If you're specifying finned heaters for fan coil units, you've probably noticed that most suppliers offer three standard sheath material options: stainless steel 304, stainless steel 316, and Incoloy. They all look similar on a drawing, and they'll all heat air. But choose wrong, and you'll be dealing with premature failures in the field-corrosion, oxidation, or outright sheath rupture-long before the heater should have worn out.
We've been making U-shape finned heaters for HVAC applications at Jaye Heater for years. Here's what we've learned about when each material makes sense, and when it doesn't.
Why Sheath Material Matters
The sheath is the outer metal tube that contains the resistance wire and magnesium oxide insulation. It's the only thing separating the heating element from the airstream. In an FCU, that airstream can be clean and dry, or it can be humid, salty, or contaminated with airborne chemicals-depending on where the unit is installed.
The sheath material determines three things: how hot the heater can safely run, how well it resists corrosion, and how long it lasts. Get any of these wrong and the heater fails. And in an FCU, a failed heater means a service call, a replacement, and an unhappy customer.
Stainless Steel 304: The Standard Choice
SS304 is the most common sheath material for tubular heaters. It's what you'll see on probably 80% of the finned heaters in the market.
What it's made of: Chromium 18-20%, Nickel 8-11%, iron balance.
Temperature limits: Maximum continuous sheath temperature is around 650°C (1200°F). In practice, most FCU applications never get anywhere near that-typical air heating runs well below 400°C.
Corrosion resistance: The 18% chromium content forms a protective oxide layer that resists oxidation in air up to about 800°C. It handles typical airborne moisture just fine.
Best for: Standard HVAC applications with clean air, normal indoor environments, and no significant corrosive contaminants. If your FCU is going into an office building, a school, a hospital (non-surgical areas), or a residential setting, SS304 is the go-to.
When to avoid it: SS304 is vulnerable to chloride attack-pitting and stress corrosion cracking-especially in humid environments with salt air or where airborne chlorides are present. Don't use it in coastal installations, marine environments, or anywhere near salt spray.
Stainless Steel 316: The Upgrade for Corrosive Air
SS316 is the logical next step when the air quality isn't clean.
What's different: SS316 adds 2-3% molybdenum to the 304 formula. That small addition makes a big difference in chloride resistance.
Temperature limits: Slightly lower than 304-around 600-700°C maximum continuous operation. Still more than enough for FCU duty.
Corrosion resistance: The molybdenum content provides far superior resistance to pitting and crevice corrosion in chloride-containing environments. It handles salt air, coastal environments, and mild chemical contaminants much better than 304.
Best for: FCUs installed in coastal areas, marine environments, or locations with poor air quality containing mild corrosive elements. Also a good choice for units exposed to outdoor air intakes that might bring in road salt or industrial pollutants.
When to avoid it: SS316 is still vulnerable to chloride stress corrosion cracking above certain concentrations and temperatures-it's better than 304, but not immune. For truly aggressive environments, you need to look at Incoloy.
Incoloy 800/840: When You Need High-Temperature Performance
Incoloy is often over-specified for FCU applications, but there are situations where it's the right call.
What it's made of: A nickel-iron-chromium alloy with 30-35% nickel content. This gives it much better high-temperature strength and oxidation resistance than stainless steels.
Temperature limits: Up to 1100-1200°C continuous operation. That's far beyond what any FCU needs-but the headroom isn't the point.
Why you might use it: Incoloy's real advantage is resistance to extreme oxidation and scaling at high temperatures. If your FCU runs at elevated sheath temperatures (above 600°C) or if the application demands extreme longevity in a harsh environment, Incoloy makes sense. It also resists carburization and sulfidation atmospheres better than stainless steels.
When to avoid it: Cost. Incoloy is roughly 3.5 to 4 times more expensive than SS304. For a standard comfort-heating FCU, you're paying for capability you'll never use. Unless you're dealing with very high temperatures or aggressive chemical environments, stick with stainless steel.
A Quick Reference Table
| Feature | SS304 | SS316 | Incoloy 800 |
|---|---|---|---|
| Max continuous temp | ~650°C | ~600-700°C | ~1100°C |
| Chloride resistance | Poor | Good | Excellent |
| Oxidation resistance | Good to ~800°C | Good | Excellent to ~1200°C |
| Relative cost | 1.0× | ~1.3× | 3.5-4.0× |
| Best for | Standard clean air HVAC | Coastal, marine, poor air quality | High-temp or aggressive environments |
What We Tell Our OEM Customers
Here's the practical guidance we give to FCU manufacturers:
Start with the installation environment. Where is this unit going? Office building in a dry climate? SS304 is fine. Coastal hotel? Go SS316. Chemical plant or industrial kitchen with aggressive airborne contaminants? Consider Incoloy.
Don't over-spec. We've seen OEMs spec Incoloy for every model "just to be safe." That's money wasted on 90% of their units. Save Incoloy for the applications that actually need it.
Consider the fins too. The fin material matters as well. Most FCU finned heaters use SS304 fins as standard. If you're upgrading the sheath to SS316 for corrosion resistance, make sure the fins are also specified appropriately-otherwise you've created a weak point.
Test if you're unsure. If you're designing for a new environment and aren't confident about the material choice, run a sample. Put a few heaters in the actual operating conditions and check them after a few months. It's cheaper than a field failure.
If you're specifying finned heaters for a new FCU line or upgrading an existing model, send us your operating environment details-air quality, expected temperatures, installation location-and we'll recommend the optimal sheath material. We've been manufacturing U-shape finned heaters for HVAC OEMs at Jaye Heater for over a decade, and we can help you get the material choice right the first time.
Click here to request a quote or email us at marsha@jaye-heater.com. We'll respond within one business day.
