Cleaver-Brooks - Complete Boiler Room Solutions

Hydronic System Design Basics

Tip Sheet: May 2018

Key Facts

  • Condensing boilers can be easily incorporated into a variety of system applications
  • Condensing boilers are designed to handle a range of operating temperatures
  • A variable-flow primary system offers the highest operational efficiency and lowest total cost of ownership

Condensing boiler applications are widespread because of their many benefits such as increased energy efficiency, and ease of incorporating them into a variety of system applications. Along with more traditional boiler systems, condensing boilers have the ability to operate in a variety of other applications since they are designed to handle a range of operating temperatures.

Below are formulas, conversions and basic piping schematics recommended for hydronic system design. 


1. Output btu/h per boiler:
   a. Input btu/h * operating efficiency = output btu/h
   b. With condensing boilers, operating efficiency depends on return water temperature. The
        lower the return water temperature, the more efficient the boiler.

2. Energy Equation:
   Q = 500 x GPM x ΔT
   Where: Q = Total heat load [BTU/H]
                GPM = Flow rate [GPM]
                ΔT = Supply and return piping temperature difference [°F]

3. Buffer tank calculation: formula


1. Btu/h to boiler horsepower:
    a. 1 BHP = 33,275 Btu/h output

2. psig to ft. hd.:
    a. 1 psig = 2.31 ft. hd.

Basic Piping Schematics

Variable-Flow Primary:
a. In this type of system, the main system pumps also pump through the boilers. This design requires high-mass boilers capable of handling varying flow rates. The minimum flow bypass, when using high-mass boilers, is often required to protect the system pumps when the load is satisfied and all of the zone valves are closed. This system design offers the highest operational efficiency and the lowest total cost of ownership – lower install and operating costs over the life of the system.

a. A primary-secondary system design has dedicated boiler pumps with separate system (often called secondary) pumps. The boiler pumps pump water through the boilers and primary loop, with a hydraulic decoupler between the primary and secondary loops. This design works with any boiler type, including low-mass boilers that require constant flow rates; however, it costs more to install and operate over the life of the equipment.

To learn more about hydronic system design, read a six-paper series authored by David Grassl, PE, principal of Dynamic Consulting Engineers, or contact your Cleaver-Brooks representative.

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