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

Methods of calculating requirements for both heating and air conditioning tend to the arcane or the very simplistic. There are too many variables to consider, e.ggram., the colour of the deck paint affects the amount of heat gain inside. The author has developed a spreadsheet application that tries to strike a balance between simplicity and accuracy. When calculating heating requirements, it ignores heat gain through southern exposure windows in the daytime and heat loss through all windows at night. It also ignores sporadic heat gain from equipment and appliances.

The spreadsheet uses the following formula to determine heating requirements invBritish Thermal Units per hour (BTU/hBritish Thermal Units per hour) [1]:

BTU = VVolt * TDraft of ship * KKelvin, SI unit of thermodynamic temperature * BBeam

where:

V = volume of the accommodation in cubic metres

T = temperature differential in degrees Celsius

K = dispersion coefficient (how heat ‘lossy’ is your boat)

B = 4 (conversion factor to BTU)

To calculate the Volume, for each living space multiply Length * Width * Height in feet as shown in the below table. Use judgement in deciding whether to list each space individually or as part of a section. The calculator will do the conversion to metric.

For T, if you need to convert degrees FFarad, SI unit of capacitance, also Freeboard to degrees C, the formula is:

C = (F – 32) * 5/9

The dispersion coefficient K is adapted from housing construction as follows:

K = 3.0 - 4.0 (Simple construction, simple windows - Not insulated)

K = 2.0 - 2.9 (Simple construction, simple windows - Poorly insulated)

K = 1.0 - 1.9 (Standard construction, double-pane windows - Moderately insulated)

K = 0.6 - 0.9 (Advanced construction, triple pane windows - Well insulated)

With K=3, the calculator yields 19 BTU/ftFoot-sq while experts recommend 20 BTU/ft-sq, so we have good agreement at one end of the range. How aggressive you should get towards the other end is impossible to say. However, with the three heating systems specified for the boat there should be ample scope for increasing or decreasing the heat without upsetting the balance of the system. In a system that is under-sized, the furnace will run for long periods. In an over-sized system, the furnace will cycle frequently and run for very short periods. In general, a heating system should be sized 154%percent of the requirement, so it runs at about 65% duty cycle.

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% 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.

Ventilation rates can be expressed in several ways:

   Cubic feet per minute (CFMCubic feet per minute) or litres per second (Llitre/sSecond) of outside air brought into the boat
   CFM per person: CFM/p
   CFM per unit floor area: CFM/ft2
   Air changes per hour (ACHAir changes per hour)

Standards for ventilation differ, and have varied over time subject to lobbying, energy efficiency doctrines and the emergence of sick building syndrome. A reasonable yardstick is somewhere in the range of 0.5-1.25 ACH or, more precisely, 1.0 ACH translating to around 1.66 CFM per 100 cubic feet of cabin volume. You can double check this to ensure at least 15 CFM/p.

For example, assume a boat having 6,000 cubic feet of volume and berths for five people. Using 1.0 ACH this yields 99.6 CFM and 15 CFM/p yields 75 CFM.

Maximum air velocity in ventilation ducts and vents should not exceed 2.6-3.3 ft/s (0.8-1.0 mMetre, SI unit of length/s) to minimise noise and differentials in air pressure. Air ducts for combustion systems can run as high as 40-66 ft/s (12-20 m/s).

Let’s work a complete example. Assume a salon of 1280 cubic feet. At 1.0 ACH this requires 21.3 CFM:

CFM = Volume * ACH/60 minutes

The corresponding vent area with a velocity of 2 ft/s is:

Vent Area = CFM/(Velocity * 60 seconds) = 21.3/120 = 0.18 sq ft = 25.6 sq in

Close enough.

In this case, we could put a 5- x 5-in intake vent at one end of the salon and a vent of the same size at the other end with an exhaust fan driving 2 ft/s.

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.)

Related Systems

Hot Water

Hot water is heated in several ways. In port in summer, the water is heated by standard electrical elements operating off the AC. In winter, it is heated by the water jacket on the diesel oven. If the oven is not in use, and there is no other source of heat, the hot water tank defaults to the electrical elements.

Under way, engine coolant circulates through the hot water tank, and hence to a water-water heat exchanger with keel cooler. Another feature of this design is that raw seawater is not circulated through the engine. There is a bypass circuit around the water heater that closes thermostatically when the heater is at temperature.

(The next article will describe a tankless design for a hot-water heater with a solar collector and engine pre-heat.)

In winter if the boat is out of the water, the engine may have to be run to charge the batteries. In this case, an optional water-air radiator in the engine room provides engine cooling.

Use an anti-scald, balanced-pressure shower valve (not a tempering valve!) on the showers to regulate the water to 120 F. This will avoid scalding people, and reduce water consumption. Bathers will be able to mix the water faster to a comfortable temperature.

Watermaker

For cold water expeditions, the water intake to the watermaker should be preheated.

Engine Cooling

Refrigeration

HVAC Scenarios