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High- vs. low-water volume design: which is right for you?

April 14, 2020

When assessing your boiler system design, it’s important to understand how much water is used relative to the amount of heat it’s putting out. Therefore, you must choose a boiler which either holds more or less water. A low-water volume boiler typically holds a small amount of water in their heat exchangers making them a good space-saving solution for more residential applications. High-water volume boilers can have twice the water capacity compared to low flow designs, making them significantly more efficient for many commercial applications.

What does this mean for a HVAC system designer? Here are a few advantages and disadvantages of high-water volume boilers:

Advantages of high-water volume designs 

Reduced system cycling

Reducing system cycling is important because boilers experience extensive wear and tear when they go through start and stop cycles. Therefore, lower cycling boilers typically means more longevity.

Heat exchanger longevity

Higher water volume boilers typically have larger tube diameters, which means you can reduce the speed at which the water flows through your system. This leads directly to the third advantage…


The heat exchangers are more reliable because high-water volume boilers have larger tube systems.

System protection

There is inherent system protection with higher water volume designs because you have as much as twice the amount of water in your system. This is a critical factor because if you experience an issue with your water source, you have more time to get it resolved.

Disadvantages of high-water volume designs 


There will always be a size trade off when it comes to design. However, many manufacturers can successfully address this issue, like Weil-McLain, by developing products with door fit in mind or products which feature specific design elements to better fit a boiler room application.


Because low-water volume boilers are typically smaller, there is an assumption that they are easier to install. However, like size, there are several approaches to boiler design which make the larger high-water volume solution more installation friendly. One example is integrated roller casters and leveling legs which make moving a larger unit easier.

Response times

Depending on the application—residential vs. commercial—response time is an important factor. If you’re relying on rapid response time to heat a large facility, then you most likely have a boiler design system flaw. On the contrary, for residential usage, response time is more important especially as you consider sharing load between all systems that can be using hot water.


There is an industry perception that higher volume designs are more expensive. However, that is on the manufacturer trying to contain cost structure and to account for all the above advantages of high-volume water boilers.

As with any boiler system, there are many disadvantages and advantages. To determine if high- or low-water volume design is right for you, it may come down to application, size, reliability, protection and system cycling.