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Old 30-03-2020, 06:19   #31
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Re: Air con for heat in winter?

For heat pumps in cold water, a bigger seawater pump helps. More water flow through the unit will reduce the temperature drop as the water goes through the coil, which means slightly more effective heating and it can buy you another couple of degrees of water temp before things start to ice up. You still won't get good results below high 30s water temp though.
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Old 30-03-2020, 06:20   #32
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Re: Air con for heat in winter?

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I'm on the Pamlico in NC with two (5Kbtu & 16Kbtu) reverse cycle units on my 35' S2. Indispensable in the heat but troublesome in the cold. The raw water circulation lines freeze up about 5AM on cold nights. Anybody else have this problem?
Our new to us boat is a s2 9.1 center cockpit. Working on a plan to make the air con work for heat in southern Chesapeake. A previous poster recommended using a fresh water tank to cycle the ac instead of raw water. Check out the posts above. Bill on the Denali Rose recommended this idea.
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Old 30-03-2020, 07:23   #33
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Re: Air con for heat in winter?

I have a serious problem of understanding how using a fresh water tank as a source of heat for a heat pump is going to work from a physics point of view. Wrwakefield's water tank is 10 to 20 degrees warmer than the sea water because he is pumping heat out of his refrigeration unit into the tank. I'm not sure how his tank is set up but because it's staying warmer than the seawater it must have some insulation from the seawater. It may just be the fiberglass hull or the hull plus an airspace. If he turned his refrigeration off over time it would find a balance temperature between the seawater temperature and the heat coming in from the boat's interior and it's not going to be anywhere near 20 degrees above the seawater. A simple calculation shows that a 110 gallon tank 20 degrees above the sea water temperature of 48 degrees has a heat capacity of 880 BTUs per degree F. So if the temperature of the water tank is 68 degrees and the minimum temperature that the AC units can work at is 41 degrees then the heat capacity of the tank is 23760 BTU. One 15000 btu/hr heat pump will reduce the temperature of the tank to 41 degrees in about an hour and a half of constant running. Now because the refrigeration unit is dumping heat in the tank it will actually take a little longer to reach 41 degrees but not much longer. The idea of using a fish tank heater to maintain the temperature means that someone does not understand the difference between temperature and heat. Aquarium heaters come in sizes rate at a couple of hundred watts. A 15000 BTU/hr heat pump would require a 4300 watt heater just to maintain the temperature of the tank. Now if you had an aluminum or steel boat with one side of the tank being the hull in contact with the seawater and that contact area was big enough this scheme might work, but it's not going to work in a fiberglass boat with an independent tank.
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Old 30-03-2020, 08:06   #34
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Re: Air con for heat in winter?

Heat pump mode doesn't inherently use more power. It may seem like more power but you have to look at the temperature differentials. If you like 70 F and it's 90 F, you need to lower the temp by 20 F. That would be the equivalent of the temp being 50 F and having to heat it up 20 F. You would probably find that power consumption is very similar in these two conditions for a heat pump system. You are just changing the refrigeration cycle to reject heat in your evaporator (inside part) rather than reject heat in the condenser (outside part). As the temps get colder, the system works harder and it becomes more difficult (and eventually impossible) to "reject the cold" from indoors to the outdoors. At that point an auxiliary resistance heating coil would kick on and that uses a TON of power. Think big toaster. I would recommend installing another form of heating for when temps get too low. Something non electric.

I saw humidity discussed briefly in this thread. A better way to understand the effect a heating or cooling coil has on the air (e.g. heat pump system) is to think of the air like a sponge. When it's hot and humid (say >70% RH (relative humidity)) and you cool the air, it is like squeezing the sponge. The air gets denser as it cools but the amount of moisture in it stays the same up until 100% relative humidity (air changes size but water does not), which you get at a cooling coil, and then it "rains." The moisture is squeezed out of the air.

In the winter time when it is 0 F and snowing, the humidity is 100%. Same as when it is 100 F and raining in the summer. When you heat that cold air it expands while still maintaining the same amount of moisture. Thus the relative humidity drops. So heating very cold air with a starting RH that is "normal" (say between 20% and 60% for human comfort and well being) will result in a warm but VERY dry air (can be much less than 20% RH depending on the starting temp and RH). This is why the air gets too dry inside in the coldest parts of winter. You're expanding air a lot and therefore reducing the amount of moisture within the same quantity of air. Humidity in these conditions is actually better maintained by "reusing" the same indoor air. Humans add quite a bit of humidity and the air is already warm.

