Experience with efficiency of marine AC for heating

[billknny]

Originally Posted by Seighlor

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.

You have two choices for getting your AC unit to work as it should:

#1 Install a new one sized to handle the load, OR
#2 Install a deck sun cover.

You will be AMAZED at the difference a sun cover makes. Really. Get one. Stop sweating.

If you are not sure the difference will matter, borrow a tarp or anything else on the next hot sunny day and rig it up temporarily to shade the deck from overhead sun, and especially the hatches. You'll be calling the canvas maker the next chance you get.

[Dennis.G]

Originally Posted by waterman46

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

So very wrong. Heat pump is much more efficient for space heating than resistance heat. It does loose efficiency as outside source temperature (water temperature in this case) drops, but will remain more efficient for most units for temperatures above something like 10 deg F. Highly doubt you will see water temperatures that cold anywhere a cruiser might be.

[David B]

Originally Posted by aybabtme

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?

We have both reverse cycle air-conditioning and a very compact ceramic resistance heater. We use the A/C to cool, and to heat, but frankly, since we bought the little heater (for when we are in the yard getting ready) we use that all the time now for heating. Easier, and saves the aircon compressor and water pump etc for cooling. The heater no doubt is less efficient, but that's still a lot cheaper than replacing an A/C unit that has been doing double time heating as well as cooling.

[MartinR]

Originally Posted by Dennis.G

like 10 deg F. Highly doubt you will see water temperatures that cold anywhere a cruiser might be.

No, hardly, this would be ice Hard to pump through a heat pump.....

My unit is efficient until about 50F water temperature. But my feed pump is marginal. I am sure, with a larger feed pump, colder water temperatures could work.

[Woodland Hills]

We have a large oil filled resistance type radiator that we use in the salon. It’s a lot quieter than the heat pump, so we run it when we watch TV. We use the heat pumps in the galley and the staterooms where the slight noise is not an issue.

[a64pilot]

Originally Posted by Woodland Hills

We have a large oil filled resistance type radiator that we use in the salon. It’s a lot quieter than the heat pump, so we run it when we watch TV. We use the heat pumps in the galley and the staterooms where the slight noise is not an issue.

These are also safe as they can’t get hot enough to catch something on fire. If you must have a heater on board these I think are the way to go.

[CarlF]

The reverse cycle air conditioner is much more efficient than the space heater because it extracts "free" heat from the ocean. In reverse cycle, heat pumps work by sucking in, say 55 degree water and returning 50 degree water over the side. That five degree drop is a lot of heat energy.

In HVAC equipment the ratio of thermal energy produced divided by the electrical energy consumed is called COP (Coefficient of Efficiency). By definition, a resistive heater like an electric space heater has a COP of 1.

A high efficiency gas furnace has a COP of about 2.

A typical air source heat pump like a home air conditioner has a COP around 3.

A water to air heat pump (which is what are in our boats) has a COP approaching 5 (it's actually not quite this good as the small water pump electricity is not included)

So our boat AC systems produce almost four times the heat energy for the electricity as does a space heater. The great efficiency is because water contains much more heat energy than air - and we have an unlimited supply of water.

But the efficiency of standard boat heat pumps do fall as the water temperature falls due to design limitations with the compressor. Starting around a water temperature of 50 degrees they will start putting out substantially less heat and at 40 degrees put out little to no heat.

Air source heat pump compressors for buildings have recently been improved so that the best will heat down to -20F outside air temperature. Massachusetts (and some other northern states) are now offering large rebates to encourage people to switch their home heat from oil furnaces to Air Source heat pumps because they save lots of money in the long run as well as greatly reducing CO2 emissions.

It's false economy to "save wear" on the compressor. Compressors will run just about forever. The part that fails are the water pumps or leaks in the refrigerant lines.

Cheap electric space heaters, on the other hand, rarely last more than about six months of constant use - often failing by catching fire (the oil ones last a good bit longer but the Chinese undersized cords are prone to overheat if the unit is run at the full 15amp power ). Most marine insurance companies prohibit the use of portable space heaters because of this fire risk and will deny a claim if a boat fire is caused by one.

[aybabtme]

Thanks, this was pretty informative! I'll put this project in the "should do" column then.

[GordMay]

Originally Posted by CarlF

The reverse cycle air conditioner is much more efficient than the space heater ...

Indeed!