A cool season changes the heater: sizing an outdoor sauna for Chiang Mai mornings

What the north's weather really is

Chiang Mai sits at 305 m. At its station the coolest month is January, with a mean daily minimum of 15.7 °C; the hottest is April, with a mean daily maximum of 36.7 °C; annual rainfall is about 1,139 mm, nearly all from May to October (Thai Meteorological Department, 1991-2020 normals). Record lows near 3.7 °C and highs near 42.5 °C come from TMD extremes compiled by Wikipedia. A cabin on a hillside lodge sees most of that range in one year.

Heat loss, to a first approximation

A cabin loses heat roughly in proportion to the difference between the inside and the outside. Taking a 90 °C cabin and comparing every outside temperature with a 30 °C day, as a first approximation:

At 16 °C, a typical January morning, the cabin loses about a quarter more heat than on a warm day; at 5 °C, about two fifths more. Wind and an open site add to it. Real losses depend on insulation and glass, so treat the table as a guide to the direction and the rough size of the allowance, not a calculation for your cabin.

What we change for the north

• A cold-weather heater allowance on top of the one-kilowatt-per-cubic-metre rule and the glass allowance; for outdoor cabins in Chiang Mai about a quarter more.
• Double glazing on the windy side and a door seal that closes properly.
• A sheltered siting. The cabin's back to the prevailing cold wind, the door on the lee side, a deck that drains.
• A vestibule or a screen so that opening the door does not dump the heat in a cold gust.

Warm-up, in plain words

Our standard is about forty minutes to working temperature. A correctly sized heater keeps close to that on a cool morning; an undersized one stretches it, and the cabin then never feels properly hot. If you want to use the cabin at six in the morning, a timer starts the heater before you walk down. See the deck and garden sauna page for the build.

And in April: the same circuit, the other way round

When the pool is the main event, the chiller is the equipment under strain: a 36.7 °C afternoon average, strong sun and low humidity. Shade the pool, insulate the shell and place the unit in a ventilated niche. The annual mean dew point at the Chiang Mai station is about 19.8 °C, against 23⁠-⁠24 °C at the coastal stations, so cold pipes sweat less than they do on the coast. The Chiang Mai page has the local calendar and chillers for Thai heat the sizing.

A cool-season and a hot-season routine

A worked example: a 4-seat outdoor cabin

On the coast a 4-seat cabin of 2.0 x 1.7 m inside takes an 8 kW heater. In Chiang Mai, with a cold-weather allowance of about a quarter, it steps up to the next size, a 10.5 kW heater (the size we use for a 6-seat cabin). That is three-phase supply either way, but the breaker and cable are sized for the larger unit from the start. The cost is a heater and a cable, not a different cabin; ask for it in the layout.

Wind, the door and the shelter

• Turn the back to the wind. On a hillside the cold air runs downhill at dawn; a cabin with its blind wall uphill loses less heat.
• Put the door on the lee side, with a screen so opening it does not dump the cabin's heat.
• Plan the walk. A lit, dry path of ten metres or less from the house to the cabin is the difference between a ritual and an intention.
• Deck, not lawn. A teak deck with drained edges keeps wet feet off cold ground.

Getting the pieces up the hill

Many hillside lodges sit at the end of a narrow, steep road, and the last stretch decides how the cabin and the pool shell arrive. We ask for the narrowest gate, the tightest bend and the steepest gradient on the route, then size the sections to pass them; where a delivery truck cannot reach, the final leg is a smaller vehicle or a carry. A short video of the approach from the main road tells us more than a plan does.