Marine Heat pump

[Marc S]

Does anyone have experience replacing AC with a heat pump? How much of the AC installation can be reused? Boat is in FL so AC is the main issue but heat is needed at times, too.

[Scubaseas]

"Reverse cycle A/C" is what they call it on boats . Works the same as a heat pump. They work fine for both A/C and heat.

[BobHorn]

Marine A/C with or without reverse cycle is basically the same unit. If you currently have A/C you can replace the unit with a reverse cycle unit in the same space.

[Scubaseas]

i just replaced my old A/C only Dometic / Marineair 16000 btu unit with a new one that is reverse cycle. $2350 or so for the new unit but plug and play as it's an exact fit. My old one was tired and starting to leak. Plus the circuit board was shot plus. plus. If you are thinking of converting what you have you might be able to but I don't know of any kits for it. By the time you got done with the solenoids and controls it may be close to what a new reverse cycle unit might cost. A lot of people use oil filled electric heaters for heat, just FYI.

[Marc S]

Thanks, Scubaseas! By “converting” I meant mainly re-using hose fittings and through hulls. The AC is showing signs of approaching death, so not looking to replace pumps, condenser, or solenoids. Mainly concerned about the actual installation which we plan to do ourselves.

[Scubaseas]

My Dometic was plug and play. Hoses, ducting, foot print, wiring to control panel was all the same.

[Marc S]

Thanks again. That's the info I was looking for.

[SailingHarmonie]

The only “gotcha” on marine reverse cycle units is that they have a minimum water temperature they can support heating. Typically around 40F. This is likely not an issue in Florida, but is in places further north.

[Scubaseas]

Originally Posted by SailingHarmonie

The only “gotcha” on marine reverse cycle units is that they have a minimum water temperature they can support heating. Typically around 40F. This is likely not an issue in Florida, but is in places further north.

Excellent point. They don't cool all that well once the water goes above 80*F either. Plus the reversing valves are notorious for sticking.

It's amazing what they're doing with land based newer heat pumps but it may be a long while until they tool that up for marine use.

[Tsuru]

We have a Dometic 16500 btu reverse cycle heat/AC and the unit comes complete, just drops in place. Needs wiring and duct in/out. In our location I winterize the unit to prevent freeze damage but it works pretty well up until that time (agreed about 40f). I havn't noticed a huge reduction of cooling with water temps over 80f but I wasn't studying the temp, just happy to have some cool air flowing. Other than my unit gets a bit skunky if it don't clean it occasionally, I've had no maintenance issues.

[SailingHarmonie]

Originally Posted by Scubaseas

Excellent point. They don't cool all that well once the water goes above 80*F either. Plus the reversing valves are notorious for sticking.

It's amazing what they're doing with land based newer heat pumps but it may be a long while until they tool that up for marine use.

It’s just physics. You cannot pull heat or of cold water, and you can not pump heat into hot water. Wishing it were so doesn’t make it so. Even if lots of people wish it so STILL doesn’t make it so.

[fourlyons]

Originally Posted by SailingHarmonie

It’s just physics. You cannot pull heat or of cold water, and you can not pump heat into hot water. Wishing it were so doesn’t make it so. Even if lots of people wish it so STILL doesn’t make it so.

That's not really true, 40 deg f water has plenty of heat. Land based heat pumps pull plenty of heat from sub 0 deg air while consuming less energy, but it takes technology that hasn't yet come to boats.
But perhaps you can't do that with water rather than air due to the physics? Some have talked about marinizing the modern air exchanger mini splits.

[Scubaseas]

Originally Posted by SailingHarmonie

It’s just physics. You cannot pull heat or of cold water, and you can not pump heat into hot water. Wishing it were so doesn’t make it so. Even if lots of people wish it so STILL doesn’t make it so.

There is a decrease in efficiency but cold climate units like are used here in Maine work down to about 13*F. And have no loss of efficiency at 98*F. It's just physics. There's a lot of heat between 40*F and absolute zero.

[Scubaseas]

Originally Posted by fourlyons

That's not really true, 40 deg f water has plenty of heat. Land based heat pumps pull plenty of heat from sub 0 deg air while consuming less energy, but it takes technology that hasn't yet come to boats.
But perhaps you can't do that with water rather than air due to the physics? Some have talked about marinizing the modern air exchanger mini splits.

The physics part is perhaps more in the refrigerant and the system design. The Dometic unit I have is essentially the same size condenser & evaporator as when they designed it to use R12 and then R22. Now they use unscented propane.

The land mounted air source low temp heat pumps use R410 , a much better compressor and evaporators and condensers sized properly.

[Pizzazz]

It seems to me that marine water-to-air heat pumps do not work well in cold waters simply because you can't make the water colder than 1C without freezing (kind of obvious). Thus, if the sea water temperature drops to 5C (typical in the Black Sea), you need to move a lot of outside water to recover meaningful heat if you only have a temperature drop from 5C to say 2C. Therefore water-air heat pumps would not work well here in the winter.

Air-to-air (the popular reverse cycle air conditioner) needs to move a lot more air than water (because the energy density of air is so much lower) but you get a much bigger temperature differential and I believe efficiency is strongly dependent on the temperature differential.

If you will be using heat occasionally and in Florida or warmer, just do a drop in replacement with reverse cycle heat. For us, in the colder climates I would say that a better heating solution would be either diesel heat or if you have the space to install a domestic air-air conditioner hidden from view. Some ideas we have discussed here before is 1) a split type air conditioner if you have the space to hide the outside body; 2) a portable air conditioner in the machine compartment where you need to duct outside air in, air out to the outside, air from cabin and air back to the cabin; 3) van roof-type air conditioner if you can hide it under some furniture on the flybridge.