Mark Robison, P.E.

Summary: This article is about trouble shooting hot water recirc systems for hotels.  For many different reasons,  hotel hot water recirc systems can be a major challenge to commission at the opening of a hotel.  My goal here is catalog all the problems and associated solutions to hot water recirc systems that I or anyone else knows.  I encourage readers to share their examples.   This will be an article that continues to grow over time.

Related Article: Hotel Hot Water Recirculation Systems

Problem:  Cold water entering the hot water system:  If you are observing cold water entering the hot water system, it is likely that cold water pressure is higher than the hot water pressure.   When this pressure difference exists,  there will be a natural tendency for cold water to migrate through mixing valves into the hot water system.  This problem often occurs when a water softener is installed serving only the hot water.  This problem is easily fixed with a pressure regulating valve for each of the hot and cold water systems.  Locate the hot water pressure regulator on the outlet side of the water softener.  Then set the pressure of the hot water regulator a few pounds higher than the cold water.  This will assure any migration of water through the mixing valves will be toward the cold water, which is a forgivable situation in most cases.

Problem:  Plumber installs more risers than originally designed.  Sometimes a plumber will decide that it is easier to install a separate riser for each stack of fixtures instead of combining risers to serve several fixtures via horizontal branch connections.  This change is OK, but it creates more risers than included in the original design.  With more risers to serve, the recirc system must be upsized to maintain the original design flow in each riser.  Generally this will involve upsizing the recirc pump and the return piping. 

Problem: Recirc lines leaking after several years of operation:  Recirc lines need to be sized for continuous flow or they will erode in a few years and develop pin-hole leaks.   The sizing criteria for recirc lines should be a maximum velocity of about 3 feet per second.  This is much slower than the criteria used for domestic water lines, because water lines operate very intermittently. 

Problem: Oversized Recirc Pump:  We all know it is better to oversize a recirc pump than undersize it.  But this is only true if balancing valves are installed.  If the oversized pump is allowed to operate without balancing, the circ lines will erode from excessive water flow velocity.  I recommend a conservatively sized recirc pump (i.e. oversized), and the installation of a balancing valve at the outlet side of the pump.  Of course, someone must actually perform the balancing, which often is the greatest challenge of all.

Problem: Hot water entering the cold water system:  There are many ways this can happen, but one example I encountered was a high rise hotel with two zones of domestic water.  Since PRVs were involved to create the two pressure zones, the recirc loop was required to circulate through a PRV.  This required the pump to have a very high head to overcome the PRV pressure drop.  The result was a back-pressure that forced hot water into the cold water system system in the mechanical room.  This problem only occurred under low flow conditions, since at that time there was no other release for the hot water system.  The solution was to install a check valve in the cold water pipe serving the boiler.  This eliminated any reverse flow caused by the recirc pump. 

Aside:  Although we solved the problem, I was never fully satisfied I understood the physics.  Here is why.  The mystery was how could the recirc pump force hot water back against the cold water source?   If you “count” molocules of water coming and going, there is no way that a hot water recirc pump can create a net increase in system water that would force hot water back into the cold water.  My only guess is that there really was no perfect separation of the hot and cold water systems due to the mixing valves throughout the hotel.  And what was really happening was that the high pressure recirc pump was recirculating hot water through the mixing valves with the resulting “appearance” of hot water being forced back into the cold water.  Regardless, the check valve solved the problem.  Howver, I felt very unsatisfied not really understanding why the fix worked.  I would be interested in hearing about similar cases.

Summary: Save money on costly high AIC rated breakers by adding length to panel feeders.

Hotels with 208/120 volt services can have very high fault currents near the service entrance.  It is tempting to locate some of the branch panels in the electrical room to serve nearby loads, including the guestrooms in that vicinity.  However, this can result in excessive fault current exposure to the guestroom circuits.

Summary:The IBC requires the top of all elevators to be vented to the outside. The size of this vent is important because in many cases it is routed through the elevator machine room.

