The list of items below are common things that I find during investigations of commercial, institutional and industrial buildings. These are related to design, installation and maintenance problems. A system can be designed perfectly, but if it is not installed or maintained properly, things can go wrong. On the other hand, a poorly designed system can make it difficult for installers and maintenance personnel to get a system to perform as intended. Here are a few plumbing issues to watch for: 

High velocity water air in the pump

I get many calls related to water flow velocity, and most of the time, the person calling does not know it is a velocity problem. The caller usually says there is a strange kind of corrosion taking place, after someone has taken water samples to determine if the water is acidic and causing this strange corrosion. I start by asking if the problem is in the hot or cold water system, or both. Most of the time it is only in the hot water system. Problems include leaks at elbows, after balancing valves and after circulating pumps.

If it is a leak that is near a change in direction or a control valve, then high velocity hot water could be the culprit. The Copper Development Association recommends a maximum flow velocity of eight feet per second for cold water and five feet per second for hot water. They also recommend a maximum velocity of two to three feet per second for hot water over 140 F. These recommendations are sufficiently vague enough to lead you in the right direction, however, I have come up with a more accurate table for pipe sizing and maximum flow velocities to maintain based on a range of system temperatures. This table has worked well for me, and keeping the hot water velocities in this range should provide a system that will work without velocity erosion issues. 

Domestic hot water above 180 F is not recommended because of the potential for scalding, and as temperatures get higher, the corrosion is accelerated. If domestic hot water temperatures can go higher than 180 F with heat exchangers or with heat recovery systems or other specialty systems, consider sizing the piping to keep velocities lower than two feet per second.  I often find that there is an oversized circulating pump in a domestic hot water return system causing velocities that are so high it is like having a high pressure power-washer inside of the pipe and eroding the walls away. This is a serious issue and will lead to leaks throughout the piping system, and eventually catastrophic failure. In improperly balanced or unbalanced systems,  high velocities can occur in branches close to the pump, while branches farther away receive inadequate flow and long wait times for hot water.

Air in the pump

I have seen domestic hot water circulator pump impellers disintegrate from cavitation. I received one peculiar case from a corrosion engineer, asking me to evaluate some pictures. The building manager had been continually replacing circulator pumps and changing impellers, and they seemed to corrode away within weeks. This corrosion was only occurring on the circulator pump impeller. I suspected cavitation, so I asked about the system layout. 

The building was about ten stories tall, with booster pumps in the basement and water heaters and circulator pumps in the mechanical penthouse. The hot water main fed down on one side of the building with branches crossing each floor to a hot water return riser on the other end of the building. The hot water system resembled a ladder. The hot water return riser went up to the mechanical penthouse and traveled to an elbow where it dropped down to the circulating pump. 

I was pretty sure air in the system caused the cavitation. When water is heated, microscopic bubbles of gas appear and travel along with the flow of hot water until they rise to the top of the pipe or combine into larger bubbles and rise to the top of the piping system. In a high rise building, risers tend to collect air or gasses at the top of the riser. These gases rise up to the horizontal hot water return pipe at the top of the hot water return riser and collect in the hot water return pipe, just before it turns down to the circulator pump.  

I suggested piping the top of the hot water return riser to a nearby plumbing fixture on tenth floor and routing the hot water return up to the penthouse from a side outlet tee from the riser below the branch. That way, the air and gas would continue moving up to the fixture that would eventually vent out the faucet on the top floor. The manager was happy that the suggestions had solved the corrosion mystery, and it was a relatively inexpensive fix with a few minor piping changes that solved the problem. It turned out to be cavitation.

Building water service pipes

I have seen many building service pipes with valves and fittings that are not approved for water service. I commonly see cast iron or unlined ductile iron valves and fittings rusting away in the building water service piping. Hospitals are required to have two separate water services, but in some cases the water services come from a loop, and they have isolation valves on each side of the tees in the loop to allow service from one direction or the other. However, the two water service lines run adjacent to and within a few feet of each other. This creates a condition where one water service line can potentially endanger the other if a water service line break occurs and the water main washes out the soil around and supporting the adjacent water main.   

Mechanical joint restraint fittings

I have also seen a significant increase in the number of water main failures associated with mechanical joint restraint fittings separating, failing or pulling out. Most of these cases occur after a shutdown when the contractor just opens up the pipe without slowly filling the system first. 

However, some cases have happened as a result of events in the piping system that should be anticipated or designed for. The manufacturers of mechanical joint restraint type fittings use set screws and, in some designs, wedges or teeth that dig into the pipe’s outer wall to hold a flange collar or mechanical joint in place. These mechanical joints should have friction clamps across the joints with tie-rods, and a good design will call for all of that and thrust restraints at changes in direction. I investigated a recent flood where one of these fittings failed in a utility tunnel.   

