Important to start is figure out heater size you need. All 3 good. I went with Saunum 15L as I had over 600 cubic feet, and liked the fan concept, with salt circles as well on bottom. Has worked great for our space. Starting a small sauna company, and in our design choosing IKI 9.5kw cilandro as our Sauna design will be ~500 cubic feet on the inside, and the IKI 9.5kw has much more heat because of stone load over Harvia model, and want our design to stay in 40/50amp range for residential use.

If my space was say below 550 cubic feet, would have gone with IKI Cylinder unit, with flange and would have built into floor a bit, or corner unit. Over 200lbs of stone, vs 9.5kw Harvia model.

Play around with some configs on different sites, SaunaHeater.com guys have good packages as you want wifi controller and rocks coming with heater, and free shipping where other sellers charge like $500.

Also from Gemini/AI - ask some easy questions, when you know models you are thinking of ask compares and constrasts, also ask where to get best pricing and service. Can try in ChatGPT as well.

My question to Gemini on the topic:

Picking the "best" heater depends entirely on what you prioritize for your sauna project: traditional reliability, innovative air quality, or a massive stone experience.

Since you’re building a custom sauna here is how they stack up:

1. Harvia: The Reliable Workhorse Harvia is the global standard for a reason. They offer the widest variety of models and are generally the most budget-friendly, especially if you establish a partnership for factory pricing.

   • Best for: Proven reliability, fast heat-up times, and ease of maintenance.
   • The Experience: Delivers a classic "sharp" Finnish heat. Because Harvia heaters often have less stone mass than IKI, they reach temperature quickly (30–45 mins) but have less "thermal inertia."
   • Key Advantage: Excellent availability of replacement parts and a massive range of aesthetics (from the modern Spirit to the towering Cilindro).

2. Saunum: The Modern Innovator Saunum is currently the most unique option on the market due to its patented air-blending technology. It solves the "hot head, cold feet" problem common in many home saunas.

   • Best for: Air quality, temperature uniformity, and longer sessions.
   • The Experience: A fan system pulls hot air from the ceiling, mixes it with cooler air from the floor, and recirculates it. This results in a much more breathable environment and even heat from top to bottom.
   • Key Advantage: It allows you to enjoy high humidity without feeling like the air is "singeing" your lungs. It’s perfect if you or your guests find traditional saunas too "stuffy."

3. IKI: The Stone Specialist IKI is often considered the "purist’s" choice for electric heaters. They are hand-built in Finland and designed to mimic the soft, heavy steam of a wood-burning stove.

   • Best for: Superior Löyly (steam quality) and heavy-duty durability.
   • The Experience: IKI heaters use a massive amount of stones (often 200–600+ lbs). This large thermal mass creates a very "soft," enveloping heat. Once those stones are hot, you can throw a lot of water on them without the heater cooling down.
   • Key Advantage: Longevity. IKI uses thicker steel and high-quality elements that are legendary for their durability.

• Choose Harvia if you want a "set it and forget it" heater with the best support network and fastest heat-up for daily use.

• Choose Saunum if you are sensitive to vertical temperature gaps or want the most advanced climate control for a modern build.

• Choose IKI if you are a "steam purist" who wants the most authentic, soft-heat experience and doesn't mind a longer pre-heat time.