Always-ready sauna heaters
An always-ready heater keeps its stones hot all the time under an insulated lid. You lift the lid, throw water and the sauna is on. Finns call it hetivalmis, “ready right away”. Here is how they work, who they suit, and what you can buy.
Full power on, lid open, and the room is ready to bathe in.
A large, insulated stone mass that stays hot between sessions.
It uses some electricity all the time to keep the stones hot.
How it works
An ordinary electric heater holds 20–60 kg of stones in an open frame and heats the room from cold every time, usually in 30 to 60 minutes. An always-ready heater has a much larger stone chamber, typically 80–100 kg, with insulation around it and a lid on top. The elements keep the stones hot around the clock, while the insulated shell stays safe to touch.
When you want a sauna, you switch the heater to full power and lift the lid. The heat stored in the stones flows into the room, and you can start throwing water almost at once. Harvia says its Forte is sauna-ready in a few minutes. When you are done, you close the lid and the heater goes back to holding the stones warm. If the lid is left open, the control panel warns you and switches the heater off.
“There is a Harvia Forte at my workplace gym, and I like it because the sauna needs no planning. Full power on, lid open, water in the bucket and in you go. You get good löyly straight away, and always-ready heaters usually give pleasantly soft löyly too. To me they suit people who like a sauna almost every day.”
Why the löyly feels soft
Löyly is softest when the water lands on a large mass of stones that are hot all the way through. A small heater with a thin layer of stones can feel sharp: the water hits a few very hot surfaces and flashes into steam at once. In an always-ready heater the stones have been heating for hours, so the steam comes off evenly and gently. The feel is closer to a heat-storing wood stove than to a compact wall heater. Heat-storing stoves →
Three heater types compared
| Ordinary electric | Always-ready electric | Heat-storing wood stove | |
|---|---|---|---|
| Ready to bathe | 30–60 min from cold | A few minutes | Several hours of firing |
| Stones | 20–60 kg, open | 80–100 kg, insulated, lidded | 100 kg or more |
| Löyly | Good; sharper on small heaters | Soft and even | The softest |
| Energy | Only when you use it | A little all the time, more when you bathe | Wood, once per session |
| Suits | Regular, planned sessions | Near-daily, spur-of-the-moment sessions | Weekends, cabins, special occasions |
Who it suits, and who it doesn't
- Good fit: you take a sauna most days, at different times, and want it without waiting. Harvia names family saunas and gyms as the typical places.
- Good fit: families with small children. The outer shell stays cool, and the stones are covered when the sauna is not in use.
- Poor fit: you take a sauna once or twice a week. Keeping 100 kg of stones hot all week is likely to cost more than heating an ordinary heater twice.
- Poor fit: a cabin or outdoor sauna that stands empty for days, or a room you only heat for guests.
What it costs to run
An always-ready heater uses electricity in two ways: a steady trickle to keep the stones hot, and full power while you bathe. Of the makers here, only Helo states the trickle: the Saunatonttu's manual lists standby settings of 100, 200 and 300 W. Left on around the clock, that is about 2.4 to 7.2 kWh a day. At the top setting a day of standby uses a little less than one warm-up and hour-long session on an ordinary 8 kW heater in a typical home sauna, about 8 kWh by the running cost calculator. Harvia and Narvi do not publish a standby figure; if you have one, check your electricity meter over a day without bathing.
The comparison that matters is how often you bathe. An ordinary heater spends most of its energy warming the room and stones from cold before every session. The more often you take a sauna, the more of those warm-ups an always-ready heater replaces, and the closer the two come in cost. The running cost calculator shows what an ordinary heater uses per session and per month.
Sizing and installation
Size an always-ready heater by room volume exactly like any other electric heater: the manufacturer's rated room range has to cover your room, with extra for glass and masonry. The calculator works it out. They are floor-standing, heavy once loaded, and need a fixed 240 V circuit in North America or a three-phase supply in Europe, connected by a licensed electrician. Restack the stones at least once a year so air can flow between them and the elements. Sauna stones →
What you can buy
United States and Canada
Always-ready heaters are rare in North America. Harvia sold the Forte in three US versions, all 240 V single-phase and ETL listed, with 100 kg (220 lb) of stones. Harvia's US site now lists all three as discontinued; some retailers still have stock.
| Model | Power | Rated room | Status |
|---|---|---|---|
| Harvia Forte AF45 | 4.4 kW | 177–283 ft³ | Discontinued |
| Harvia Forte AF65 | 6.5 kW | 247–424 ft³ | Discontinued |
| Harvia Forte AF100 | 9.8 kW | 353–530 ft³ | Discontinued |
Buying remaining stock: check the ETL label, the rated room range and that the retailer still offers the warranty. Ranges from Harvia's product pages; the AF65 and AF100 ranges converted from 7–12 m³ and 10–15 m³.
Finland and Europe
In Finland the big hardware chains carry always-ready heaters next to ordinary ones. These are 400 V European models and are not sold for North American circuits.
| Model | Power | Rated room | Stones |
|---|---|---|---|
| Harvia Forte AFB4 / AFB6 / AFB9 | 4 / 6 / 9 kW | 5–8 / 7–12 / 10–15 m³ | 100 kg |
| Narvi Löylyvalmis 6 / 9 | 6 / 9 kW | 5–9 / 10–14 m³ | 80 kg |
| Helo Saunatonttu 3 / 4 / 6 / 8 | 2.0–8.0 kW | 3–6 up to 9–17 m³ | about 100 kg |
Questions
Can I leave the lid open while I bathe?
Yes, that is how you use it. Close it when you finish; if it stays open too long, the control panel warns you and switches the heater off.
Can I switch it off between sessions?
You can, but then it behaves like a slow ordinary heater: 100 kg of cold stones take a long time to heat. It makes most sense left on for regular use.
Is the löyly really different?
In my experience it is softer than a compact wall heater, because the water lands on a large mass of evenly hot stones. It is not quite the deep, gentle heat of a heat-storing wood stove, but it is close for an electric heater.
Sources: Harvia, Forte Ever-Ready heater and Forte product pages and manual; Narvi Löylyvalmis product card; Helo, Saunatonttu installation and use manual. Checked October 2026.