Multifunction Full Condensing Units | My Business | Mississippi Power | A Southern Company

Multifunction Full Condensing Units

Multifunction, full-condensing systems are vapor compression cycle units that have several operating modes, including a water-heating-only mode. Terms such as "trifunctional" and "triple-function" are used by some manufacturers to describe these units. Their refrigerant-to-water heat exchangers are large enough to fully condense the refrigerant, and special controls are used to manage refrigerant inventory. Annual water heating COP ranges from 2.0 to 4.0, but can be much higher if waste heat from a cooling function is the primary heat source. Their primary disadvantage is their high initial cost.

Multifunction full condensing units may operate in some combination of the following modes:

  • Dedicated space cooling
  • Space cooling and water heating
  • Dedicated space heating
  • Space heating and water heating
  • Dedicated water heating

Multifunction, full-condensing systems (MFFCs) have two condensers, one for space heating and cooling and one for water heating. The water heating condenser is used when water heating is required; it is usually plumbed in parallel with the space heating condenser, although many variations are possible. Some systems are plumbed and controlled to allow the water heating condenser to be used either for full condensing or desuperheating.

Multifunction, full-condensing systems are referred to as "full-condensing" because, unlike desuperheaters, they can apply all the waste heat energy produced in the cooling mode to water heating, not just the 10 to 30% contained in the superheated vapor. The ability to utilize all available waste heat, combined with the water-heating-only mode, means multifunction, full-condensing systems can supply much more high-efficiency water heating annually than desuperheaters. Supplementary water heating capacity is still required; but it is generally infrequently used.

mffc

Terms such as "trifunctional" and "triple-function" are confusing because they are used by some to describe desuperheaters, and by others to refer to multifunction, full-condensing systems. The terms "trifunctional" and "triple-function" are avoided here and in other EPRI publications.

Advantages of MFFCs

  • Water heating output is higher than that of desuperheater water heaters.
  • Annual COP is higher than that of a conventional electric water heater or desuperheater. Annual water heating COP ranges from 2.0 to 4.0. Higher efficiency is possible if waste heat from a cooling function is the primary heat source.
  • Waste heat recovery rates in the space cooling/water heating mode are higher than for desuperheater water heaters.
  • Overcooling of interior spaces can be avoided by switching from indoor air to outdoor air for the heat source.
  • Diversified electric demand is lower than for conventional electric resistance water heaters.

Disadvantages

  • First cost is higher than for desuperheater water heaters and some dedicated HPWHs
  • Controls are more complex than those in desuperheater water heaters, dedicated HPWHs, or many vapor-compression systems that do not heat water
  • "All-in-one" design means that high-efficiency water heating capability is lost if malfunctions occur elsewhere in the vapor compression cycle. (Backup water heating would still be available but would be less efficient.)

There are a variety of multifunction, full-condensing units available for different applications.  All are capable of operating in two or more of the following modes, including at least one water heating mode.

  • Dedicated space cooling
  • Space cooling and water heating
  • Dedicated space heating
  • Space heating and water heating
  • Dedicated water heating

MFFCs have two condensers, one for space conditioning and one for water heating, usually plumbed in parallel.  Heat is rejected through the water heating condenser when water heating is required.  Unlike desuperheaters, MFFCs can apply all of the condenser heat to the water heating load, not just the 10 to 30% available by desuperheating.  Their ability to do full-condensing water heating and their common dedicated water heating mode of operation allow MFFCs to provide a much greater portion of the annual water heating load than desuperheaters.

Applications

Multifunction, full-condensing units are best applied where there is a need for space conditioning and where the water heating load is large enough to permit regular operation of the system's high-efficiency water heating functions.

Best applications

  • Residences, indoor pools, and restaurants. 

Possible applications

  • Where there are significant water heating and space conditioning loads.

Efficiency

Because they are capable of many combinations of operating modes, it is difficult to define ratings that describe the performance of multifunction, full-condensing heat pumps. There are no established standards for these systems. One manufacturer obtained a waiver from US DOE to define a combined cooling performance factor or CCPF combined operation for cooling and water heating. A combined heating performance factor was defined for combined space heating and water heating. The CCPF and CHPF are similar to SEER and HSPF, respectively, with the water heating output of the heat pump added to the space conditioning output.

Contact us for a detailed list of manufacturers for this equipment.

Other information

Multifunction, full-condensing units are not a well-defined product category. ARI Standard 290-96 establishes definitions, classifications, and testing and rating requirements for air conditioning and heat pump equipment incorporating potable water heating devices. This standard applies to air-source units with rated capacities less than 65,000 BTUH.