CTM vs Dometic vs Velair: Which Self-Contained Marine Air Conditioner?

If you are replacing a tired air conditioner or adding one to a cabin, three self-contained lines cover almost every boat we see: the CTM CT series, Dometic’s GT and Turbo GTX, and the Velair VSD units. All of them are single box systems that cool with seawater, and all of them come in the 6,000 to 16,000 BTU range most cabins need. They differ in what they are built to do best, and that is what this comparison uses, based on each manufacturer’s published specifications.

The short answer

• Replacing an old unit and keeping the existing control and cables: CTM CT, built as a drop-in replacement with published cross-references to Dometic, Cruisair, Marine Air, Webasto and Mermaid models.
• Want the Dometic name with a control display in the box: Dometic GT, which ships with an integrated digital display and uses R454B refrigerant.
• Want Dometic with a titanium condenser and more sizes: Dometic Turbo GTX, from 6,000 to 27,000 BTU. The control is not included, so plan for one separately.
• Running on a small generator or weak shore power, or want the quietest start: Velair VSD, with an inverter compressor that ramps up and down instead of starting at full load.
• No generator at all: Velair i09, a 9,000 BTU unit that runs directly on 12V DC.

Whichever brand you choose, the capacity comes first. If you have not sized the cabin yet, start with our marine air conditioner BTU sizing guide.

Side by side

CTM CT: the replacement specialist

CTM built the CT series around the most common job on an older boat: pulling out a failed self-contained unit and dropping a new one into the same space. Each CT product page lists the original models it substitutes. The CT-12, for example, lists the Dometic DTG12 and DTU12, the Cruisair STQ12 and STX12, the Marine Air VTD12K, the Flagship FM12 and the Webasto FCF 12,000 BTU units. CTM also states that the CT series works with Dometic, Cruisair, Marine Air and Micro Air controls, thermostats and cables, which is often what makes a replacement quick: the display on the bulkhead and the wiring behind it can stay.

Construction is built for salt water, with titanium circulating coils, stainless steel bolts and a stainless structural pan with an ABS tray. The range runs from the CT-6 through the CT-10, CT-12 and CT-16 up to 25,000 BTU, in 115V and 230V, with soft start and NMEA 2000 versions in several sizes. If you need a new control, the CTM Elite II is sold separately. Some CT sizes are built to order, so check the availability shown on each product page.

Dometic GT and Turbo GTX

The Dometic GT comes with an integrated digital display for temperature control and system monitoring in every box, so there is no separate control to choose. Dometic designed it as a drop-in replacement for older systems too, with a blower that rotates up to 270 degrees to line up with existing ducting, a composite drain pan with three drain locations that keeps draining when the boat heels, and cushioned vibration mounts for quieter running. All GT units provide reverse cycle heating and run on R454B, a lower global warming potential refrigerant.

We carry the GT6, GT10 and GT16 in 115V and the GT16 in 230V.

The Dometic Turbo GTX is a different buy. It adds a titanium condenser and more size steps: 6,000, 8,000, 10,000, 12,000 and 16,000 BTU in 115V and 230V on R454B, plus 18,000 and 27,000 BTU options in 230V only that list R410A. Unlike the GT, the Turbo GTX does not come with a control, so budget for one separately. Send us your current setup and we will tell you which control fits.

Velair VSD: the inverter option

A conventional air conditioner starts its compressor at full load, which is what trips breakers on weak shore power and strains a small generator. Velair’s VSD units use a compressor with a brushless DC motor driven by an inverter that constantly adjusts its speed: full power to pull the cabin down, then lower speed and lower consumption to hold the set temperature. Velair states the result is no start-up current spike, steadier temperature and humidity, less noise and vibration, and proper operation on smaller generators and weak shore power.

Every Velair unit ships with a digital control and a 26 ft cable, and the Smart models can be controlled from an iOS or Android app. We carry the i10VSD in 115V and 230V, the i16VSD in 115V and 230V, and the i21VSD for bigger cabins at 21,000 BTU.

For boats without a generator, the Velair i09 runs directly on 12V DC from the battery bank, with no inverter in between. Velair lists a maximum draw of 45 amps in cooling and 18 amps in Eco mode, so plan the battery bank and charging around those numbers.

Things to check before you order

• Voltage and frequency. Match the unit to your boat’s AC system, 115V or 230V, and 60 Hz in the US.
• Footprint and ducting. Measure the space and the supply duct size of the old unit. Velair lists 4 inch ducts on the i10 and 5 inch on the i16; the CTM CT-12 calls for a 5 inch minimum supply duct.
• Seawater flow. A new unit may need more water than the old pump delivers. Our marine air conditioner pump guide shows how to size it.
• Control. Decide whether you keep the existing control (the CTM route) or move to the control that comes with a Dometic GT or Velair unit. The Dometic Turbo GTX needs a control bought separately.

Can a CTM unit replace my old Dometic or Cruisair air conditioner?

CTM designs its CT series as a drop-in replacement for self-contained units from the major brands and lists the models each one substitutes. The CT-12, for example, lists the Dometic DTG12 and DTU12, the Cruisair STQ12 and STX12 and the Marine Air VTD12K among others. CTM also states the CT series works with Dometic, Cruisair, Marine Air and Micro Air controls and cables.

What is a VSD or inverter marine air conditioner?

VSD stands for variable speed drive. The compressor runs on a brushless DC motor controlled by an inverter, so it speeds up to cool the cabin quickly and slows down once it reaches the set temperature. Velair says this removes the start-up current spike and lets the unit run on smaller generators and weak shore power.

Is there a marine air conditioner that runs on 12V batteries?

Yes. The Velair i09 is a 9,000 BTU self-contained unit powered directly by 12 volts DC from the battery bank, with no generator or inverter. Velair lists a maximum draw of 45 amps in cooling and 18 amps in Eco mode, so the battery bank and charging have to be sized for it.

Which refrigerant do these marine air conditioners use?

The Dometic GT and the Turbo GTX from 6,000 to 16,000 BTU use R454B, a lower global warming potential refrigerant, while the 18,000 and 27,000 BTU Turbo GTX options list R410A. The CTM CT units in our catalog list R410A. Velair uses R32 on its 115V Generation 2 units, so check the spec sheet of the exact model if refrigerant matters for your service plan.

Still deciding?

See every unit in Self-Contained Marine Air Conditioners, or browse by brand: CTM, Dometic and Velair. Send us the model on your data plate and we will tell you which unit replaces it. In Pompano Beach and across South Florida we also install and service marine air conditioning: call (786) 277-2596 for a quote.