A thought on compressor fridges..

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A thought on compressor fridges..

April 12, 2022, 11:53:52 am

When considering the relative advantages of replacing an ammonia-cycle propane-powered fridge with a compressor model, the most common points mentioned are faster cooling and better efficiency. I want to examine the efficiency issue.

A propane fridge on AC uses about 350 Watts, and if it needs to be running 30% of the time, this is a rate of 105 W-hr per hour. The 12V compressor uses about 5A, so at the same 'on' duty cycle, that would be 18 W-hr per hour. Since the fridge off-grid would run on either propane or battery, and propane 'power' for the ammonia-cycle fridge should be about the same as on AC, this points to the compressor model being about 6 times as efficient.

Here's the rub:

In 24 hrs, the compressor fridge would consume 18 X 24 = 432 W-hr from the battery, or 432 / 13 = 33 Amp-hrs from a 13V LiFePO4 battery bank. This is the fridge alone, assuming an average of 30% on-time, which will depend on many factors. Replacing that daily will either require adequate solar and sunny days, or running the generator and converter. And, of course, this is not the only demand on the battery.

In the same 24 hrs, the propane fridge will consume 105 X 24 = 2520 W-hr, or about 8570 Btus of propane energy. Propane contains 91,500 Btus/gallon, so ignoring combustion efficiency, a gallon of propane will last about 10.5 days - sunny or not.

Point is, fridge efficiency should not be a deciding factor from the RVers point of view, though the full-hookup park might prefer it was...

Steve

Re: A thought on compressor fridges..

Reply #1 – April 12, 2022, 04:55:40 pm

Thanks for a very informative post, Steve! From your numbers I can draw a few broad conclusions:

1. Running a propane fridge on electricity takes significant power. With 120 VAC shore power, 350 watts is no big deal. But doing it on battery power via inverter means 30 amps at 12 VDC, and that will run down batteries very rapidly. Having done that accidentally a few times myself, I can testify to that. These fridges should be run on propane unless shore power is available.

2. A compressor fridge uses much less electrical power, but its requirements are not trivial. When shore power is not available, that means a significant investment in solar panels and batteries. Of course it uses no propane... but as you pointed out, propane goes a long way when running a refrigerator.

3. Since most RVs don't come from the factory with sufficient solar and battery capacity to support a compressor refrigerator, propane fridges are a good choice for most users.

Re: A thought on compressor fridges..

Reply #2 – April 12, 2022, 07:53:43 pm

Agree with Steve's analysis. There are so many factors to be taken into considerations when deciding whether to replace a factory installed propane fridge at the end of its life, whether to stay with propane or switch to a compressor driven unit. Lifestyle in an important factor. When we were doing a lot of traveling in the west US and a lot of boondocking, we did not dare to switch to 12v DC fridge even we had plenty of solar energy (600W), we rather save the solar electricity for other use such as induction cooktop, microwave, and electronic devices. Now as we have changed to the "snow bird" lifestyle, staying in the campgrounds most of the time except during travel, it make more sense to replace the end of life propane fridge with a 12v DC compressor unit. We have plenty of solar charge and engine alternator charge to be stored in the 440AH battery set for using at night. Our rig does not have an auto leveler, we had to spend time to level the LD to protect the fridge, once to change to the 12v unit, we will no longer have to worry.

Re: A thought on compressor fridges..

Reply #3 – April 13, 2022, 12:50:15 am

"we did not dare to switch to 12v DC fridge even we had plenty of solar energy (600W), we rather save the solar electricity for other use such as induction cooktop, microwave, and electronic devices."

Yes, it's a matter of choices. :-) Like you, I travel the Southwest and have generally had at least 400 W of solar panels... but I don't use a microwave oven or induction cooktop. Instead, I cook with propane, which I find uses very little propane even when baking, and save the solar electricity for my computer and compressor fridge. That's not better or worse than what you've been doing; it's just a different set of priorities.

Actually, I got through this past winter with only 200 watts of solar panels, although it was dicey a few times. The trick was using panels laid on the ground, tilted to the correct angle for the season (about 30°), and turning them two or three times a day to face the sun. I did this because, procrastinator that I am, I haven't yet gotten around to installing the 400 watts of panels that will go on the roof of my rig. Obviously what I've been doing isn't the best way to go in the long run, but it does show that one can get by with surprisingly little solar input if one is willing to work at it. (200 watts on the roof would not suffice, since the two panels need to be tilted, and re-aimed multiple times each day.)

