How to load and stack heater stones

Load loosely, and leave gaps. A heater breathes through its stones, and a tight pile chokes it. Both Harvia and HUUM say so in their guides.

One thing first. Hard-wired heaters are licensed electrical work. HUUM says only a certified electrician is allowed to connect the heater to the power supply¹. Wood-fired stoves are a separate matter. Flues, clearances and permits come from the maker's manual and the rules where you live. Stoves also bring a carbon monoxide risk, so never run one in an unvented room.

Before you start

Make sure the heater is off and fully cool. Then:

1. Wash the stones. Harvia says to start by washing the dust off before placing them².
2. Let them dry completely first³.
3. Clear out old dust and debris from the heater¹.
4. Spread the stones out on a mat or paper near the heater, so you can pick sizes as you go⁴.
5. Wear gloves. Sauna from Finland calls this a good idea⁴.

The don'ts

These come straight from Harvia's tips for piling stones²:

• Don't drop stones into the heater.
• Don't form a high pile on top.
• Don't place stones on top of the heater frame.
• Don't wedge stones between the heating elements.

HUUM agrees on the last one: never force stones between heating elements¹.

The do's

• Go sparse. Harvia says to arrange stones evenly and sparsely so air can flow between them². HUUM says to place them loosely rather than packing them tight, and to keep air gaps¹.
• Big at the bottom. HUUM stacks larger stones low for airflow and smaller stones on top for even steam³.
• Stand long stones up. Harvia says to place elongated stones vertically².
• Let stones hold each other up. In electric heaters, Harvia says to pile the stones so they support each other instead of lying their weight on the heating elements².
• Support the elements. Harvia's tip is to support the heating elements with stones so the elements don't bend².
• Cover the elements. HUUM says to make sure heating elements remain covered by stones¹.

New stones: the first heat-up

Fresh stones can smell a little at first. Harvia says new stones may give the sauna a natural stone scent during the first heating².

Makers differ on the first heat-up. HUUM recommends tempering new stones before first use to increase their durability. To do it, heat them once to the usual sauna temperature and let them cool down slowly, without throwing water on them³. Harvia says the opposite. After piling the stones, heat the sauna and throw enough water to douse all of them. That disperses the unwanted stone scent². Follow the manual that came with your own heater.

Wood-fired heaters

The same ideas hold, with a twist in size. Sauna from Finland says that in wood-fired stoves the larger stones sit at the bottom, and the higher you fill, the smaller the stones should be⁴. Harvia adds that smaller stones are used on the top layer of wood-burning heaters².

Check your work

Look through the heater from the side. Can you see gaps? Good. Is anything pressing an element out of line? Take it out and rearrange.

Different heater shapes need different piles. Harvia says the best guide is the manual that came with your heater². Follow it for your own model, and use the size page and weight page to get the right stones in the first place.