Comparison Of Steels Used In Outdoor Boiler Construction

Advantages Of Certified Boiler Plate Steel
1) Modern Industrial and Commercial Boilers are still made from boiler plate
2) Expands 30% less then 304 S.S. when exposed to heat
(less expansion = less likely to fail)
3) Is the more thermally conductive then stainless steel
(transfers heat more efficiently)
4) Boilers are typically made of 1/4" or thicker steel

Disadvantages Of Certified Boiler Plate Steel
1) is less resistant to corrosion then stainless steel
2) Expands more then 409 S.S. when exposed to heat

Advantages Of 409 Stainless Steel
(commonly called titanium blend or enhanced)
1) Expands half as much as 304 S.S. when exposed to heat
(less expansion = less likely to fail)
2) Is twice as thermally conductive as 304 S.S.
(transfers heat more efficiently)
3) Is more resistant to corrosion then mild steel or boiler plate

Disadvantages Of 409 Stainless Steel
1) 409 S.S. is less resistant to corrosion then 304 S.S.
2) Boilers are typically made of 10 or 12 gauge (1/8" or thinner) steel

Advantages Of 304 Stainless Steel
1) 304 S.S. is more resistant to corrosion then other steels

Disadvantages Of 304 Stainless Steel
1) Expands twice as much as 409 S.S. when exposed to heat
(more expansion = more likely to fail)
2) Is half as thermally conductive as 409 S.S.
(transfers heat less efficiently)
3) Boilers are typically made of 10 or 12 gauge (1/8" or thinner) steel

Disadvantages Of Mild Steels
1) Expands more then Boiler Plate when exposed to heat
(more expansion = more likely to fail)
2) Is less thermally conductive then Boiler Plate.
(transfers heat less efficiently)
3) Is less resistant to corrosion then Boiler Plate.
4) Boilers are typically made of 7 or 8 gauge (3/16" or thinner) steel


The resistance to corrosion of the type of steel used in the construction of the boiler is only
1/3 of the decision of which steel to use, because there is no steel that is the best in all
off the three categories relevant to the steel performance when used in boiler construction.

In summary of: there is no steel which is best when you compare:
1) steel features, 2) heat transfer and 3) corrosion.

They all have their pro's & Con's.

Athough if you base the steel on best average - "Certified Boiler Plate" apears to be the best choice.

Keep in mind the majority of all commercial boilers are constructed out of certified boiler plate, and
that there are still locomotive steam engines around that were constructed out of certified boiler plate.

Ultimately if you are using the minimum recommended 30% propylene glycol, combined with corrosion
inhibitors as the heat transfer fluid in your boiler, combined with your due diligence of testing
and maintaining the heat transfer fluid (recommend semi-annual testing) you should be able to maintain
a balance of a) freeze protection, b) lubrication of pumps (as the majority of pumps are lubricated
by the fluid they pump) c) heat transfer (reduced by the glycol) and d) neutralizing any potential
corrosion in the heating system, including the boiler.