Efficient hot water systems

Engineers should work to design the most efficient water heating system or boiler possible. Understanding the codes/standards is paramount, followed by knowledge of the key equipment functions of boilers and similar systems.

05/20/2015


This article has been peer-reviewed.Learning objectives

  • Learn about the components that make up hot water systems: boilers, pumps, and coils.
  • Understand system control strategies for efficient system operation.
  • Learn to maximize the efficiencies of these components.

Designing efficient systems of any kind requires consideration of the individual system component efficiencies as well as the overall system operation. In the case of hot water systems, the individual components are the boilers, pumps, and coils.

Boilers

The boiler is the primary energy-consuming piece of equipment in a hot water system. The costs associated with its energy use over the lifetime of the equipment will far outweigh the initial equipment cost. A careful comparison of the fuel-to-water efficiencies should be made as part of the equipment selection process and economic analysis. The fuel-to-water efficiency is a measure of the overall boiler efficiency that includes burner and heat exchanger performance as well as the thermal losses through the boiler casing. Minimum boiler efficiencies are required by the Dept. of Energy, energy standards such as ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings, and state energy codes.

Boiler efficiencies vary with the type of boiler and operating parameters. Traditional noncondensing boilers have efficiencies in the 80% to 85% range. These boilers are designed to operate at vent temperatures above 127 F, which is the dewpoint of the combustion gases. This results in hot water system operating temperatures of 160 to 180 F supply and 135 to 140 F return to ensure that the return water temperature remains well above the combustion gas dewpoint. Operating this type of boiler below the combustion gas dewpoint will significantly shorten the life of the boiler. Noncondensing boilers are most efficient when they are firing in the 60% to 80% range. This makes them good candidates for base load or peak load operation.

Figure 1: This figure illustrates the effect of return water temperature on the efficiency of a condensing boiler at different firing rates. Lower return temperatures result in higher efficiencies for all firing rates. The highest efficiencies occur withCondensing boilers have efficiencies in the low- to mid-90s. The efficiency increase is primarily due to latent heat recovery from the combustion gases. This results in the boiler operating below the combustion gas dewpoint in the condensing range. As a result, hot water systems using condensing boilers have lower operating temperatures, 140 to 160 F supply, and 100 to 120 F return. Lower return water temperatures result in more latent heat recovery from the combustion gases and increased boiler efficiency. Condensing boilers are most efficient at lower firing ranges of 20% to 50% (see Figure 1). This makes them ideal for part-load conditions.

Due to the potential higher efficiencies, condensing boilers are seeing increased use. However, the traditional considerations employed in noncondensing systems must be revisited when using condensing technology. The first of these is turndown. 

Figure 3: Condensing boilers serve two athletic buildings on a university campus. This project centralized the heating plant in one of the existing boiler rooms. There are two 3,000 MBH input boilers and one 1,500 MBH input boiler in parallel. The plant c

It is a widely held belief that the higher the turndown, the better. Let us compare a constant flow 5:1 turndown system to a 20:1 turndown system. In this example the system has a 1,800 MBH output boiler, a 90 gpm constant speed pump, and a 130 F hot water supply temperature. Ignoring losses, the boiler input must be equal to the output to the hot water system:

Q = mCpdT

Where Q is the boiler net output

m is the mass flow rate

Cp is the specific heat capacity of the fluid

dT is the temperature difference of the fluid (Ts-TR)

For water the equation simplifies to Q = (GPM)*500*dT

Table 1: This summarizes the calculations described above for full-fire, low-fire for 5:1 turndown, and low-fire for 20:1 turndown, and the resulting efficiency when using a constant-speed pump. Courtesy: P2S EngineeringAt full fire the temperature difference is 40 F, resulting in a return water temperature of 90 F (see Table 1) and a boiler efficiency of 94% (see Figure 1).

Using a 5:1 turndown, the system will operate at 20% of capacity with a temperature difference of 8 F and a resulting return water temperature of 122 F (see Table 1). Even though the temperature difference has decreased significantly, the boiler will continue to operate in the condensing range and can maintain a high efficiency at low fire (see Figure 1).


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