Since I'm talking about solar panels, I'll also mention that I've found that a 100 W panel laid flat on the ground is more than adequate to power my truck fridge, a 47-quart (1.6 cu. ft.) Dometic CFF 45. The panel feeds into a group 27 AGM battery via a Victron 75/15 MPPT solar controller. Now that I have a topper on my truck, I'll be installing the solar panel on the topper, so I don't have to lay it on the ground. Having a full-time fridge in the truck is not only extremely convenient, but it serves as backup storage for my rig's 4.6 cu. ft. Vitrifrigo C130 compressor fridge. (There isn't space in my 19' Airstream for the 7 cu. ft. propane fridge used in Lazy Dazes.)

Re: A thought on compressor fridges..

Reply #4 – April 13, 2022, 01:08:34 am

"Our rig does not have an auto leveler, we had to spend time to level the LD to protect the fridge..."

I can't resist putting in a word for LevelMatePRO. I know I've mentioned this device before, but in a nutshell: it tells (via an app on my phone), in quarter inch increments, exactly how much I need to change--front to back and side to side--to get level. The display updates in real time, so when driving onto ramps, all I have to do is go until it reads "0.0" in both directions.

I've seen a number of electronic leveling aids over the years, but none of them was a tenth as good as this. I can't begin to tell you how much time and frustration it has saved me.

Re: A thought on compressor fridges..

Reply #5 – April 13, 2022, 11:52:16 am

I’m putting a 2.0 cu ft Norcold 12vdc/120vac fridge under the existing lazy daze fridge combo. It fits perfect in the space and I think actually has more storage space then the drawers it replaced. Looking at the specs it is very doable having a very low amp draw for its size and being just a extra fridge. I already had the 12v fridge on hand so no extra cost.

Re: A thought on compressor fridges..

Reply #6 – April 13, 2022, 04:16:38 pm

Love this analysis, thanks Steve!

Given my pending solar setup, can’t wait to have my fridge replaced when it gives up the ghost, which probably won’t be for another 10 years or until I get tired of making propane runs.

Thanks for the recommendation on LevelMatePRO Andy. Needed something for level readings, on the way home used the iPhone Measure app, placing it on the floor next to the drivers seat, figured It was better than nothing.

Re: A thought on compressor fridges..

Reply #7 – April 13, 2022, 06:57:29 pm

Quote from: Andy Baird - April 13, 2022, 01:08:34 am

"Our rig does not have an auto leveler, we had to spend time to level the LD to protect the fridge..."

I can't resist putting in a word for LevelMatePRO. I know I've mentioned this device before, but in a nutshell: it tells (via an app on my phone), in quarter inch increments, exactly how much I need to change--front to back and side to side--to get level. The display updates in real time, so when driving onto ramps, all I have to do is go until it reads "0.0" in both directions.

I've seen a number of electronic leveling aids over the years, but none of them was a tenth as good as this. I can't begin to tell you how much time and frustration it has saved me.

Thanks for the tip, Andy! There have been some times when leveling has taken way too long so hoping this device will help. Just ordered!

Re: A thought on compressor fridges..

Reply #8 – April 13, 2022, 07:44:38 pm

Electronic levels seem pretty clever. Personally, I’ve always used a $5.00 Bullseye Bubble placed on my kitchen sink countertop. Simple and works every time.

I’ve also made a small notebook of sites in many CG’s. It includes the LD tires and the levelling blocks with their height required to level the rig. Works great.

Andy’s tool is no doubt accurate. With that, there is a large price difference between these two methods. One day I may shell out the cost for Andy’s tool. I like upgrades.

Just sayin’

Kent

Re: A thought on compressor fridges..

Reply #9 – April 13, 2022, 07:50:09 pm

I hear you, Kent, but what I like about Andy’s solution is the real-time aspect of it which allows leveling while sitting in the driver’s seat. And measurements accurate to 1/4” is quite nice. I’m all for saving time doing set-up chores when traveling which allows more time for the fun stuff!

I like your idea of detailed notes on the different sites. Might start doing that also!

Re: A thought on compressor fridges..

Reply #10 – April 13, 2022, 08:38:36 pm

Quote from: Kent Heckethorn - April 13, 2022, 07:44:38 pm

Andy’s tool is no doubt accurate. With that, there is a large price difference between these two methods. One day I may shell out the cost for Andy’s tool. I like upgrades.

