HVAC Question

Ok, I tried to post this HVAC question from my mobile phone, and either I had phone problems, or the adminstrator thought it was so dumb, no one would reply. So here goes try No. 2.

Why is it that boats (particular stern drives and inboards) do not use automotive type compressors, evaporators and closed systems like cars? My boat essentially has a 6.5 Litre General Motors engine. This engine has plenty of torque to turn an AC compressor, and a Leece-Neville Alternator (if this was needed) to run a fairly large AC set up. If it can cool a 25 long Suburban, it should be able to cool/heat my 28 foot boat. What am I missing???

Thanks, Mark.

Mark P said:

What am I missing???.

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Heat exchange

The problem is mounting the front of the suburban to the bow of your boat to provide airflow over the radiator.

They are pretty commonly used for cold plates in sailboats for ice boxes. No reason it needs to be unique to sailboats but it is just more common in their electrical power setups. The condenser is either a sea water cooled heat exchanger or a thru hull with a substantial amount of bronze that serves as a heat sink.

For the same reason RV class A’s have roof A/C machines. They can’t cool that much space. Your Suburban is maxing out what a car A/C can cool.

Not the same machine to cool dockside as it is to cool under way. So if at the dock and shore power available, or anchored and gennie available, do you want to run your main engine for cooling?

On my ride, I almost never run the AC when under way. The breeze is good enough to keep it pleasant. I want AC when at dock or anchor in the evening to cool things down for good sleep. A main engine driven AC is not optimal for my use.

An AC driven off the main engine certainly can be done. Just does not fit many uses.

To cool a Suburban in a hot climate, there usually an evaporator in the dash and another in the back.

There are salt water cooled condensers.

In addition to the compressor, you'd need a raw water pump for the condenser and an evaporator matched to the compressor. Unless you'd build a system from wrecking yard parts, you'd probably spend more money than buying a proper AC system.
And as others, I think it's better to run a generator to run the AC. An option is a big alternator and inverter to run the AC when running the main engine.
There are chilled water systems that can be run by a main, generator or shore power, but usually on much bigger vessels.

Gang, all of this makes sense, thanks for the information. What do folks do when cruising in 95 degree weather? Our practice boat is great (Sea Ray 280 SLX) but she is almost unusable in July and August when the temps are very high. When underway at 25 mph or so, the breeze is nice, but the admiral likes cruising around 10, so we sweat like dogs on our evening cruises in these two months (and freeze to death in Jan, Feb even on Hilton Head), and she is not happy - so you know how the saying. Our next boat will be a trawler so 10 or 11 knots may be top speed, so I need to figure out how to cool/heat the cabin while underway. Is gennie the only way to go??

On a trawler, a gennie is not the only way to go, but probably the most practical. Aircon can run off gennie when cruising/anchored. Then on shore power at the dock.

Lots of ways to get aircon. Lots of creative engineering can be done. But the industry at this point has settled on electric aircon driven by AC power, either from gennie or shore.

Our planned use is to Marina hop and be on shore power while docked or otherwise be underway. Has anyone set up an alternator (pick up the AC current from it) to run HVAC and other AC power requirements while underway? It would seem to me that a diesel running at between 900-1250 RPM would have more than enough torque to turn an alternator to generate the power needed to run the HVAC units on a boat. If we build one, we may end up getting a gennie for resale value (if it helps), but want to understand options of maybe not getting a gennie with the boat. Thanks.

Mark P said:

If we build one, we may end up getting a gennie for resale value (if it helps), but want to understand options of maybe not getting a gennie with the boat.

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I can’t speak for others, but I would not buy a boat without a genset. From a resale perspective you are limiting your prospective buyer base to those with similar plans...those that find it acceptable to hop from marina to marina. I can’t imagine going to the Bahamas and hanging out in marinas.

As Ski notes above, a gennie is not the only way to power your A/C but it is the most practical because you can use it underway and at anchorage and for power to recharge batteries using your shore power charger and other AC devices like your water heater.

But to answer your question, yes you can set up an alternator and inverter to run your A/C but it will take a big one of each. You will need to install at least a 200 amp alternator on your engine which is a large case, high output one with external voltage regulation. That gives you a nominal 2,400 watts of DC. Most 16,000 btu A/Cs need about 1,800 watts. Any smaller alternator and you will be overloading it or running your batteries down.

You can't just hook up an AC alternator to your propulsion engine because it will not operate at the right frequency.

You also need a big, 2,500 watt minimum inverter to convert the DC to AC to run your air conditioner.

Years ago there were packaged systems to do this, can't remember the brand and not sure that they are still in business as I haven't read about them recently. But they weren't cheap, about $5,000, and more for installation.

You can buy a small NextGen 3.5 KW generator for about $5-6,000 and a better Northern Lights 5 KW for 2-3 thousand more.

David

The only reason an engine powered compressor would not work is , what would you do at the dock?

Underway a belted system would be far more powerful than a 120V 15A unit .

A heat exchanger , instead of a radiator would be no big deal to install.

Coldplate refrigeration system

I have a 36 grand Banks trawler that has a original Technicold Cold plate Refrigerator and freezer system. I converted it to 134 and my pressure settings are 15 on the suction and 150 on the high side. Is this where I should be if not what should the readings be. It’s cooling but will not get below 50° in the refrigerator