by Rob Spence (Mechanical Engineer) and Abigail Cantor (Chemical Engineer)

Introduction

This article is part of a series discussing the growth of microorganisms in plumbing systems.  As previously noted, microorganisms can grow and thrive in a plumbing system when water that enters a system is not used for a long time.   This article looks at sizing pipes and tanks in order to minimize the volume of water stored on-site and the residence time of water in the plumbing system.

Current plumbing and home designers are at odds with this desire to reduce system volume.  New homes and bathroom remodels are incorporating multiple head showers, body sprays, and large whirlpool tubs.  This drives up the plumbing system requirement with larger pipes, water heaters, water softeners, expansion tanks, and filtration units.  Many times the added capacity is only utilized a fraction of the time, which results in very long residence times.

Fixture Units

Residence time can be addressed first in design by considering “fixture units”.  The Plumbing Code assigns a fixture unit to each type of device in the plumbing system so that pipes can be properly sized with adequate flow and pressure to each fixture.  The plumbing designer must tally up all the fixture units serviced by each segment of the piping system (hot, cold, combined, branch, and mains) and size the pipes accordingly.  Therefore, as we add bathrooms, whirlpool tubs and extra shower heads, the pipe sizes must get larger.

Let’s contrast a typical home with 2 ½ baths utilizing single shower heads to a luxury home with 3 full baths and two ½ baths.  One of the luxury baths will have a rain head shower and a hand-held spray plus six body sprays, as well as a 75 gallon soaking tub.

In our example, the luxury house fixture count would require 1¼” supply line and 1” hot and cold mains with a 1” branch just to the master bath.  The shower alone could have a fixture count as high as 15.  In contrast, our standard home would only require a ¾” main.  Note that the 1¼” supply pipe in the luxury house has over six times the volume of water per linear foot than the standard ¾” supply pipe.  The Code doesn’t allow us to reduce the pipe size in these cases, but there are things we can do to reduce the load on the pipe so that the number of fixture units are lowered and the pipe sizes are calculated to be smaller.

Consider using diverting valves in shower systems rather than volume controls or separate valves on each device.  This lets the user choose only one of the multiple heads at once rather than the ability to have all functions at the same time.  Some of the pre-assembled shower towers utilize this functionality as well as low flow body sprays, so they only require ½” supply lines.  Contrast that with a luxury shower with individually mounted body sprays, which would require a 1” equalization loop going to all six locations. As you can see by utilizing a shower tower or diverting valve you only have to count one of the devices in the shower, which reduces the pipe sizes in the shower, branch circuit, and even the mains.  This choice reduces piping system volumes as well as labor for installation!

One other plumbing option to consider for reducing pipe volume is a “home run system”.  This is a design where manifold pipelines host connections to groups of fixtures instead of each fixture connection branching off from the main piping all through the building.  This type of system utilizes individual ½” or ¾” lines to each fixture or bath group.  The design will typically increase the overall linear feet of piping, but because it is smaller diameter pipe, the system volume is less.

Hot Water Tank Volume

As with considering fixture units, the Plumbing Designer must also adequately size a hot water heating system to handle all the potential flow. The Code states that the design “must provide adequate hot water for peak load”.  With traditional tank-type systems, our luxury home might need to be designed with several hundred gallons of storage.  We’ve all heard it:  “This is a high end customer and they don’t want to run out of hot water”.  So, the system is designed with multiple large tank-type heaters or a boiler system with several indirect tanks.  All of this hot water is sitting and waiting for one or two days per year when it is really required.  It makes sense to consider tankless water heaters that do not store any hot water.  It is true that in some cases, it will require two or three tankless water heaters.  But, the risk for damaging biofilm growth is greatly reduced.

Educate the home owner of the risks of over-sizing the domestic hot water package.  By utilizing the diverter valves in the shower system, we can get by in our luxury home with only two tankless water heaters.  In addition, the Plumbing Code now gives an alternate method for sizing tankless heaters.  It allows the designer to base the heater size on 65% of the peak fixture load.  Using this method, our typical home only requires one tankless heater.

One other method of reducing water heater tank volume is the use of an indirect water heater mated to a boiler system.  Typically, these systems have higher recovery rates (rates of heating and re-heating water), so many times a smaller hot water storage tank can be used than in direct heating.  For example, a standard 75 gallon gas water heater has a first hour delivery of 120 gallons per hour (GPH) compared to a 30 gallon indirect water heater that is rated at 183 GPH. In this comparison, there is an increased first hour delivery rate for indirect water heaters with a tank volume reduced by 45 gallons.

For water systems with private water sources, utilizing a constant pressure well pump will also decrease system volume.  A typical expansion tank for a standard well pump is 50 to 75 gallons.  However, that drops to 5 or 10 gallons on a constant pressure pump system.

