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Old 01-03-2019, 04:39   #1
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Ac water during cold weather

Hi All .. my question is around winterizing in freezing weather. My boat will remain in water year round but periodically it can get below freezing. Regarding that a/c heater unit. This required water in operation. When not in use how to avoid that water from freezing. Does it naturally drain ? Should I loosen the intake water filter ? Thanks Dennis
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Old 01-03-2019, 06:42   #2
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Re: Ac water during cold weather

I would disconnect the hose from the thru hull and blow it out with compressed air. Not too forceful as you could I guess damage the pump if you pushed a huge volume of HP air through.
Iíve never had to winterize a boat though, so Iím not speaking from experience.
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Old 01-03-2019, 06:44   #3
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Re: Ac water during cold weather

The A/C system will drain a little bit not completely. Youíll either need to blow out as a-64 suggests, fill with antifreeze, or leave the heater on so that water is always moving.
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Old 01-03-2019, 07:55   #4
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Re: Ac water during cold weather

Agree with the others that you need to drain the water heat exchanger/coolant line. Not a real big fan of just blowing air through a system and hope it gets all the residual water out. Have seen too many times, lines freezing in low points from residual water pooling then causing cracked line/blow outs.

IMO less worries to fill with antifreeze. Little bit more of a pain to fill, but not too bad. I'm sure there are other ways, but we attached a hose to the heat exchanger (that is long enough to give you good head pressure) and w/a funnel on top filled with antifreeze.


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Old 01-03-2019, 08:36   #5
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Re: Ac water during cold weather

Tell us more about that "periodically it can get below freezing" statement.

If it just drops below occasionally, you probably needn't bother. Think of it this way; if a bird bath on shore would freeze solid, then it's time to start worrying. Just a skim of ice over the water in the bird bath, no problem.

Also, what's the water temperature? Assuming that's above freezing, it'll take a longer time for whatever's inside the hull to get down to freezing - if ever.

Finally, salt water (which is what would be in the air conditioner hoses and heat exchanger) freezes at a much lower temperature than pure water.
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Old 01-03-2019, 09:02   #6
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Re: Ac water during cold weather

I did not have a chance to test that this year, since the boat is hauled. My thought had been to keep it running in reverse-cycle all winter. As a test, I’ve plumbed the output into a T in the lavatory drain, so that it discharges below the water line. This would avoid a big icicle forming at the through-hull and might have some ice-bubbler activity.

To fill the loop with antifreeze, the simple method I use for the engine would probably work. A cheap garden hose Tee (barb x barb x 3/4 hose) with a screw-on cap is inserted near the through-hull. A 4-foot hose is kept coiled nearby. To winterize, simply unscrew the cap, screw in the hose, and turn on the pump for a moment to suck antifreeze solution from a bucket. To spend a little more money, one could replace the Tee with a 3-way valve, but you know, it’s 50 cents vs 30 dollars.
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Old 01-03-2019, 09:12   #7
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Re: Ac water during cold weather

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Originally Posted by CaptTom View Post

Finally, salt water (which is what would be in the air conditioner hoses and heat exchanger) freezes at a much lower temperature than pure water.
Not as much as you might think. The salt content of seawater will reduce the freezing point to 28 Deg. F which isn't that much lower on which to base a decision for winterization.
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Old 01-03-2019, 09:48   #8
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Re: Ac water during cold weather

I'm on the Columbia River. We get freezing temps and snow. I have a reverse cycle heat pump and cannot use it because the water temp is 37 degrees F. 40 degrees F is the minimum. I'd just installed the unit, turned on and was anxiously awaiting a warm boat. About 20 minutes later as I watched the tubes that were supposed to be hot, frost over, I saw the discharge freeze solid. Immediately shut the unit off. Had I not done so, it would have destroyed the brand new unit. Called the manufacturer and learned I should have read the directions a bit more carefully. As the water goes through the unit, it drops 5 degrees F in temp which is why 40 degrees is the number. A buddy has one in a different area here in Portland and his water is a few degrees warmer. he reports it doesn't heat hardly at all. I've not had any issue with it freezing and have done nothing to winterize it. I use an infrared temp gun and shoot temps in the engine room which is not heated. Even on 20 degree nights, in the morning the temps in there are around 37, same as the water.
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Old 01-03-2019, 09:48   #9
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Re: Ac water during cold weather

