HVAC System Selection

Systems Selected

FURNACE

Furnaces are the most commonly used residential heating system in the United States.  They are more often powered by natural gas but can be powered by other means such as wood, coal, heating oil, liquefied petroleum gas, or electricity.

A furnace consists of four major components: the burner, heat exchanger, controls, and blower.

This system was chosen as it is very readily available. Because it is so widely used is relatively easy to maintain.  Additionally, because the ducts would already be in place for the cooling system, it saves money by not having to install radiant heating pipes in the floor or along the baseboards.  It was also determined that the forced air heating could more easily maintain a comfortable temperature for the entire restaurant.  A condensing furnace also has a very high fuel utilization efficiency (90%-97%) when using natural gas.

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CHILLER

Chillers work by cooling water and then either distributing the chilled water to other minor A/C units or by directly circulating air over the chilled water tubes and then throughout the system.

Chilled water is effective for systems that have multiple cooling zones.

A graph can be used to approximate the sizing for the air ducts.

Alternative Systems

HEAT PUMP

A heat pump moves heat from one location to another.  The key components of a heat pump system are the refrigerant, the condenser coil, the evaporator coil, the compressor and the reversible valve.

The refrigerant absorbs heat when it vaporizes in the evaporator coil and releases heat when it condenses in the condenser coil.  Because the system is a loop, the heat pump can be used for both heating and cooling.  To switch from heating to cooling, the heat pump just has to reverse the flow of the refrigerant.

The problem with heat pumps is that they are not efficient in extreme weather.  If the temperature fluctuates too greatly throughout the year, a heat pump will neither cool nor heat effectively.  Heat pumps would be impractical for a place such as Tennessee.

The following chart shows the coefficient of performance for various heat pumps in different conditions:

Pump type and source Typical use case COP variation with Output Temperature
35 °C

(e.g. heated screed floor)

45 °C

(e.g. heated screed floor)

55 °C

(e.g. heated timber floor)

65 °C

(e.g. radiator or DHW)

75 °C

(e.g. radiator & DHW)

85 °C

(e.g. radiator & DHW)

High efficiency air source heat pump (ASHP). Air at −20 °C 2.2 2.0
Two-stage ASHP air at −20 °C Low source temp. 2.4 2.2 1.9
High efficiency ASHP air at 0 °C Low output temp. 3.8 2.8 2.2 2.0
Prototype transcritical CO2 (R744) heat pump with tripartite gas cooler, source at 0 °C High output temp. 3.3 4.2 3.0
Ground source heat pump (GSHP). Water at 0 °C 5.0 3.7 2.9 2.4
GSHP ground at 10 °C Low output temp. 7.2 5.0 3.7 2.9 2.4
Theoretical Carnot cycle limit, source −20 °C 5.6 4.9 4.4 4.0 3.7 3.4
Theoretical Carnot cycle limit, source 0 °C 8.8 7.1 6.0 5.2 4.6 4.2
Theoretical Lorentz cycle limit (CO2 pump), return fluid 25 °C, source 0 °C 10.1 8.8 7.9 7.1 6.5 6.1
Theoretical Carnot cycle limit, source 10 °C 12.3 9.1 7.3 6.1 5.4 4.8

Source: http://en.wikipedia.org/wiki/Heat_pump#COP_and_lift

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