New A/C for next year

[Ex Sailor]

After this brutal hot summer in Ontario, we have decided to install A.C. in our boat for next year. I intend to remove our diesel heater and install a Heat Pump unit in it's place. I hope to re use the present duct work @ 3" with maybe some mods. at the A.C. discharge.


The Boat: 40' Trawler, main salon, galley, forward V and aft cabin.

I have a line on a 16000 BTU unit by Dometic @ about 2,300.00 CAD. just for the unit ( no pump ) . Is 16000 BTU enough ? and does anyone know this brand or is there an alternate to look at ?



Tx in advance, f

 

[menzies]

What areas are you going to be covering - Salon, Cabins, PH? If you were only going to install one unit I would make it the cabin, especially the aft cabin which probably doesn't get much air circulation.

How are you going to run the duct work, does that currently go to all rooms, or just the salon? And don't forget you will need an evaporator as well, which will have to drain into your sump.

Then you need a through hull for the RW and an above the waterline through hull for the discharge.

 

[DavidM]

16,000 btu would be about half of the recommended capacity for a boat your size. But you can probably get by with dampers installed in critical ducts. Route the output to the salon during the day and back to your sleeping cabin at night.


If you want more capacity, you will have to consider power supply and installation. You will need 240/120V, 50A service to power two 16,000 btu units.


David

 

[sunchaser]

Our vessel resides in BC. We have three ACs, one 16K and two 12K for each end of the boat. During hot days the main cabin 16K AC is all we need. Cool air sinks forward and aft. During cooler days the two 12Ks warm the vessel up from the lower extremities.

We also have diesel heat which negates, most of the time, the need for the AC heat pumps in cooler weather.

 

[Ex Sailor]

16,000 btu would be about half of the recommended capacity for a boat your size. But you can probably get by with dampers installed in critical ducts. Route the output to the salon during the day and back to your sleeping cabin at night.


If you want more capacity, you will have to consider power supply and installation. You will need 240/120V, 50A service to power two 16,000 btu units.


David

Thanks ... I know 16K is marginal at best. There is physically NO room to install more than one. The main issue in my mind would be that some marinas may not have more than 30 Amps AC at the dock and this would be the largest I can go with a single supply and still have power left for my charger etc. My current Espar ducting is 3" and I hope to re use most of it since it's already routed. ( I may have to insulate as much of it as I can get to ) The discharge at the unit is 6" so I will have to install a piece of 6" to act as a plenum ( and minimize back pressure ) and then " Y " off to existing ductwork. The aft cabin will be the priority and I will have to damper outlets as I need it.

 

[Comodave]

I like Mermaid A/Cs. Have installed them in 3 boats now. I have a 41’ sundeck with 2 16k units and that works well for us. The problem for you is 3” duct work is way too small. Mermaid requires a 6” duct and a 6” vent within about 5 to 6 feet of the unit or else the unit will tend to ice up. I suspect that is true of any manufacturer since physics apply to all of them. In our sundeck we have one unit under a dinette seat and one in the bottom of the aft cabin hanging locker. I would look around your boat and see where else you might get a bit more room to mount one and run proper sized duct work. Only having one 16k unit is marginal in a boat your size. Having undersized duct work is marginal. Having both only one unit AND undersized duct work will make you very unhappy with the results.

 

[South of Heaven]

A single 16k BTU unit won't cool a 40 footer. I have a single 16 in my 30 foot boat...I suppose you could supplement that with a myriad of good quality oscillating fans but it's just not the same.

Didn't your boat originally have AC? Is all the ductwork still there?

 

[Nopistn]

I have a 16k in the rear and 9k in the front of a double cabin tub. Each unit has a vent in the salon and in its berth.
I JUST installed the new marinair unit in the aft cabin closet and that will cool the aft cabin and salon pretty well unless it's over 90deg and the sun in hitting the windshields (in texas). I only use the front 9k unit on the hottest days or if I have company staying in the V berth. If you want to cool a berth while you sleep you need to have a duct into that room and a way to restrict the vent in the salon.

 

[LarryM]

Or, you could spend a little bit of time measuring the actual spaces to be air conditioned and use a calculator to get better idea of what your system requirements would be.

Ocean Breeze AC has one here. Regardless of the manufacturer, the BTU requirements will be the same.

 

[Nopistn]

Or, you could spend a little bit of time measuring the actual spaces to be air conditioned and use a calculator to get better idea of what your system requirements would be.