The engineer of record, the owner and the contractor were relying on the restraint joint with no thrust block or restraint type supports to hold the piping together in a tunnel. A ductile iron tee was added in a water main in a tunnel, and the restraint type joints were used. The joint failed a couple of years later, and the tunnel flooded and so did several mechanical rooms, which caused tens of millions of dollars in damage. 

During the investigation, I talked to the mechanical joint restraint fitting manufacturer, and he gave me the maximum pull-out force that each size of fitting will withstand. I calculated the thrust force for the design condition fire flow for the building and determined that the water hammer force would be one and one-half times the pull out force rating of the mechanical joints. 

As a former volunteer firefighter of 34 years, I can assure you there were many times during a fire event when someone came on the radio yelling at the pump operator to shut down the water flow due to a fire hose bursting or loosening. The pump operator typically goes to the butterfly valve on the discharge of the fire truck and slams it shut. This causes all the fire hoses to move violently; a massive water hammer pressure wave goes through the pump and fire hydrant, into the underground water main, and travels miles away from the fire source until the pressure wave dissipates. The water hammer spike pressure can be about one and a half times the pull-out resistance pressure rating for most mechanical thrust restraint type fittings. The moral of the story is: use concrete thrust blocks or engineered thrust restraints if you have not calculated the potential thrust forces for a given joint. 

Valve access

Another issue I have found when reviewing drawings for critical facilities like a hospital, research lab or other critical building is with the water utility service loops around the facility and isolation valves. There needs to be isolation valves and access to the isolation valves around a large university campus or hospital building. Hospitals require at least two sources of domestic water, so most have two water service entrances. Make sure the isolation valves are located properly and are accessible. If a water main or service line breaks, the building maintenance personnel should have quick access to the valves for isolation to minimize flood damage and restore water service to the building.  

Dead legs

Dead legs should be limited to five pipe diameters or less. Isolation valves should be within five pipe diameters of the water main. The five pipe diameter distance was determined by observing colored water flowing past a tee in a glass piping system. There are vortex and eddy currents that mix water treatment chemicals about five pipe diameters into branches. A branch to a seldom-used fixture, like a hose faucet, should be flushed in periodic intervals based on water testing that establishes when water treatment chemical residual levels fall below the levels that can control the bacterial hazards in the water system. Then, a schedule to flush the branches of stagnant water can be developed based on the dissipation rate of the water treatment chemical levels.

Bladder tanks

Many bladder tanks are dead-end style tanks by design. Dead-end bladder-type tanks, for both thermal expansion and hydro-pneumatic tanks, create large dead legs with ideal conditions for growing Legionella bacteria on the bladder surfaces. During pressure fluctuations, they inject the bacteria back into the water stream, which is a significant source of Legionella and other biological pathogens in the domestic water system. There are new designs of thermal expansion tanks and hydro-pneumatic tanks on pressure booster systems that have flow-thru designs. A solution to this is to install new flow-thru style expansion tanks. 

Pre-heaters, heat recovery devices, heat exchangers and temperature controls

I have investigated many new energy recovery systems in buildings that are utilizing some form of heat exchanger, to recover waste heat from a process or boiler flue to pre-heat domestic hot water. On some of these systems, I have found that, in the rush to install a heat exchanger to recover heat and get points for a building certification, the engineer overlooks the safety controls to protect the domestic hot water system from an over-temperature situation. I have observed the pre-heated domestic water being delivered to the water heaters, at temperatures well above the water heater thermostat setting, where there were no thermostatic temperature controls for the water delivered to the building’s domestic hot water distribution piping system.  

Many hospitals are installing heat exchangers energy recovery devices to recover waste heat from boiler stacks and other heat generating equipment. The United States Green Building Council, which developed the LEED, is giving points for these energy recovery systems without realizing they are creating Legionella incubation systems.  

I am seeing an increased number of heat exchangers being installed in facilities for energy conservation, where I have found the heat recovery heat exchangers, (u-tube or plate and frame design) were not made of materials suitable for potable water. 

In other cases, I have found energy recovery systems that are installed in seasonal systems, like boiler stacks for heat recovery units, where the heat exchanger only recovers heat in the winter months when the boiler is firing. If the designer does not use a closed loop heat transfer system with a separate heat transfer fluid and a double-wall heat exchanger, they end up running the domestic water through long pipes to get to remote heat exchangers that are often shut down during the summer months, creating a rather large dead leg that sits stagnant in the off-season. When the domestic water heat recovery system is turned back on, it doses the system with very high bacteria counts, far beyond the capability of any normal water treatment chemical to provide control of bacteria and pathogens. Many of these systems operate at temperatures in the ideal growth temperature range for Legionella bacteria. 

In many systems I have investigated, the hot water system relied solely on the water heater temperature controls to maintain the hot water system temperatures. It is dangerous when heat recovery systems are added later. In many of these, heat recovery devices were installed, and there were no additional temperature controls. 

In some cases, I found that the heat recovery pre-heater heated the cold water higher than the water heater set point, so the burner or the steam valve did not come on, but the system heated up anyway. 

