"Boiler Thermal Shock" can be loosely defined as sudden thermal changes that occur within the boiler causing rapid and uneven contractions of the boiler's cast iron or steel material. An example is placing a cold glass under hot water -- the glass cracks because of the extreme temperature change. In thermally shocked boilers, the fractures or cracks occur where the temperature difference is greatest -- usually in the back of the boiler near the nipple joints or the furnace area where the cold water enters. Surfaces exposed to cold water are contracting while surfaces exposed to fire are expanding.

Causes of Boiler Thermal Shock

Several conditions can contribute to boiler stressing and eventual boiler shock. All involve introducing excessively low temperature water, or cool temperature water at high flow rates, into the hot boiler:

Returning water at too low a temperature

Cool return water at too great a flow

Firing the boiler and heating up water before system circulator is turned on

Moving the burner into high fire with boiler water at too low a temperature

Influence of System Designs

Systems incorporating night setback and/or weekend shutdown are designed to save energy, but turning down or shutting off the building's temperature causes problems when all the zone valves and pumps come back on delivering room temperature water to a hot boiler.

Dual temperature changeover systems can experience boiler problems when the system tries to change over from a cooling demand to heating. The piping system and terminal units are filled with 50 - 60°F water and the boiler may contain 180°F water.

Heat pump loop systems typically require some form of supplementary heat to maintain supply water loop temperatures when the outdoor temperature approaches design conditions. Boilers are the common source for this additional heat, but design loop temperatures are as low as 70 - 85°F, while most commercial cast iron boilers don’t operate below 140 - 150°F.

Heating systems that have boilers maintaining temperature without flow are susceptible to thermal shock by sudden changes in flow due to pump operation.

The most common cause of thermal shock is a system that incorporates outdoor reset with 3way valves while the boiler maintains temperature. The boiler is at 180°F, but based on outdoor temperature, the system may require only 100°F. The return temperature can be as low as 90°F, which can cause a 90°F differential across the boiler. (Remember the cold glass and hot water!) Most cast iron boiler manufacturers would like to see no more than a 40 - 50°F temperature difference between the boiler's return temperature and leaving temperature.

Preventing Thermal Shock

Waterside thermal shock can be prevented by controlling the load imposed on the boiler. Boiler load is a function of flow rate and temperature difference, and one of the most effective methods is to create a boiler loop separate from the system and pump it with its own circulator. Since the flow rate is constant, the temperature difference across the boiler becomes the measurement of the boiler's load, and if the boiler is maintaining temperature, the return water's temperature will determine the boiler load. Control against "boiler shock" involves control of the incoming cold water flow rate so that the boiler's temperature is changed slowly. By installing the 3-way valve in the boiler loop, the outdoor reset can control the amount of hot water that is introduced into the system based upon a reset schedule. More importantly, the reset controller can measure the return temperature entering the boiler. If water temperature becomes too low for the boiler manufacturer's recommendations, the 3-way valve will close off the system loop. Hot water from the boiler will then be pumped right back into the return, raising the water temperature entering the boiler. The 3-way valve and controller will float back and forth, resetting the supply water to the system while protecting the boiler from cold water.

The best method for interconnecting this boiler loop with the system loop is through primary/secondary pumping techniques. By keeping the supply and return tees close together, the pressure drop in the common piping is kept to a minimum. This allows different size pumps to coexist in the system without affecting each other as well as preventing ghost flows from occurring from one loop into the other.