I had always thought heat pumps were the perfect answer for heating/cooling on a boat, but never considered one due to their cost. When I saw a two-year old one on craigslist for less than 1/4 the retail cost, I jumped without hesitation.
(An aside to those who do not know how a heat pump works: A heat pump is almost exactly like an air conditioner, but with the addition of solenoid valves which allow reversal of the freon flow. Did you ever notice that a running window air conditioner blows hot air outside? That is the heat which was removed from the cooler inside. If you were to physically turn the unit around in the window, it would become a heater. The solenoid valves allow this reversal of function without having to physically move the unit. And yes, the heat pump is also an air conditioner. One more thing: on a marine heat pump, the heat source/sink is not the outside air, but is instead the water. In Puget Sound, the water temperature ranges from about 55 degrees in the summer to about 48 degrees in the winter, so it is perfect for this.)
Here on the boat, we live on 30 amps. That is, the total amount of power available to us is a little less than what you can get from 2 household circuits. This requires us to be conscious of our electrical usage. With the current price of diesel, it is cheaper to heat the boat with electricity than using diesel in our diesel heater. To do this, we have used 3 space heaters (forward cabin, main saloon, aft head), each set on a 500 watt setting. Thus we were heating with 15 amps, leaving another 15 amps to run the water heater, refrigerator, espresso machine, microwave, lighting, etc. Tho this sounds like not very much heat, it was sufficient until outside air temps dipped into the low 40's, at which time we had to fire up the diesel heater too to make up the difference.
Those 3 500 watt heaters were pulling about 15 amps, and delivering about 5100 BTU/hr. The heat pump, when in heating mode, draws 12 amps, and delivers 18,000 BTU/hr. That is, the heat pump delivers more than 3 times as much heat, and uses less power to do so. That is because not only do we get the energy from the 12 amps drawn from the electrical circuit, but this energy is used to MOVE heat, so we also get the heat captured from the sea water. We also get air conditioning, but that is seldom needed in this climate.
The heat pump is about the size of a window air conditioner, but without the external housing. Unfortunately I didn't take any pictures before I got it into the fwd settee compartment. Because of its size, it was necessary to remove both the door and the bezel from the large opening. It just fit. Next, the unit requires a supply of sea water for its pump. For now, given that we are in the heating season, I stole the connection for the generator, and I made use of an abandoned above-the-waterline thru hull for the discharge. The initial installation just had the giant 6" hot air discharge hose led back out the door. And although this kept the main saloon toasty warm, it left the ends of the boat cool.
So to string 4" flexible aluminum ductwork (think dryer hose) from the compartment to carefully chosen locations for heat registers... I rented a big (BIG) electric drill and a 4.5" hole saw. This is a monster. It takes two hands and all my strength to keep this thing going in the right direction and not jumping out of the hole and chewing up everything in sight. And given that all the duct locations are run thru the backs of the small compartments along the port side of the boat, running the hole saw to make the holes can best be compared to wrestling with a pit bull while under your kitchen sink, with the door closed.
There were a lot of holes to be cut; each took between 30 and 60 minutes to make.
I found the 4 - 4" grills for the cabins and heads on the internet, but the 6x10" grill for the main saloon is made of a conventional grill, with a teak overlay bezel (that I made) in place. Thankfully, the giant silver snake is gone from the saloon.
We are warm and cozy, and when you hang up a towel in the head, it is dry the next time you use it, instead of just cold. The boat has never been this warm, and yet the heat pump runs just a fraction of the time, and is far quieter than the fans in the 3 space heaters. We also now have a smart programmable thermostat which allows us to have the boat cool when we are absent or asleep and warm when we are here, automatically.
OK, you people in houses have had these for a long time. For us it is a new luxury.