Ocean Breeze AC has one here. Regardless of the manufacturer, the BTU requirements will be the same.

For gulf coast heat and humidity I say get the biggest that will fit.
When my boat gets a lil chilly a crack a window and welcome the fresh air

 

[Gmarr]

I’d contact the Dometic Service guys. I did pre install of one of their units and found them very helpful in talking about specs, plenum and ducting requirements as well as deciphering their tech lingo.

 

[O C Diver]

For duct work, you absolutely can't skimp on size. Two things happen.

First, the reduced size reduces airflow frow across the evaporator coil which reduces the output of the unit. Think about the output of a car air conditioner. When you turn the fan speed up, you get more cooling with the increased airflow.

Second, if it's really humid and you reduce airflow, the evaporator runs colder which can start to form ice on the evaporator. Once the ice starts forming, less air flows through the evaporator, making it colder, and forming more ice. Eventually the entire evaporator is a block of ice.

Airflow is your friend when it comes to air conditioning a boat. Every air conditioniner manufacturer talks about how critical proper flow is because the evaporator coil is so small to make the unit as compact as possible.

If you must make long runs with smaller duct, spit the output into multiple ducts with Ts or Ys when going from 6 or 8" to smaller size ducts. It's all about airflow!!!

Ted

 

[psneeld]

A 16K probably isnt enough for the saloon, and too much for foreward or aft by itself.

I have a 16K in the saloon and it works overtime to keep things barely comfy on a 90 degree day working full blast.

The aft stateroom has a 9K which is too much and the fwd area has a 6K which is too much unless cooking.

So I play with temp control and blow hot air up and down so the fore and aft units keep the saloon one from working full time.

I probably could havd gotten away with a 6K in the aft stateroom for cooling, but it is nice fow heat on a blustery cold day.

My 3 new units fit in thd same places the old air handlers did, and now there are no compressor units in the engine room to deal with.

Finding room for them is always a challenge.....I would also reconsider removing the diesel heater unless moving the boat south permanently.

 

[twistedtree]

For duct work, you absolutely can't skimp on size. Two things happen.

First, the reduced size reduces airflow frow across the evaporator coil which reduces the output of the unit. Think about the output of a car air conditioner. When you turn the fan speed up, you get more cooling with the increased airflow.

Second, if it's really humid and you reduce airflow, the evaporator runs colder which can start to form ice on the evaporator. Once the ice starts forming, less air flows through the evaporator, making it colder, and forming more ice. Eventually the entire evaporator is a block of ice.

Airflow is your friend when it comes to air conditioning a boat. Every air conditioniner manufacturer talks about how critical proper flow is because the evaporator coil is so small to make the unit as compact as possible.

If you must make long runs with smaller duct, spit the output into multiple ducts with Ts or Ys when going from 6 or 8" to smaller size ducts. It's all about airflow!!!

Ted

This is really important. Heating ducts are typically a lot smaller than AC ducts, and AC ducts on boats are almost always smaller than they should be. Check the installation manual for the AC unit for guidelines. You probably need at least 5" ducts, kept as short as possible.


Also, it's really important that the ducts be insulated 100%. It's partly about heat loss (OK cooling loss), but even more about condensation control. Uninsulated ducts will sweat and drip water into the interior of your boat. If that finds it's way to woodwork, you will have rot, staining, and lifting finish. And wherever it goes, you will get mold growing.


In all honesty, if you can't work out a proper installation, I wouldn't install AC, or would try to do some sort of free standing unit, window unit, or roof mount unit. I think you will otherwise get poor performance and damage to your boat.

 

[HiDHo]

Ex-Sailor, we added another 16,000 btu Dometic package unit to our boat in addition to an old Cruiseair 16,000 btu split unit, both are heat pump type units. The addition of a Dometic “smart start “ devise works great with two 30 amp shore power feeds and our 5kw NL generator underway.
Another plus to the new Dometic unit is a digital thermostat that can be set to dehumidify your boat. Ours keeps the humidity level at 50% +- 5%.

 

[FF]

If you can find the room for it outside , I would install a more modern mini split system.

Heat & cool and no water pumps or thru hulls.

 

[CaptTom]

We just lived through a couple of brutal heat waves on our 36'er. Here's my experience, for what it's worth:

We have two 16K BTU units. Each has a duct to one cabin (fore or aft) and to the main saloon. I can run both on a 30A supply. They pull about 10A each when running, including the separate water pumps. Starting doesn't seem to cause any problems.

