Why do some people run hot and others cold?

Two people sit in the same room, at the same air temperature, wearing broadly similar clothes. One is too warm. The other is reaching for a cardigan.

Neither is being difficult. They are producing different amounts of heat.

This is the next instalment in our series on thermal neutrality, and the variable in question is metabolism.

The body as a heat engine

Metabolism is the energy your body produces from processing food. Some of it powers essential functions — keeping organs running — and some powers everything else you do with your day.

The awkward part, from a comfort point of view, is that heat is the by-product of all of it. Energy in, work done, heat out. Produce more energy and you produce more heat, and you need to shed more of it to stay in balance.

That is the entire mechanism behind the thermostat argument.

Basal metabolic rate

Your basal metabolic rate is the energy needed to keep you alive at rest. Four things drive it: age, sex, height and weight.

Age. Metabolic activity generally slows with age, largely through loss of muscle mass and shifting hormone levels. The same diet that maintained you at 25 will not at 50.

Sex. Studies consistently find women have a lower BMR than men, even after correcting for age, weight, height and activity. The gap is often modest — a few per cent — but it is persistent.

Height and weight. Both raise BMR, because there is more mass to maintain. Composition matters more than the number on the scale: fat-free mass — muscle and skeleton — drives BMR considerably harder than fat mass does. It is why more muscular people eat more to hold their weight.

The Harris–Benedict equation estimates it:

• Men: BMR = 13.397W + 4.799H − 5.677A + 88.362
• Women: BMR = 9.247W + 3.098H − 4.330A + 447.593

With W in kilograms, H in centimetres and A in years. Two examples:

• A 28-year-old man, 90 kg and 179 cm: about 1,884 kcal/day.
• A 42-year-old woman, 60 kg and 165 cm: about 1,260 kcal/day.

A gap of 600 calories a day between two ordinary people, before either of them has done anything.

Then activity multiplies it

BMR is the floor. Total daily energy expenditure applies a multiplier for how you actually spend your day — from about 1.2 for a sedentary office worker to 2 or more for a manual worker or an athlete.

Apply that to the same two people:

• The man, sedentary: about 2,260 kcal. Training five times a week: about 3,391 kcal.
• The woman, training one to three times a week: about 1,890 kcal. Manual work or professional sport: about 2,520 kcal.

Two further factors sit on top. NEAT — non-exercise activity thermogenesis, the fidgeting, posture shifting and trips to the kettle — can account for up to 300 calories a day on its own. And the thermic effect of food, the energy spent digesting, is typically 5 to 10% of what you eat.

From calories to watts

Heat output is measured in MET. One MET is 58 W/m² of body surface, representing a seated person. Average body surface area is about 1.8 m², so 1 MET is roughly 104 W of heat — about the same as an old incandescent light bulb, which is a useful thing to remember when sizing a room.

Depending on activity, a person ranges from around 60 to 480 W/m². Someone with a higher metabolic rate is running a bigger heater.

That is why the same room reads as pleasant to one person and freezing to another. On the ASHRAE comfort chart, a higher metabolic rate shifts the comfort zone to the left — toward cooler temperatures — and a lower rate shifts it right.

What to do about it

Two useful conclusions.

Stop trying to find the one correct temperature. It does not exist. In any room with more than a handful of people, some will be uncomfortable at any setting, and the usual response — pushing the thermostat further in one direction to satisfy whoever complains loudest — makes it worse for everyone else while burning energy.

Design for control instead. Fast-responding, zoned systems let people set their own space rather than negotiating over a building-wide number. A radiant system helps here specifically, because it can be zoned room by room and responds in minutes rather than hours, so local control is actually meaningful rather than theoretical.

For larger spaces, the same logic argues for deliberately providing a range of thermal environments and letting occupants choose where to sit.

Next in the series: clothing insulation, and the surprisingly large role it plays in all of this.