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Explain the differences between a zoned bypass heat pump and "hybrid/dual fuel HVAC"

I have been wanting a dual fuel HVAC system for my new house. Recently I toured the Bright Green "Kincaid"Concept House at Briar Chapel and discovered the Carrier Infinity air purification system and the Bryant Hybrid Heat HVAC system. I would like guidance into selecting the best one but not necessarily the most expensive one. Is there a"Consumer Reports" for these systems?

Asked by Melissa Rives
Posted Thu, 05/28/2009 - 20:10

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Melissa,
The "Bryant Hybrid Heat" system is an air-source heat pump coupled with a gas-fired furnace. It is used for residential heating and cooling. During the summer, it acts like a central air conditioner. During the winter, you have a choice of two fuels: electricity (used to fuel the heat pump) and gas, which is more economical in very cold weather.

The "Carrier Infinity Air Purifier" is not used for heating or cooling. It's just an air purifier. According to Carrier's explanation, it includes an air filter and a system of purification using "ionization charge particles" and "ion bombardment" to capture air-borne pathogens.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 05/29/2009 - 04:24

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The hybrid heating systems just replace the electric emergency heating strip that is used for back up in most heat pumps with a small propane or natural gas burner. They don't do anything to address the indoor humidity problems we have here in the southeast, just shift fuel from electricity to propane during extremely cold weather.

Ionization air filters seem fussy to me, I prefer simple extended media filters which can be coupled with an ultraviolet light duct sanitation system if additional purification is needed. Replacement filters for either of these systems are not cheap, the Carrier cartridges are $85 and the five-inch pleated extended media filters I use are $35 each.

Zoned-bypass heat pumps are typically used in very well insulated homes with the tendency towards excess humidity problems during the summer cooling season. All the zones of the house are served by a single air handler with zone valves controlling which part of the house gets conditioned air.

When one of the zones reaches it's desired temperature it sends a signal to close an electric damper to stop air from being sent to that zone. When this happens a weighted passive "barometric damper" opens allowing the excess air to flow through a "bypass duct" back to the return air manifold. This cooled air mixes with the air returning from the return air ducts in all the zones and pre-cools it before it reaches the cooling coil. The cooling coil is thus better able to drop the temperature of the air to the dew-point and extract more humidity from it so it greatly enhances the de-humidification and comfort of the air. By pulling return air from all the return air registers it mixes the air in the building minimizing stagnant areas. In the winter the returning air is pre-heated when the bypass is activated leading to warmer air at the supply registers.

These systems are much more difficult to install than conventional multi air handler systems and need an integrated project management approach to be successful with cooperation between the builder, the designer, the plumber, and the HVAC installer. The installation is much more prone to error as it involves enhanced implementation of the electronic controls incorporating a frost sensor on the cooling coil, the damper controls, and simply routing all the different zone ducting which can be challenging unless the designer builds the room for it into the plans. (Good thing I'm married to my architect, therein lies true integrated project management.)

Answered by Michael Chandler, GBA Advisor
Posted Sun, 05/31/2009 - 23:19

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