Through my position as a plumbing code columnist, I have been asked to serve as an expert witness in legal cases concerning plumbing. This has opened a new perspective for me on how the written codes and standards — and the design, installation and maintenance documents — are translated and applied in the real world.

However, based on my education and experienced opinion, there is ignorance of or disregard for plumbing safety. This would require some understanding of how plumbing systems function to, for example, deliver domestic hot water through the system to appliances, valves, piping and faucets, as well as a mindset of profit-saving and cost-cutting. An “always-present” danger or foreseeable risk of injury doesn’t mean an injury will always occur; however, when these injuries do occur, they can be serious, devastating and deadly.

For example, although our technology and written codes, if followed, should practically eliminate all tap water scalding incidents in the building, I find that serious scalding incidents in bathtubs, showers, sinks, lavatories, shampoo bowls, pedicure sinks and bidets still occur — in old and new construction, in big and small buildings, and for the rich and poor. 

These scalding incidents were always preventable had safe and knowledgeable plumbing design, installation and maintenance been implemented. Litigation does not legally require safe installation and knowledgeable maintenance going forward.

One type of installation I have encountered far too many times, and which is the focus of this column, is when the domestic hot water (DHW) system in a large institutional and multifamily building consists of a heat exchanger with no storage tank or code-required and approved temperature controls anywhere in the system — from the hot water source all the way to the hot water delivery at the point of use. 

Yet, when asked why the systems were installed this way, the answers provided by owners, managers, and maintenance personnel are typically deflective of any responsibility: 

“I thought it was code-compliant.”

“The inspector didn’t say anything.”

“I don’t know, I just work here.”

“This is how they do it around here.”

“We have been doing it this way for decades.”

“No one has ever been scalded/injured before.”

“It’s the original installation [so, it’s grandfathered].”

Site-Built Heat Exchanger Water Heaters

I have mostly seen this type of DHW installation in the northeastern United States, and in a few cities scattered around the country where HVAC engineers design the plumbing. Site-built heat exchangers generally rely on controls not intended for the application. Heat exchangers are assembled on-site with HVAC or process control valves on the heating hot water or steam system piping. No storage tanks or temperature-actuated mixing valves exist in the DHW distribution pipe downstream of the heat exchanger. 

These systems rely on an assembly of components, relief valves and aquastats that are not manufactured, assembled and tested by one manufacturer. The systems that I inspect are often missing components and safety controls that must be set or adjusted by the installing contractor and maintained thereafter. 

The use of HVAC or process control valves is an immediate and obvious cause for concern because of construction materials and temperature ranges. It also can be a code violation, as the code requires equipment to be listed and tested for its intended purpose. 

When a heat exchanger is installed as a site-built “water heater,” missteps can arise because the manufacturer’s installation instructions will only address the heat exchanger. 

The instructions do not address the other system components and operating requirements such as temperatures and capacities for a DHW system. The heat exchanger installation instructions also don’t mention things such as sensors, control limits for stable hot water distribution temperatures, high-temperature limit or energy cutoff to prevent a steam explosion upon a failure, storage capacity to meet peak demand and provide temperature buffering, piping between the heat exchanger and other components, circulation pumps, control monitoring locations and wiring, control types, system set temperatures, temperature gauge ports, anode rods for corrosion control if necessary, vessel drain and piping, relief valve ports, and a temperature and pressure relief device to prevent a steam explosion. 

These other system components and operating requirements are necessary to ensure that the DHW system is safe with respect to temperature control, water heater construction, control of Legionella bacteria growth, scald risk control and explosion prevention. 

Most states require an engineer’s seal on the drawings and plans of all commercial, institutional and industrial buildings — and residential buildings over a certain size. This means that an engineer must review and sign off on the design of the mechanical systems, including the DHW system. 

This would include verifying system capacity, tank sizing, safety controls, temperature-limiting controls, pressure relief, high-temperature limit controls and other requirements listed in Chapter 5 of the model plumbing codes, which addresses requirements for water heaters. 

Therefore, when an unlisted site-built water heater is installed, there should be engineering plans and drawings showing the DHW system design and specifications and performance requirements typically in a schedule.

