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Old 24-11-2019, 04:28   #1
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Experience with efficiency of marine AC for heating

Hey folks,
I'm looking at cooling and heating options, and I'm considering adding a marine AC with reverse cycle.

So far we're not really needing any AC for cooling, but we might one day as we plan to cruise further south this year. Right now we run a resistive heater at night, year round, because it's a bit too cold to be comfortable. I was thinking, the heat pump on an AC is probably more efficient at heating up the boat a little bit, than the space heater with resistive elements.

If you've used both, what's your experience with that regard? Is my assumption right?
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Old 24-11-2019, 08:48   #2
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Re: Experience with efficiency of marine AC for heating

A reverse-cycle heat/cool unit will cool a boat well, but it has some limitations when it comes to warming the boat. As the water temperature drops towards about 40F, the system begins to lose itís ability to heat (just when you need it most).
If, however, your local water temperatures remain between about 40F and 90F (which I'd expect in Ca), it could be a good solution.
See ➥ http://www.cruisersforum.com/forums/...te-123829.html
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Old 24-11-2019, 08:58   #3
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Experience with efficiency of marine AC for heating

Heat pumps are far more efficient as they move heat and not create heat.
That is why you see them down South, but not up North, most of the time, down South itís not too cold for them to work, while most of the time up North it is.
However often times a heat pump moves a lot of air but provides a gentle warming, where a furnace etc often move less air but at a much higher temperature.
People from up North that move South often donít like heat pumps, they want to feel that hot air, and a heat pump often doesnít do that.

But for electric heat and energy conservation, they are the answer.
Many heat pumps are equipped with resistance heating strips for when the air is too cold for a heat pump to work efficiently.
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Old 24-11-2019, 09:02   #4
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Re: Experience with efficiency of marine AC for heating

Gord, that thread you linked to seems to be an air to air heat exchanger heat pump. Not an air to water like our boats are.
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Old 24-11-2019, 09:47   #5
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Re: Experience with efficiency of marine AC for heating

Quote:
Originally Posted by a64pilot View Post
... However often times a heat pump moves a lot of air but provides a gentle warming, where a furnace etc often move less air but at a much higher temperature.
People from up North that move South often don’t like heat pumps, they want to feel that hot air, and a heat pump often doesn’t do that.

But for electric heat and energy conservation, they are the answer.
Many heat pumps are equipped with resistance heating strips for when the air is too cold for a heat pump to work efficiently.
The ideal heating/cooling system would not be noticeable.
Not hot/warm, not cold/cool, just "comfortable".
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Old 24-11-2019, 10:26   #6
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Re: Experience with efficiency of marine AC for heating

A reverse cycle AC is not nearly as efficient as running a direct heating element such as found in the space heater. Running AC in reverse cycle puts a lot of un-necessary hours on an expensive piece of gear.

You didn't mention what type of AC you have. Normally the outlets of installed AC units are located high up in the cabin so the cool air can sink down as it cools the interior. Running the reverse cycle for heating puts warm air in the top of the cabin interior, where it wants to stay and won't make you feel warmer. In this case you may need a small fan near the overhead to force some warm air down to floor or bunk level.

The little space heaters are great;we have two of them. Move them around to exactly where you need them, keep your feet warm when sitting down in the cabin.
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Old 24-11-2019, 11:18   #7
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Re: Experience with efficiency of marine AC for heating

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A reverse cycle AC is not nearly as efficient as running a direct heating element such as found in the space heater.

You have a reference for that, because I believe you are incorrect.

Now before you state that electric resistance heating is 100% efficient you need to know that a heat pump is up to effectively 300% efficient.
I know thatís not possible this article explains how.

https://help.leonardo-energy.org/hc/...s-a-heat-pump-
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Old 24-11-2019, 11:53   #8
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Re: Experience with efficiency of marine AC for heating

Personally I don’t care about “efficiency”. What I want is the boat temperature to be controlled. Reverse cycle AC is GREAT to have and definitely worth having in hot zones. If you are going to live in the frozen tundra world you need diesel heat.
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Old 24-11-2019, 12:14   #9
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Re: Experience with efficiency of marine AC for heating

I just winterized the AC system, when the water temperature got down to 37į it wasn't putting out much heat. I use the reverse cycle units to warm up the boat and then the built-in electric heaters will maintain it pretty well. It was 6į overnight about 2 weeks ago and the boat stayed 65į to 67į with the electric heat, 1 in solon and a 2nd in forward berth with a small 12v fan to help circulate heat.
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Old 24-11-2019, 13:30   #10
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Re: Experience with efficiency of marine AC for heating

