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Old 10-16-2020, 08:16 AM   #1
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Heat Pumps

Or more correctly: Reverse cycle air systems.

Another thread got into it a little bit, but I have a question that, I believe, deserves it's own thread.

As discussed there the efficacy of the heating function decreases with water temperature. When the water is cold, there is little heat to extract. At around 40 the results are about nil.

So there is a plethora of solutions to heat the boat beyond that point, the details of which I do NOT want to get in to here. What they all have in common though, is that on a large(r) boat they will need considerable additional installation of plumbing and/or wiring to distribute to the far corners of the vessel.

Which is kinda sad as the heat pump(s) that were working up to this point were already there..... (4 in my case)

So the idea I had was to simply provide the system with the warm water it needs. I have ONE pump that circulates water through the four compressors. If I close off the through hull and the overboard discharge and create a closed loop with a heat source that keeps the water (or glycol) at, say, 70 my heat pumps would be back in business.

Now the how...... On demand heater running diesel? Electric? Small tank with large electric elements and, of course, thermostatic control?

I have room in the engine room next to the pump.... The installation could be compact and accessible.

Anyone done this? Anyone with the physics/math on the tip of their tongue want to throw some meaningful numbers out?

Thanks.
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Old 10-16-2020, 08:33 AM   #2
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Interesting thread currently on this site about using engine exhaust to do this. Other thought is most boats already use engine heat to heat domestic hot water. Water is a decent but not great heat sink. Fe is a bit better. Could pump a simple loop from hot water tank and use that source to heat the boat. Add controls to continue to extract heat from engine block via its coolant loop even after engine is off. Bypass the energy needs of reverse AC.
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Old 10-16-2020, 09:16 AM   #3
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Oscar, I think I was part of the discussion to which you refer. I would just like to offer my experience with reverse cycle. You stated that at around 40 degrees of water temp heat production is nil. Nil to me means zero, nothing. My three units are able to keep the boat at 70 degrees down to 38 degrees water temp with the air output still 15 - 20 degrees above the input temp. That's my boat, my experience. Others will have differing results.

Yes, the output certainly decreases as the water temp decreases but there is still, in my systems, enough heat to be extracted at 38 degrees to make it worthwhile to run the reverse cycle without the units freezing up. But, our boat is a "warm" boat. The engine room in which the units reside stays relatively warm as we are full time live aboards and I keep the engine room door open. That is why maxims difficult with anything boating.
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Old 10-16-2020, 09:27 AM   #4
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I assume you want a heating source that works with or without the engine running and mostly without. And I appreciate your desire to utilize the existing heat pumps and ducting systems throughout your boat.

Electric heating of the water circulating loop is a poor choice as it will take a lot of watts from a generator or shore power keep up with 5 systems. Let's assume that you have 50,000 btu/hr of reverse cycle systems, some 16,000 btu/hr, some smaller ones and lets assume that these have to run at a 50% duty cycle to keep it warm. That is 25,000 btu/hr or about 25,000/3,400 = 7,400 watts.

A better way, but much more expensive is to install a hydronic boiler such as used in boat hydronic heating systems. You use this to heat water and circulate it to all of your reverse cycle units. Again it will take 25,000 btu/hr but I would size it for a maximum heat output of 50,000 btu/hr to match the full output of the reverse cycle units. That won't be cheap, maybe $5,000+ for just the boiler, much less installation and plumbing.

That 25,000 btu/hr average output will take quite a bit of diesel. Diesel has a heat content of about 140,000 btu per gallon and that system at best will be 80% efficient so figure 25,000*24/.8*140,000 = about 5 gallons per day.

You might be thinking, reverse cycle units produce 2.5 times the electric energy input with warmer water, so why not use the 2.5 figure. It doesn't work that way since the 2.5 figure doesn't include the energy in sea water. But you have to add that energy in the form of electricity or diesel. The first law of thermodynamics rules. Energy in = energy out.

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Old 10-16-2020, 09:36 AM   #5
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Oscar,
The way I see it: you would be using energy (likely shorepower?) to heat a fluid and then try to suck the heat out of the fluid and move it to air. Every transformation of heat from one medium to another comes at an efficiency cost, pick a number.

Seems to me you would keep the boat toasty using the energy you'd put into the fluid to directly heat the air in the boat, eliminating at least one transformation of heat energy. I can appreciate that you don't want to lay more hoses and bus heaters all over the boat, for example.

If this is for at-anchor then might be different solution path.

Edit: looks like David beat me to the punch with his post#4!
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Old 10-16-2020, 09:38 AM   #6
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OK, thanks for all the input.

Two takeaways.

The heat pumps may do better than I thought. I have not run THESE units at low water temps yet. I'm basing my opinion on experience with a previous vessel with a single unit, which may, or may not have been operating correctly. So I will definitely wait until I can see for real what these units do.

Electric is a non starter. Yeah, I did some math and came up with 45A of 240VAC for some of the tankless electric heaters. Somethings gonna end up smoking......

So diesel it is..... Not worried about the gallons, we are not liveaboards and if we're on a two week trip running diesel heat I probably won't notice what with the two Detroit Diesels and all that.......

I do like the idea of using the engine as a heat sink. I could run it for 30 minutes to get it warm and then slowly extract the heat......hmmmm.
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Old 10-16-2020, 10:10 AM   #7
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OK, thanks for all the input.

I do like the idea of using the engine as a heat sink. I could run it for 30 minutes to get it warm and then slowly extract the heat......hmmmm.
I recall this rule of thumb taught to me by Tony Athens: 1/3 of the heat in diesel goes to produce power in a diesel engine, 1/3 goes out the exhaust, and 1/3 goes out the cooling system.

So if you are only using the cooling system heat you are wasting 2/3 of your diesel and going back to my previous post you will use as much as 15 gpd to heat your boat. You would be running your engine in gear while tied to the dock to absorb the power, and blocking in the raw water system so you could circulate your coolant to your reverse cycle units.

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Old 10-16-2020, 10:27 AM   #8
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That makes sense. So only when running to get somewhere would this be a good idea...
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Old 10-16-2020, 10:34 AM   #9
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Know from prior experience having both reverse AC and a wesbasto as a long term cruiser. Our fueling intervals decreased when we used reverse AC anytime except for the rare occasions we were on shore power. Using reverse AC to heat a boat at ANY ambient water temperature is LESS efficient than a hydronic heating system at any temperature. That’s even with the electrical draw of the wesbasto. In fact depending on fuel costs and shore power costs its often cheaper to heat the boat with the wesbasto rather than reverse AC even when shore power is available. Also the wesbasto heat is more pleasant. No AC noise nor drafts and boat is evenly heated. Plus you get the advantage of having multiple zones. Would think this would be even more apparent with a drip heater as there would be no electrical draw.
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Old 10-16-2020, 10:43 AM   #10
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Couple of useful points:

I have 3 reverse cycle units on my boat and a geothermal heat pump in my house. The takeaway from both is water flow makes all the difference. My geothermal unit will produce heat down to about 10 degrees Fahrenheit. It's able to do this because the loop has an antifreeze solution. While it's nice to know it has that capacity, it's never gone below 40 degrees. While the efficiency is better the warmer the ground around the radiators is, it still has the capacity.

With boat reverse cycle units, everyone fixates on how small a pump they can get away with. The reality is more flow allows the unit to produce more heating or cooling. The last AC water pump I put on my charter boat was a spare 900 GPH pump that came with my trawler. The pump was pushing between 10 and 15 GPM through the AC unit. Not only was the stream impressive, but the unit was able to produce more cooling because it lowered the gas temperature further.

The water pump on my trawler is larger also and produces more heating and cooling. It can still produce heat without freezing up with mid 30s water temperatures.

If I were going to take your boat and try to utilize the existing reverse cycle units, I would experiment with taking one of the units, feeding it with it's own water pump (10 to 15 GPM) and seeing how low it will go without freezing up and how much additional heat it produces with the extra flow.

Ted
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Old 10-16-2020, 10:48 AM   #11
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Couple of useful points:

I have 3 reverse cycle units on my boat and a geothermal heat pump in my house. The takeaway from both is water flow makes all the difference. My geothermal unit will produce heat down to about 10 degrees Fahrenheit. It's able to do this because the loop has an antifreeze solution. While it's nice to know it has that capacity, it's never gone below 40 degrees. While the efficiency is better the warmer the ground around the radiators is, it still has the capacity.

With boat reverse cycle units, everyone fixates on how small a pump they can get away with. The reality is more flow allows the unit to produce more heating or cooling. The last AC water pump I put on my charter boat was a spare 900 GPH pump that came with my trawler. The pump was pushing between 10 and 15 GPM through the AC unit. Not only was the stream impressive, but the unit was able to produce more cooling because it lowered the gas temperature further.

The water pump on my trawler is larger also and produces more heating and cooling. It can still produce heat without freezing up with mid 30s water temperatures.

If I were going to take your boat and try to utilize the existing reverse cycle units, I would experiment with taking one of the units, feeding it with it's own water pump (10 to 15 GPM) and seeing how low it will go without freezing up and how much additional heat it produces with the extra flow.

Ted
Don't know the GPH on my pump but it's substantial. It feeds 4 units and in cooling mode they work VERY well. But yes, I'm going to see how low it will go. As mentioned my previous experience was another boat and it may well have had a relatively small pump.....So good point.

