Choosing the right stone size for your heater

Use the size range your heater's maker names. Different makers name different ranges, so there's no one size that fits every heater.¹

What you need

• Your manual, or the maker's page for your model.
• A ruler or tape, to check the stones you already have.
• The sizes printed on any stones you're considering.

Sizes makers list

Harvia's stones come in 5-10 cm and 10-15 cm diameters.²

HUUM's range is round stones with a diameter of 3-5 cm and 5-10 cm.³

The ranges overlap but differ. HUUM's CLIFF heater, for example, needs approximately 75 kg of stones with a diameter of 3-5 cm.³

Why size matters

Harvia says smaller 5-10 cm stones are usually used in electric heaters because they fit well between heating elements.²

It says larger 10-15 cm stones are used in woodburning heaters, and in selected Harvia electric heaters with larger stone capacity.²

So "usually" is the word to watch. Some electric heaters take the larger size, and your manual will say if yours does.²

Mixing sizes

HUUM says you'll need smaller and larger stones to fill the heater evenly without noticeable gaps.³

Harvia's rounded stones come in both sizes too. It says to finish the top layer with the smaller 5-10 cm stones if you use the larger rounded ones.²

A short rule of thumb

Don't guess from the stones you already have. Harvia says to always check the size and amount in the manual included with the heater.²

If the manual's lost, the maker's website or support is the next place. Harvia also says to always check the stone size against the manufacturer's recommendations for your heater.¹

Decorative stones

Decorative stones have their own limits. Harvia says its white and red granite stones go on the top layer only, at a maximum of one box per heater.²

Reading a product page

Product pages often list size and weight together, like "5-10 cm, 20 kg". The first figure is the diameter of each stone. The second is the weight of the box. Harvia's pages use that pattern.²

Boxes and wood stoves

Harvia lists most of its 5-10 cm and 10-15 cm olivine diabase stones in 20 kg boxes. Its rounded 5-10 cm stones come in a 15 kg box.²

HUUM's boxes are 15 kg for both of its sizes.³

Wood stoves are a different case. HUUM's HIVE Wood stove calls for stones of 5-10 cm, the same size as several of its electric heaters.³

For stone weight, read the page on amounts: how much does my heater need. For rock names, see stone types.