Marine Air Conditioning Not Heating
Reverse cycle heating works by running the refrigeration cycle backwards, taking heat out of the sea and putting it in the cabin. Which means it depends on the sea being warm enough, and on a valve that only gets used a few months a year.
No heat: First Checks
Not urgent. What is the seawater temperature? Below about 7 °C a reverse-cycle unit legitimately gives up, and nothing is broken. In Australia that is Hobart in August and almost nowhere else.
Where this sits on the seawater loop
- Seacock
- Strainer
- Pump
- Condenser
- Evaporator
- Discharge
- Condensate
The reversing valve sits inside the refrigerant circuit, which is the half you are not licensed to open.Marine air conditioning is water-cooled, and that is what makes it different from a house. The full cutaway is in the guide.
No heat: Diagnostic Steps
Work down it rather than picking the step that sounds most likely. The order is chosen so that the cheap, common causes are eliminated before the expensive, rare ones — which is also the order that stops you paying somebody to clean a strainer.
Confirm the unit is actually a reverse-cycle model.
Not every marine unit heats, and a surprising number of owners discover this in their first cold snap. Check the plate, not the brochure.
Check the seawater temperature.
Below roughly 7 °C there is not enough heat in the sea to move. This is physics, not a fault, and it is why southern boats often carry a diesel heater as well.
Listen for the reversing valve when you switch to heat.
It makes a distinct clunk and a whoosh as the refrigerant changes direction. Silence means the valve is not being commanded or is stuck.
Give it ten minutes before judging.
Many units delay the changeover, and some run the fan without heat for several minutes first. Impatience diagnoses a lot of healthy systems.
Feel the overboard discharge in heating mode.
In heat the discharge should be cooler than the sea, not warmer — the boat is taking heat out. Warm discharge in heat mode means the valve has not changed over.
When to Stop and Call a Marine Technician
A reversing valve that is stuck or has failed is a refrigerant-circuit component. Diagnosis is licensed work.
Handling refrigerant in Australia requires an ARCtick licence under the Ozone Protection and Synthetic Greenhouse Gas Management Regulations, and fixed 240 V work on a vessel is licensed under AS/NZS 3004.2. Both exist for good reasons. Everything above the line is yours; everything below it is not.
Possible Causes of No heat
Seawater too cold for the cycle
Nothing to fix. A diesel or reverse-cycle-plus-heater combination is the answer in Tasmania.
The unit is cooling-only
Check the plate. Replacement is the only route to heat.
Controller not commanding the changeover
Check the mode setting and any seasonal lockout in the controller menu.
A stuck reversing valve
Licensed work. Sometimes freed, often replaced.
A failed reversing-valve solenoid coil
The cheap half of that fault — the coil is electrical and replaceable without opening the circuit.
Ranked by how often each one is the answer on Australian boats, not by how often it appears in a service manual. The seawater side dominates this list because our water is warm, our marinas are full of growth, and the strainer is the part nobody opens.
Reporting This Marine Air Conditioning Fault
Tell us the boat, the unit if you know it, and what you found at each step above. We will tell you what we would check next, and whether it is worth a technician's call-out or a new machine. We do not charge for that and we will say if the honest answer is that the unit has had its life.