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The Hybrid HVAC System, Part I: Concepts and working systems
The Hybrid HVAC System, Part II : Typical s

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We've been preaching the benefits of hybrid systems for several decades; in fact our go to system today evolved out of the late 70's and early 1980`s with the market introduction of heat recovery ventilators (HRVs) and radiant floor heating - which has become the benchmark in preferred systems for discerning buyers.

Below you will find three hybrid schematics incorporating radiant cooling and heating for thermal comfort and a dedicated outdoor air system for ventilation, de(humidification), deodorization and decontamination with optional second stage cool/heat. Shown only: outdoor air /make up air unit. Not shown: exhaust air / heat recovery.

Figure 1. Dedicated outdoor air system w/ radiant space heating. This arrangement will serve almost any transitional to terrific building where the radiant floor can handle 100% of the thermal comfort load with the air handler taking care of the make up ventilation air. Shown here is the supply air side only meaning exhaust fans located throughout the building are not shown. Outdoor air is brought in to the air handler and conditioned to a discharge temperature deemed comfortable by codes and the occupants (typically ≈ 75F (24C)).

Figure 2. Dedicated outdoor air system w/ radiant space heating and parallel control valve for second stage heating. This arrangement serves the traditional or terrible home where supplemental heat may be required during design conditions due to low to poor thermal performance of the enclosure. In this setup, radiant handles all but the extreme conditions with the thermostatic controller (marked "T") with discharge air sensor maintaining constant air temperature for seasonal conditions. When design conditions occur the second stage of the two stage thermostat opens control valve 1 (marked CV 1) to allow air temperatures to increase freely based on the water temperature of the boiler (typically ≈ 110F-130F (43C-54C)). Note due to the single coil and common distribution the entire building will be supplied with high temperature air under maximum loading. If this could cause overheating in critical zones then use Figure 3 below. This is the system the author uses at home where the air handler is also fitted with a steam humidifier.

Figure 3. Dedicated outdoor air system w/ radiant space heating and reheat coils for second stage heating. In this arrangement every process is similar to Figure 2 except the second stage is a remote coil or coils for only those zones needing a second stage of heat. This is useful where the majority of the building can be conditioned with radiant cooling and heating for thermal comfort and ventilation air can be distributed throughout the building at a constant temperature except where there may be one or two zones which need a second stage of cool or heat.

Hybrid for cooling


The moisture problem in high performance homes in hot humid climates has more to do with interior moisture generation and lower sensible heat gain and less to do with ventilation and the cooling system. Separating dehumidification from comfort cooling as a hybrid system is an effective strategy.

All of the above schematics can be implemented for cooling, the only difference is the fluid temperature becomes chilled instead of heated and the control and equipment strategy has to accommodate the heat/cool changeover and dehumidification process. There are other items to consider in the changeover systems which can be studied in our dedicated outdoor air system (DOAS) and radiant cooling sections.

See: The Hybrid HVAC System, Part I: Concepts and working systems

Suggest study:

Dedicated outdoor air systems (DOAS)
The Hybrid HVAC System, Part I: Concepts and working systems

Additional reading:

  1. Lstiburek, J., Residential Ventilation and Latent Loads, ASHRAE Journal, April 2002


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