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Make the Switch to Hot Water

Key Points
  • Converting a steam system to hot water heating can save up to 25 percent in energy costs.
  • Hot water systems provide comfortable heat and have efficiencies up to 98 percent.
  • Conversion requires a significant investment in system modification and new equipment.

boiler
Is your low-pressure steam system on its last legs? Is the boiler short cycling too much? Are fuel and maintenance costs increasing? If so, you may want to consider converting your steam boiler to a hot water boiler as long as it's used solely for space heating. Since hot water boilers operate at lower temperatures than steam boilers, they have a higher fuel conversion efficiency. Hot water boiler efficiencies range from 80 to 98 percent, while steam boilers top out at about 84 percent. 

A steam system also has higher energy losses as there are more places for heat to escape, including boiler surfaces, pipe systems, steam traps and other components. Hot water boilers also typically cycle less frequently, although they do require circulating pumps. Energy savings of up to 25 percent are possible after a conversion. Potential applications include greenhouses, apartment buildings, schools and on the larger scale, university campuses (district heating).

The following table compares the two types of heating systems:

Comparison of Heating Systems

Steam Hot water
Advantages

High heat transfer

Pressure depends on temperature

Self circulating

Less plumbing

Smaller pipe

Higher efficiencies

Comfortable heat

Low transmission loss

Good control of temperature

Lower operating costs

Safer for building occupants

Disadvantages

Moderate efficiency

Requires water treatment

Pump cavitation

Leaks from failing traps

Higher maintenance costs

More difficult to control flow

Produces high temperature condensate

Requires more pumps

Pump seals are a source of leaks

Larger heat transfer equipment (lower heat capacity)

More susceptible to thermal shock

Slower to respond

Large volumes of water required

 Source: Cleaver-Brooks

Switching to hot water heating usually requires a significant investment in planning, system modification and new equipment. For multifamily buildings, one study found conversions of two-pipe systems had higher average energy savings (27 percent) than single-pipe conversions (19 percent).

Other considerations include:

  • Steam boiler ratings are expressed in horsepower: 1 boiler HP is about 33,000 Btu
  • Pressure test pipes before conversion to locate any steam leaks
  • Two-pipe distribution systems are less expensive to convert than single pipe
  • To properly size a new boiler, heating load must be calculated
  • Use a circulating pump with an electrically commutated motor, which uses up to 90 percent less power
  • Add variable frequency drives to the pump's motors for additional savings
  • Use a modulating condensing boiler, which adjusts its firing rate to compensate for outdoor temperatures

Remember, if you already have a well-designed and maintained steam system, switching to hot water will improve energy efficiency by 5 percent at the most. Consider other upgrades, such as installing an economizer or insulating steam pipes, for saving energy.

Hot water conversion passes the test

At a school with a steam heating load of 5 million Btu and 8,000 annual operating hours, the steam system was costing about $334,000 per year for operations and maintenance. After switching to hot water heating, the school reduced heating energy costs by about 35 percent, or $118,000 a year. Overall system efficiency was improved from 75 percent to 95 percent.

 

Image source: iStock