We’ve been talking about decreasing system volume to reduce the risk biofilm growth, which is the primary goal of this article.  However, if it is determined that the system must have large hot water storage tanks, the design temperature of those tanks can be elevated to a much higher temperature (160 to 180 degrees F) to prevent biofilm growth.  This system must be fitted with an anti-scald valve to reduce the temperature to a safe level for the distribution to the house.  Indirect water heaters are a good choice for this type of system since they typically have a much lower energy loss as the water sits in the storage tank.

The Tub Dilemma

The required hot water tank volume also depends on the fill rate of bath tubs.  Large whirlpool tubs typically have tub fillers capable of 12 to15 gallons per minute (gpm).  Most designers would recommend an additional hot water storage tank at least as large as the tub volume, since no tank-type heater will keep up with 15 gpm fill rate.  Again, this requires large piping and major domestic hot water heating equipment that can lead to the growth of biofilms.

However, plumbing designers can educate homeowners about this dilemma.   If a homeowner can accept a slightly lower fill rate for the tub, one or two tankless water heaters can be used instead of a large tank-type heater with hot water storage.  In addition, the tankless heaters will maintain outlet temperature indefinitely so that there will be no drop in temperature as the tub slowly fills.  With a large tank-type water heater and additional large storage tank, a 75 gallon tub could potentially fill in 5 minutes.  With one tank-less water heater, the max fill time would be, at worst, 15 minutes.

There are a few issues that must be considered when applying a tank-less heater.  There is a phenomenon called a “Cold Water Sandwich.” That is, the first gallon or two of water is room temperature because it has been sitting in the room’s piping.  Then, the cold influent water comes in and has not had adequate time for heating, so there is about 3 to 5 seconds of cold water.  Then the properly heated water arrives.  A person can’t jump right into a shower or a bath tub and can’t turn the water on and off repeatedly.  But, a 2 to 5 gallon buffer tank can be used to solve the cold water sandwich problem.

Another issue is that there is higher pressure drop in tankless water heater units which must be considered in the plumbing design.

Water Treatment Equipment

The water softener and other water treatment equipment are also sites of large residence times.  As plumbing features of a luxury home are added, the peak flow rate increases.  This large flow rate alone given to a water treatment designer may result in very large water softening and treatment system.  However, the homeowner can describe the intent of how the luxury plumbing features will be used.  If the use of multiple luxury features is only going to occur for a short period, say 10 to 15 minutes every few weeks, consider installing a smaller softener and any other required water treatment equipment.  As long as the connections to the softener and other water treatment equipment are sized to handle that flow, the momentary surge will only result in partially softened/treated water going down stream for that short period.  Size the water treatment equipment for every day flow.

Hot Water Recirculation Lines

Hot water recirculation lines are needed in larger buildings so that hot water is immediately available throughout the building instead of having to travel from the hot water tank first.  However, hot water recirculation lines are notorious for biofilm growth because of added residence time of water and because the recirculation serves as a means to spread microbiological colonies to previously unaffected parts of the hot water system.

The “home run” plumbing system discussed in the section about “Fixture Units” can eliminate the need for a hot water recirculation line.

Summary

In summary, it is difficult to lower water residence time and water volumes in modern plumbing systems, but it can be done.

Using diverting valves, shower towers, or home run system design can decrease the number of fixture units in a plumbing design. This will result in smaller piping and decrease the size of heating and water treatment equipment.

Homeowners can be encouraged to choose tank-less water heaters and to accept lower tub fill rates to drastically reduce system volume.   Where tank-type water heaters are used, indirect heaters require less hot water storage than direct heaters.

Water softeners and other water treatment equipment can be sized for typical daily flow if peak flows are minimized by the homeowner.

Volume can be reduced further on systems with private water sources by using constant pressure well pumps.

We can’t expect plumbing designers and consumers to eliminate all the luxuries that we have grown accustom to.   But, as a plumbing community we need to be smart with our designs and educate our customers on the consequences of over-engineering.  With very small concessions, consumers can enjoy substantial energy savings as well as lower risks to health and to pipe integrity.

Responsibilities in preventing microbiological growth in plumbing systems
by Abigail Cantor P.E. (Chemical Engineer)

This article describes the responsibilities in preventing microbiological growth from occurring in plumbing systems.  Those responsibilities are shared by the Plumbing Designer, the Building Contractor, the Municipal Water Utility Manager, and the Property Owner.

The Plumbing Designer

The prevention of microbiological growth in plumbing systems starts with the plumbing designer.  Two new design requirements must be met in order to prevent microbiological growth – the volume of water residing on-site must be minimized and the surface area that the water contacts must be minimized.  These design requirements must be balanced against the property owner’s list of desired plumbing fixtures and the plumbing code’s design criteria.