But, the smaller the indoor environment, the more impact the outdoor environment has on the inside (greater ratio of radiated heat to floor area and overall larger percentage of air leakage). For example, the indoor air of a house is much more affected by outdoor conditions than would be the indoor air of a 20 story high rise. By that same logic, the indoor air of a boat is more affected by outdoor air conditions than the indoor air of a house would be. So in a boat you are really battling outdoor air conditions, with respect to temp and humidity, more than one might expect even if you are only drawing air into your system from inside. Hence, very dry indoor conditions despite only using pre conditioned air.
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Old 30-03-2020, 09:24   #35
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Re: Air con for heat in winter?

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Originally Posted by Luckyknot View Post
Heat pump mode doesn't inherently use more power.
I don't mean to be argumentative but I am just going by the data specifications. For example Dometic says that a 12,000btu unit uses 25% more current draw in heat mode compared to cool mode.

https://www.dometic.com/en-us/us/pro...specifications
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Old 30-03-2020, 09:40   #36
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Re: Air con for heat in winter?

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Originally Posted by transmitterdan View Post
I don't mean to be argumentative but I am just going by the data specifications. For example Dometic says that a 12,000btu unit uses 25% more current draw in heat mode compared to cool mode.

https://www.dometic.com/en-us/us/pro...specifications
I was of the same impression but it's one of those things tucked in memory and the source? I think it is relevant the coil sizes and efficiency whether being used as a condenser or evaporator?
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Old 30-03-2020, 10:08   #37
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Re: Air con for heat in winter?

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I don't mean to be argumentative but I am just going by the data specifications. For example Dometic says that a 12,000btu unit uses 25% more current draw in heat mode compared to cool mode.

https://www.dometic.com/en-us/us/pro...specifications
No problem. I'm happy to have a discussion . The numbers you are comparing are for maximums. A heat pump system is usually built to be able to still heat in freezing temperatures. This requires more energy as the temperature differential is greater than what would normally be seen in cooling mode (say 100 F down to 70 F = 30 F differential in summer but 25 F up to 70 F = 45 F differential in winter and can be much greater depending on the region). That is where auxiliary heaters (among other methods) within the aircon unit are used that would consume that additional power you noted. The actual heat pump proper (compressor/refrigerant) should not use any more power.

This is not 100% true as there are other things at play which may account for different power consumptions in heating vs. cooling mode. Different manufacturer's have different methods, controls, and capabilities. Generally speaking though I would say that the refrigeration cycle of a heat pump is probably typically sized for max cooling with auxiliary heating systems added on to be able to provide increased heat capacity for colder temps.
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Old 30-03-2020, 10:11   #38
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Re: Air con for heat in winter?

I can confirm that my Dometic / Marine Air Vector Turbo units consume somewhat more power in heat mode than A/C mode. They don't have any form of auxiliary heating. My best guess is that there's the added power draw for the reversing valves and then other factors might make the system a bit less efficient in heat mode. They sound a bit different in heat mode as well, FWIW.
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Old 30-03-2020, 10:34   #39
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Re: Air con for heat in winter?

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Originally Posted by rslifkin View Post
I can confirm that my Dometic / Marine Air Vector Turbo units consume somewhat more power in heat mode than A/C mode. They don't have any form of auxiliary heating. My best guess is that there's the added power draw for the reversing valves and then other factors might make the system a bit less efficient in heat mode. They sound a bit different in heat mode as well, FWIW.
I would be interested to know what the difference in power consumption is at a 15 F difference in temperature meaning:

power consumption in cooling on an 85 F day with interior temperature at 70 F
vs.
power consumption in heating on a 55 F day with interior temperature at 70 F

With a unit properly sized for the space, that should keep it well within the compressor's capabilities without any supplemental anything. I don't think those numbers will be exactly the same but certainly much less than a 25% difference. The mention earlier of differences in size and efficiency of the evaporator and condenser may be quite valid for one cause of the difference in power consumption at these conditions I think.
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Old 30-03-2020, 17:36   #40
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Re: Air con for heat in winter?

