HVACIntegration
HVACHeating, ventilation, air conditioning Integrated Design
Summary
The heating, ventilation and air-conditioning systems are a blend of loosely coupled systems to provide maximum energy efficiency and redundancy. This example works through the considerations in designing HVAC for year-round living on a 50-foot boat.
Design Considerations
Design Goal
AAmpere (amp), SI unit of electrical current basic heating, ventilation and air conditioning system is described below. In this case the design goal is a year-round live-aboard in north-eastern North America. For completeness in understanding the trade-offs made, the engine cooling system, hot water and refrigeration and watermaker are also shown. Some of the design considerations are:
* One single fuel type on board
* Minimize ACAlternating current loads
* Minimize sound transmission
* Pre-heat water for the water maker
* Minimize energy usage
* Maximize efficiency
* Minimize dependencies
* Provide redundancy
The requirement for a single fuel type effectively eliminates propane heating in favour of diesel. Diesel is anyway much safer. It is also more efficient, providing around 140,000 BTUBritish Thermal Unit (British Thermal Units) per gallon, compared to 91,000 for propane.
Distribution System
The first major issue is whether to use forced air or circulating water to distribute heating and cooling. In the beginning, memories of cold radiators in grade school in the dead of a Canadian winter, and the comfort of humidity control with forced air in modern homes predisposed me to forced air. Over time, I changed my mind several times. In the end, circulating water was chosen to:
* Reduce the size of ducts in the insulated space * Eliminate a path for airborne noise and dirt from the engine room * Reduce the general level of air-borne dust * Eliminate the need for a cold-air return * Minimise openings in watertight bulkheads * Deliver a more even heat by reducing stratification * Eliminate cold drafts on start-up
Like electric heating, hot-water heating is very dry. This is offset by ventilation, which introduces fresh air. A programmable thermostat is located in the forward passageway. In each living area, opening/closing individual radiators will control temperature manually.
In addition, to provide backup in the case of failure in a severe cold spell, a diesel bulkhead fireplace in the salon, such as the Kabola Old English Diesel Room Heater [4] or the Harworth Bubble [5] is also plumbed into the distribution system. Other types of bulkhead heater are available from Dickinson [6], Refleks [14] and Sigmar [15].Initially a fireplace was desired for lifestyle reasons, but as the design evolved it became a backup system. The Dickinson Bristol Diesel Cook Stove [6] in the galley can also heat the forward accommodation, but it is not part of the main distribution system. The main distribution system also routes through the towel rails in various compartments. These are switched out of the circulating water system in summer and heated with AC elements.
Heating
Ventilation
Fresh air ventilation is required to replenish oxygen removed by people and sources of combustion, and to dilute odours and pollutants. Local exhaust ventilation is required in heads and the galley to remove airborne odours before they spread through the boat. From a ventilation viewpoint, the most effective method is an integrated HVAC system with air distribution and local controls in each cabin. Such a system can include an air-to-air heat exchanger to precondition the temperature of the air and recover energy, and a humidifier/dehumidifier to control levels of indoor moisture. Humidity control is especially important in hot humid climates where unconditioned ventilation can deliver 1-lbPound weight of water per cubic foot of intake air.
Excess humidity causes condensation on windows and water pipes. It can blister paint, rust metal and warp wood, and cause electrical faults. Dust mites, fungus, mildew and mould thrive in humid conditions, aggravating allergies and sometimes damaging lungs. Insects like clothes moths, cockroaches and fleas also like high humidity.
People prefer a relative humidity of 30 to 50%percent and find anything much higher to be very uncomfortable.
Unfortunately I decided against an air distribution system in favour of a water system for heating and air cooling. This was to minimise the scope of pass-throughs in water-tight bulkheads but like many design decisions this had further consequences. It made an integrated ventilation/humidification system impossible.
The alternative to running fairly large air vents the length of the boat is local ventilation in the main zones of the boat. This is far from ideal. In both summer and winter the air intakes will be working against the air conditioning and heating systems, respectively, and deck-mounted dorades for intake and return air are multiple hull openings. The ventilation system must be designed carefully to minimise these risks of water entering.
Humidity control is also difficult with local ventilation; although it may be possible to incorporate small electronic dehumidifiers into the vents. Electronic dehumidifiers use small peltier heat pumps but consume a fair bit of electrical energy. For small vents, mechanical dehumidifiers don’t scale down, and desiccated dehumidifiers are overly complex.
If you plan to spend your time in hot humid climates, you should consider a solution that incorporates a dehumidifier.
Air Conditioning
A water-based chiller provides air conditioning. The chiller circulates chilled water through a water distribution system to the cabins, to cool them in summer. All pipes should be insulated to prevent condensation. (Similarly, if you opt for forced air, the ducts should be insulated.)
The heat exchanger can be water-air or water-water. A water-air exchanger would have to work against the heat in the engine room, so it makes more sense to use a water-water heat exchanger with a keel cooler as a heat sink. This is overall more efficient (the temperature differential is higher with water), and avoids generating extra heat in the engine room.
Additional cooling for one zone is provided by the Glacier Bay cold-plate refrigeration system [8]. (A high-efficiency 12-VDCVolts direct current old-plate design was chosen for the refrigeration to reduce AC loads, while not imposing a continuous DCDirect current load. Excess capacity may be used for air conditioning.)