Introduction

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Many homeowners and installers often wonder if it is possible to share an exhaust flue between a high efficiency furnace and a tankless water heater. This article explores the practical considerations and safety concerns associated with sharing exhaust flues between these powered venting appliances.

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Understanding the Risks of Sharing Exhaust Flues

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First and foremost, sharing an exhaust flue between a high efficiency furnace and a tankless water heater is not recommended and, in many cases, is strictly prohibited. Simply put, never share a flue between powered venting appliances.

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A shared flue can pose significant safety risks. For instance, category III and IV gas-fired appliances, which include high efficiency furnaces and certain tankless water heaters, operate with a positive pressure venting system. This design ensures that exhaust gases are directed away from the appliance and household. When these appliances share a flue, the following risks arise:

r r r Backdrafting: If only one appliance is operating, the exhaust gas from the other appliance could be pushed back into the non-operating appliance, and potentially into the house, leading to dangerous levels of carbon monoxide (CO) and other combustion byproducts.r Reduced Venting Efficiency: The positive pressure venting system designed for each appliance may not function optimally when shared, leading to inefficient venting and other performance issues.r Chimney Pressure Fluctuations: The pressure within the chimney can fluctuate unpredictably, leading to safety hazards and potential damage to the venting system.r r r

The Reality of Sharing Exhaust Flues

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Despite the general prohibition, there is some flexibility in certain scenarios. However, it is important to note that this flexibility is limited and should only be considered under specific conditions.

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Most conventional appliances have developed standards that allow for combined venting. However, high efficiency furnaces and tankless water heaters are not conventional in these aspects. Each has unique and specific installation requirements that must be strictly followed:

r r r Tankless Water Heaters: These units typically vent at the top of the appliance and may have specific hot gas flue requirements. Anything that interferes with these requirements can compromise the safety and efficiency of the unit.r High Efficiency Furnaces: These appliances often "vent" via a plastic pipe, which is designed to maintain proper suction to expel exhaust gases. Sharing this flue with a tankless water heater can disrupt this efficient exhaust system.r r r

Safe Venting Solutions

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Typically, the safest approach is to install separate exhaust flues for each appliance. However, in some cases, sharing a flue might be feasible under specific conditions. Here are some considerations and safety recommendations if you decide to proceed with a shared venting solution:

r r r Proximity and Vertical Alignment: The two appliances should be as close as possible and on the same vertical plane to minimize the risk of backdrafting. Ensure that the flues join together in a manner that maintains appropriate spacing and clear air flow.r Exhaust Fan Placement: If the two appliances must share a flue, the fan from the high efficiency furnace should be installed above the point where the flues join together. This placement can help prevent the exhaust from the furnace from blowing back into the tankless water heater.r Check Valves: Installing check valves on both appliances can prevent backflow and ensure that exhaust gases are expelled in the correct direction.r r r

Conclusion

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While sharing an exhaust flue between a high efficiency furnace and a tankless water heater may seem like a cost-effective solution, it is important to prioritize safety and compliance with local code regulations. Consult with a professional HVAC technician and ensure that all safety measures are in place to avoid hazardous situations and potential equipment damage.

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