Hot water heaters and reverse return piping
I can always tell from the questions I get from our guys in the field when there is an issue that is poorly understood and improperly or insufficiently expressed or detailed on project construction documents. Those issues usually warrant discussion in an article due to their seemingly pervasive misunderstanding. The specific dilemma over the past week or so had to do with the piping of traditional water heaters and boilers and the concept of a reverse return piping arrangement.
For starters, water heaters are for the most part pretty unintelligent pieces of equipment. They contain a thermostat with set points. When the temperature in the heater reaches its low temperature set point it turns on. When the heater reaches its upper temperature set point it turns off. Pretty basic. There are larger, more sophisticated heaters with staged firing such that burners turn on in sequence as a function of demand, but even those staged boilers benefit from reverse return piping.
What is reverse return piping? To use a term popular in both manufacturing and asset-management, it would correlate to LIFO — last in, first out. With respect to water heaters or boilers, this term only pertains to systems with multiple heaters. For a single water heater or boiler it would not apply. When you have multiple heaters serving the same area or pressure zone of a building, the reverse return concept comes into play.
The opposite of LIFO is FIFO — first in, first out. This is the simpler, and seemingly more intuitive, way of piping water heaters and boilers but, in practice, it creates problems. Water, we all know, seeks its own level and follows the path of least resistance. Since water heaters piped in parallel generally do not have alternating controls the way most pump sets do, heaters should be piped such that the path of friction through each heater is equal and balanced. If the path through one heater has less resistance than the path through the others, then that heater will always act as the “lead” heater and will fire more frequently, leading to uneven wear of the heaters.
Figure No.1 shows what is referred to as a direct return piping arrangement (FIFO). As stated, at a glance it is seemingly more intuitive. But a second glance will obviate the fact that heater No.1 has less friction than heaters Nos. 2 and 3. Accordingly, during low and medium flow conditions, most of the water will flow through heater No.1, and it will carry the vast majority of the low-demand water heating requirements. Heater No. 2 will fire less often, and heater No. 3 will only fire during peak demand and as required to maintain standby losses.
Figure No. 2 shows a reverse return piping arrangement. It is slightly less intuitive by nature, but closer scrutiny reveals that the friction losses are balanced, or more nearly balanced, than the direct return system. Heater No. 1 will have the least amount of friction on the inlet but the most on the outlet. Heater No. 3 will have the most friction on the inlet but the least on the outlet. Heater No. 2 will have a comparable amount of friction balanced between the inlet and outlet. In this arrangement, the flow through all three heaters should be nearly the same regardless of demand, and each heater will fire for approximately the same number of hours.
It should be noted that some manufacturers will set the thermostats on their heaters to help compensate for the imbalance when their heaters are installed in a direct return configuration. If the heaters have precision digital controls, in the case of Figure No. 1, heater No. 1 might be set to fire at a temperature of 118 F, heater No. 2 might be set to 120 F, and heater No. 3 might be set to 122 F. As such, the heater that receives the least flow (No. 3) will also be the first to fire, based on the higher set temperature. By adjusting the thermostats in this manner, the individual heater controls will help compensate for flow imbalance associated with the direct return system.
Reverse return piping systems also have relevance in certain pumping and hydronic systems. For domestic water pumps, the benefits of reverse return are negligible, since flow rates are generally relatively low and pump pressures are usually fairly high. Since pumps alternate based on the pump controller, rather than based purely on hydraulics, reverse return can be ignored.
For condenser water or chilled water systems, flow rates are relatively high and pump pressures are relatively low. In this case, reverse return can have greater benefit such that each pump will run at the same operating point or on the same flow curve (for VFD systems). For high horsepower pumps with high flow and low head, this balanced operation can make a difference in long term overall system performance.
In hydronic systems where flow rates are relatively low and system balance is essential, minimizing hydraulic imbalance is important. Even though the hydraulic loops are balanced with control valves or small circ pumps, often of fractional horsepower, the hydraulic balance of reverse return helps the system stay in balance and demands less of the control valves or pumps. Secondly, after such a system shuts down and later starts up, the reverse return hydronic system will establish balanced operation in a shorter period of time.
The only downside to reverse return systems is additional piping. The added cost of this pipe length is usually negligible compared to the benefits. But in systems where achieving reverse return requires long additional pipe runs, or where the piping is so expensive that the cost of even small pipe runs is substantial, then the extent of the benefit has to be evaluated against the added cost.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at email@example.com.