On really hot, sunny days, they run continuously, and keep the inside temps down to the high 70's or low 80's when it's 90s or 100 out. Barely. But they lower the humidity enough that it's comfortable. If I had the option, I'd want a smaller, dedicated unit for each cabin, and one 16K for the saloon. The cabins stay cool enough, it's the saloon which heats up the most.

Moving the air around with fans really helps. Thermostat placement is critical. It's tough because the right location for cooling isn't the same as for heating. Likewise, a location suitable for the saloon leaves the cabin cooler. In cooling mode, I close the master cabin vent about 90% or it'll be like a deep freeze down there.

Pull-down blinds in the saloon help. Outside sun covers on the windows would help, as well as sun shades or canopies over the decks. However, in the afternoon as the sun lowers, there's not much I can do to shade the sides of the boat itself, and you can feel the radiant heat coming in. Cooling a boat is nothing like cooling a home.

 

[refugio]

I intend to remove our diesel heater and install a Heat Pump unit in it's place. I hope to re use the present duct work @ 3" with maybe some mods. at the A.C. discharge.

I think everyone has done well to point out that 3" ducts - particularly with long runs - will not work. And pointed out you're going to need two thru-hulls for the pump (and a strainer, and power to the pump) - make that EACH pump since everyone is also saying that a single self-contained unit will not be sufficient.

But removing that diesel furnace has additional issues. You'll have to cap off diesel supply (and possibly return, maybe just goes to a bottle). And you will have to significantly upgrade the wiring. And then you're left with unused combustion air input and exhaust. And, while this will indeed warm the boat I'm pretty sure you're going to run into a water temperature issue in the winter and heating won't work at all. I suspect you're going to want that furnace after all.

And the ducting is truly different for heating versus cooling - hot air rises and cool air sinks. And all of that grillwork you see for A/C on boats is balanced by equivalent fresh air intake - almost certainly this was not planned for in the existing furnace install.

I suggest you start with sunscreens for the windows and possibly awnings to keep the sun load off of the boat. And put a self-contained unit in the aft cabin. And leave the furnace in operation.

 

[O C Diver]

Looking at your avatar, a possibility might be to install the one unit in the saloon. If you can utilize an existing cabinet with near zero duct work (unrestricted airflow is everything!!!), the unit will run extremely efficiently to cool the saloon. In the evening, to move the air to the lower cabin and V berth section, free standing fans could be used to push cooler air to the berthing areas with the warmer return air rising to the saloon. The added advantage of having the unit in the saloon area would be reduced noise in the berthing areas. Also, dumping all the AC in the saloon would make that area cooler during the day which is likely where you spend more time.

If you did this, I would leave the existing heating system in place.

Food for thought.

Ted

 

[Boat]

Good thread, good read.

Four principles for anyone aspiring to understand AC:
Subcooling, Superheat, Sensible Heat and Latent Heat of Vaporization.

In those, find that much of the work done by AC is dehumidification. With your space limitations, an AC unit assisted by a dehumidification unit might more likely meet your basic needs.

I also would keep the diesel heat in some form.

 

[Mrwesson]

If cost is an issue then you could do a 16k "standard" A/C and supplement it with a rooftop RV/Coleman 16k btu.

Other than being loud mine has stood up for a decade and works great.. The previous owner cut a hole to install but I wouldn't do that. I'd install over a hatch if possible.

 

[Deckape]

Would not get the heat pump as your cold water in winter will affect the out put of warm air, they do not work well in water/air below 40 degees. Keep your heater. Would also use insulated duct work thru out your boat. If the install is too hard , look in to window unit on deck or hatch type a/cs..

 

[Gmarr]

The AC guys talk about square inches of supply and return. Here are the duct equivalents when you get that far:

The “metal box” mentioned is the “splitter” box that affixes to the blower (supply side) and splits into a few hoses.

From Dometic to me:

Your figuring it out. Yes, the combination of hoses existing that metal box needs to equal the 70 square inch requirements. Here is the breakdown of the various round duct sizes.

4” duct = 12.5 square inches
5” duct = 19.5 square inches
6” duct = 28 square inches
7” duct = 38.5 square inches
8” duct = 50 square inches
9” duct = 63.5 square inches
10” duct = 78.5 square inches
11” duct = 95 square inches