Heat pump - Reverse cycle A/C experiment update

[robert644]

So I received my Heat pump - Reverse cycle A/C unit on Tuesday and set it up this morning in the boat to see how it would go.

Well, after 6 hours, not much. I made a quickie test installation and got the thing going. I expected this thing to blast me out with heat, but not so much. The boat was warmer than outside, but not as hot as I wanted it to get.

I'm going to leave it on all night on auto mode, which heats to 70 and then switches to dehumidify when that has been reached. The unit may be working well and may be adequate, but under today's test circumstances not so much.

I'll really know once I start living on the boat, for now there is still alot of work to get even there.

http://i.imgur.com/uRT114d.jpg

[Greenhand]

In general, the heat pump system does not make a drastic difference between the air temp and what it puts out. If you are within its operating parameters, it will slowly put out warmer and warmer air. Blasts of warm air are just not a thing for heat pumps.

[robert644]

Well, I have the hatch open in front, for drying out, but I will close up the boat tonight and see if it gets to temp in the morning. It's a 37 foot boat and temps are supposed to get to 34 in the night. 32 right now. It's supposed to rain, or be cloudy or sunny tomorrow, so I'll see what happens.

Maybe I'll crank it up to 80 and see what that does overnight. Might help the boat out, I was at it with a brush and hose this morning inside.

[Greenhand]

Is your heat pump even rated to work at those temps? They generally aren't very helpful at anything less than ~40F.

[robert644]

The operating temperature for heating is listed as 23f to 89f. I suspect older units didn't go as low.

[s/v Jedi]

If it's water cooled, then it is the water temperature that counts, not the air temperature.

Basically, the heat pump takes the heat out of the cooling medium and puts it inside the boat. You need the water to be at least 40F for it to work good. For air cooled, the air needs to be 40F. Below those temps, one can argue that it still works, but it won't be practical/enough.

[Talbot]

When I looked at these, the consensus was that water temperature needed to be at least 50 deg F, and preferably + 56 deg F.

[captainbri]

Originally Posted by robert644

The operating temperature for heating is listed as 23f to 89f. I suspect older units didn't go as low.

ive never seen a heat pump work worth a damn under 40.. ive been in hvac for 7 years. If a customer is adamant about switching from a gas furnace to a heat pump. We install electric heating coils because heat pumps suck when its cold.

[CarlF]

As usual, marine equipment is well behind the state of the art. Residential air-air and water-air heat pumps are now available that will operate below -15 C (5 F) input temperature. They also are much more efficient than, for example, my MarineAir unit. Typical COP's of the new units exceed 4 (COP is a ratio of electricity to BTUs produced - the COP of an electric space heater is 1.0).

http://www.daikineurope.com/docs/ECP...re%20split.pdf


The unit in your picture is clearly not a marine unit so it could well be using the newer technology.

Two things to keep in mind:

1. Heat pump designers have to pick a range of input temperature and output temperature. If you want to design for cooler inputs you also design for cooler output. Thus, the air doesn't seem very hot on your hand.

2. Heating is a much tougher task for heat pumps than air conditioning. With air conditioning you are trying to lower cabin temperature from 32c to 25c (90F-78F) - just 7 degrees C. When heating you are raising temperature from 0c to 21c (32F-70F) - 21 degrees C. Your unit may simply not have this capacity especially if your boat is not well insulated.

[robert644]

Thanks for the replies, I haven't checked on the unit today with the soup de jour being water from sky. However when I brave it outside I'll post an update.

The unit is a new model. No older a design than 2010 I am sure. I knew it would have trouble heating, I remember the old reverse cycle A/C units in hotels. I just want to see what '14,200' BTUs of heat look like after 12 hours in the boat.

-----------------------

After 12 hours of heat, the boat is colder that it started out at. Outside temp is 32 degrees. Humidity is 99.99%. Since I just described 180 days of the year here in the PNW, I think that tells me I need a different heat source.

Although I wonder if I have the two hoses too close together. Perhaps I am collecting cold air in the cockpit and I need to move the intake hose higher.

[CarlF]

That moisture is a problem. You might want to consider buying an inexpensive dehumidifier and running it too. It will exhaust quite a bit of heat into the cabin as it condenses the moisture out of the air. Two thirds of energy used by air conditioners is for dehumidification (the drip) because an air to water phase change requires much more energy than just cooling air. With an air conditioner, the heat from dehumidification is exhausted outside but a room dehumidifier just dumps the warm air in the room.

The heat pump will work better too with dryer air too.

[robert644]

I turned the thing back on after running a 1500 watt for about 4 hours. The 1500 didn't do much and when I turned the heat pump back on I could tell a big difference in warmth. 9000 more BTUs than the little 5000 BTU electric heater. Also this machine has a water reservoir.

It's advertised to exhaust water outside, but that in some situations where there is too much to exhaust it will collect in a pan or something. So there was an alert on the control panel and I drained the water. I'm guessing what I really need is a hose from the unit to the bilge.

[tspear]

Originally Posted by CarlF

That moisture is a problem. You might want to consider buying an inexpensive dehumidifier and running it too. It will exhaust quite a bit of heat into the cabin as it condenses the moisture out of the air. Two thirds of energy used by air conditioners is for dehumidification (the drip) because an air to water phase change requires much more energy than just cooling air. With an air conditioner, the heat from dehumidification is exhausted outside but a room dehumidifier just dumps the warm air in the room.

The heat pump will work better too with dryer air too.

Please explain the highlighted part.

Tim

[robert644]

So the conclusion is, with high humidity both inside and out, the heat pump isn't worth anything. It raised the temp 5 degrees from 40 to 45 over a 10 hour period. I haven't given up on it yet, but in these conditions it isn't working.

[ranger58sb]

Originally Posted by robert644

I turned the thing back on after running a 1500 watt for about 4 hours. The 1500 didn't do much and when I turned the heat pump back on I could tell a big difference in warmth. 9000 more BTUs than the little 5000 BTU electric heater. Also this machine has a water reservoir.

It's advertised to exhaust water outside, but that in some situations where there is too much to exhaust it will collect in a pan or something. So there was an alert on the control panel and I drained the water. I'm guessing what I really need is a hose from the unit to the bilge.

I guess I haven't understood your installation. Our reverse cycle AC units had the boat toasty yesterday, in conditions starting at about 40°F outside air temps and 45° water temps. Took about an hour. Inside temps easily hit 70°F to the point where I couldn't stand it any longer... and I toasted the place when outside air temps have been down in the 30s, and water temps were right at 40°F.

When you say "it's advertised to exhaust water outside" do you mean you don't always have water flow? A typical reverse cycle installation will have constant flow of raw water in, and then out. Does yours not?

When you're talking about watts... does that mean you've got an auxiliary resistance heat source, in between reverse cycle AC unit and the air handler? Something like described in post #28 from this thread? http://www.cruisersforum.com/forums/...on-122662.html

-Chris