A simple dynamic solution for balancing multi-loop domestic hot water systems
One of the major headaches for engineers and contractors in today’s complicated construction projects is properly balancing a multi-loop domestic hot water system (DHWS). Getting these systems properly balanced initially and having them stay balanced over the life of the building has, until now, been nearly an impossible job.
Furthermore, with today’s emphasis on more efficient building design and better operation and management, many practices of the past are no longer adequate or acceptable. No process or system can be ignored when looking for improvements. Even the seemingly simple task of supplying domestic hot water to all areas of a building requires another look to ensure that the most current and efficient methods are being employed.
In this article, we are going to take a look at DHWS, how balancing a system works, and what some of the newest trends and technologies have to offer.
DHWSs are designed to keep the hot water supply at the desired target temperature and to minimize wait times and the wasting of water at the fixtures waiting for the hot water to arrive.
Note: The correct hot water design temperatures is a separate subject as to scalding verses Legionella control and is not a part of this article.
In order to have hot water at all locations, the circulator pump must send water through the system fast enough to overcome any heat loss of the circulating piping system. To ensure this target is maintained, the engineer selects the pump and piping so that the flow rate through the system is high enough to maintain the necessary temperature, but not so high as to cause excessive noise and potential pipe erosion due to excessive velocity in the piping.
Furthermore, the engineer must remember that the temperature drop across the system is dependent on the quantity of water circulated. See the ASPE Domestic Water Heating Design Manual for further data.
Anyone who has experienced the arduous task of balancing a DHWS in a multi-branch building knows how challenging it is to do right the first time let alone maintaining balance under constantly changing conditions due to modifications in the building. It’s common to hear stories of many call-backs to a building to re-adjust balancing valves since one or more branches are still not getting hot water soon enough, or not at all.
Traditionally, the flow through each branch of a DHWS is controlled using manual balancing valves (See diagram). These valves are manually adjusted to set the flow in each branch to achieve the desired hot water temperature in that branch. Since the branches of a DHWS are interdependent this is a formidable and time consuming process. In some cases these balancing valves are only basic ball valves which lack the refinement to be accurately adjusted to the low flows required for this demanding task and to remain set at the proper setting over time.
In addition, many of these balancing devices are not pressure independent and, therefore, change flow with the constant changing pressure within the system. As many balancing technicians did not know the exact flow through the balancing valve, they sometimes set the valves at higher flow rates than required, resulting in larger pump circulation and higher fluid velocities. This compromises the efficiency of the system and can even lead to premature failures caused by velocity induced erosion in the piping.
Since the line pressure in various branches changes as hot water is used and manual balancing valves are not able to compensate for these changes, therefore, the “automatic” balancing valve was introduced. Automatic valves provide a fixed flow across a range of pressure drops using interchangeable cartridges that are installed into the valve body. While this is an improvement and allows the installer to set the branch flows closer to the flow specified by plumbing engineers, these are still fixed flow devices and are not able to respond to changing conditions. They require the plumbing designer to calculate necessary flows on a branch-by-branch basis. However, rather than performing all of the calculations, often a rule of thumb is used with a high safety factor, which can lead to excessive and constant overpumping. Since these valves have a small orifice they are also prone to clogging and scaling, as well.
The problems presented with these flow based solutions made it obvious that a better option was needed. A valve that responds to changing conditions to dynamically and reliably control the flow through each branch would improve both performance and efficiency of a DHWS. This need brought about development of thermostatic balancing valves, such as the ThermOmegaTech Circuit Solver Thermostatic Recirculation Valve. These valves use a paraffin wax actuator powered by changing temperatures in the water.
By responding to water temperature it is able to adapt to changing conditions in a building. This results in a system that will self-balance and completely removes the cumbersome balancing process that has always been required. Shifting the burden of balancing a system from the installer and engineer and places it on the valves themselves. The function of these valves is simple; when the water temperature in the branch is below the specified return temperature the valve opens allowing water to flow through the line.
As the temperature rises and approaches the required return water temperature the valves begin to close until the full return temperature is met. At this point the valve will be in its “closed” position. However, the “closed” position has a designed bypass opening to ensure that there is always sufficient flow through the return system so as to never starve the pump and always keep a minimum flow in that branch.
As each branch is satisfied more water is diverted to the rest of the branches until the system is fully balanced. Once the system is balanced the thermostatic valves maintain temperature by dynamically positioning themselves and only returning water to the pump that is below the desired temperature thus creating a highly efficient, demand based system. It is a temperature controlled device solving a temperature problem.
Furthermore, the thermostatic balancing valve is able to prevent hot water from “short circuiting” through close loops if someone inadvertently or improperly readjusts a Manual Balancing Valve or if the pump quantity of water is reduced. Thermostatic valves have been gaining popularity in recent years because of their versatility and convenience, qualities which really show on complicated retrofit projects.
The constantly decreasing costs of the technologies encompassing “smart” circulator pumps and control systems have made these items increasingly common. These systems have built-in sensors and controls that enable them to monitor temperature, pressure, usage, and/or flow rate and adjust the pump to better match the changing system demands. As these types of pumps come into wider usage the differences in water flow between the various balancing valves become even more critical. Since traditional balancing valves do not adapt to changing required DHWS return water requirements, the pump alone cannot determine the fairly uniform circulation rate. However, the thermostatic balancing valves, by constantly adjusting branch flow based on actual water temperatures, works together with the smart pump to provide a dynamically responsive recirculation system.
The growing use of building automation systems to further improve the efficiency of DHWS is another significant factor that can affect the choice in balancing valves. For example, in cases where the recirculation pump and hot water heater are turned down during periods of low or no occupancy, the DHWS must be able to respond to these new conditions. Experiences have shown that fixed flow balancing valves in such systems require significantly longer start up time to bring the DHWS back to full operational temperatures, which negates some of the potential benefits of these automation systems.
In other words, “when occupancy increases in the morning, a system using fixed flow balancing valves has to be restarted significantly earlier than a system using thermostatic balancing to come back into proper balance.” This is because the thermostatic balancing valve limits flow in a branch when that branch comes up to temperature and then forces the water to other branches.
As you can see, technological developments in areas as routine as DHWS balancing that normally does not get much of a second thought have come a long way over the years and can make a big difference in building efficiency and customer satisfaction. Choosing the right balancing valve and circulator pump for an application ensures a smooth installation and many years of efficient, trouble free operation. Also, always be sure to see the manufacturer’s installation instructions for required valving and strainer requirements.