Additionally, for an unlisted site-built system, the owner must have the engineer of record review, inspect and commission the system after installation and provide a protocol for testing in compliance with the industry standards listed in the codes. A certification test should be performed after installation to verify temperature control in conformance with the industry standards. It is difficult and costly, but not impossible, to test a site-built system. 

Heat exchangers are instantaneous water heaters, which generally cannot accurately control the DHW system temperatures without additional controls as found in many packaged systems. An understanding of how a heat exchanger operates to heat water reveals the pitfalls and dangers of using a site-built heat exchanger as a water heater without a storage tank or temperature-actuated mixing valve in the DHW distribution pipe downstream of the heat exchanger. 

An understanding of the heat source flow (steam or heating hot water) through the heat exchanger and the domestic water flow (water to be heated) through the heat exchanger is necessary to design, install and maintain a safe system.

Domestic Water Flow Through Heat Exchanger

The flow rate in a DHW system fluctuates between less than 1 gallon/minute of the circulating pump flow rate during off-peak hours and the peak flow rate of all hot water fixtures (showers, laundry, dishwashers, sinks and lavatories) flowing during the peak morning period using the fixture unit method. 

This wide disparity of flow rates creates problems for most instantaneous heat exchangers because as the flow increases, the ability to transfer heat in the heat exchanger decreases. Therefore, higher flows will cause the outlet temperature to drop. When flows slow down, the outlet temperature rises. The outlet temperature of an instantaneous heat exchanger is directly proportional to the flow rate of domestic water through it. 

Because the DHW temperature at the outlet of a heat exchanger will fluctuate wildly with variations in domestic hot water system flow, a heat exchanger must be paired with a storage or buffering tank and a downstream temperature-actuated mixing valve with circulation when required and downstream point-of-use domestic hot water system temperature controls. 

Steam or Heating Hot Water Flow Through Heat Exchanger

The domestic water flow rate through an instantaneous heat exchanger is independent of the steam or heating hot water flow rate through it. Unlike DHW flow rates, the steam or heating hot water that flows through a heat exchanger is at a generally fixed flow, controlled by an open or closed throttling a modulating control valve or the use of a three-way valve not listed to ASSE 1017.

I have seen the use of three-way control valves in circulated heating hot water systems and open/closed control valves in steam systems that sometimes modulate to try to control the heat source delivered to a heat exchanger serving as a domestic water heater. 

The location and distance of the temperature sensor or aquastat in the piping system relative to the heat exchanger will affect the reaction time of these controls and, consequently, the system’s ability to properly control the DHW system delivery temperatures. This leads to the potential for overheating and scalding, especially when the control valve leaks or sticks open and there is no flow in the DHW system. 

Steam or heating hot water control valves can and do get stuck, and when the valve leaks or gets stuck open, it allows heating fluid to continue flowing through the heat exchanger. This causes the DHW system temperature to reach the full temperature of the heating hot water or steam system. In steam systems, this means that the DHW temperatures can reach more than 210 F or flash into steam. 

Any time you use steam or heating hot water, both at a very high temperature, the control valve can stick open and cause a runaway temperature situation. To guard against this, use a high-temperature alarm in the distribution piping that will shut off the steam or heating hot water with a separate shut-off valve ahead of the control valve and sound a high-temperature alarm. 

Further, these control valves are sometimes the same that would be used in a heating hot water or steam system for building heating coils if the engineer or installer is unaware of the code requirements for water heaters in Chapter 5 of the model plumbing codes. That is, the components used to make up the DHW system are often a copy of the building heating hot water temperature controls. 

However, DHW and building heating systems differ greatly in temperature, flow and risk of scalding. Whereas flow is relatively constant in a heating system coil because of airflow or heating water flow, flows vary in a DHW, and there can be low or no flow for very long periods in DHW systems. 

Using HVAC or process controls is an unsafe practice. They are often not made of the proper materials for a potable water system and are often individual components not part of a packaged water heater designed and tested for accurate temperature control for the application.

Next month, I’ll discuss DHW temperature regulation and how it was determined that 120 F was the sweet spot for bathing and washing fixtures.