Boat HVAC Considerations

So, the boat definitely needs an HVAC unit. The original was in place, which as some of you may know, can be challenging to even find out the model and other information - when your boat is several hours away.

Boat HVAC systems are interesting beasts - some have either cooling only, or cooling and heating, but for the most part, are miniature heat pumps like residential units, although in this case, use water instead of ground temperatures (yes, there are also residential water-sourced heat pumps). Of course, they also come in 110/120v and 220v variants - hint: if you’re a US boater unless you specifically know you have 220vAC, assume you have 110v shore power.

OEM unit

The original unit for our 1998 Sundancer 270, and many others, was a 7000 BTU 120v/60Hz Cycle Cruise Air, model SXR7-M127. The model names in most units do imply their BTU (heating and cooling capability), e.g. the R7 = 7K BTU, and this holds true for most replacement units as well.  The dimensions of the OE unit are 21”L x 13” H x 11” D.

You still need to be careful on the models, as there are 110/115v, 210v models, and some with options with and without heat, or having electric strips only. Figure out what you’re replacing and it’s configuration, as well as what you think you’ll need - people up in the midwest or western US may not need both cooling and heating, while here we’re less likley to use heat, but it’s possible as we sort later how later into ~November we decide to keep the boat in the water.

Who makes replacement units?

Cruise Air was bought out by Dometic some time ago, so direct replacement units were a no-go, but Dometic does make a more or less drop-in replacement unit. What does this mean? Well, primarily that the blower outliet is facing the right direction, the unit physically fits in the typically installed location, and it’s compatible with the existing wiring going to the display - which shows current temperature and allows you to set the temperature and use the fan controls, etc. If not compatible with existing wiring, you’ll nee dto re-run new wires, or find a way to make the existing ones work.

The Dometic is available in various output capacities, with the 10K likely being the most appropriate - you don’t want to go up wildly in output capacity as just like for homes, it may become less efficient, but anywhere in the 10-12K BTU range would likely do OK to replace the OEM 7K. A pic of the retrofit model is below, although no, I am not an online store - so the Add to Cart button won’t work.. ;)

This seemed like a good deal at first, basically a drop-in replacement. But - I like to research on bigger $ expenses before purchasing, and from what I saw consistently online, like so many other things today - many claimed to expect to get a couple of years out of the ‘new’ units before they themselves would need replacement. That seems like false conomy right there to me, so I went looking for other options.

Dometic does make a ‘turbo’ version of the same general model line, For approximately another $1000 USD - which somehow seems to not come with the control panel/display, or possibly even the turbo ‘shield’ (a $300-$400 ‘cover’ that quiets the unit). I also couldn’t find defintively if the components were any different or improved that might lead to a better service life on the now-really-not-inexpensive unit, while I did find Dometic installers and similar saying they share many components. Might be a great unit, but I wasn’t biting on that from what I was reading.

Compatibility for replacement units

  • Power and Frequency

    • Most US units for smaller cruisers and below will be 110/120vAC and 60Hz.

  • Physical dimensions

    • Boats are pretty cramped for appliance fitting. The HVAC is under the aft dinette seat on my 98 270 SDA, and on many others. The OE unit dimensions are 21”L x 13”H x 11”D.

  • Blower Outlet Positioning

    • You may find units saying left, right, or top outlets. This is the HVAC unit’s outlet into the ductwork for delivery of conditioned air. If unsure which way you need, a growing number of units offer ‘clockable’ outlets, meaning they can be loosened and repositioned to suit in order to mate up with any existing ductwork.

  • Output levels in CFM (Cubic Feet per Minute)

    • You want to ensure a new unit is at least moving as much air as the unit it’s replacing, with more being generally ok and even desirable, but considering the next bullet as well

  • Output and vent considerations

    • Are you connecting to a 5” duct line, 6”? How many vents are fed off of that? This is in the system specifications for the HVAC system. Generally if keeping the same size or to ‘one step up’ in size, should generally be OK, but consider if you need to buy or make adapters in connecting to your existing duct lines.

  • Power Consumption

    • Electric motors generally have a startup surge current as well as sterady-state power requirements. If you are plugging a bunch of things into shore power, it may make sense to ensure what level of service is provided, and consider before increasing startup or steady state current/power needs significantly. This also applies if trying to run the HVAC off of small generators.

    • There are devices called ‘cold start’ or ‘smart startup’ boxes for a few hundred dollars(USD) that will help tame some of that startup surge current. Some are simply capacitor banks, while others are more complicated. These can help if popping your shore power breaker or unable to start on generator. The Dometic unit seems to be the best option at the moment.

  • Water requirements from your seawater pump

    • Line sizes are typically any of 1/2", 5/8”, 3/4” coming from the seawater pump - can work around this within reason via an adapter, but obviously if your boat has a 1/2” water line and you’re trying to install an HVAC requiring a 1” feed line - your sea pump itself and line are likely not a match. 

    • GPH - Gallons Per Hour requirement for the HVAC unit. If going from a tired sea pump or one suitable for e.g. a 6K HVAC and moving to a 12K BTU HVAC, you may want to make sure your pump can push the right volume of water for the unit.

Nothing beats having the boat in front of you to confirm the sea pump rated GPH, line size, blower size, and even blower outlet location and lines, as well as possibly most importantly, the physical space you have to work with if not able to use an OEM direct replacement.

We’ll look at some replacement contenders in the next HVAC entry..