running your reverse- cycle heat pump below 40 degrees?

[MitchM]

some of us have the air cons w reverse cycle heating capacity. the temptation is to run the unit in heat mode when temps get chilly, especially if your marina forbids plug in electric heaters.


if you do use reverse cycle heating when temps fall in the 40's, -- here' s a scary article on what could happen to you and your $$$ heat pump...

A Heated Conversation

Using a reverse cycle marine air conditioner to keep your boat warm is a fine idea, but when the water temps get to freezing temperatures, can you still use this heat pump?

coastalclimatecontrol.com

 

[jssailem]

Nice post Mitch. Helpful for the winter boaters.

 

[kloudie1]

We can use the heat pump here in winter in the marina because the water in our bayou rarely falls below 50 F ish.

 

[Terry Cox]

Hi Mitch, our boat came with dual heat pumps that come in very handy during these cold winter days while connected to shore power. It is amazing how these systems can efficiently extract heat from 38-40 degree water to warm the boat.

Our waters seldom go below 40 degrees, but do occasionally by a degree or so, but not down to the temps the NE experience. If water flow is interrupted for a period of time (especially when someone forgets to open the seawater valve before starting the system), that will allow the evaporator to freeze up and cause equipment failure. Even on the coldest of days, our sea water might develop a thin sheet of ice, maybe a quarter inch at the most, but the water temps below at the through hull level will remain somewhat warmer.

Checking the sea water temp sensor display located at the nav station before starting the system informs me whether to turn the system on or not. Normally our winter sea water temps range in the 45-50 degree area.

 

[Don S/V ILLusion]

I’m trying to figure out how a heat pump can depress the water column temp being drawn into the system sufficient to create the problem postulated. It seems to me to be an impossibility that 40degree water circulating through the system at some significant flow rate depending on pump size could ever freeze in the coil.

 

[kloudie1]

Problem is boundary layer.. a thin layer of ice will form in the boundary layer of slow velocity water then it gets thicker until the velocity is high enough in the tube to keep the ice from getting thicker.. at that point, the layer of ice may be insulating the heat exchanger so that it is very compromised and can't further extract a useful amount of heat from the water.. Probably wouldn't freeze solid.. Same as taking something out of a freezer on a warm day.. ya get frost on the outside in the boundary layer of air..

 

[capta]

When I first bought Skipping Stone I decided to spend the winter in the water in RI, getting things done that I'd not get done playing in the tropics.
Fortunately, one of my winter dock mates warned me that in my reverse cycle heating wouldn't work once the water got cold enough, or I'd have spent days trying to fix something that wasn't broken.

 

[Don S/V ILLusion]

Problem is boundary layer.. a thin layer of ice will form in the boundary layer of slow velocity water then it gets thicker until the velocity is high enough in the tube to keep the ice from getting thicker.. at that point, the layer of ice may be insulating the heat exchanger so that it is very compromised and can't further extract a useful amount of heat from the water.. Probably wouldn't freeze solid.. Same as taking something out of a freezer on a warm day.. ya get frost on the outside in the boundary layer of air..

not laminar flow unless by complete coincidence

 

[kloudie1]

Boundary layers exist in both flow regimes.

 

[Scott T-Bird]

I used our R/C quite a bit in November on nights that went into the 40's. Water temp never dropped below 50 while we had the boat in the water until early December. On the cooler nights, it had to run a lot to keep the cabin at mid 60. The thermostat seemed to kick on when the temp dropped 2 d F from the setting. The blower is loud. I can't see how it is a good way to heat as air and water temps drop much further. But it did feel like a very nice luxury while I was using it!

 

[All U Get]

Electric heating pad for the bed, my last chance for a smoking hot body.

 

[Terry Cox]

Electric heating pad for the bed, my last chance for a smoking hot body.

Hi Tom, got the admiral a 12V DC heating blanket for the boat while she remains in bed. Only takes about four amps and gets warm, not hot. Nice while on the hook during those winter nights.