May 10, 2019

Tidbit: Fresh water as coolant for air conditioning and refrigeration systems

This is one of a series of brief, no nonsense posts that we call aTidbit:
noun; small and [possibly] particularly interesting item of gossip or information...
The purpose is to share succinct posts about lessons learned, or things we use or do that work [or don't...] that are common to most of us boaters. 
Our goal is to garner feedback from those of you having first-hand experience with a different approach/ solution/ product/ or additional useful information to share...  
We never assume what we are sharing is the ideal or only; it just seems to best suit our needs [and/or habits and/or budget] from our experiences thus far...

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Many boats have pump driven raw water cooling loops for air conditioners [A/C] and refrigeration systems. 

Water cooling is required on the A/C units when the compressor is runing for either cooling or heating [e.g., reverse cycle heating.] 

Water cooling also boosts the DC powered refrigeration system efficiency [beyond the electric fan most have...] when ambient temps in the compressor location exceed 90°F.

Our 3 air conditioners serve two functions: cooling and heating [using either reverse-cycle heat or resistance coils.] We often use the A/Cs for heat when at the dock in cooler weather. [We are currently at 56°N in SE Alaska.] 

Why use electricity?  Where we are, electricity costs about the same as the amount of diesel we would burn if we used our Espar heater, and this way it keeps the hours off the Espar...

We are lucky in that the Pacific waters usually stay warm enough for reverse-cycle heat to work well. [Water temps needs to be above ~42°F for reverse cycle heat to work well...] Therefore our A/C units also have resistance heat coils as a back-up. [i.e., just like a portable electric heater...] But resistance heating is not as efficient and requires more electricity to produce the same amount of heat as reverse cycle does, so if using electric heat, we prefer running reverse cycle on the A/C compressors...

However, since we also enjoy venturing to higher latitudes with even cooler water temps, we experimented with using a potable water tank for the cooling water loops because the water in those tanks [even though they sit low in the hull...] averages 10-20°F warmer than the water we are floating in; Perfect for using reverse cycle heat in colder waters...

This approach of is not new or unique: The previous owner of our boat did just this with one of the refrigeration water cooling loops. Great idea. Lets extend this to the air conditioners/ heat pumps...

For a couple of years now we have been using one of the boat's potable water tanks for all the cooling water loops— instead of raw water. [Our two potable water tanks— 110 gallons each— are low in the hull, but stay warm enough for efficient reverse-cycle heat— even in freezing water.] We dedicated one tank to this use. It is also still a back-up potable tank if needed...

The cooling plumbing can be easily switched back to raw water again if needed via 3-way valves. [We never intend to switch back to raw water, but can if necessary...] If we switched back and forth, we would have to clean and sanitizing the raw water loop[s] before switching back to potable... [More below...]
We should mention all our drinking water runs through a .5µ filter...
The advantages of using fresh water in cooling loops include:
  • Greatly reduced maintenance [eliminated really...] on the cooling water pumps and loops with consequent longer lifespan of those components [fresh vs. salt water]
  • Fewer open through-hull valves
  • Reverse-cycle heat [more efficient than resistance heat] works when it otherwise wouldn't when in cooler raw water temperatures
Some disadvantages are:
  • Coolant loops that were initially used with raw water need to be hyper cleaned/sanitized before switching to potable water [if the dedicated tank is also a back-up potable tank...]
    • If installing new there is no problem connecting to potable water tankage...
  • The potable tank used for these coolant loops is now 'emergency' only back-up for potable water— or we need to remember to turn off the refrigeration water cooling loop, and not use the A/C heat pump[s] 
    • We placed a check list next to water tank selector valve as a reminder
To keep the water fresh in this potable water tank used for recirculating cooling water, we routinely pull water from, and immediately refresh this tank when doing laundry while running the watermaker or when at a dock.

This set-up works well for us, and has the added benefit of saving kind souls everywhere from feeling compelled to urgently inform us our bilge pump is running continuously... [Our raw water cooling discharge is above the waterline...]

Can you think of other advantages or disadvantages to this approach?  It has been working well on our boat for years...

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