The Ductless Mini-Split Heat Pump's Big Brother - CITY MULTI

In my last four HVAC Design jobs, I've specified ductless mini-split heat pumps. I have four more such jobs underway now.

Why mini-splits?

• Efficiency (up to 26 SEER)
• Variable Refrigerant Flow
• Versatility
• No Ducts!

In fact, some say they're threatening to replace the standard forced air HVAC systems that we've grown to lo...well, that we've grown accustomed to. Allison's post about the advantages of these mini-split heat pumps explains some of the other reasons why they're becoming so popular. 

Diamond Designer

At a recent Mitsubishi Diamond Designer training, I learned more about the mini-split's big brother, the CITY MULTI system by Mitsubishi Electric HVAC, and how to design it for multi-family and commercial buildings. It uses the same efficient technology as mini-splits, but on a larger scale. In fact, we recently looked at a 250-unit high rise condominium retrofit project where CITY MULTI would replace a tired, inefficient boiler and chiller system from the 1960's. CITY MULTI's pipes are no bigger than 3" with insulation, so the lines will be small enough that the team won't have modify the structure much at all.

The WHAT and the HOW

CITY MULTI is a 2-pipe, modular heat pump system that takes advantage of special compressors that can feed up to 50 different zones. It also can do simultaneous heating and cooling.

The 2 pipes are the refrigerant lines that run from an outdoor condensing unit to either a BC Controller or Branch Header. From there, soft copper, pre-insulated line-sets are used to deliver the cold or hot refrigerant to individual fan coils that are placed within each zone that requires heating or cooling.  With the simultaneous heating and cooling system, the same refrigerant which is used to heat one zone can be pumped to another zone with a need for cooling.  This offers both increased energy efficiency and comfort over other types of HVAC systems.

Depending on the series (R2, WR2, Y, WY, H2i, & S), systems can be designed for up to 30-tons of capacity, using  6, 8, 10, or 12-ton modular condensers piped  together. These systems can also have over 100% connected capacity, in some cases up to 150% of their condensing unit rated capacity because of the systems greatest asset: Variable Refrigerant Flow/Volume (VRF or VRV).

VRF/VRV allows a system to adjust the amount of refrigerant delivered by the compressors to provide only the amount of heating or cooling to a zone that is required at any given time within the limits of the designed peak load. It does this through the use of a Linear Expansion Valve (LEV) that controls the refrigerant flow to each indoor unit. Through 3 on-board thermistors, the indoor unit constantly monitors the space and refrigerant conditions and communicates to the LEV and the condensing unit to dial up or down the amount of refrigerant delivered to the coil.

Controls vary as well but are very simple and integrated with the system. The indoor units can be controlled individually, by a central master controller, both, or by a PC-based system. Because the thermostats are within the indoor units, or heads, temperature is controlled remotely and the controls can be placed anywhere within the zone. They can be used to read the temperature in the room but are NOT a thermostat.

The Advantages

• Certain models offer 100% heating capacity down to 5° F.  All the units are heat pump models that provide substantial low ambient heating capability typically eliminating the need for additional heating systems in most of the Southeast and even much farther north if properly designed.
• The inverter driven compressor uses only as much or as little energy as needed at any given point in time to maintain room conditions.  This results in energy savings that have been in the range of 10% to 40% over other systems (based on actual retrofit results and project energy models)
• R-2 and Y series systems are compatible with both air cooled and water cooled configurations, and the water-cooled equipment CAN be used with geothermal applications
• Indoor fan coil options range from floor, wall, and ceiling mounted to floor, wall and ceiling concealed (ducted).
• Using their new Hydronic Heat Exchanger and Booster units, hot water can be created directly or through the use of recovered energy from cooling for large potential energy savings.
• Offers completely integrated ventilation options between 300 and 1200 CFM.
• First cost is competitive with water source heat pump systems and is better than four-pipe and geothermal systems.

In addition to commercial and multi-family projects, this could potentially be a great application in larger homes that have loads greater than 5 tons (and available 3-phase power). 

 

- from the desk of Energy Vanguard's "Diamond Designer"