Cold heat

Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Hi all,

Since we have had some chilly mornings we've resorted to using the AC reverse heater in the aft cabin. We seldom use the cooling cycle so getting some heat is appreciated. At what water temperature do they stop creating heat? Since we are taking a BTU from seawater and returning it to the sea, I'm thinking there has to be a limit. :waycool:
 
Feb 10, 2004
3,918
Hunter 40.5 Warwick, RI
I am able to get heat from seawater as low as 45 degrees. I think there is a fixed rise specification, but I don't have my books with me right now.
 

capta

.
Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
Yep, there surely is. I spent the first winter in RI after I bought the boat, figuring that if I sailed directly to the Caribbean I'd never get the upgrades I wanted to do done. As the temperature dropped, so did the efficiency of my reverse cycle AC's heating. A day or so before I was going to jump into repairing it a dock mate mentioned that it would not work once the water got down to around 40 degrees or so. Whew, what a relief! What was an old tropical bird like myself to know of anything less than 75 degrees, water or air!
 
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
Cruisair says "Once you get below 40°F, your unit’s heating will be running at 50% capacity."
I've never tried mine much in low temps.
 

Gunni

.
Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
You will know when the compressor stays on most of the time trying to pull heat from the seawater, and your boat still isn’t warming up.
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
In my townhouse, the heat pump would only "heat" when the outside temps were above a certain point- somewhere in the 30s, IIRC. I learned, and put a remote temperature sesnsor next to the outsideheat pump unit, and when it show the low temp, I just went to the backup 110 volt heating element. If there is no heat to pump, it just ain't gonna happen.
 
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Feb 14, 2014
7,399
Hunter 430 Waveland, MS
It is the "size and cleanliness" of your "Refrigerant to Seawater" heat exchanger.
Heat is removed from seawater, but falls off when...

Cabin/outside air temperature versus Seawater temperature approach each other.

Most reverse cycle units [landlubber too] are sized for the cooling temperature differences. The heating side normally needs...
backup 110 volt heating element
_____
You are still getting heat from seawater.;)
Jim...
 
Dec 25, 2000
5,702
Hunter Passage 42 Shelter Bay, WA
During the winter months our sea water temp drops to around 40 degrees, maybe a bit lower when we get a snap that causes the water surface to freeze with a thin layer of sheet ice. Even then our heat pump puts out enough warm air to heat the inside of the boat.
 
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NYSail

.
Jan 6, 2006
3,048
Beneteau 423 Mt. Sinai, NY
In the fall the heat works great..... early spring the water is too cold. Our boat just using the aft unit is very warm tonight!

Greg
 

TomY

Alden Forum Moderator
Jun 22, 2004
2,758
Alden 38' Challenger yawl Rockport Harbor
This always surprises me. Residential heat pump efficiency has become very high. We use an Air Source Heat Pump to heat our old house on the coast of Maine along with a pellet stove. The ASHP, while it loses efficiency as the temperature goes well below freezing, it still will produce heat at below zero (F) temperatures. The ASHP is used solely during the shoulder seasons when air temperatures are below freezing as it is so much more efficient than any fuel burning appliance.

I'm curious if there are marine salt water source heat pumps that are more efficient for heating?
 
Mar 3, 2003
710
Hunter 356 Grand Rivers
I can hold 68 inside in mid 30 outside temperatures inside my H356 with water temps in the low 40’s. The coldest we have used our boat was 18 outside. We were moving our boat in early December. We never got the boat above mid 60’s and the water temp was about 45. That is the only time we were pretty cold and never got too comfortable. We find that once the boat has warmed up the inside surfaces and mass, you can keep it very comfortable with temperatures in the mid 30’s outside with low 40 water temps. Trick is not having a cold soaked boat and having to use those BTU’s heating the boat mass.
 
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Dec 25, 2000
5,702
Hunter Passage 42 Shelter Bay, WA
I'm curious if there are marine salt water source heat pumps that are more efficient for heating?
Hi Tom, not sure about efficiency between heat pump brands, but our 1991 original MarineAir units still work quite well on R22.
 

DougM

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Jul 24, 2005
2,242
Beneteau 323 Manistee, MI
I came from northern Michigan where we had good old fashioned warm forced air heating to Virginia where the house is equipped with a “heat” pump. The poorest excuse for a heat source ever invented by man.
 
Feb 14, 2014
7,399
Hunter 430 Waveland, MS
to Virginia where the house is equipped with a “heat” pump. The poorest excuse for a heat source ever invented by man.
Since Heat Pumps have an economical Temperature Difference Design, Latitudes of the world make big difference in performance.

Virginia's Latitudes are at the upper edge for landlubber Heat Pumps, so they are still good, but not as good as Gulf Coast Latitudes.

Most heat pumps, even marine type, use 40-45°F as the maximum difference between the heating source Air or Water AND the desire Home/Cabin for economical sizing their heat exchangers.

Simple example for home:
Desired inside temperature 75°F
75° - 40° = 35°F
So a heat pump will begin to struggle when the outside Air temperatures drop below 40°F.
As they struggle, the desired 75° will begin to drop too.
But they still remove "free heat" from the outside Air.;)
The cost for supplemental heat by fuel or electricity is still much reduced.:biggrin:

Marine systems
work much the same, using water temperature variations, but they are designed more for the Summer time cooling than Winter heating.
Since Water temperatures are LESS variant than Air, the Latitudes that work well are much further North than Virginia.;)
_____
It must be the new refrigerants.
The newest ones claim 10-35% efficiency improvements, but it from the Compressor designs innovations allowed.
Jim...

PS: How do I know this? I just replaced my R-22 compressor [17 yr service]. R-417a was supposedly the newest replacement refrigerant. I talked directly with Tecumseh®, which is MOST marine refrigeration compressors makers. After full consideration of efficiency versus $$, I picked....R-22. ;) Longer story and no thread :hijack:.
 
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Nov 6, 2006
9,884
Hunter 34 Mandeville Louisiana
The real limit on the water cooled units is the freezing point of water.. The heat exchanger must operate below the water temperature in order to make the heat move from the water into the boat (via compressor and refrigerant).. In a bayou, like where my boat lives, that is very close to 32F.. which means that the water must be 8-10 degrees F above the freezing point..to avoid forming a boundary layer of insulating ice inside the exchanger, limiting the movement of heat to the boat... Salt water freezes lower so on the ocean or a salty bay, the heat pump can heat better than it can in fresh(er) water.. The neat thing about a heat pump is that most of the inefficiency in the compressor (both electrical and physical from the refrigerant and its compression) comes out as usable heat in the cabin! Way much better for heat than a resistance heater. Great where they work, but limited by physics.. I love mine because our bayou water rarely gets below about 50F..