special facility and very little cost. It works by trapping your body heat and your sweat, creating a hot microclimate against your skin.

That is also precisely why it is the most dangerous method described on this site.

A heat suit works by preventing your body from cooling itself. If you overheat, you will continue to get hotter after you stop, and you will not cool down if you collapse. Three collegiate wrestlers died in 1997 while exercising in vapour-impermeable suits in hot environments. I have had some close calls using this technique. Start with very low intensity, build up very gradually, and make sure somebody knows what you are doing and will check on you. Read the safety and first aid sections of Heat Acclimation Training before using this.

Before you build one

• Never use a heat suit for weight loss or weight cutting. Sweat-based weight cutting in impermeable suits is the practice that has actually killed athletes. The NCAA banned vapour-impermeable suits, saunas and steam rooms for weight management as a direct result. The weight lost is water, not fat, and it returns within a day or two. Everything on this page assumes you are training for heat adaptation and drinking normally.
• Never do intervals or high-intensity work in a heat suit. Core temperature can rise faster than you can detect or reverse.
• Have cooling ready before you start, not after you need it. See first aid in Heat Acclimation Training.
• Do not use one if you are ill, feverish, sleep-deprived or dehydrated.
• Prefer somewhere you can be found. A heat suit lets you train anywhere, which tempts you to train alone and remote. Resist that. The failure mode here is collapse while still being heated.

Why two waterproof layers

A traditional sweat suit uses a single waterproof layer to trap sweat and stop it cooling you. The problem is that the sweat soaks through whatever you are wearing underneath and ruins the insulation, so in cool conditions the wet clothing eventually makes you cold rather than hot.

The Fellrnr approach uses two waterproof layers, with the insulation sandwiched between them. The inner waterproof layer traps sweat in a thin layer next to the skin; the insulation outside it stays dry and keeps working; the outer layer stops your heat escaping. The result is far more effective and much more consistent than a single-layer suit.

Construction

Four layers, over both torso and legs.

Layer 1: close-fitting thermal base layer

Layer 1: a close-fitting thermal layer next to the skin. Its job is to hold the sweat and keep your skin reasonably comfortable. I wear a Halo headband to keep sweat out of my eyes, plus a thin hat so the waterproof hood does not stick to my ears.

Layer 2: inner waterproof layer

Layer 2: a waterproof layer that traps the sweat in the base layer. Ideally non-breathable: breathability is the enemy here.

Layer 3: insulation

Layer 3: an insulation layer, such as fleece, to stop heat escaping. Because it sits between the two waterproof layers, it never gets wet and never loses its insulating value.

Layer 4: outer waterproof or windproof layer

Layer 4: a second waterproof or windproof layer to trap body heat in the insulation. Add a hat, gloves and ideally a neck warmer or face mask.

This combination prevents most of your heat escaping. That is the point, and it is also the hazard.

Using it

• Start very low. My first sessions should have been shorter and easier than they were. Twenty to thirty minutes of genuinely easy effort is a sensible first session.
• Add duration before you add layers. Build the time up first, then make the suit more effective.
• Low intensity throughout. In the published elite protocol, heat suit sessions were 50 minutes at low power output in temperate conditions, producing a rectal temperature of about 38.9c in the first week settling to 38.5c thereafter, with fluid losses of roughly 1.6 to 1.7 litres per session. That study reports no external financial support and no conflicts of interest.
• Drink. Fluid loss of over a litre and a half in 50 minutes is normal. Include sodium; I use about a quarter teaspoon of salt per litre, and drink to thirst.
• Plan how you will get out of it. Four layers take a while to remove, which matters if you need to cool quickly. Practise this when you are not in trouble.
• Stop at the first warning sign, and get the outer layers off immediately rather than finishing the session.

Heat suit versus heat chamber

Both work. In the elite comparison study, a heat chamber and a heat suit produced similar increases in haemoglobin mass of around 2.4% and 2.6% respectively.

• The suit is cheap, portable, needs no space, and works outdoors or in a cold garage. It is less comfortable and considerably more dangerous, because the heat is held against you and cannot be escaped quickly.
• The chamber takes space, money and building work, and only works where you built it. It is much easier to escape, easier to adjust mid-session, and more comfortable for long sessions.

If you are doing occasional heat blocks, or travelling, the suit is the practical choice. If heat training is a regular part of your year, see Fellrnr Heat Chamber.

See Also

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