Specifying a Trawler: 2010

HVAC Systems

The heating, ventilation and air-conditioning systems are a blend of loosely coupled systems to provide maximum energy efficiency and redundancy.

2009 © David Shaw
N.B. This article references an Excel spreadsheet available from the author.

Design Considerations

The basic heating, ventilation and air conditioning system is described below. Remember, 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 AC loads
Minimize sound transmission
Pre-heat water for the water maker
Maximize energy usage
Maximize efficiency
Minimize dependencies
Provide redundancy

The requirement for a single fuel type effectively eliminated propane heating in favour of diesel. As discussed elsewhere, diesel is anyway much safer. It is also more efficient, providing around 140,000 BTU per gallon, compared to 91,000 for propane.

The Solution

Distribution

The first major issue was 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.

Fig 10-1 – The Harworth Bubble stove is one of the few diesel fireplaces available– Photo with permission © Harworth Heating, http://www.bubble-stoves.co.uk/

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.

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-lb 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.

Fig 10-2 – The combined HVAC system

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-VDC old-plate design was chosen for the refrigeration to reduce AC loads, while not imposing a continuous DC load. Excess capacity may be used for air conditioning.)

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.

HVAC Scenarios

With this integrated design, the following scenarios apply:

Fireplace in use:

Central furnace is turned down

Oven in use:

Central furnace is turned down
Central hot water AC is turned off

Main engine in use:

Central hot water AC is turned off

Central furnace fails:

Fireplace and oven provide central and space heat

Central furnace and distribution system fail:

Fireplace and oven provide space heat

Shore AC power fails:

Oven provides hot water
DC-AC inverter provides electrivity to hot water elements

Heating Requirements

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.g., 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/h) [1]:

BTU = V * T * K * B

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 F 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/ft-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% of the requirement, so it runs at about 65% duty cycle.

Accommodation Space Calculation
Area	L (ft)	W (ft)	H (ft)	V (cu ft)
Forward
Aft
Pilothouse
Salon

Ventilation Requirements

Ventilation rates can be expressed in several ways:

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

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 m/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 Requirements

Calculating air conditioning is more complex and so the answers are more varied. The next table gives three sets of estimates to illustrate the issue.

Column A gives a series of BTU values derived from the buyenergyefficient.org web site [2].
Column B is based on an expert rule of thumb of 14 BTU per cubic foot, plus an extra 1000 BTU for good measure.
Column C uses the spreadsheet calculator.

Except for the last two data points, methods A and C are in good agreement, but I leave you to your own judgement.

This spreadsheet calculator is adapted from Air Conditioning Your Home [3], published by the Energy Office of Natural Resources Canada (NRCAN) and available from its web site. It appears to fall within the general range of the other methods, based on area alone. Most rules of thumb are designed for single rooms, or two rooms joined. The author's calculator considers numerous more factors:

Number of occupants
Area of each accommodation
Area of windows and degree of sun exposure
Energy efficiency of windows
Shading of windows
Degree of insulation in the boat
Heat gain through the engine room bulkhead
Heat gain from AC machinery in the accommodation
Heat gain from DC machinery in the accommodationHeat gain from DC lights in the accommodation

Several approximations were made in adapting the NRCAN model. For example, houses have a fixed position, allowing us to calibrate the different heat gain from windows facing any compass quadrant. Boats are mobile, allowing windows to face any direction at any time. The calculator assumes the worse case, with one full side of the boat having maximum southern sun exposure, the other minimum, i.e., it is moored east-to-west.

The degree of insulation is set with the K factor in the heating calculation. The factor for heat gain through engine room bulkheads is a pure guess. The heat gain from AC and DC equipment is factored at 3.4 - 4.3, while NRCAN suggests 3.0 for AC appliances in a house.

Recommended Cooling Capacity (BTU/h)
Area (ft²)	Method (ft)
Area (ft²)	A	B 14 BTU/ft²	C Calculator (K=0.7)
100 - 150	5,000	3,100	3,465
150 - 250	6,000	4,500	5,775
250 - 300	7,000	5,200	6,930
300 - 350	8,000	5,900	8,085
350 - 400	9,000	6,600	9,200
400 - 450	10,000	7,300	10,395
450 - 550	12,000	8,700	12,705
550 - 700	14,000	10,800	16,170
700 - 1,000	18,000	15,000	23,100
A. Based on http://www.energyefficient.org/ [2] B. Expert rule of thumb C. Sharina's Excel spreadsheet [3]

Zones

For heating, ventilation and air-conditioning distribution and control purposes, Sharina is divided into the zones in the next table. With a K=1, Sharina requires approx. 37,810 BTU/h of heating. The main diesel furnace supplies this, sufficient for the coldest weather.

HVAC Zones
Description	Zone	Distribution	Air Conditioning BTU (K=1)	Heating BTU(K=1)
Forward cabin	1	44%	24,349	16,637
Aft cabin	2	17%	9,408	6,428
Pilothouse	3	18%	9.961	6,806
Salon	4	21%	11,621	7,940
Engine room	5	-	-	?

But what happens in an emergency? In the event the furnace fails, the Bristol Pacific model diesel stove in the galley can provide 6,500-16,250 BTU to heat the forward accommodation. At the lower heat setting it could maintain a temperature differential of 21 C, while the higher one maintains Sharina’s design differential requirement of 55 C in the forward compartment.

At the lower setting, water pipes, etc., are protected down to -20 C, a not infrequent winter temperature, which is why the design requirement is the higher 55 C differential. Because the galley stove alone cannot heat the whole boat in the event of a furnace failure, additional heat has to be supplied by the diesel fireplace in the salon. A fireplace such as the Bubble produces only 3.5 kW (11,946 BTU), good for a 17 C differential overall. So it will only heat the pilothouse and salon, not the aft cabin.

Therefore in an emergency in the coldest weather we have a heating shortfall of 21,560 BTU (6 kW). This is not critical above deck in the salon and pilothouse, since there are no water pipes there. But it is critical in the aft head.

Finally, some heating has to be provided to the engine room to keep water tanks and pipes from freezing. Obviously some further development is required in the design of the back-up heating. Increasing the output of the diesel stove is not a good option, as this would tend to make it less useful as a cook stove. Perhaps the Bubble should be re-located to the aft cabin, but this negates its lifestyle purpose. More practical solutions are to shut off the water to the aft head and run the engine to keep the engine room warm. Another solution is to have an aft engineroom and a contiguous forward accommodation space.

Summary

The HVAC system uses a blend of loosely coupled systems to provide maximum energy efficiency and redundancy for a year-round live-aboard. Fresh-air ventilation uses small zone-based air vents but this makes humidity control difficult. The heating and cooling systems use a shared circulating-water distribution system to minimise bulkhead pass-throughs. Heating is by a diesel furnace with backup from a diesel fireplace. Cooling is by a chiller with keel cooler, with backup from the cold-plate refrigeration system. Hot water is heated by the engine, the diesel oven, a solar collector or AC elements using shore power or the house bank. The provided calculator gives heating and air conditioning requirements in BTU/h.