(Hot Water)

Water is heated by the diesel fired burner system and circulated through a closed loop of tubing or hoses to the area requiring heat.

In the area to be heated you can install convection type radiators* or the most common installation is the fan forced radiator system similar to the heating system in your car.

The fans on each heat exchanger can be controlled with a room thermostat to give you automatic zone controlled heating or with manual fan speed control switch.

These units can also be installed with special heat exchanger units to heat domestic hot water for showers or the galley.

Another option is to install a water to water heat exchanger between the main engine and the heating loop. This arrangement can allow the boat to be heated by the main engine when underway. Using the Boat Electric designed control system the operation is totally automatic.

When the engine cooling system has excess heat available it will provide heat and the Espar unit will go to standby. Other systems may take too much heat away from the engine, this could result in less efficient operation.

To see more information on specific Espar models, just click on the model number

Model Btu output Amp Draw 12v  (includes pump except where noted) Maximum Boat Length
Hydronic 4

13,700 Boost
11,300 High
5,500 Low

3.3 Boost
2.9 High
1.5 Low
27 32
Hydronic 5

17,000 High
8,200 Low

4.4 High
1.9 Low
30 40
Hydronic 10

32,400 Boost
25,600 High
10,900 Medium
5,100 Low

4.5 Boost
2.4 High
1.4 Medium
1.2 Low
44 50

Hydronic 12

41,000 Boost
33,000  High 
17000  Med1
11000  Med2 
5100  Med3
4100 Low

8.4 amps (Boost)
6.6 amps (High)
3.5 amps (Med1)
3.0 amps (Med2)
2.6 amps (Med3)
2.4 amp (Low)

48 55
Hydronic 16


2.1** 24V
4.4** 12V
50 60
Hydronic 24


3.3** (24V Only) 55 70
Hydronic 30


3.95** (24V Only) 68 80
Hydronic 35


5.0**(24V Only) 72 90

B = Boost Output (intermittent rating) 
H = High Output (continuous rating) 
M = Medium Output (when maximum is too much & low is not enough)
L = Low (Minimum Output)
* Convection type radiators are large and are not practical on most boat installations
Heater only + Pump Current

  Forced Air

Air is heated by the diesel fired burner system and carried via high temperature flexible ducting to the areas requiring heat.

The distribution of heat is controlled by using various duct sizes and in some areas adjustable heat registers may be useable to regulate hot air flow.

The forced air systems utilize one centrally located thermostat to control the operation of the heater, very similar to a forced air system in a home.


Btu output rating

Amp Draw 12v

Boat Size in Feet in
the Northwest
Airtronic 2

7,500 Boost
6,150 High
4,100 Medium
2,900 Low

2.8 Boost
1.9 High
1.0 Medium
0.7 Low

24 30
Airtronic 4

13,650 Maximum
10,200 High
 6,800 Medium
 3,400 Low

3.3 Boost
2.0 High
1.1 Medium
0.6 Low

27 36
Airtronic 5

18,800 Boost
16,400 High
 9,200 Medium
 4,100 Low

6.7 Boost
6.7 High
3.3 Medium
3.3 Low

38 40



46 50

Boost = Intermittent rating. 
High = continuous rating  
Medium = when maximum is too much & low is not enough
Low = Lowest Output (If the temperature continues to increase at low the heater will cycle off to standby.)
(All hot air models turn off when no heat is required to maintain cabin temperature)


Sizing Guide for your application

Method 1

Cubic Foot Area to be heated - Measure the boat interior and use the guide below to determine your multiplier.

Caution - when using this method of estimating the heat requirements, it is important to 

Accurately measure the INTERIOR VOLUME to be heated.
     Do not measure the box that the boat came in.
     Do not measure the space taken up by cabinets, bunks or other permanent structure.


Geographic Location

Limited Winter

btu's per cubic foot


btu's per cubic foot

Live Aboard
All Year 

btu's per cubic foot

Southern California 7 8 9
Northern California 9 10 11
Oregon & Washington 11 12 13
South East Alaska 14 15 16
Alaska 19 20 21

Example: cubic foot area to be heated = 1000 
 Live aboard in Washington = 11 

Estimated btu's: 1,000 x 11 = 11,000 -   Heater size = Airtronic D4 hot air or  Hydronic 4


929 N Northlake Way    #300A      Seattle,  WA. 98103

Est.  1981