Summary: Example of how to apply ASHRAE 62 to achieve a LEED credit. 

Applying ASHRAE 62 to a hotel to get LEED credits is very easy to do because hotel spaces generally have separate air handlers serving each type of space.  Therefore, to achieve ASHRAE 62 ventilation is as simple as setting the air handler minimum outside air to the ventilation requirements for the respective zones computed according to the ASHRAE 62 method.  This includes an area factor and a occupancy factor, of course.  The only situation that truly challenges the designer is where a single outside air unit is used to serve several air handlers each serving zones with different ventilation requirements.  For those cases, the ventilation minimum outside air setting must be computed according to the involved formulas.  ASHRAE has a spreadsheet available to help with these calculations, or you can use a design program such as Design Master which has this feature integrated into the load calculations.

Below is an example of an ASHRAE 62 computation performed for a simple building with diverse zones served by common air handlers.  The example is offered to assist in understanding what is involved.  The computations are performed using the ASHRAE 62 spreadsheet and Design Master for comparison.

Sample Analysis:

ASHRAE 62 ventilation concept is a method for setting the minimum outside air for an air handler, which serves rooms that have different percentage outside air requirements.

The building consists of five zones, each zone served by a separate HVAC unit. Within each zone are a number of rooms ranging from one room to eight rooms. The single room zones are a trivial case of the ASHRAE 62 concept. For those zones, the minimum outside air setting is simply the ventilation rate prescribed by the ASHRAE 62 ventilation amount based on occupancy and room area.

For the zones with more than one room per zone, there are three possible methods of calculating ventilation rates that could be used, each with a different effect on air quality and energy consumption. The first method is to simply add up the individual room ventilation rates and set the HVAC unit to a minimum outside air based on that sum. This results in an amount of outside air at the low end of the spectrum. This is the most energy efficient selection, since this minimizes the outside air heating and cooling load on the system. However, it may not provide sufficient ventilation for the room, which requires a higher percentage of outside air compared to other rooms in the zone. For example, if a conference room and an office are both served by the same HVAC unit, and the conference room requires 50% outside air and the office requires 20% outside air, then the office will not receive sufficient ventilation based on this method.

One method to absolutely guarantee that every room receives the required ventilation is to set the outside air at the HVAC unit to the percentage matching the room with the highest percentage ventilation requirement. The drawback to this approach, however, is that the rooms such as offices compared to conference rooms would receive far more ventilation than necessary. Thus, energy would be wasted.

The ASHRAE 62 approach is to find a middle amount of ventilation between the two extremes described above. Based on extensive research and analysis, the ASHRAE approach gives credit to the mixing effect of return air from one room to another. For example, if an office needs 20% outside air, but it is being served with 25% outside air, then a portion of the air returned to the HVAC unit is still “fresh” and could be counted as outside air when recirculated. The trick is to determine just how to translate this general concept into actual usable outside air values. This is what the formulas of ASHRAE 62 accomplish. ASHRAE 62 computations result in a setting for the outside air that is a proper compromise between the most efficient energy setting, and the maximum case ventilation setting.

The following table shows the five zones, the supply air for each room, and the amount of ventilation required for each room within the zones. In the least ventilation amount case, the sum of the room ventilation cfms is shown for each zone. In the maximum ventilation amount case, the ventilation for each room is computed based on the percentage of ventilation of the worst case room applied to all the other rooms. This percentage is then applied to the zone to show the worst case outside air setting. The column between the least and the worst-case ventilation rates is the ASHRAE 62 rate as computed using the ASHRAE 62 formulas. Note that this amount is between the two extremes in all cases except the cases where the zone has only one room. In those cases, the ventilation rate simply matches the minimum amount required for the room.

The ASHRAE 62 calculations were performed using Design Master HVAC software, which combines load calculations, ductwork design, and the ASHRAE 62 calculations. As a further check of this method of computing the AHSHAE 62 values, the official ASHRAE spreadsheet was used. The values matched and confirmed the correctness of the solutions. 

 The following exhibits are included for reference: