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Old 18-07-2016, 06:24   #1
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Preheat seawater for AC/heating pump

I am considering living on my 53' Carver in the New York area. I'm also looking into using a diesel fired hydronic heater to warm the seawater going to the reverse cycle AC/heating system. Anyone have any thoughts? I'd like to keep it simple.
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Old 18-07-2016, 06:36   #2
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Re: preheat seawater for AC/heating pump

Why wouldn't you just use your hydronic heating to heat the boat?
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Old 18-07-2016, 06:39   #3
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Re: preheat seawater for AC/heating pump

To avoid having to run additional plumbing lines and drilling more holes in the boat. I would like to use the existing heating and duct already on the boat.
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Old 18-07-2016, 06:43   #4
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Re: preheat seawater for AC/heating pump

Sounds misplaced to me. I would think the piping and heat exchanger for the seawater are going be a lot more complex than just getting a heat exchanger for the hydronic system and using it as it was designed for.
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Old 18-07-2016, 06:55   #5
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Re: preheat seawater for AC/heating pump

All I want to do is to heat the seawater going to the existing heating system to allow the water temp to be above the 45 degree for heating. I want to avoid running water pipes to heath the rest of the boat.
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Old 18-07-2016, 07:11   #6
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Re: preheat seawater for AC/heating pump

You can check with the hydronic heater manufacturer to be certain but I think running sea water directly through a hydronic furnace is a really bad idea. Heating sea water increases its corrosion properties significantly. I think the hydronic heater will quickly disintegrate. Most hydronic heaters specify use of antifreeze mixtures to help prevent corrosion.

So about the only way you can make this work is to run the sea water from the A/C pump through one side of a liquid/liquid heat exchanger. Let the hydronic heater run with its normal antifreeze solution through the other side. Someone will have to help you design the heat exchanger I suspect. It is not a simple problem because the flow rates of the A/C pump are pretty high.

How big is your hydronic heater? It takes a lot of heat to raise the water temperature in the A/C water flow. Let's assume you have 10 gallons per minute of A/C water flow (that might be low for a 53 foot boat with 3 A/C units). Let's also assume the water temperature is 40C and you want to raise it to 45C. That would require 12.5kW of heating power. If the water is colder or the flow rate is higher then it takes proportionally more heat. It also requires careful design of the heat exchanger to get the right amount of heat transfer.
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Old 18-07-2016, 07:25   #7
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Re: preheat seawater for AC/heating pump

Many thanx for your analysis. I have not chosen the heater as of yet. Merely exploring the options. As I indicated, I want to avoid running more piping and drilling holes if I can use the present system. I am open to suggestions with a good rationale.
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Old 18-07-2016, 07:31   #8
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Re: preheat seawater for AC/heating pump

You going to be on shore power?
If so then a second hot water heater with fresh water would work, just recirculate the water through the AC evaporator to the hotwater heater in a closed loop. You would need to be able to have a thermostat that would keep the water from getting too hot though, I'd shoot for 90F. I'd run a mix of antifreeze, both for freeze and for corrosion protection.
You would not want to heat water, then pump it overboard, that would be very inefficient
No need to have sea water.
This is a little Rube Goldburg, heating water to remove its heat, to "make" heat, but I don't see why it wouldn't work
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Old 18-07-2016, 07:48   #9
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Re: preheat seawater for AC/heating pump

Interesting approach. Thank you...
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Old 18-07-2016, 08:02   #10
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Re: preheat seawater for AC/heating pump

What a heat pump does of course is move heat around, takes heat from outside and moves it inside, with some loss of course, nothing is 100% efficient. But when water is cold, like below about 50F, there isn't enough heat to make the pump effective, same with cold air of course on an air to air heat pump.
Another very likely thing to do is find out if you can install "heat strips" in your AC and just heat the air with inductive heating, I know this is done in camper Airconditioners, why not marine units?
How ever many calories your heat source can make will determine how much heat the pump can move so you may need a decently powerful water heater, and likely 50 amp shorepower.
More efficient of course to heat the water and extract the heat directly from radiators, school bus heater etc.
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Old 18-07-2016, 08:37   #11
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Re: preheat seawater for AC/heating pump

I'm envisioning limited use of the extra heat source for when the water is around 45F. The standard system does work as advertised with the seawater temp is around 45F. It seems like it would be efficient to heat that water as it comes through the hull. I have plenty of time to sort this out.
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Old 18-07-2016, 09:33   #12
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Re: preheat seawater for AC/heating pump

Here is how I did it living through three winters in Rochester NY on my 40-foot pacemaker MV.
My boat had two 30-amp shore power cables. I added a properly fused third 30-amp cable along with additional receptacles dedicated to the new shore power cable. Snaking the wiring was very easy, there is quite a bit of room between the hull and the living compartments that is reasonably accessed. The result was 90 AMPS available in the boat. Each live aboard slip has its own electric service paid to the utility company not to the marina. I paid to have the utility company upgrade the power on my slip to 200-amp service. I purchased one of those tiny cube shaped ceramic heaters for each room. These heaters vary the fan speed and current flow to maintain the set temperature. I also put a 40-watt light bulb under each engine, generator, and next to each overboard through hull. The light bulb idea made daily checks in the engine room easy. This setup worked very well. One storm knocked out the power in the town for a day, that day I ran the generator and since the marina bubblers were knocked out I ran the main engines in gear at idle occasionally to keep the slip free of ice. It was a wonderful experience, the wild life that you will see during the winter is entirely different that when the summer has the marina full of people. Deer swimming the Genesee river, beaver making houses, 13 different kinds of ducks, Birds and fox. Electric heat has no carbon monoxide, expensive but safe.
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Old 18-07-2016, 09:36   #13
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Re: preheat seawater for AC/heating pump

Can you put a hot water coil in your air handler?
Use the diesel boiler to supply the hot water.
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Old 18-07-2016, 10:04   #14
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Re: preheat seawater for AC/heating pump

....but you said "keep it simple" in your first post.

A heat pump system is a recovery system in that it recovers heat in the water, and you want to preheat that water to make recovery easier, but the energy used in the preheating will have losses of energy and that energy would be better spent directly into the living space.

I think your barking up the wrong tree, in engineering terms.

If I was you, I would rent/borrow/buy a thermal imaging camera. Do a thermal loss survey, and spend the saved money on insulation and correction. I am sure that there are HVAC companies that can carry out the survey for you.

In my experience, it is far more efficient to reduce losses, than to add more heat.
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Old 18-07-2016, 10:53   #15
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Re: preheat seawater for AC/heating pump

Sounds like folly. Heating for a marginal recovery. Conventional heat pumps have heat tapes for a reason. Sounds like me wrapping an electric blanket around my condensing unit. In NY you are to far north for a heat pump other than spring and fall for heat.
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