Reliable and Affordable DC air-conditioning options
It seems that there’s an ongoing quest in some circles for the holy grail of boating comfort: DC air-conditioning, either 12v or 24v. Yet few seem to be aware of the more practical, but far less complex and expensive alternative - 115v AC air-conditioning powered by batteries through an inverter.
An inverter is a device that changes battery power into the mains power that you have in your home or the shore power on your boat. So many boats these days already have inverters installed, many of which will be combination inverter/chargers, and there’s a wide range of inexpensive models available that are suitable for powering small marine air conditioners.
This article will focus on small air-conditioning systems that are suitable for sleeping cabins or for small boat applications. With a small air conditioning unit, the DC current draw is low enough that they can usually be powered by an engine alternator whenever the engine is running, as well as just from the batteries when required. This is true of both straight DC air-conditioners as well as 115v or 230v AC units powered through an inverter from batteries.
Of course, any sized air-conditioner can be powered by batteries through a suitable inverter, even a 16,000 Btu model, but that will be drawing around 120 amps from a 12v battery, so it’s just a matter of practicality.
To better understand the differences between the two concepts, some comparisons are shown below between the Climma Compact 4,200 Btu 115v AC unit and the Dometic Cuddy II 3,500 Btu 12v DC model. Both models’ Btu output is specified at ABYC rating conditions, and both come from well-respected manufacturers.
There are other DC compressor-based units available, but either their specifications and documentation give insufficient information, or they are not suitable for small marine applications.
1. Component complexity - The Climma 115v AC unit is comprised of rugged, well tested, off-the-shelf components that are made by the gazillion and are very familiar to service technicians. In contrast, the Cuddy II 12v DC components are highly specialized, especially the compressor that requires a sophisticated electronic controller in order to operate. This item alone, we’re told, will cost around $1,500 to replace, if you can find one.
2. Power draw - The Climma 115v 4,200 Btu unit requires an inverter with minimum power output of 600 watts, but a 1,000 watt model is recommended. Even taking into account losses through the inverter, the Climma unit uses only slightly more power than the much smaller Cuddy II DC system. Both systems use small 12v DC sea water pumps drawing one amp or less, with the Climma model powered through its own AC to DC converter.
3. Power supply wiring - The Cuddy II 12v unit requires large gauge battery cables to handle the heavy current demand and to prevent excessive voltage drop. The further the unit is from the battery connection, the larger the cables have to be. Now, battery voltage is never constant (unless there is a charger operating), and as the battery voltage falls, the current draw has to increase to compensate and keep the power level steady, (remember, watts = volts x amps). That increase in current results in more voltage drop through the cables, which in turn speeds up the rate of discharge from the battery in an accelerating voltage death spiral. Installing large DC cables is the answer, but this is typically both tricky and expensive, with the cable alone costing in the region of $75 for a 15’ wiring run.
The Climma 115v unit however, requires only small gauge AC wiring between the inverter and the unit. This is easy to run, inexpensive, and as it carries AC power, its size is not dependent on the length of the cable run, as with DC cabling. Inverters should be mounted close to the battery in order to keep large cabling runs to a minimum, and consideration should be given to the fact that an inverter will maintain a steady 115v AC output unaffected by falling or varying battery voltage.
4. Heat/Cool - The Climma 115v unit is available as cool-only or as a reverse cycle heat-pump that produces great heat down to a water temperature of 40F. The Cuddy II, along with most other DC units, is simply available as a cool-only unit.
5. Starting surge - The Cuddy II boasts of having a reduced compressor starting load, which is a factor of the compressor design and the electronics that drive it. But the diminutive compressor on the Climma 4.200 Btu unit has such a low starting surge that this is not an issue. The fact that a pocket-sized 600 watt inverter has no problem starting the Climma compressor is testament to that.
6. Cost - Now we’re down to the nitty-gritty. DC air-conditioning systems come with a hefty price tag. The cool-only Dometic Cuddy II retails for just under $5,500.00, and that’s just for the unit and control, without the pump, cables, hoses, ducting, etc.!
By contrast, the Climma Compact 4,200 cool-only with control retails for less than half that, under $2,000, and just slightly more with reverse-cycle heat (check here for current pricing). If the vessel doesn’t have an inverter already, a good 1,000 watt pure sine wave model will cost less than $300, and with the added benefit that it can also power other 115v AC devices like a blender, microwave, coffee maker, etc.
So, DC air conditioning may at first seem to be the coolest thing around, but not so hot when price, performance, practicality, and serviceability are taken into consideration.