It’s a matter of perspective.

To me, that leveling tool is far less expensive than hydraulic jacks and less weight too.

Then again, I could keep using the iPhone measure app and be closer to getting that compressor fridge I want.

Re: A thought on compressor fridges..

Reply #11 – April 14, 2022, 06:32:29 pm

I bought the Level Mate Pro a couple years ago, the thing works great and you can be as picky as you want when it comes to leveling. It's all right there on my phone, no more arguments with the wife.

Jon

Re: A thought on compressor fridges..

Reply #12 – April 14, 2022, 07:20:06 pm

A pair of small levels, one mounted on the inside of the driver's door and the other on the dash is a low-tech method of determining the level from the driver's seat.
This allows slowly driving around a campsite, watching the levels to find the most level spot, one requiring the least amount of leveling blocks or none at all.
Amazon.com: Hopkins 08525 Graduated Level : Automotive

To install, level the RV, using a torpedo level inside the refrigerator to determine when the rig is level.
Sit in the driver's seat and find a spot on the door and dash that are visible while driving. Use these spots to mount the short, dash levels, each in the perfectly level position, they come with peel-off adhesive backs. Mount the door level with the door closed.

Larry

As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.

Re: A thought on compressor fridges..

Reply #13 – April 14, 2022, 08:08:40 pm

Thanks for the tip Andy. I just ordered it from Amazon and used a $20 off coupon that’s being offered now.

I’ve used bubble levels for years and decided to treat myself. A bubble level and the cheap electric versions will tell you when you’re level but at times it can be difficult to judge how much extra thickness is needed at a particular wheel. I use a set of ramps plus a little collection of 2x6’s and cut sections of horse stall matts for my leveling needs. It will be so nice to know what thickness needs to be added rather than all the guessing that I typically do.

Re: A thought on compressor fridges..

Reply #14 – April 22, 2022, 01:18:49 pm

Just received the LevelMatePRO and was going to install it but will have to wait until another day. Happy Spring?

Re: A thought on compressor fridges..

Reply #15 – April 22, 2022, 03:02:51 pm

Brrrr, looks like you could ice skate on Lake Newton.

Re: A thought on compressor fridges..

Reply #16 – April 23, 2022, 05:49:16 pm

I have the 2 bubble levelers-one mounted on the dash and one on the door. To double check our level, I have downloaded a free Clinometer App on my cell phone. I open the app and then lay my phone flat on the floor in front of the fridge or on the bottom shelf of the fridge. It's simple to read and very accurate!
Mottleycru

Re: A thought on compressor fridges..

Reply #17 – May 02, 2022, 02:33:15 pm

For those interested in the LevelMate Pro, I found a pretty good install video from Techno RV. His install is on a trailer so some of it doesn't apply to a motorhome but helpful nonetheless. Getting ready to install mine tomorrow.

https://www.youtube.com/watch?v=7UDVe-0tSJY

Re: A thought on compressor fridges..

Reply #18 – May 02, 2022, 06:20:08 pm

That's a very complete installation tutorial. For those who don't have a LevelMatePRO, the process may look complicated, but that's just because the presenter goes into so much detail. It really isn't hard at all. Mount it, type into the app your RV's width and wheelbase, level the rig, and tell the app "This is level"... and you're done.

For the record, there's a competing device, Rhino Storm RV Leveler, that costs about half as much. It looks like a knockoff, and I'm a little distrustful of such. But the one comparison review I found said that it works OK, except for much shorter range than the LevelMatePRO. If somebody wants to save a few bucks, it might be worth a try.

But as the reviewer says at the end, LevelMatePRO is "tried and trusted," so I'm sticking with that. Knowing that each LevelMatePRO is individually calibrated, and is tested for a couple of hours at varying temperatures in an environmental chamber, gives me a good feeling about the company.

Re: A thought on compressor fridges..

Reply #19 – May 03, 2022, 08:34:26 am

I like the idea of the LevelMatePro. this has been a very good read this morning. I would like to find the coupon before ordering if it is still available.

Re: A thought on compressor fridges..

Reply #20 – May 03, 2022, 09:10:37 am

Quote from: spartan11 - May 03, 2022, 08:34:26 am

I like the idea of the LevelMatePro. this has been a very good read this morning. I would like to find the coupon before ordering if it is still available.

Yeah, there was a $20 coupon available when I purchased from Amazon but it’s gone now. TechnoRV may give a discount if you sign up for their email list.

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