In other cases, the heat recovery systems operate in the ideal Legionella temperature growth range. 

Plumbing codes and heat recovery/pre-heaters

The plumbing codes have not caught up with many of these heat recovery/pre-heater health and safety issues. It is important that the engineering community understands these issues and provide protection against pre-heaters that have pre-heated water that may be in seasonal loops with hot water systems that are in the ideal Legionella bacteria growth temperature range, or can exceed the water heater set point temperature, causing a scalding hazard. The code bodies have already let the water conservation proponents poison the cold water system with recirculated hot water with on-demand hot water circulators that circulate hot water back through the cold water pipes. This is a serious health and safety issue that got swept through the code process under the guise of saving water. I’m all for saving water, but I am not for contaminating it.  

The codes need to mandate temperature control valves, or an approved method, to prevent an over-temperature situation.

There are ways to design a closed loop with two heat exchangers and a heat transfer fluid to allow the system to sit idle without having dead legs that will promote bacteria growth. The heat transfer fluid should also flow through a double-wall heat exchanger with a vented interstitial space to protect the domestic water system. A temperature probe in the domestic hot water tank can turn on a circulator pump when the temperature in the tank falls below the set point (a few degrees above the water heater thermostat set point, to minimize burner operation). If there is a failure of either wall of the heat exchanger, it will show up as a leak on the floor rather than allowing heat transfer fluid to flow into the domestic water supply.  

Master mixing valves 

If the water heaters are supplied with water from pre-heater that is too hot, the water will pass through at the elevated temperature. This is why there needs to be a temperature actuated mixing valve downstream of any water heater with a pre-heater, unless the water heater has a built-in mixing valve to control the outlet temperature by mixing in cold water to maintain the desired temperature. There is a significant risk of scalding without a master mixing valve conforming to Standard # ASSE 1017 Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems. The master mixing valve should be located after the water heater to provide a safe and stable hot water distribution system temperature. 

I would personally recommend a digital mixing valve (DMV) to control the discharge hot water temperature to within about 2 F of the desired set point temperature. DMVs can mix circulated tempered water return temperatures that are very close to the tempered water supply temperatures and provide very accurate control of mixed or tempered water supply temperature. The hot water supply and hot water return temperatures get very close during peak usage periods when hot water demand pulls the hot water out to the end of the system. 

Some styles or designs of thermostatic mixing valves that rely on bi-metallic elements, or very small thermal elements, require a significant temperature differential in order to move the thermostatic element and the accuracy may not be there. It is for this reason that some manufacturers have come out with thermostatic balancing valves to shut down the flow when the temperature is satisfied. 

I always caution to use variable speed pumping or leave the balancing valve wide open on the farthest balancing valve in each wing to assure the constant speed circulating pumps are not dead-headed. The cold water supply to any mixing valve should be supplied with the same water source that is supplied to the water heater if the water heater requires softened or treated cold water or if the tempered water is recirculated. The tempered water return must be split after the circulating pump and routed to the cold water inlet or tempered water return connection, on the mixing valve and to the cold water inlet of the water heater. This is so the mixing valve can have flow from both inlets of the mixing valve when there is no hot water usage in the building and the circulating pump is trying to circulate water.  
Hard water can cause problems with scaling and mineral build-up on temperature sensing elements and on moving parts. DMVs have a bit of an advantage because they provide more accurate temperature control. Some manufacturers of DMVs have integral temperature sensors on the inlet ports and outlet port. Some DMVs can sense the temperatures, digitally record the temperatures for conformance with water management programs and be connected to the building management computer system. They can also send texts, emails and control water temperatures to within 2 F of the set point. There is a digital mixing valve controller that can connect to the building maintenance system to sound an alarm in an over-temperature condition and allow temperature data logging to have accurate temperature data for the entire domestic hot water system by the second, minute or hour for system monitoring, maintenance and water management program record keeping requirements.

One manufacturer has taken the DMV to the next level by including a software monitoring option that monitors and records the hot water system temperatures and notifies appropriate parties if there is an issue that needs attention.  

The plumbing codes have not caught up with many of these pre-heater health and safety issues. It is important that the engineering, contracting and inspection communities understand these issues and provide protection against pre-heaters that have pre-heated water that may be in seasonal loops with hot water systems that are in the ideal Legionella bacteria growth temperature range or pre-heaters that can exceed the water heater set point temperature causing a scalding hazard. The codes need to mandate temperature control valves when pre-heaters, heat exchangers, or any heat recovery device is adding heat the cold water feeds to a water heater to prevent an over temperature situation, and to know when the temperatures are stored and distributed in the ideal Legionella growth temperature range. 

Thermometers/temperature sensors

All mixing valves, water heaters, heat recovery devices or heat exchangers should have thermometers or a temperature sensing well to be able to monitor system temperatures.  

Sampling valves

Consider adding sampling valve stations at key monitoring and control point locations to assist with collecting water samples and getting water temperatures for building water management programs.