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Old 04-08-2019, 01:37 PM   #1
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Hydronic air conditioning

Hi All,

A boat I'm considering already has a Webasto hydronic heating system aboard. Thus, the fluid lines and air handlers are already installed throughout the boat. I'm interested in adding a water chiller (chiller, control valves, etc.) to provide air conditioning as well. Does anyone have experience with such a system? I'm early into this quest, and can only guestimate at this point that I'd need something like 8K BTU for the forward cabin, 16K BTU for the salon, and another 8K BTU for the aft cabin. And the boat only has an 8KW generator for power, so start-up loads are an issue. Boat will be primarily used off the dock at anchor.

Given I'm in the PNW, use of this system would be rather infrequent, as there really aren't that many really hot days afloat. However, there's been several instances in the past where we've been driven indoors by heat, BLACK FLIES, and no breeze. No bueno combination.



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Old 04-08-2019, 02:11 PM   #2
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Suggest you contact Sure Marine in Seattle. If this is being done with Webasto, they will know about it. I've never heard of such a setup myself....usually a dual-roll system like this is based on a heat pump, electric-based.

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Old 04-08-2019, 02:45 PM   #3
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Chiller-based AC systems a common in larger boat, and they can be combined with diesel or electric heat for hydronic heating using the same plumbing and air handlers. But the devil is in the details.

First, all the systems I've seen are principally AC system with an added ability to heat the water and boat via reverse cycle, electric resistance, or diesel. The implication is that everything is sized and plumbed for AC, and that's very important. For AC, the plumbing needs to be meticulously insulated or it will sweat and soak interior parts of the boat possibly causing mold, mildew, and rot. Heating system plumbing is often not insulated, and insulating it after the fact is probably not practical.

Second, the air handlers for AC are much larger than the air handlers for a heater, so trying to use handlers designed for heat is not going to have good results. The big difference is the temp differential. Hydronic heat operates around 100F over room temp, where AC chilled water operates maybe 30F below room temp. So you get 3x the effectiveness out of the heat exchanger with heat vs cooling.

Third, and back to condensation, an AC air handler will condense a lot of water, and they all have drip pans with drain fittings that get plumbed into the bilge, a sump tank, or other tank. Heating air handlers don't have any condensation collection mechanism, and will just dump water on the floor below them.

Forth, the control systems for a hydrid system can get complicated. I helped a friend design one for a recent boat build, and it was not simple. And he was starting with an AC chiller system, and adding diesel to heat the water for heating. Probably the trickiest part is that the chiller system can't handle the normal temp of a hydronic system, so you need a heat exchanger and careful temp regulation.

Bottom line, I think you would be much better off installing whatever AC you need/want as a separate system.
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Old 04-08-2019, 04:05 PM   #4
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Most major AC companies have chilled water systems. In most small systems a heater/cooler, similar to a car heater, is mounted inside a cabinet and the water is circulated either thru a boiler or the chiller. I've installed these in yards and been on ships with chilled water systems. Search "chilled water systems".

I built my system and set it up for the addition of a chiller, but I avoid hot weather and haven't sen a need for AC. All the pipe runs need to be insulated. Many hydronic boat systems aren't insulated or poorly insulated. The heat loss is still contributing to the boat heat. But cooling needs the insulation because the chilled water temperature difference is closer to the air temp needed cooling. Cooling lost in the piping doesn't contribute to the cooling of above deck cabins.

I use individual forced air heaters in each space. Some in cabinets, under beds or in built in couches. Fans are 3 speed. My plumbing is zoned so the circulating water only goes where it's needed. Valves controlled by thermostats open sections of the piping as needed. If I add a chiller, the valves to switch from boiler to chiller would be controlled by setting the thermostat to heat. You can't have both heat and cooling at the same time, in different parts of the boat without double the piping.
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Old 04-08-2019, 04:37 PM   #5
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To add to the spot on comments from TT and Lepke, traditional air handler systems are great dehumidifiers. Both in the heat and cool mode. Right now I'm enjoying 90 degrees in AZ, it is quite comfortable. 90 degrees in Houston would very uncomfortable.

Industrial design liquid chiller coolers oFten have a large dehumidifier as part of the system to achieve the dryer air, to provide the cool dry feeling. Mini split designs for a boat are not out of the question for an aftermarket setup in the PNW. With a couple of portable fans you could nicely cool the entire vessel from the highest point.
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Old 04-08-2019, 05:15 PM   #6
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TT and Lepke's comments are spot on. But there may be a simpler way to get what you want.

First you will probably be able to get by with a total of 24,000 btu of cooling in a, what 40' boat in the PNW. Your 8 KW genset will easily start that system.

It will take some custom design and engineering. You will probably have to scrap the existing air handlers because as noted you won't have enough air flow or coil area for A/C. Replace the ductwork as well as it also will be too small.

One simplification (maybe) is to use two separate coils for heating and cooling in each air handler and use hot water in the heating coil and freon in the cooling coil. That is what we have in or home heating/cooling system.

I know it is quite possible to use two air handlers with a split, ie separate compressor and evaporator, but I am not sure about three. Maybe use one 12,000 BTU system for the main salon and split a 12,000 btu system for the two cabins.

I am not sure if anyone makes packaged freon/hydronic air handlers so you may have to build it up from separate parts like my home system. It will be bulky and may not fit, which is the big negative alluded to above.

Why not scrap your hydronic system, install a 2-3 way split heat pump system and run the genset for heat?

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Old 04-08-2019, 05:20 PM   #7
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Originally Posted by Lepke View Post
I've installed these in yards and been on ships with chilled water systems. Search "chilled water systems".
Any specific recommendations for models/setups or yards that do it? I've been interested in this as well, but only find large boat systems, or building systems. Nothing that was obviously easily adapted to a 30-50 foot boat.

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