How many stones does a sauna heater need?

Ask the maker. The number comes from them. It's printed in the manual, or on the maker's page for your model. Harvia gives the same advice from its side.¹

What you need

• The manual, or your heater's model name so you can find it.
• A way to weigh stones if you're reusing old ones, such as a bathroom scale and a bucket.
• Boxes of stones in the size the manual names.

Where the number is

Look in the stones or loading section of the manual. If it's gone, look on the maker's website for the model. Harvia says to follow your heater's guidelines.²

Examples from one maker

HUUM publishes a list of stone amounts by model. These are the maker's figures, not a general rule.³

The spread is big. One small wall heater lists 15 kg, while a larger model with an air tunnel lists ten times that, and nobody could guess either number from the heater's looks.³

A guess from another heater won't work. Read your own model's line. If it isn't on any list you can find, ask the maker. The amount depends on the heater's design, and a wrong load can block the air the elements need.

Boxes, wood stoves and cages

Stones are sold by the box. HUUM's boxes hold 15 kg, so a heater that needs approximately 55 kg is a little under four boxes. Harvia's standard boxes are 20 kg.³¹

HUUM lists wood stoves too. Its HIVE Wood 13 needs approximately 90 kg, and the HIVE Wood 17 approximately 135 kg. Its stone cage takes approximately 30 kg.³

Too many, too few

Overload a heater and air can't move well inside it. Harvia says that can lead to the elements overheating.²

Go the other way and there's a cost too. Harvia also says too few stones will not provide the best sauna experience.²

Counting what you have

If you're working from a weight, you'll want a simple routine. Weigh the stones you plan to load, in batches, with a bathroom scale. Write the total down. Compare it with the manual's figure before anything goes in the heater.

HUUM lists its amounts as "approximately" and gives no tolerance. Follow the manual's figure and stacking notes, and if in doubt, ask the maker.³

Reusing stones

If you're reloading old stones, weigh them and drop any that are cracked or crumbling. You may need to top up with new stones to reach the manual's weight. Don't skip that step. A short load is a different problem from a full one. What counts as a bad stone is on the crumbling stones page.

For layout, see how to stack the stones.