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Quotes - Engineering - Sales| DUTY | TYPICAL SERIES | ADDS | CONTROL |
| Recirculation | SU / WU / DSU | Continuous small load | Turndown matters |
Domestic Hot Water Recirculation
The Load That Never Goes Away
A recirculation loop circulates hot water continuously through the distribution system so that any tap delivers hot water quickly. It saves water and improves service, and it changes the heater's duty profile substantially.
It adds a continuous load. The loop loses heat through the pipework all day and all night, and the heater has to make that up. On a building with long runs and modest insulation, that standing load can be a significant fraction of the design duty.
Turndown becomes the design problem. The heater now has to satisfy a large morning peak and a small overnight recirculation load with the same control valve. A valve sized for the peak operates near the bottom of its range for most of the day, which produces hunting and temperature swing.
Legionella control interacts with this. Recirculation systems are usually required to maintain a minimum temperature throughout the loop, and that requirement sets the heater's setpoint and the acceptable temperature drop around the circuit rather than comfort alone.
Scaling accelerates with continuous operation. A heater running continuously at a high wall temperature scales faster than one that cycles. Lowering the medium temperature and adding surface is the usual remedy.
Recirculation Selection
The peak draw and the recirculation load are two different duties the same heater has to serve.
Duty Inputs
- Peak draw: GPM and duration
- Recirculation flow: GPM around the loop
- Loop heat loss: BTU/hr standing
- Minimum loop temperature: Set by policy or code
Series
- SU or DSU on steam
- WU or DWU on a hot water loop
- Storage with TCS or TCW where peaks are sharp
- Double-wall where potable
- Sized for peak and checked at turndown
Design For
- Wide turndown ratio
- Continuous overnight load
- Minimum loop temperature
- Scaling at continuous operation
- Control valve range
Related
APPLICATION • STORAGE
Storage Tank Heating
Storage absorbs the peak so the heater handles recovery.
+ Learn More
APPLICATION • POTABLE
Potable Water Heating
The construction requirement on the domestic side.
+ Learn MoreTurndown Is the Recirculation Problem
One Valve, Two Very Different Duties
The heater has to deliver the morning peak and then hold a small overnight recirculation load. A control valve sized for the first operates near the bottom of its range for the second, and near the bottom of the range is where valves hunt and outlet temperature swings.
Where the ratio is wide, either a smaller valve in parallel or a storage vessel that lets the heater cycle rather than modulate is worth designing in. On steam, wide turndown also brings the risk of stall.
Continuous Operation Accelerates Scaling
A heater serving a recirculation loop never fully stops, so the tube wall is hot all the time. On hard water that scales faster than a heater that cycles and cools between draws.
Lowering the medium temperature and adding surface reduces the wall temperature and slows the scaling. It is more effective than treating the symptom with more frequent descaling.
