A double wall vented coil is a heat transfer coil constructed with two separate tube walls rather than a single tube wall between the heating or cooling fluid and the fluid being heated or cooled. The space between the two walls is vented to atmosphere. If either the inner wall or the outer wall develops a leak, the fluid escaping through the failed wall exits through the vent rather than migrating across the barrier into the other fluid circuit. The two fluid streams never come into direct contact with each other regardless of which wall fails.
This construction is required in applications where the consequence of cross-contamination between the two fluids is not an efficiency loss or a maintenance problem but a public health risk. When one side of the heat transfer surface contains a fluid that cannot under any circumstances be allowed to contaminate the potable water on the other side, single wall construction is not an adequate safeguard regardless of the tube material or the operating pressure differential. Double wall vented construction is the code-required solution.
Where double wall vented coils are required
The primary application for double wall vented coils is domestic hot water heating where the heating medium on the primary side is a fluid that would be hazardous if it entered the potable water supply. This includes systems where the primary heating fluid is treated boiler water containing chemical inhibitors, glycol solutions used in freeze protection applications, steam condensate from systems where the condensate return carries chemical treatment residue, and any other non-potable fluid where cross-contamination into the domestic water circuit would create a health risk.
Plumbing codes in most jurisdictions require double wall construction for heat exchangers where a toxic or otherwise hazardous fluid is used as the heat source for potable water heating. The specific requirement varies by jurisdiction and by the specific fluid on the primary side. Engineers specifying a domestic hot water heating coil where the primary fluid is anything other than clean, untreated water should verify the applicable code requirement with the authority having jurisdiction before specifying single wall construction.
Pool heat exchangers are a second application where double wall vented construction is commonly required. The pool water circuit carries chlorine and other chemical treatments that cannot be allowed to contaminate the potable water or the building heating system fluid on the other side of the heat exchanger. Many jurisdictions require double wall construction for pool heat exchangers regardless of the specific chemical concentrations involved.
Solar thermal systems that use glycol antifreeze in the collector loop are a third common application. The glycol loop and the domestic hot water storage tank cannot share a single wall heat transfer surface under most plumbing codes because the glycol is toxic and cannot be allowed to enter the potable water supply.
How the vented gap works
The double wall construction consists of an inner tube carrying one fluid and an outer tube surrounding it, with a small annular space between the two tubes that is open to atmosphere at one or both ends. In a coil configuration, both the inner and outer tubes are formed into the coil geometry, maintaining the annular space throughout the coil.
The vent opening is sized and located so that any fluid leaking through either wall exits visibly rather than silently migrating to the other fluid circuit. A leak in the inner wall allows the inner fluid to enter the annular space and drip or flow from the vent point, which is a visible indicator that the inner wall has failed and the coil needs replacement before it is returned to service. A leak in the outer wall allows the outer fluid to enter the annular space from the outside and exit through the vent, which is a visible indicator that the outer wall has failed.
The key characteristic of the vented gap is that it works passively without sensors, controls, or monitoring equipment. The physical gap and the vent path provide the cross-contamination protection by making it physically impossible for a single failure event to allow fluid to cross from one circuit to the other. Monitoring can be added to detect leaks earlier, but the protection does not depend on the monitoring working correctly.
Construction and materials
Double wall vented coils are fabricated with the same tube materials used in standard single wall coils, with the additional requirement that both the inner and outer tube materials be compatible with their respective fluids and that the annular space remain clear and unobstructed throughout the coil’s service life.
Copper inner and outer tubes are the standard construction for domestic hot water and pool heating applications where the fluid chemistry on both sides is within the range that copper handles correctly. For applications where the primary fluid is aggressive toward copper, the inner tube material can be specified in a different alloy while the outer tube remains copper, or both tubes can be specified in compatible alloys.
The vent configuration needs to be accessible for inspection during the coil’s service life. A double wall vented coil installed with the vent points inaccessible cannot be visually inspected for leaks, which defeats one of the primary functional advantages of the construction. For coils installed in confined or difficult-to-access locations, the vent lines can be extended to an accessible inspection point so that the annular space condition can be checked without accessing the coil directly.
Specifying a double wall vented coil
The specification for a double wall vented coil needs to confirm several items beyond the thermal performance requirements. The fluids on both sides of the heat transfer surface and their operating pressures and temperatures. The applicable code requirement for double wall construction in the specific jurisdiction. The vent configuration and the location of vent points relative to the installed coil position. The inner and outer tube materials selected for compatibility with both fluids. And the inspection access requirements for the vent points in the installed configuration.
For domestic hot water applications where the primary heating source is a GP Energy Products boiler plant, the GP Energy team handles the boiler plant side of the application and can confirm the primary fluid chemistry for the double wall coil specification. Visit gpenergyproducts.com for more. For the pump systems serving domestic hot water heating applications, the Merion Pump Company team handles domestic hot water recirculation and primary heating circuit pump selection. Visit merionpump.com for more.
HX Coils fabricates custom double wall vented coils for domestic hot water, pool heating, solar thermal, and other applications requiring separation between potable and non-potable fluid circuits across the Mid-Atlantic region. Reach out before the coil is specified and we will confirm the construction requirements for the specific application and jurisdiction.
References
1. International Plumbing Code. Section on cross-connection control and double wall heat exchanger requirements for potable water heating applications. iccsafe.org
2. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers double wall heat exchanger construction requirements and application guidance for potable water contact applications. ashrae.org
3. ASHRAE Standard 188. Legionella Risk Management for Building Water Systems. Covers domestic hot water system design requirements applicable to heat exchanger selection for potable water heating. ashrae.org
4. NSF International. NSF/ANSI Standard 61. Covers material certification requirements for heat exchangers in direct contact with potable water including domestic hot water heating coils. nsf.org