One point I disagree with is that your Geo thermal unit works in lower temperatures because there's Glycol in it. It works because it goes far enough into the ground that it reaches the strata where the temperature is 50-ish degrees. Always.
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Old 10-16-2020, 10:55 AM   #12
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Have two 250’ deep closed loops to run our geothermal HVAC house system. Water/antifreeze mix comes out at 55 F all the time. Ambient air temp is irrelevant. How deep the ground is frozen is irrelevant. Same 55F 24/7 365. Totally different set of circumstances than with a boat. Water temperatures a boat is working with is even higher than the thermocliine the boat sits in.
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Old 10-16-2020, 11:16 AM   #13
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Don't know the GPH on my pump but it's substantial. It feeds 4 units and in cooling mode they work VERY well. But yes, I'm going to see how low it will go. As mentioned my previous experience was another boat and it may well have had a relatively small pump.....So good point.

One point I disagree with is that your Geo thermal unit works in lower temperatures because there's Glycol in it. It works because it goes far enough into the ground that it reaches the strata where the temperature is 50-ish degrees. Always.
You're wrong about my geothermal. I did an exhaustive amount of research before installing and have a loop temperature gauge that measures the loop solution coming out of the ground including retaining high and low absolutes. While the ground temperature is 59 degrees in my area, the earth within 5 to 8' of the loop cools during the winter and heats during the summer. For undersized systems, the radiators generate popsicles around them as the ground water freezes when the loop goes into the 20s. My loop temperature right now is in the mid 60s from heating the ground around the radiators up to the mid 70s during air conditioning this summer.

While I'm sure you think your water pump is substantial, is it producing 60 GPM (4 units at 15 GPM)? If working at the extremes for heating or cooling, I would be more inclined to go with individual pumps for each unit. Wouldn't want a pump failure to eliminate all the heating in the boat.

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Old 10-16-2020, 11:21 AM   #14
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Have two 250 deep closed loops to run our geothermal HVAC house system. Water/antifreeze mix comes out at 55 F all the time. Ambient air temp is irrelevant. How deep the ground is frozen is irrelevant. Same 55F 24/7 365. Totally different set of circumstances than with a boat. Water temperatures a boat is working with is even higher than the thermocliine the boat sits in.
So you have a gauge that measures loop liquid temperature coming out of the ground? How many tons is your system?

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Old 10-16-2020, 12:06 PM   #15
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While I'm sure you think your water pump is substantial, is it producing 60 GPM (4 units at 15 GPM)? If working at the extremes for heating or cooling, I would be more inclined to go with individual pumps for each unit. Wouldn't want a pump failure to eliminate all the heating in the boat.

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I carry a spare. But you made me look and that's good. March AC-5D-MD

Max flow 17GPM.

So maybe an upgraded pump is first order of business.
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Old 10-16-2020, 12:07 PM   #16
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Yes temp measured in and out. Along with sensors measuring air temperature/ humidity through out the house. Also measure barometric pressure in/out of house. House kept at slight positive pressure and what ever humidity/temperature you want in each zone you want. Also heat exchangers so internal air is cycled but filtered (13)
Temperature of earth once you get that deep is constant. How deep You need to go depends on geology. Nothing else
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Old 10-16-2020, 12:49 PM   #17
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I carry a spare. But you made me look and that's good. March AC-5D-MD

Max flow 17GPM.

So maybe an upgraded pump is first order of business.
A 17 gph pump with a ten degree temp rise at the condensor (or fall at the evaporator while heating) which is typical of reverse cycle systems, will carry 17*8*60*10= 80,000 btu/hr. But the flow is probably much lower, maybe half of that due to pressure drop in your system, so it may be marginal. And lowering the temp drop (while heating) by increasing the flow will help keep it from freezing up.

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Old 10-16-2020, 03:56 PM   #18
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So, is it safe to conclude that the more raw water one can push through the units, the better cooling/heating? For example, Oberdorfer makes an AC pump that puts out 1,440 GPH. My pump puts out 900 GPH.
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Old 10-16-2020, 04:54 PM   #19
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On those larger Hatteras motoryachts like the 53 (we owned and lived on a 56), there is in my opinion a great option when at the dock: immersion block heaters on the engines. Cover the ER vents, open all the ER doors and heat 'er up. Keeps everything dry and toasty, no pumps running, and great for the health of the engines. We loved this set up.
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Old 10-16-2020, 04:58 PM   #20
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So, is it safe to conclude that the more raw water one can push through the units, the better cooling/heating? For example, Oberdorfer makes an AC pump that puts out 1,440 GPH. My pump puts out 900 GPH.

For the most part, yes. There's likely a point of diminishing returns, however.
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