We occasionally get asked whether it is possible to run a reverse-cycle marine air conditioner (heat-pump) when the water surrounding the boat is at freezing temperatures. The answer to that is “yes, it is indeed possible”, but is it a sensible, safe, and reliable way to heat a boat in those conditions?
Most of these inquiries come from live-aboards up north, although we may well be hearing from some in Florida and other southern states in the days to come once this arctic blast hits down there also. We hear that many marinas are now stipulating that plug-in heaters are prohibited, citing insurance issues, including even the ceramic and oil-filled radiator types, which seems a little overly precautious to my mind.
For a really cozy cabin you could install a diesel-fired forced air or circulating water system, but these are expensive and complex installations that require strict adherence to the manufacturer’s instructions in order to prevent potential ventilation, exhaust, and noise issues. Then, they sit idle for half the year.
So, your vessel is sitting in freezing/frozen water and you want to use your reverse-cycle air conditioner to heat the boat. You won’t get that much heat out of it, but once it’s up and running and you have a warmish cabin, you should get around 30F higher air temperature at the air outlet than at the inlet. Just don’t stop it once it has started, as it will take forever for the system to start producing any useable heat again. A cold compressor will suck up all the heat being produced by the system for the first 15 to 30 minutes until that heavy hunk of metal has had a chance to heat up.
Manufacturers of marine air conditioners typically advise not using their reverse-cycle products in heat mode when the water temperature is below 40F. At that stage there is not much heat left in the water, and by taking heat from the water to heat the cabin (that’s why they’re called heat pumps) we’d be reducing the temperature of the water further and getting close to its freezing point.
If the water happens to freeze in the coils of the water-to-refrigerant heat exchanger it could possibly expand and rupture the cupro-nickel tubing, which would be a terminal and most inconvenient and unwelcome event. And if the heat exchanger somehow manages to cheat death, even though the water in it is frozen solid, the already-low suction pressure of the compressor will be lowered even further, which can lead to dangerous and possibly fatal stresses on the compressor due to an elevated pumping ratio, lubrication issues, excessive heat, etc.
There is one little issue to consider when running water-cooled reverse-cycle heat-pumps in heat mode. If the water flow stopped for any reason, i.e. blockage, pump failure, freeze-up, etc., the system has no way to detect such a potentially calamitous situation unless a low pressure cut-out is installed, which is rare.
The high-pressure cut-out, standard on all air conditioning units, is only effective in cool mode, and there will be no increase in current draw that might otherwise trip a breaker. The result will be that the compressor will just keep running and running and running until either it self-destructs or someone comes home and investigates the cause of icicles in the companionway.
If you were paying attention during science classes at school you will remember that water is at its densest at 4 deg C (39F) and is the least dense at 0C (32F). For dense, read heavy, so when the water at the surface is freezing or frozen, the water at the bottom of the creek will be 4 deg C or thereabouts. So if we could get at that water on the bottom, then we could use it for our heat pump at much less risk of equipment failure than using water at or near the surface, and there are two common ways to achieve this.
One method is to lower a submersible pump down to the bottom and pump the water up from there, but this is fiddly and problematic as the pump is prone to ingesting growth and debris without adequate protection.
A better method is to dangle a submersible de-icing motor/propeller combo under the boat that will gently waft that warmer water up from the bottom and around your boat and into your air conditioning pump intake while keeping ice off the hull as a bonus.
There are other possible solutions:
1) The “Power Plumber” approach. I have heard that there are some enterprising folk out there that have modified an electric water heater to make a closed loop system thereby negating the need to use sea water altogether. This is a complicated and technically challenging approach that is not for your average live-aboard. Don’t try this at home (i.e. boat).
2) Install air conditioning units with electrical resistance heat, or add a Duct Heater or Auxiliary Heat Module to an existing reverse-cycle unit’s ducting system - For live-aboards in frigid areas where there is strict enforcement of the no plug-in electrical heater rule, installing an air conditioning unit with an electrical heating element in place of the reverse cycle mechanism may be a workable solution.
The same may be true of add-on heat options. The down side is that when heating in less-than-Arctic conditions, electric heat will be less efficient and effective than reverse cycle heat; around three times less heat for the same current draw as a reverse-cycle unit. This is especially noticeable at the end of the summer season when water temperatures drop far more slowly than air temperatures and reverse cycle units will be kicking out boat-loads of heat.
Whichever way you try to beat the freeze on board this winter, just be aware of the potential drawbacks and dangers, and always have an emergency plan ready to implement should the elements win. Like when the power goes out on the dock during a blizzard...
Image graciouly provided by Rob Lamb, Ontario Canada of his ketch Goshawk. See more from him on YouTube.