Hot water heating renewal series: Domestic hot water
By Mike Zongor
Hot water is used for a number of different purposes such as showering or bathing, doing laundry, washing dishes, cleaning or sanitizing around the house, or even killing weeds. Unfortunately, hot water can present a significant health risk; exposure to 60°C (140°F) water can cause first degree burns in as little as 2 seconds and third degree burns in as little as 5 seconds.
By Mike Zongor
For this reason, the Ontario Building Code dictates maximum temperatures that can be supplied to plumbing fixtures. With some exceptions, in residential occupancies the hot water temperature should not exceed 49°C (120°F). This and other code requirements exist to ensure the safety of the public.
We are all probably familiar with the water tank heaters, either gas/oil-fired or electric that are commonly found in our homes. There are many different system types and configurations that you may be less familiar with that are more commonly found in commercial applications. Typical system types are:
1. Domestic boilers and storage tanks (conventional)
2. Heating boilers, heat exchangers and storage tanks (indirect)
3. Heating boilers and heat exchangers (semi-instantaneous, indirect)
4. Domestic boiler only (instantaneous, or “on-demand”)
5. Solar thermal
Designing domestic hot water systems is tricky because the design has to be able to handle the wide range of operating conditions; from almost no load overnight to very high loads in the morning when showers are taken. It is because of this load diversity that sizing a domestic hot water system should provide a balance between heating capacity and water storage. The storage is used to supplement the heating capacity during periods of high load and reduce the overall heating capacity that needs to be installed; saving on initial cost. The best domestic hot water system for each application is one that satisfies the peak load, provides high operating efficiencies during high and low use periods by adapting to the load diversity, suits the operating hours of the building, can handle the local water quality, is easily maintained, and achieves the owner’s goals in terms of installed cost.
Recall from the first article in the series on boiler efficiency that the type and design of a boiler has a lot to do with the system efficiency, but there are many other factors that can reduce the efficiency of a heating system. The same is true for domestic hot water systems where storage and insulation losses, as well as standby losses from boiler cycling, can have a drastic impact on the overall system efficiency. Instantaneous or semi-instantaneous systems may be suited to lower load or low diversity applications such as office buildings. For residential applications where large load diversity is experienced, conventional systems with storage capacity make more sense. Even though large storage volumes can contribute to system losses through the tank insulation, they can also provide a huge benefit during low load periods by extending boiler cycle times attributable to the added thermal mass.
Retrofit projects carry additional challenges. Many older systems have very large tanks, even over 1,000 US gallons, where the building was built around the tank. Many of these tanks are in poor condition due to their age and replacing them is difficult. Since they are too large to fit out the door, they will have to be cut up and removed in pieces while new storage is brought in and put in place. If mechanical room space is limited, this may not even be possible and temporary services must be arranged. Another option is to call in a specialty contractor to re-line your large tank using a concrete product. This lining will typically last for 15 years and is a very cost effective approach compared to replacing the storage volume directly.
As is illustrated there are many factors influencing the choices for domestic hot water system design and retrofit options. Building owners and managers should consult an experienced domestic hot water plant designer to help you determine what system type is best suited to your application and to optimize your objectives for code compliance, energy efficiency, and project cost.
Relevant Hot Water Heating Plant Codes
• National Building Code
• Ontario Building Code
• Ontario Fire Code
• National Fuel Gas Code
• Canadian Electrical Code
• Ontario Hydro Electrical Safety Code
• CSA B.149 Natural Gas and Propane Installation code
• Canada Plumbing Code
• CSA B51 –[M1991] Boiler, Pressure Vessel, and Pressure Piping Code
• CSA-B214 Installation Code for Hydronic Heating Systems
• SMACNA Guidelines
Mike Zongor, EIT, Senior Project Manager with Efficiency Engineering Inc. in Cambridge, specializes in energy efficiency and control of central plants. He can be reached at firstname.lastname@example.org.