Efficiency of Reverse cycle a/c

[67hat34c]

Use the Detroit heaters, no wear on ac compressors and exercises the engines.

[TedZ]

Well lets see. At 100% efficiency, if - and that is a big if - you have 35 degree water and you would like a measly 15,000 BTU/hr out of your heater
that is going to require 5000 pounds of water per hour or 600 plus gph...10 gpm. At 50% efficiency, 20 gpm. The assumption here is that water freezes at 32 degrees. I know if kept moving you might get it to 31. And that is one heater. Many have more than one all serviced by the same pump. Fact is the equipment doesn't have the capability to make heat in those conditions.

The problem with the numbers is we rarely have 35 degree water that has a boat in it. If it does it has bubblers and they are pulling 40+ degree water from lower depths. I go with the folks that say without red heat somewhere either in the system or stand alone heaters, below 45 degree water, your heaters don't make much heat.

The COP is the inverse of the efficiency in our systems. They are both "what you get"/"what you bought" one way or the other and throwing in 100 to get percentages if you like them.

Ted

[stormchaser]

Although your numbers make sense...I personally know several people who are getting 90* air out of their reverse cycle at our marina when water temps are in low 30s (surface has skims over with ice at night, defrosts in the sun).

[krush]

To get an exact answer, it is a lot more complicated than ya'll will read on wiki. Operating pressures, coil service area, mass flow, etc all come in to play when figuring out your temps and efficiencies.

Below a certain temp it just becomes too difficult to pull heat from one medium and transfer it to another. Home heat pumps will have a COP curve that is determined a large part by outside temp. As the temp drops, the effective COP will, all else equal, go down.

As mentioned above, if your system isn't charged correctly and not operating correctly, then you have no chance of getting good heat. Also, wear and tear on compressor is almost nil when it is running--starting and stopping them is much harder on them. They are sealed and live in a bath of oil.

I'd be skeptical of claims of super hot air coming from 30 degree water. Are ya'll positive there are no aux resistance coils in there?

Overall, a heat pump is going to use a lot less power than resistance heaters.

[jiminhixson]

Could you not use a heating tape wrapped around a copper coil and run the supply water through it. You can buy a unit that can control the temp and use that to raise the supply water to above 45. This could be turned on only when needed for extereme temps.

Wouldn't that eliminate the need for space heaters if the Reverse-cycle system can keep up better? Even if you just raise the temp of the water 5-10 degrees, I would think it would be much more cost effecient using two or three space heaters through out my boat.

I really have no idea, it was just a random thought that started to grow on me. Any input would be appreciated.


Thanks.

[REBrueckner]

Could you not use a heating tape wrapped around a copper coil and run the supply water through it.

It would be far more efficient just to use the heat tape inside to warm your boat...most of heated water would be discharged plus you are running the compressor and pump.....5% efficiency would be about what I would guess...

for long term reverse cycle heating, you'd want a unit that is efficient as possible (has the highest COP) and that switches to a resistance heating coil when COP drops to about one (1).

The major advantage of diesel or propane or any other fuel based system is that if shore power is lost it takes only minimal electric energy....only battery power with many systems....as most of the heat is derived from the energy content of the fuel itself....

[stormchaser]

I'd be skeptical of claims of super hot air coming from 30 degree water. Are ya'll positive there are no aux resistance coils in there?

I am absolutely positive that there was no additional electric heat. In the case of my previous boat, they were Mermaid brand units and they don't even offer electric heat and even in the middle of a cold snap, we could keep the boat like a sauna if we wanted. In the case of our friends 40' sailboat...they are geting 90* air out as of yesterday and the water is very cold (skim of ice). Most of the live aboards at our marina seem to still be using reverse cycle (see the water coming out).

[MikeP]

90? WOW, that's impressive! I thought that the two "better" units on our boat were pretty good but when I was there in December they ran a LOT to keep the boat in the mid-70's.

But then again, perhaps they could do better, the POs records, which are extremely complete, do not show that the units were serviced at all during the 14 years he owned the boat and it seems unlikely they were serviced in it's first 10 years. So they have yet to be touched (other than some trigger replacements) which is not bad for 29 year old units. This year they will be checked/charged but I would never expect them to be able to keep our 53 comfortable in the middle of jan/feb. But it doesn't matter because fortunately, I'M NOT THERE!!!

[stormchaser]

Oh, his boat is in the 60s but the air output is 90*. He has a very leaky/drafty boat.