Parts of the metal expand or contract faster than their neighbors, setting up stresses that crack cast iron sections, loosen rolled tubes and crack tube sheets and refractory. Typical causes are cold return water dumped into a hot cast iron boiler, firing a cold boiler straight to high fire, or feeding cold water into a boiler that ran low. Bypasses, mixing valves and slow warm-ups prevent it.
Big cold slug of return water every time the zone valve opens, that's thermal shock, it's why section four cracked.
Warm a cold boiler on low fire; the steel needs time, and refractory needs even more.
Also heard as
- temperature shock
Related terms
Cast iron sectional boiler
A boiler built from cast iron sections joined in a row, used widely for residential and commercial heating.
Low-fire start
The rule that a burner must light off at its minimum firing position, proven by a switch, before increasing rate.
Dry fire
To run the burner on a boiler that has lost its water, overheating and often destroying the heating surfaces.
Tube sheet
The thick flat plate at each end of a firetube boiler shell that the tube ends are rolled or welded into.