Antoine,

We have several different heating systems:
  • Hydronic from engine
  • Forced air diesel [Espar Airtronic D5]
  • 3 Heat Pump air conditioners [AC] with resistive heating elements [for when the water is too cold (~42*F) for the heat pump to operate efficiently... more below]
  • Portable electric forced air resistive heat units [These 2 units by Calframo satisfy our insurance company... Engine room; living space heater]
  • Portable, low profile oil filled electric radiators- convection heaters with built-in timer [very efficient, and our favorite; best for fringe seasons and spot supplementing during cold spells when at the dock]

They are not all used at once, and each have their merrits and efficiencies in various use cases. e.g., During a cold snap [say single digits Farenheit] our diesel forced air keeps the boat comfortable at anchor. If at the dock [or at anchor running our 10KW generator] we will use electricity [50A 230V AC service] and run the AC heat pumps because they cost about the same to operate as the diesel, and we aren't putting hours on the diesel heater, nor having to refuel...

If the heat pumps don't cut it, we switch to the resistive heating elements in the AC units. While on paper the resistive elements are less efficient per KW of heat produced vs. a heat pump, resistive actually likely costs less to run [ittermittant cycling] than a heat pump running continuously because it cannot produce enough heat in colder water temps.

To ameliorate the affects of cold water on the heat pumps a couple of years ago I experimented with plumbing the AC raw water coolant loops to one of our two potable water tanks [110 gallons each]. Since the tank is low in the hull, but still exposed to the heated portion of the cabin, the temerature of water in that tanks stays warm enough to maintain heat pump efficiency... [We are currently in Pacific waters @ 56įN in SE Alaska. The ocean water rarely freezes around here...]

If you are interested, there are more details about using potable water tanks for this purpose in this post, which also includes a reference section to several other related posts on the topic of living on a boat in cool climates...

Best wishes sorting it all out to best suit your needs.

Cheers! Bill
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Old 24-11-2019, 13:51   #11
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Re: Experience with efficiency of marine AC for heating

Electric blanket to pre heat the bed n balnkets!
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Old 24-11-2019, 15:05   #12
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Experience with efficiency of marine AC for heating

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Originally Posted by Disailor View Post
Electric blanket to pre heat the bed n balnkets!


A twin size electric blanket uses about 25 to 40W on medium, less on low, and low is plenty with a blanket on top of it.
25 W is 2 amps, most of us can carry a 2 amp load overnight, especially as my fridge isnít working nearly as hard in the cold, so I figure itís only another amp.
We run one when itís cold.
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Old 24-11-2019, 17:09   #13
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Re: Experience with efficiency of marine AC for heating

This from a dumb Florida bystander: When the sun is up, the A/C on my boat does a miserable job of cooling; nighttimes are a bit more comfortable.

But when it's cold... wow... The heat cycle will make you want to open the hatches.
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Old 24-11-2019, 18:18   #14
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Re: Experience with efficiency of marine AC for heating

Make sure you size it correctly. Too small is better than too big. We had a 24,000 BTU unit on our Catalina 30. It was bigger than I wanted, but they did not make a 20,000 unit at the time and I had heard a 16,000 did not cool a 30 at all in the summer.

The 24,000 would cool the boat 20 degrees in 20 minutes, but it cycled too fast. Since it had a minimum run time, it often went way beyond the target temperature (both heating and cooling) and did not dehumidify very well.

We eventually replaced it with a 16,000 BTU unit expecting to need a separate 9,000 for the v-berth. The 16,000 takes about 4 hours to cool the boat to 72 degrees when it is in the high 90's, but once there it can maintain it. I have to be more careful about keeping the companionway closed now however. A boom tent also helps when it is really hot out.

The above post is correct, just like a home heat pump there are limits as to how cold (or hot) the water can be and still heat/cool the boat. Plus unless you have a genset, you will only be able to run the A/C when you have shore power. I did see an ad once for a 12v A/C, but it not have many BTUs and was still an amp hog.

We are in Atlanta, GA.
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Old 24-11-2019, 18:28   #15
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Re: Experience with efficiency of marine AC for heating

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Originally Posted by waterman46 View Post
A reverse cycle AC is not nearly as efficient as running a direct heating element such as found in the space heater. Running AC in reverse cycle puts a lot of un-necessary hours on an expensive piece of gear.
This is just plain WRONG. Basic thermodynamics allows a heat pump to be a lot more efficient--within the temperature ranges it is designed for. Sorry Waterman, but physics is NOT on your side on this one.

Any decent AC unit should easily last 10,000+ hours so the "extra" hours as a heater really aren't a major issue. I mean, you bought it to keep the boat comfortable, right? If you didn't run it in cooling mode OR heating mode it might last forever!

But... on a boat who really cares? You are either plugged into the shore power grid, and the difference in cost between heat pump and resistance heat is tiny, if the marina meters the power at all, or you are running a genset, and the difference is again lost in the other costs.

Nobody is going to run a heat pump or AC or resistance heater from a normal cruising boats batteries for any significant time, so efficiency really isn't the issue here.
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