Table of Contents
- A run-around system places a coil in the exhaust airstream and a matching coil in the supply airstream, then connects them with pipes carrying a transfer fluid, usually water or water-glycol. The exhaust coil gives up heat to the fluid, a pump circulates it to the supply coil, and that coil hands the heat to the incoming make-up air. Crucially, the two airstreams never share a device or a medium, they are coupled only through the sealed fluid loop, so supply and exhaust are fully isolated, exactly like a heat pipe but over an arbitrary distance. That makes the run-around coil the right call when ducts are far apart, when a wheel or plate cannot physically span the gap, or when contamination rules forbid any shared matrix. A run-around versus heat-pipe piece is worth reading alongside this one.
- The transfer fluid is almost always water with an inhibited glycol, ethylene or propylene, to suppress freezing and corrosion. The glycol concentration is the key knob: too little and the loop freezes in winter, too much and the fluid gets viscous, the pump works harder and the heat-transfer coefficient drops. A 30 to 50 percent glycol mix covers most climates; colder sites push toward the higher end. The fluid also needs an inhibitor package matched to the coil metallurgy, copper, steel or aluminium, to avoid galvanic corrosion over years of service. The penalty you pay for all this isolation is pump power: the loop consumes electricity to move fluid, which a heat pipe or wheel does not, so the net recovered energy must clear that parasitic load to be worthwhile.
- The run-around coil and the heat pipe solve the same separated-streams problem, and the choice is mostly about layout and effectiveness. A plate exchanger is cheapest for adjacent streams, but between distant ducts the heat pipe needs the two banks reasonably close and oriented for the loop, while the run-around coil can route its pipes anywhere. The heat pipe wins on effectiveness, often 70 percent plus sensible with no pump, and zero operating cost; the run-around coil typically lands at 50 to 65 percent effectiveness but offers unlimited layout freedom and easy maintenance, you can service a coil or the pump without touching the airstreams. For retrofit plant rooms and long duct runs, that flexibility usually decides it. Criterion Run-around coil Heat pipe Stream separation Full, fluid loop Full, sealed pipes Layout freedom Unlimited pipe routing Coils must be co-located Typical effectiveness 50 to 65 percent sensible 65 to 75 percent sensible Parasitic load Pump power None Maintenance Coil and pump serviceable Core sealed, long life
- Sizing a run-around loop follows a few rules. Match the two coil face areas and face velocities so neither starves the other. Set the fluid temperature lift from the exhaust-to-supply temperature difference and target a realistic 50 to 65 percent effectiveness. Size the pump for the loop length and glycol viscosity, not just the heat duty, and budget its kWh against the recovered energy. Insulate the connecting pipes to avoid losing the recovered heat in the plant room. Finally, add a bypass or shutoff so the loop can be taken out of service for cleaning without disrupting ventilation. Our selection guide walks through the calculation, and we can size a loop from your duct layout.
- QIYU supplies run-around coil packages and heat-pipe cores for separated-stream recovery, with fluid loops matched to your climate and coil metallurgy. Send us your duct layout, airflow and temperature lift and contact us for a sized quote. Reach Yang Manager on WhatsApp +86 15753355505, US tel +1 915 295-3666, or email kuns913@gmail.com. We help you select the right model or engineer a custom recovery package.
When the supply and exhaust ducts are in different parts of a building, or when you need absolute stream separation with no rotating parts, the run-around or coupled coil loop is often the only sensible recovery option. It uses two hydronic coils, one in the exhaust and one in the supply, joined by a pumped fluid loop that carries the recovered energy between them. This article covers how the loop couples the streams, the fluid choices and their trade-offs, how it stacks up against a heat pipe, and a practical design checklist.
A run-around system places a coil in the exhaust airstream and a matching coil in the supply airstream, then connects them with pipes carrying a transfer fluid, usually water or water-glycol. The exhaust coil gives up heat to the fluid, a pump circulates it to the supply coil, and that coil hands the heat to the incoming make-up air. Crucially, the two airstreams never share a device or a medium, they are coupled only through the sealed fluid loop, so supply and exhaust are fully isolated, exactly like a heat pipe but over an arbitrary distance. That makes the run-around coil the right call when ducts are far apart, when a wheel or plate cannot physically span the gap, or when contamination rules forbid any shared matrix. A run-around versus heat-pipe piece is worth reading alongside this one.
The transfer fluid is almost always water with an inhibited glycol, ethylene or propylene, to suppress freezing and corrosion. The glycol concentration is the key knob: too little and the loop freezes in winter, too much and the fluid gets viscous, the pump works harder and the heat-transfer coefficient drops. A 30 to 50 percent glycol mix covers most climates; colder sites push toward the higher end. The fluid also needs an inhibitor package matched to the coil metallurgy, copper, steel or aluminium, to avoid galvanic corrosion over years of service. The penalty you pay for all this isolation is pump power: the loop consumes electricity to move fluid, which a heat pipe or wheel does not, so the net recovered energy must clear that parasitic load to be worthwhile.
The run-around coil and the heat pipe solve the same separated-streams problem, and the choice is mostly about layout and effectiveness. A plate exchanger is cheapest for adjacent streams, but between distant ducts the heat pipe needs the two banks reasonably close and oriented for the loop, while the run-around coil can route its pipes anywhere. The heat pipe wins on effectiveness, often 70 percent plus sensible with no pump, and zero operating cost; the run-around coil typically lands at 50 to 65 percent effectiveness but offers unlimited layout freedom and easy maintenance, you can service a coil or the pump without touching the airstreams. For retrofit plant rooms and long duct runs, that flexibility usually decides it.
| Criterion | Run-around coil | Heat pipe |
|---|---|---|
| Stream separation | Full, fluid loop | Full, sealed pipes |
| Layout freedom | Unlimited pipe routing | Coils must be co-located |
| Typical effectiveness | 50 to 65 percent sensible | 65 to 75 percent sensible |
| Parasitic load | Pump power | None |
| Maintenance | Coil and pump serviceable | Core sealed, long life |
Sizing a run-around loop follows a few rules. Match the two coil face areas and face velocities so neither starves the other. Set the fluid temperature lift from the exhaust-to-supply temperature difference and target a realistic 50 to 65 percent effectiveness. Size the pump for the loop length and glycol viscosity, not just the heat duty, and budget its kWh against the recovered energy. Insulate the connecting pipes to avoid losing the recovered heat in the plant room. Finally, add a bypass or shutoff so the loop can be taken out of service for cleaning without disrupting ventilation. Our selection guide walks through the calculation, and we can size a loop from your duct layout.
QIYU supplies run-around coil packages and heat-pipe cores for separated-stream recovery, with fluid loops matched to your climate and coil metallurgy. Send us your duct layout, airflow and temperature lift and contact us for a sized quote. Reach Yang Manager on WhatsApp +86 15753355505, US tel +1 915 295-3666, or email kuns913@gmail.com. We help you select the right model or engineer a custom recovery package.