Easy steps to reduce cast iron boiler failures due to thermal shock

Thermal shock leads to many cast iron boiler failures, but boiler manufacturers set operational limits to avoid the cumulative conditions that lead to thermal shock. By following manufacturer’s specifications and scheduling consistent, coordinated servicing, cast iron boiler owners can reduce the risk of thermal shock.

The risk of significant downtime and expense

A recent study we conducted found that 90% of the cast iron boilers monitored were, at some point, operating outside of the manufacturers specifications – sometimes significantly.

Cast iron boiler systems typically require 1-4 weeks to repair, and if a boiler is damaged, there’s significant value associated with the downtime required to make necessary repairs.

There is considerable incentive for business owners to avoid the possibility of having to shut down their facility for an extended period of time, risk losing business, having freeze losses or the extra expenses associated with rental equipment should such a failure occur.

Manufacturers of cast iron boilers agree on these operating parameters:

  • The return water to the boiler should stay above 140° Fahrenheit and rapid drops in return water temperature should be avoided.
  • The difference in water temperature between the return water and the supply water should not be greater than 40° Fahrenheit.
  • The boiler should not short cycle (fire-up more than approximately 4 to 6 times per hour).

Even if a cast iron boiler has been serviced, it’s owner may have one or more of the conditions above and not be aware of it. For example, a boiler technician may tune up the boiler, then an Energy Management Systems technician may set up the system controls to optimize the performance and save money on fuel cost.

This often involves lowering the return water temperature. Sometimes the temperature – or other settings – is inadvertently set lower than the manufacturer specifies, which may lead to thermal shock.

What can an owner do to prevent this?

  • If the return water is less than 140°F, the internal sections should be examined for fireside corrosion.
  • If your system has monitoring equipment, review the data to assure that it is running as designed.
  • Make sure that all of technicians coordinate their actions to set up the equipment to the boiler manufacturer’s specification each time the equipment and controls are serviced – on an annual basis at the minimum.

This may result in adjustments needing to be made to one or more of the following:

  • Number of zones that are actuated at one time.
  • The sequence of zones that are actuated.
  • The temperature set points of the system.
  • The turn down rate of the boiler.
  • The use and maintenance of temperature mixing valves.

A well-tuned and controlled boiler and coordinated system is more energy efficient and will maximize the life of the equipment.

HSB customers can either call our Inspection Hot Line toll free, at 1-800-333-4677 from 8 a.m. to 8 p.m. EST, Monday through Friday, complete our online Inspection Request Form or email us at NSCInsp_Hotline@HSB.com. Learn more about our inspection services here.


© 2015 The Hartford Steam Boiler Inspection and Insurance Company. All rights reserved. This article is intended for information purposes only. All recommendations are general guidelines and are not intended to be exhaustive or complete, nor are they designed to replace information or instructions from the manufacturer of your equipment. Contact your equipment service representative or manufacturer with specific questions.

William P. Desrosiers

Director of Risk Management at HSB and President of RIMS, CT Valley Chapter, Bill has been living and breathing risk management for the past 35+ years. From insurance to facilities to firefighting, Mr. Desrosiers has seen risk from all angles, and has some of the keenest insight into risk mitigation you’ll find.


  • On a cast Iron boiler you stated that the inlet water can never get below 140 degrees? Is there a cold water start or run design that you might recommend for cast iron boilers?

    • Hi Joe! During start up, the boiler burner should have a low fire hold point. Once the boiler water temperature reaches the set point temperature (140 degF for example) the boiler is released for high firing. This prevents the temperature from rising quickly, which helps avoid thermal shock. As always, consult your operations manual for specifics of your boiler make and model.

  • Thanks for your steps to reduce thermal shock in cast iron boilers, William. You make a great point about how the return water should never go below 140*F. Cast iron is quite brittle. Repair on boiler units can lead to significant downtime, like you mentioned, but it’s best to avoid the problem in the first place if you can. Thanks for your informative post.

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