Considering the Number of Fixtures and Pipe Sizes

The property owner may have a long list of desired plumbing fixtures and showers with multiple sprays.  The plumbing code will base the size of the piping on the number of fixtures that can possibly operate at the same time.  The property owner needs to understand that there is a trade-off between having extra plumbing fixtures versus minimizing the volume of water stored in the piping.  The use of diverter valves as well as the property owner’s acceptance of fewer fixtures may assist in keeping pipe sizes smaller.

Considering Water Conservation Fixtures

Water conservation fixtures are becoming a necessity in some areas where drinking water resources are scarce.  With both new plumbing systems and the modification of existing plumbing systems, the lower water usage must be considered and the on-site storage of water be reduced accordingly.

Considering Tubs

The property owner’s selection of bath tubs and hot tubs will greatly influence the quantity of hot water that needs to be prepared and stored on-site.  The property owner needs to understand the trade-off between the number of tubs and the filling rate of each tub versus minimizing the volume of water stored on-site.

Tankless water heaters can be used to assist in lowering the volume of hot water that needs to be stored.  Acceptance of lower filling rates by the property owner will also help to lower the volume of hot water storage.

Considering Water Softening

With hard water, water softening is required before sending water into a hot water system.  Calcium carbonate from hard water will scale up the heating surfaces in the hot water system which will, in turn, increase the quantity of energy and the cost to heat the water.  The life of the hot water heating equipment will also be reduced with the build-up of scale.

However, water softeners increase both the volume of water stored and the surface area that the water is exposed to on-site.  They can become incubators for microorganisms.  If less hot water is required, then less water needs to be softened and the smaller the softener tank.  Again, the property owner’s acceptance of lower filling rates for tubs will lower the hot water volume requirement.

In addition, cold water does not need to be softened except in a few specific cases.  Water softeners can be smaller when only the water for the hot water system is softened.

Water softeners can also be outfitted with the dosing of chlorine and other biofilm-fighting chemicals to the brine tank so that disinfection of the media inside the softener tank can occur routinely and prevent the development of biofilms inside the tank.

Considering Other Water Treatment

A previous article pointed out that water should not be treated on-site unless absolutely necessary.  The article listed possible contaminants in water and steps to take in deciding whether or not removal of certain contaminants is necessary.

For every treatment device, water volume and surface area is increased in the plumbing system.  Each device must have a means of routine cleaning and disinfection, as was discussed with water softeners, in order to prevent them from becoming incubators for microorganisms and inoculating the downstream piping.

 

Considering Chemical Dosing into Piping

Water softening and other on-site water treatment devices can remove existing disinfection from the water.  Long residence times for water such as in hot water storage tanks also deplete the disinfection in the water.  Designers of commercial buildings where large volumes of water are required need to consider re-injecting chlorine or other biofilm-fighting chemicals approved for potable water after water treatment devices and before storage tanks.  A previous article described chemical dosing systems.

Some property owners, such as those with private wells, do not want to introduce disinfection because of its taste and smell in the water.  Disinfection chemicals can also form harmful by-products if dosed incorrectly.  But chemical disinfection, especially chlorine, is the most significant protection against the growth of microorganisms in modern water systems because of its disinfecting power at a reasonable cost and its ability to remain in the water to the far reaches of the plumbing system.

Considering Pipe Flushing Features

If high-volume, complicated plumbing systems are inevitable in a building, automatic pipe flushing should be considered.  Automatic valves can be placed at the ends of long pipe runs to move fresh water through the water line.  Such a valve needs to be slow-opening and closing so that water hammer does not occur as it does with a solenoid valve.  Also, this strategy wastes water which cannot be tolerated in areas where water resources are scarce.  Nevertheless, in some cases, automatic pipe flushing may be the only answer to keeping water fresh on-site.

Considering Sampling Taps

Small valves and faucets are inexpensive to install in pipelines.  Consider installing such taps before and after any water treatment or storage tank in the plumbing system so that water samples can be taken and the quality of water, as it changes throughout the plumbing system, can be monitored.  A previous article described critical locations in plumbing systems where microorganisms can grow.  This can be a guide for locating sampling taps.

The Building Contractor

 

The growth of microorganisms in plumbing systems begins during the building’s construction period when water pipes are filled with water.  The water usage, at that time, is not what the plumbing system was designed for and the water sits stagnant in the pipes for a long period of time.  Any disinfection in the water is quickly depleted.  Microorganisms can now thrive.  Building contractors should ensure and document that this does not occur on their watch.

Before a project begins, review the plumbing plans for the design features that have been discussed in these articles.  Every pipe run, water treatment device, and water storage tank has the potential for microbiological growth during the construction period.  Is there a way to monitor, flush, and disinfect each plumbing system feature during the construction period?  If not, work with the plumbing designer to provide the means to do so.

Before filling new pipes with water, make sure that they have been cleaned of debris and disinfected.