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Originally Posted by Captain Bill View Post
I have a serious problem of understanding how using a fresh water tank as a source of heat for a heat pump is going to work from a physics point of view. Wrwakefield's water tank is 10 to 20 degrees warmer than the sea water because he is pumping heat out of his refrigeration unit into the tank. I'm not sure how his tank is set up but because it's staying warmer than the seawater it must have some insulation from the seawater. It may just be the fiberglass hull or the hull plus an airspace. If he turned his refrigeration off over time it would find a balance temperature between the seawater temperature and the heat coming in from the boat's interior and it's not going to be anywhere near 20 degrees above the seawater. A simple calculation shows that a 110 gallon tank 20 degrees above the sea water temperature of 48 degrees has a heat capacity of 880 BTUs per degree F. So if the temperature of the water tank is 68 degrees and the minimum temperature that the AC units can work at is 41 degrees then the heat capacity of the tank is 23760 BTU. One 15000 btu/hr heat pump will reduce the temperature of the tank to 41 degrees in about an hour and a half of constant running. Now because the refrigeration unit is dumping heat in the tank it will actually take a little longer to reach 41 degrees but not much longer. The idea of using a fish tank heater to maintain the temperature means that someone does not understand the difference between temperature and heat. Aquarium heaters come in sizes rate at a couple of hundred watts. A 15000 BTU/hr heat pump would require a 4300 watt heater just to maintain the temperature of the tank. Now if you had an aluminum or steel boat with one side of the tank being the hull in contact with the seawater and that contact area was big enough this scheme might work, but it's not going to work in a fiberglass boat with an independent tank.
Hi Bill,

I, too, wondered about the efficacy of such a set-up when I did some rough calcs before giving it a try.

Since I was cleaning and replumbing the raw water circuits anyway, it was way too easy to connect to the second potable tank since all the hoses were right in front of me as I sat there ripping old hose out... So I installed a 3-way valve [so I could switch back to raw water if the experiment failed...] and gave it a try with one 16k btu unit. It worked at freezing [outdoor air] temps that winter, so I plumbed up the other two heat pumps the same way.

I suspect we are lucky and get away with this partly because of our current cruising grounds [SE Alaska] Here the Pacific Ocean water just doesn't get that cold S of ~59N [high 30s, low 40s F in winter; ignoring glacial fjords, etc. where we wouldn't be using heat pumps anyway...] And, if we were planning not to venture to higher latitudes, I didn't even need to try this experiment. I did so in case we venture further N where the water would be too cold to winter with the heat pumps in a harbor slip [one eventuality for us...]

So why has it worked so far? [Why doesn't the thermal reservior get too cold when the heat pumps are running?]

One thought [which you already voiced] is the one small refrigerator compressor is adding a modicum of heat to the tank. The other thought is the heat pumps quickly heat the interior and do not run continuously even when they are the only heat source. [They may cycle on maybe up to 40% in cool spells, but usually less than 10 mins/hr.] And ambient waters rarely [if ever] drop below 40F where we have wintered the last couple of years during this experiment.

I haven't thought about this for a while, and your questioning made me curious.

I just measured the water temps from both water tanks [at the galley faucet after running water for a couple of mins...] Water from both tanks measured 49F. Ambient raw water temp [measured from the dock using the same simple dial face thermometer] is currently 40F. Outdoor air temp today and this past week has been hovering around freezing, and we've been heating with the forced air diesel. Nothing has been recirculating water in that 2nd tank. [The refrig that uses that tank has optional water cooling, and that has been off all winter...]

So apparently the water tanks are both getting some heat transfer from heated occupied space [galley and lower settee area] through the uninsulated tops; +9F by today's measure. [N=1 so none of this is difinitive, but it does jive with the few times I have spot checked during this experiment the last couple of winters...]

The tanks are inline low against the hull on the centerline; large rectangles not a foot tall, so the largest ratio of surface area exists on the tops and bottoms. The sides are insulated, tops are not. I cannot see underneath, but will assume there is some foam there since it is on the sides and the centerline of the hull is not flat. FRP hull averaging 1+" thick- based upon through-hull cores from adjacent areas. And no heat source is near the tanks.

So this experiment must be continued in colder ambient waters to demonstrate viability for its intended purpose; support heat pump use should we winter further N in waters approaching freezing temp. [I'm not sure we are up to that challenge at the moment...]

