Efficiency of Reverse cycle a/c

[REBrueckner]

Stormchasers recent "loss of power" thread reminded me I've been meaning to pose several issues related to reverse cycle a/c efficiency and get myself updated ...I investigated about ten years ago but don't remember details.

(1) I believe that when water temps get down to about 45 degrees or so, reverse cycle is about useless....you are likely running at lower efficiency than if plain old electric heaters are being used. ..true?

(2) at least some reverse cycle air conditioners have internal heating coils....electric heat....and I'm unsure when those activate if the compressor and water pump shuts off...if so, that's not a bad arrangement.

(3) running a/c in reverse cycle mode DOES cause wear and tear on the a/c unit compressors. Hence I prefer electric space heaters.

For these reasons and because reverse cycle has a big power surge when turning ON, I have never liked using them for heat unless its a sudden cold snap in warm waters. Space heaters have always seemed an economical alternative.

 

[Maynard Rupp]

I did not realise that using reverse cycle is tough on our compressors. I do know that 41 degree water is the minimum for usable heat from those systems. We solved that problem by raising the water temp in winter. Yes, the water temp. in Miami never gets near 41 degrees.

 

[stormchaser]

It depends on the system. My trawler had 2 Mermaid brand systems and they worked very well down to water temps in the LOWER 30s...and they did NOT have electric coil heat...jsut GOOD reverse cycle...the air coming out was HOT not just luke warm and could keep the boat in the upper 70s or lower 80s if you wanted...no problem. That's why I'm frustrated with what I've got now...

 

[fissioneng]

I use my Cruisair reverse cycle heat all winter with water temps in the low 40's. I always have PLENTY of heat such that the thermostats turn off the compressors. I don't leave it on during the week, so it takes awhile to heat the boat up (I have engine room heaters also), but once it is warm, it is very comfortable. I don't know anything about the efficiency. I would expect it to be similar to cooling efficiency, but you're right, probably not as efficient as a plug-in resistance heater. Air circulation wouldn't be as good as the Cruisair though, maybe you could just run the fan?

Many houses in our area are heated with heat pumps (usually with auxilary resistance heaters) and they're pulling heat out of 15 degree air at this very minute. That has to be much less efficient than pulling it out of 42 degree water.

 

[MikeP]

No problem with heat and water temps in the hi 30's - no experience below that. As I said in the other thread, air distribution/stratification is a bigger issue to me than the actual heat produced.

Refrigerant charge will affect heating, just like it does cooling. So one thing to check if there is not much heat is the state of the charge. Two of my units will run you out of the boat either heating or cooling. The other two are adequate but not impressive. A quick check last summer revealed that those two are low in refrigerant and should be recharged. It's another of my spring projects.

 

[fissioneng]

(3) running a/c in reverse cycle mode DOES cause wear and tear on the a/c unit compressors. Hence I prefer electric space heaters.

I don't think reverse cycle (heat) puts any more wear and tear on the compressor than running the cooling cycle. If anything, it exercises the reversing valves which have been prone to sticking (see other posts on this) and keeps them working.

Just running the compressor puts wear and tear on it, regardless of which direction the fluid is going.

 

[REBrueckner]

"Many houses in our area are heated with heat pumps (usually with auxilary resistance heaters) and they're pulling heat out of 15 degree air at this very minute. That has to be much less efficient than pulling it out of 42 degree water."


At that low temp, they are running as pure resistance heaters...just like an electric space heater. They are expensive to run at low temperatures!!! Air heat pumps work efficiently in the Carolinas for example, not in Maine.


To determine the heating efficiency of a heat pump use the COP measure COEFFICIENT OF PERFORMANCE for your geographic area with sea water. It varies with source temperature characteristics (air, sea water, underground water,etc) since it is efficient at say 50 degrees but not much at 30 degrees.

Let's say COP is 1.5. that means its 50% more efficient that electric heat which has COP = 1 anywhere.

See: http://en.wikipedia.org/wiki/Coefficient_of_performance for COP details.

 

[Boatsb]

I think the water temp can be down to about 35 degrees before the surface are on the heat exchanger is too small to get enough heat to make a difference. YMMV as the surface area of different units is not all the same ratio to its BTU rating.

I just re insulated the wall in my salon and plan on doing the other this weekend. Problem is the heat from the engine room in he summer would be welcome in the winter. Warm up 2 tons of cast iron for about $1.00 of diesel and keep the boat warm for the night.

 

[Boatsb]

I think the water temp can be down to about 35 degrees before the surface are on the heat exchanger is too small to get enough heat to make a difference. YMMV as the surface area of different units is not all the same ratio to its BTU rating.

I just re insulated the wall in my salon and plan on doing the other this weekend. Problem is the heat from the engine room in he summer would be welcome in the winter. Warm up 2 tons of cast iron for about $1.00 of diesel and keep the boat warm for the night.

 

[Trojan]

i

I use my Cruisair reverse cycle heat all winter with water temps in the low 40's. I always have PLENTY of heat such that the thermostats turn off the compressors. I don't leave it on during the week, so it takes awhile to heat the boat up (I have engine room heaters also), but once it is warm, it is very comfortable. I don't know anything about the efficiency. I would expect it to be similar to cooling efficiency, but you're right, probably not as efficient as a plug-in resistance heater. Air circulation wouldn't be as good as the Cruisair though, maybe you could just run the fan?

Many houses in our area are heated with heat pumps (usually with auxiliary resistance heaters) and they're pulling heat out of 15 degree air at this very minute. That has to be much less efficient than pulling it out of 42 degree water.

I run the same on My 43DC and it also does a good job. For weekends I some times use 2 small propane heaters. I have been going to install a small electric fireplace in the wall on the starboard side of the salon where there is a sliding door over some shelves. The P/O had a dehumidifier installed there.
I was thinking that maybe a person could install a grill or register in place of the salon center hatch and place a heater in the bilge and use it to heat the entire boat. Just a thought.

BILL

 

[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.