After filling new pipes with water, begin monitoring for and documenting chlorine concentration routinely and frequently.  A previous article describes how to do this with a relatively inexpensive field test kit.

Less frequently, monitor for microbiological activity.  A previous article described how to do this with water samples for Heterotrophic Plate Count (HPC) analysis.  Since that article was written, a better test has become commercially available.  It is a test for ATP and is described in a sidebar to this article.

Run treatment devices through their cleaning cycles on a routine basis during construction and refresh the water in piping and tanks.

The amount of plumbing system flushing can be reduced by dosing the water system with chlorine and biofilm-fighting chemicals used in potable water systems.  (See a second sidebar for a description of chemicals that can be used.)

Strategies to carry out these tasks in an economical fashion during construction need to be developed.  This is a new concept for the building industry, but it can and must be done based on the current knowledge about microbiological growth in water systems.

The Municipal Water Utility Manager

 

The role that microorganisms play in water systems is a new topic in the drinking water industry as it is in the plumbing industry.  Municipal water utility managers are just becoming aware that microorganisms and their biofilms can help to transfer piping metals like lead, copper, and iron to water.

Municipal water utility managers need to ensure and document that the water throughout the water distribution system stays properly disinfected and that microbiological activity is low.   Throughout the distribution system, managers need to:

• Monitoring for disinfection and microbiological activity
• Minimize water residence time (water age)
• Ensure a proper disinfection residual
• Perform uni-directional flushing on water mains to properly clean piping

Managers might want to refer to a Water Research Foundation report that demonstrates techniques to control water quality at the consumers’ taps (Project 4286: Distribution System Water Quality Control Demonstration).

Ironically, because of modern plumbing design practices, the growth of microorganisms can occur whether or not there is a high microbiological activity in the water entering a building.  However, water utility personnel should document that the municipal system did not contribute significantly to any building’s plumbing problem.

The Property Owner

 

Property owners need to understand the dilemmas in designing and installing modern plumbing systems as have been discussed in these articles.  That way, they can make better decisions in planning their buildings’ plumbing systems.

If a property owner uses a private well instead of municipal system water, that person must take on the responsibilities of a water utility manager.  They need to know what contaminants should be removed from the water and the best method of removal.  They also need to consider how to keep the water and the system disinfected.  They must monitor for disinfection and microbiological activity and ensure that their water system is operating safely.

Conclusions

 

Preventing the growth of microorganisms in plumbing systems is a new reality for modern plumbing systems.  There has not been a lot of experience in designing systems with minimal volume and surface area while still utilizing the new plumbing fixtures that are available today.  But, the plumbing industry can start by being aware of the microbiological growth issues and design dilemmas.  Eventually, people in the plumbing and construction industry will figure out how to accomplish this task economically and efficiently.

This pushes people away from techniques that they have always used and what they have always known to be true.  But, the industry must move in this direction or else the future will be filled with ruined plumbing systems in new buildings, angry property owners, and liability lawsuits for plumbing and building contractors.

This series of articles does not provide all the answers needed to tackle this problem; they were written to raise awareness of the problem and to encourage the plumbing profession to consider the repercussions of the volume of water stored on-site and the surface area that the water is exposed to.  The growth of microorganisms in plumbing systems can be prevented with everyone’s participation.

 

A New Method of Testing for Microbiological Activity

 

In a previous article, a method of measuring microbiological activity in water was presented.  It was a method to measure the number of heterotrophic bacteria colonies that can grow from a water sample (HPC analysis).  But, there are other types of bacteria in water besides heterotrophs.  Nevertheless, the HPC analysis was used because it was the only economical and commercially available test.

A new type of test now meets the economics of the HPC analysis.  It is a test for adenosine triphosphate (ATP), the energy molecule found in living organisms.  This test will measure the presence of any type of living organism in the water sample, not just heterotrophic bacteria.

Unfortunately, there are only a few laboratories and professors that have the equipment to run this test.  For example, only one small laboratory in Wisconsin can run the ATP test.  Nevertheless, it is worthwhile to pursue the use of this test by inquiring at commercial laboratories and by contacting major manufacturers of the equipment (http://www.luminultra.com/Industry-Applications/water-treatment.html).

New Chemicals to Clean Out Biofilms

Chlorine is the most common chemical used for disinfection of drinking water.  The chemical is effective in preventing biofilms from forming.  However, once the biofilms have formed, it is difficult for the chlorine to penetrate the biofilm and remove it from surfaces.  In addition, many modern plumbing materials cannot be exposed to high concentrations of chlorine, so dosages that can be used are limited.

There are new chemicals on the market that can penetrate and destroy biofilms.  For use in drinking water, the chemical must be approved by NSF International (http://nsf.org) as an acceptable additive.  When determining new chemicals to use for biofilm removal, look for the NSF approval and follow-up on references of organizations that have used the product.