I cut my fridge power by 75%. Really. No joke, no trick, no compromise.

[sailingharry]

So I think I've commented in other threads about my fridge/freezer baseline consumption. I had been running about 160Ah/day, which is a lot of power. Since the freezer keeps the fridge cold, the fridge compressor almost never turns on. The compressor burns about 6A, and so 160Ah/day is about 24 hours of run time a day. It basically NEVER turns off. Worse, I only get the freezer down to about 20F. I think this is really not cold enough for hamburger, but the admiral thinks it is awesome for ice cream. Hmmmm.....


So I did some digging. The results are too good not to share!


WARNING -- ENGINEERING MUSINGS. PROCEED AT YOUR OWN RISK!



First, some engineering. In refrigeration, Pressure = Temperature. Lower temperatures result from lower pressures, higher pressures lead to higher temperatures. So refrigeration gauges actually have numbers for Bars, KPA, PSI -- and Temp. Really, same gauges. Keep this in mind.


Another detail. Cheaper systems (like a little dorm fridge, or even your car) use a capillary tube as the control from the compressor high side to the fridge. This drops the pressure, and as indicated above, the temperature. Crude, but it works. Well, on my Sea Frost, it has a Thermal Expansion Valve, or TXV. It senses the temperature going into the cold box, and adjusts the flow (ie, pressure) to get the desired temperature. Keep this in mind.


Well, my box, as I got it at purchase, had the TXV set for about 15F. I was unaware of this. I set my thermostat at 10F. So, the TXV is controlling the freon going into the icebox at 15F, and it won't shut off the compressor until the box gets to 10F -- never gonna happen! The result is I run my compressor 24/7, sending 15F freon into the box, and the box holds nicely at 20F -- while burning 160Ah/day! Crap.


So I took the cap off the TXV. On my boat, it's a red vinyl tube, about 1" in diameter and 1" long. There is a nut under it. I turn it clockwise to drop the pressure/temperature. I tweaked it down until my gauges read -5F on the low side. and hot wired the thermostat to let it run non-stop. The next day, the box was -2F. YAAAY! Too cold, of course (remember the ice cream?), but if you CAN get to -2, you can CONTROL at 19 (actually, we have agreed on 15). So I fiddled with the thermostat (for some weird reason, boat fridges control cold plate temp, not fridge temp), and am currently holding around 18F.


The acid test. I sat on shore power to keep the batteries at 100%, and let the fridge run for a few days to stabilize. Then I turned off everything but the freezer (fridge off too), and turned off shore power and solar. After about 4 days, I had burned 155Ah. I used to burn that in a SINGLE DAY! The freezer is holding around 18F, the fridge at around 33F (actually too cold, but I'm not sure how to fix that since the compressor never runs). This is a daily burn of 40Ah for our freezer/fridge.


I do realize it's still early spring. Highs are in the 60s and 70s, lows in the 40's. I'm sure consumption will rise in warmer climates (but not for us -- we are going north! LOL). But the current results are just staggering!


If anyone finds this MORE than just interesting (ie, you think it applies to you), I can provide more details


Harry
Circe -- 1998 Saga 43 #19
Mill Creek, Annapolis

[rslifkin]

Add more insulation between fridge and freezer and you should be able to maintain a bigger temperature difference between the 2.

[sailingharry]

I'm thinking about 1/2" of construction foam should do it. I can easily cover about 75% of the dividing panel, which should be enough (there are assorted interferences that keep me from 100%). I only need a "tweak," since I only need to bring the fridge side up about 3-5 degrees. Of course, if I get even more, then the fridge thermostat might actually DO something! LOL

[rslifkin]

Or you'd gain the option to set the freezer colder if needed at some point.

[GordMay]

Troubleshooting Thermostatic Expansion Valves
➥ https://icemeister.net/backroom/wp-c...ooting-TXV.pdf

TXV troubleshooting tips ~ Danfoss
➥ https://www.danfoss.com/en-us/servic...shooting-tips/

Tech Tips for Thermostatic Expansion Valves
➥ https://www.achrnews.com/articles/85...pansion-valves

[Monepit]

Very interesting post. I wish I could try this but my boat has separate fridge and freezer. I thought that was a great set up because we have more space for food but we anchored out for a week and the panels won’t keep up. Even with full sun. Makes me think though that the freezer is set on 10 and the fridge at 35. Maybe if I set it at 20 and 40 the compressors, and yes I have 2, wont run as much and that could help. May also look into adding insulation. Just had the boat 6 months and the solar for a month so there is still a lot of testing and tweaking to do. I have 630 watts of solar and 660 amp hour bank.

[rha9]

How did you determine what the existing TXV setting was and know how you were adjusting it?