So I'm shrugging my shoulders right along with you because I agree, it didn't seem like it should work...

It will be interesting to hear how this works out for others.

Thanks for your feedback.

Cheers! Bill
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Old 31-03-2020, 12:55   #41
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Re: Air con for heat in winter?

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Originally Posted by CamWrenRoo View Post
Hi Bill, any thoughts on how much water volume we would need to circulate? I'm thinking I could install a 50 gallon tank under my aft berth. If you have 3 aircons (2 15000 btu, 1 9000 btu) and a fridge running on 110 gallons in Alaska, I'm thinking 50 gallons ought to do it. What do you think? I highly doubt the water would go below 50 or above 90 if it's under a berth with insulation between the tank and the hull. Thanks!
Hi CamWrenRoo,

I wrote a reply yesterday, but don't see it here so I fubared that somehow...

Since you have a new A/C unit that has never circulated salt water, I would consider tieing it into your existing 40 gal water tank as an experiment before adding a new tank dedicated to that purpose...

There are too many variables involved to predict the outcomes, so this is one case where a bit of experimenting is easier, and isn't costly... [You have to install the A/c unit anyway...]

Let us know what you decide and how things turn out...

Best wishes for success. Bill
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Old 31-03-2020, 13:10   #42
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Re: Air con for heat in winter?

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Thank you so much to everyone who replied! I think we'll try the heat pump, and if it does not cut it, we'll use some plug in space heaters. If we're still cold, we'll use either a wood or diesel Dickinson heater. Thank you!
Think and plan carefully where your air outputs are, how you'll get it there and where is the return. In my cat. Each master berth has a 4 inch vent and I have a 6 inch duct into the main cabin. These vent runs require you make an air f iui distribution box. Takes space.
My return is 3 feet above the sole in 1 if the companion ways. Thing that made a LOT of difference in efficiency was my main cabin 6x9 vent has permanent louvers pointed down. Great for heat but awful for ac. So when its warm, I rotate the vent 180 degrees. When its 100 out, this makes all the difference
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Old 31-03-2020, 13:24   #43
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Re: Air con for heat in winter?

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Originally Posted by CamWrenRoo View Post
Hi Bill, any thoughts on how much water volume we would need to circulate? I'm thinking I could install a 50 gallon tank under my aft berth. If you have 3 aircons (2 15000 btu, 1 9000 btu) and a fridge running on 110 gallons in Alaska, I'm thinking 50 gallons ought to do it. What do you think? I highly doubt the water would go below 50 or above 90 if it's under a berth with insulation between the tank and the hull. Thanks!

Water exiting my unit is 5 to 7 degrees colder than when it went in. My water pump pumps a lot. Check your manual. Fir fun, say it's only 3 gpm although I'm pretty sure that's low. That's 180gph. Or, cycling your tank water almost 4 times inan hour. Eventually, that water will chill. How far in hiw long maybe someone else will answer. You'll be cooling the tank water faster than ambient heat will warm it. Aquarium heater is an interesting idea.
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Old 31-03-2020, 14:52   #44
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Re: Air con for heat in winter?

Thank you everyone! I think we'll try the water tank idea and see what happens. There are also 2 thru hulls we can use if we put a y valve on the engine intake and plumb the head to only go to the holding tank. And the more I research the Dickinson heaters, the more I'm thinking we should get one. Does anyone have opinions on the Dickinson solid fuel heater? Thanks!
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Old 03-04-2020, 07:49   #45
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Re: Air con for heat in winter?

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Originally Posted by CamWrenRoo View Post
Thank you so much to everyone who replied! I think we'll try the heat pump, and if it does not cut it, we'll use some plug in space heaters. If we're still cold, we'll use either a wood or diesel Dickinson heater. Thank you!
I live aboard a 42 sailboat in the Middle Chesapeake. During the coldest few months, Nov-Feb, I use a small oil filled radiator, with a USB gooseneck computer fan blowing across it and it maintains the boat to about 50F. If I need additional heat, I use a West Marine Cabin Heater and it gets warm fast. Nothing is installed in the boat, so I can stow em when the weather breaks. My neighbor has a diesel heater and says its almost too hot sometimes. An electric blanket is a have to have at night! Regardless, run a dehumidifier in winter. I rigged it so it drains right into the bilge.
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