We're liveaboards in Annapolis, MD and we'll use the reverse A/C until the water temperature hits about 40 degrees (F). That's when I'll shut down and winterize the system for the remainder of the winter. I blow air through the lines (fwd and aft exits) and then pour antifreeze into the lines. I don't like to let a lot of antifreeze sit in the lines so I blow out the antifreeze. This ensures there's mostly air in the lines and if I have any low spots any remaining antifreeze in the lines will pool there. I then connect a hose to the seacock, blow air through it and seal it off. We rely on two oil-filled "radiator" like heaters over the cold months for heat. Why do we do this each winter?.... we never know when we may have to leave the boat for an extended period of time and that just gives us some assurance that if we get a cold snap, we're not going to ruin the A/C system. A/C units are not cheap to replace.
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Old 01-03-2019, 10:41   #10
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Ac water during cold weather

Even in a house Iíll only use the big oil filled heaters that look like old fashioned radiators.
This after having a bathroom heater fall over and almost catch the rug on fire, if I hadnít smelled it, it would have.
Yes it did have the tip over off switch, that apparently stuck on.

The big oil filled radiators canít catch anything on fire, they canít get hot enough, plus I think due to their larger surface area and lower temp, they are more efficient as in more heat for the same power
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Old 01-03-2019, 10:53   #11
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Re: Ac water during cold weather

If you leave a heater (A/C reverse heat or a small engine room heater) going while away then the interior will not freeze. If the A/C is not running, then close the seacock and undo the lowest connection, that will drain most of the water (into your bilge).

We leave our A/C running in defrost mode while we're away in the winter. Helps keep the mildew down and the boat warm enough to not have to worry about freeze damage. Although where we are the air gets COLD (teens) but the water stays above freezing all winter.
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Old 01-03-2019, 10:56   #12
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Re: Ac water during cold weather

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Originally Posted by Cpt Mark View Post
I'm on the Columbia River. We get freezing temps and snow. I have a reverse cycle heat pump and cannot use it because the water temp is 37 degrees F. 40 degrees F is the minimum.
I knew that number existed, I just didn't know what it was. Probably could have figured it out with a little more thought...

But part of the tortured justification for putting in the AC unit was, "This will replace the heater and the dehumidifier so it's no net increase in cargo!"

Oil-filled heater is back in there now.

BTW: I was out of the country during the unexpected cold snap in Nov 14 and did no winterization at all. Everything came through OK. But of course, although the air temperature was 10F, the water hadn't cooled that much yet.
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Old 01-03-2019, 11:03   #13
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Re: Ac water during cold weather

On our last boat we had a reverse cycle A/C heat system with an electric heat coil. My temp. limit for reverse cycle to pull usable heat out of water is 45-50F. After the temp was too low I closed off the outside water supply and winterized the heat exchanger and only used the heating coil. With the heat option, one could still use the built in thermal controls in the unit and the blower to circulate the air to more evenly distribute the heat. Not all units have this heat option, but was glad I learned about it before purchasing the unit.

Made it through several cold winters with this set up.


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Old 01-03-2019, 11:50   #14
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Re: Ac water during cold weather

The discussion mostly covers the raw water AC loop. Up here on Lake Ontario where winters can dip into negative degrees Fahrenheit, we also winterize the condensation loop. There is tray under the compressor that gathers condensate, and another pump that discharges it through a different thru-hull. So if you determine you need to winterize, don’t forget that part.

I think you just need to pour some antifreeze in the condensation tray. There’s usually a pressure sensitive pump that kicks in and takes care of the rest.
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Old 01-03-2019, 16:02   #15
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Re: Ac water during cold weather

I know this is not really an answer to your question--but it is a good idea if you live in areas where freezing is a problem, to use keel-cooling for both engines and air conditioners. One can use separate pipes for each.

The closed circuit keel-cooled systems contain antifreeze solution, which is pumped around the cooling systems of the air conditioner and the engine via impellers which are usually belt (for engine) or by electrically driven vane pumps.

Keel cooling has so many advantages, it is probably why commercial vessels rely on it.
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