[jen1722terry]

In our little 31, our fridge compressor runs, maybe 2 hours/day total.

We had an old top-loading ice box that was of the double wall design. We drilled holes in the inside shell of the box and pumped in closed cell foam to fill the void between the two walls. This, and the fact that the box is a top loader, enabled us to use very little juice. Our two Grp 31 AGMs and our 130W solar panel keep everything running well. Cold food, topped up batteries.

Now, if you have a box with less insulation and a front-opening door, things will be very different.

[flyingnut40]

Following

[bill99]

I was told by a fridgologist that fridge's don't like keeping air/nothing cold. So I always kept empty 2 litre and other plastic bottles that I filled with water, and would put in the bottom underneath everything else to fill up the void, so no empty space. My fridge motor would definitely cycle much less often. Saved power and made fridge more efficient.

[patprice]

Hello jen1722terry
I have a question please and apologies to all as slightly off topic.
It is my intention to drill and pump in closed cell foam to fill the void between the two walls.
But! I am concerned the pressure of the expanding foam may implode my FG box liner?
Should I position some temporary wooden structs in the box to prevent implosion?
How many holes did you make for the foam intrusion?
Did you do the job sort of slowly or quickly. Too slow I guess would prevent further ingress of foam?
I am in Tasmania but in a week or two will finally get to Greece to sail again on our Bavaria 42 after 21/2 years.
Which raises another issue. I wonder if the fridge has enjoyed being unused for all that time?
Any comments please.

[sailingharry]

Originally Posted by rha9

How did you determine what the existing TXV setting was and know how you were adjusting it?

I have (and know how to use) full AC gauges. They give a direct readout on the low side pressure, and the gauge is calibrated both in PSI and Degrees F (and C, I think). So, I hooked them up -- and almost immediately read 15F. This also allowed direct adjustment of the nut on the TXV -- wait 5 minutes, read, repeat.


If you don't have gauges, you can do it the hard way.
* Short the thermostat wires, so the unit runs full time.
* Come back in 24 hours and read the fridge temp (a simple residential fridge thermometer is fine). That will be pretty close to the TXV setpoint. If it's at least 10F below your target fridge temp, call it done. If not, start adjusting, maybe 1 turn per try.


Oh, to short the stat? On my boat, I added a feature, a toggle switch to turn on the freezer. The idea was that when motoring with excess amps, I could flip the switch and supercool the freezer. Wasn't much use last year, 'cause it was already running full time. May get a chance to use it this year! LOL

[StoneCrab]

Thanks for posting this. This is great practical info and you explained it well.

[carstenb]

Originally Posted by Monepit

Very interesting post. I wish I could try this but my boat has separate fridge and freezer. I thought that was a great set up because we have more space for food but we anchored out for a week and the panels won’t keep up. Even with full sun. Makes me think though that the freezer is set on 10 and the fridge at 35. Maybe if I set it at 20 and 40 the compressors, and yes I have 2, wont run as much and that could help. May also look into adding insulation. Just had the boat 6 months and the solar for a month so there is still a lot of testing and tweaking to do. I have 630 watts of solar and 660 amp hour bank.

Wow - this is close to our set-up We have 670 watts of solar and 660 house bank and we have a separate fridge and freezer. But we run the entire boat on the solar without any problems. The only time we run into issues is on passages if we are using the autopilot in heavy seas, then we run out of juice.

But if we use the windvane, we rarely have any issues. You must be burning juice somewhere else - or you should be looking hard at your set-up

OUr keel cooled fridge used about 1.5 to 2 amps per hour (during the day) in the tropics. Our freezer about the same - at night they use very little

[RaymondR]

Originally Posted by patprice

Hello jen1722terry
I have a question please and apologies to all as slightly off topic.
It is my intention to drill and pump in closed cell foam to fill the void between the two walls.
But! I am concerned the pressure of the expanding foam may implode my FG box liner?
Should I position some temporary wooden structs in the box to prevent implosion?
How many holes did you make for the foam intrusion?
Did you do the job sort of slowly or quickly. Too slow I guess would prevent further ingress of foam?
I am in Tasmania but in a week or two will finally get to Greece to sail again on our Bavaria 42 after 21/2 years.
Which raises another issue. I wonder if the fridge has enjoyed being unused for all that time?
Any comments please.

I built a smaller box inside the box which came with the boat. To fill the voids behind the glassed over plywood I used a 16 mm hole saw to place holes about 160 mm apart around the inside walls and made a big syringe out of poly downpipe to inject the 2 pack foam into them. I injected small amounts into each hole and gave the foam 15-30 minutes to expand before putting any more into each hole. I used short lengths of 16 mm dowel to plug the holes and after all the filling epoxied them in and trimmed them off flush with a renovator tool.