Ductwork for ERVs, Dehumidifiers, and Forced-Air Heating Systems - GreenBuildingAdvisor (2024)

Is it possible for an ERV, a whole-house dehumidifier, and a forced-air heating and cooling system to share ductwork? The answer is yes — but these systems need to be designed with care.

This article describes what I believe to be the best ways that these three appliances can share ductwork. Since there are many different products in these three appliance categories, and since each product has specific operational requirements, HVAC contractors should always follow manufacturer’s installation instructions. (In this article, references to a “dehumidifier” always refer to a whole-house dehumidifier.)

The gold standard: each appliance has separate ducts

We cannot discuss integrated HVAC systems without first speaking about unintegrated systems. By “unintegrated,” I mean that the ERV, dehumidifier, and air handler all operate independently of each other, and that each system has dedicated ductwork that isn’t shared with another appliance (see illustration below).

Using separate dedicated ductwork means that simultaneous operation of the blowers from two different appliances won’t cause pressure problems, and ensures that there won’t be any need for larger-than-normal ductwork. This approach may be the gold standard in terms of system performance, but often is also the most expensive.

The air handler and the dehumidifier can share ducts

It’s possible to design a system in which the air handler and the dehumidifier share ductwork (see the illustration above). This is the most common way to integrate a dehumidifier with a forced-air heating system, with or without an ERV. With this approach, the dehumidifier and the air handler share the same supply ductwork, while the ERV uses a completely separate duct system.

With this approach, the supply ductwork must be sized to account for the extra flow of the dehumidifier. Note that if the air handler’s supply ductwork is not properly sized, it’s possible that when both the air handler and the dehumidifier operate together, one or the other may exceed the maximum operational static pressure, decreasing performance. The lower the operational static pressure when both are running, the better the performance from both pieces of equipment.

I don’t recommend supplying the air from the dehumidifier to the return side of the air handler, because during times of simultaneous operation the cumulative latent capacity is severely affected and the performance is unpredictable. (For more on this topic, see the FSEC report titled “Investigation of Energy Impacts of Ducted Dehumidifier Duct Configurations and Location.”)

The ERV and the dehumidifier can share ducts

In some situations — for example, in a house with ductless minisplits, or in a house with forced-air heating and cooling with a high static duct system, or in a house with a radiant cooling system — you might want your ERV and your dehumidifier to share supply ductwork (see illustration above). These ducts are not shared by the forced-air heating and cooling system. The supply air from the dehumidifier and ERV should go to the bedrooms, while the dedicated return for the dehumidifier should pull from a large central portion of the home. The ERV should pull stale exhaust air from potentially contaminated areas. This setup requires a backdraft damper on the dehumidifier supply duct prior to connection with the ERV, in order to prevent supply air from the ERV from being supplied out the return of the dehumidifier.

The static pressure of the shared supply duct will change based on whether the dehumidifier is on or off. Depending on the ERV, this change in static pressure may change the flow rate of the outside air provided to the house by the ERV.

During commissioning, there are two ways to balance the ERV:

1. Balance the ERV with the dehumidifier on. If you do this, the ERV will go slightly unbalanced and will positively pressurize the house when the dehumidifier is not on. (This is my preferred method).
2. Balance the ERV with the dehumidifier off. If you do this, the ERV will go slightly unbalanced and will depressurize the house when the dehumidifier is running.

Correctly sizing the shared supply ductwork and terminations will greatly reduce the amount that the ERV goes unbalanced, one way or another.

A tandem arrangement: ERV plus dehumidifier

There are positives and negatives to all of the above approaches. As stated above, when a dehumidifier and ERV share ductwork, there is the potential that the ERV will become unbalanced at certain times. If this concerns you, and you are willing to turn the dehumidifier blower on whenever the ERV is on, the setup shown above may be better for you. In this case, we are avoiding the unbalanced situation; the trade-off is that the ventilation system will have a higher operating expense. The approach shown above requires the supply duct leaving the ERV to be connected to the outside air collar on the dehumidifier. It is important that the dehumidifier still maintain a return from the house, since the flow from the dehumidifier will not match the flow of the ERV supply. (There are other reasons as well – but that is a topic for another article).

A motorized damper is required in the duct from the ERV to the dehumidifier to prevent the dehumidifier from drawing in outside air when the ERV is off. This is important during outdoor pollution events (for example, during wildfires or skunk sprays) or unoccupied times (for example, during vacations or work hours). When the ERV is being commissioned, the dehumidifier should be on in order to properly balance the ventilation system.

The fully integrated “injection” approach

The last option that we will discuss is a fully integrated application (see illustration above). Both the dehumidifier and ERV are installed as injection devices in which the return and supply of these appliances are connected to the same piece of ductwork. This “injection” design creates a near zero operational static pressure for the injection devices and has no impact in terms of increased air flow (and thus static pressure) on the air handler. So there is no need to upsize the ductwork. This provides predictable performance of all the different components of the system. However, bath fans would be required with such an installation as the ERV can no longer provide removal of pollutants.

For this approach to be effective, the air handler blower must be on whenever the ERV or the dehumidifier is operating. The ERV should be installed on the return side of the air handler so that the ERV doesn’t add or remove BTUs from the supply side of the forced-air heating and cooling system. The dehumidifier ducts should be located on the supply side of the air handler so as to not reduce overall latent capacity of the system.

This approach increases latent performance (moisture removal) when the air conditioner and dehumidifier run simultaneously, as the air entering the dehumidifier is closer to the dew point. (Check with the dehumidifier manufacturer to ensure this is an acceptable application as it often is not within manufacturer’s specifications.) Yes, the sensible BTUs of the air conditioner are impacted (mechanically changing sensible BTU removal to latent BTU removal), but during peak conditions the air conditioner should be able to maintain acceptable indoor humidity levels. Therefore, during non-peak hours we have extra capacity that can be exchanged for better latent control. It should be noted that the dehumidifier will have little to no latent capacity if it operates simultaneously when the heat source for the home is on and being distributed through the supply side of the air handler.

I hope this article sparked some talking points. Integration of different systems by different manufacturers can become tricky, as each manufacturer has only their own system in mind. We all have the same goal, however: a healthy indoor environment supported by the predictable and efficient performance of the home’s HVAC equipment.

David Treleven is an industry manager at Therma-Stor where he concentrates on innovation and technical applications.