Fridge duty cycle test

[Mark26]

We are told that our fridges have a 50% duty cycle, but I have seen some people claiming a 20% duty cycle overnight when the door isn’t opened and the ambient temperature is reduced.

Sitting on board in a very quiet spot I decided to test mine.

The fridge is half full of food and drink, all of which are nice and cold.
The fridges thermostat is set to max (lowest temp inside the fridge)
It has been a hot day 27C, and it was 21C on board when tested at 23:00
The door was last opened 1 hour before the test.

Test1 Fridge off for 4 minutes 23 seconds
Test2 Fridge on for 5 minutes 23 seconds
Test3 Fridge off for 4 minutes 23 seconds
Test4 Fridge on for 5 minutes 11 seconds

Bearing in mind, it’s been a hot day and the fridge is set to max, for the purposes of calculating power consumption, I’m happy or go along with a 50% duty cycle.

It would be interesting to repeat the test sometime with the fridges thermostat at a lower setting.

If I was really bored or wanting an accurate test, I’d carry out the test again first thing in the morning. But I’d rather just cast off and go boating.

 

[Mark26]

At 05:00 the next day, after being on and untouched all night it was on for 4 minutes 25 secs and off for 5 minutes 15 seconds.

averaging out the morning after with the night before, yep 50% duty cycle. Now where’s me coffee?

 

[Daverw]

Just to add comment as a fridge engineer, those cycles seem very short, when they are this short the system has little time to get settled and the first part is not really doing any duty. Can the thermostat differential be widened? i replaced our mechanical stat for a digital one when I got the boat 5 years ago, we have a cycle of on 10 minutes and off 40 minutes

 

[vyv_cox]

My condenser is water cooled using once- through seawater. In Greece at this time of year the sea temperature is somewhere around 27 C and the ambient in the galley where the fridge is might be 35C. The fridge normally runs for 33% of the time, usually when settled, on for around 5 minutes, off for 10 or 11.

The fridge compressor consumes about 4 amps at the start of its cycle, reducing to about 3 amps at the end. The seawater pump consumes 0.5 amps, a 12 volt motor running on 6 volts using an Isotherm voltage reducer.

With 3 x 110 Ah batteries and 125 watts of solar it will run 24/7 quite happily but by mid August the solar input begins to be insufficient and some motoring is needed.

 

[Neeves]

I became excited about the same topic years ago - and the data is on the boat and I'm at home (it is winter here and refrigeration has dropped well down the priority list). Our concern is evening heat as temperature drops to single digits, not unpleasant but a bit chill.

We have a catamaran - which makes many of the fixes I describe much easier than on a mono.

Our fridge was then a Dometic unit with a remote compressor. The compressor was fitted 'behind' the fridge under some upholstery with a simple vent in the mouldings (for the seating) into the cabin. The air where the compressor was located was possibly one of the warmest places on the boat (though everywhere gets warmer in the summer (obviously). I ducted air from the anchor/chain locker using domestic kitchen extractor fan ducting to the compressor and added a computer fan at the compressor end of the ducting, so that it sucked from the anchor locker and blew this fresh air over the compressor. I also ducted the air from the location into the engine room (where there is another fan doing much the same for our deep freeze (completely separate unit).

After 20 years we replaced the firidge with a new Dometic unit, exact same external size - except the compressor is now housed in a recess in the top of the unit. We have used, with a bit of frigging about, the same ducting etc.

The computer fan that sucks from the anchor locker is wired into the supplied fan on the compressor unit (so only comes on when the fridge cycles on).

When I did all of this originally the power consumption went up, because I had a tiny computer fan added (0.1 amp), but the change was insignificant but we did make an appreciable difference in cycle time (but as I said at the outset I don't have the data - and it was a long time ago).

My point is, sorry its a longwinded explanation - if your compressor is in a confined space - you can make positive changes.

Computer fans are cheap as chips, I recycled mine from old computers (so free). Its technically not complex - you just need to think how to exhaust the warm air and how to introduce cooler air to the location. The coolest air is usually outside and if you are at anchor - from forward.

The other thing to do is fill the fridge (just water will do, or Australian white wine ) as when you open the door the cold air does not all fall out as you rummage about, to be replaced by warm air - that the fridge then needs to cool down - but you know all this .

Vyv mentioned running the engines - which reminds me. When we run our engines, for the same reason as Vyv, we will charge everything up that needs charged (and if offshore - use excess power for the desal unit and bake bread (we are lotus eaters and have an electric bread maker ). We normally would not run the fridge at its coldest but would run it more cold when the engine is on, so getting that white wine so cold its undrinkable (it soon warms up when you need it). If we have the space we would chill the gin in the fridge as well (I prefer malt whisky 'not cold').

Jonathan

 

[nigel1]

I have a frigo boat unit.
Chnages I have made have been to improve the door seal, reduce the opening in the back of the firdge where the thermostat wire and evap plate pipe comes in.
Added a better fan to the compressor, and an additional fan to blow cool air from the bilge into the locker where the compressor is. Added a vent to the top of the locker to allow warm air out.
Added a fan to the inside of fridge and put some vent hole in the solid acrylic fridge racks to improve circulation.
Removed original mechanical thermostat and fitted an electronic stat. This is set to stop the compressor when fridge at 2 C, and come back on at 5 C.

I bough a remote bluetooth temp sensor which can be placed in the fridge and will send the data to a smart phone/tablet.
Couple of weeks ago, I logged the temps over night, and on average, the compressor was on for 15 minutes, and off for between 40 to 60 mins.

During the day, cycles are reduced due to increased outside temps, and opening of the door.
I noted that opening the door for about 10 seconds caused fridge temp to rise about 0.5 to 0.7C

 

[PaulRainbow]

My fridge is top loading, with a remote compressor in a cupboard under the sink, not too hot in there, it's well vented. It switches on roughly every two hours and runs for 10-12 minutes.

Answers on a postcard.....

 

[Mark26]

My fridge is top loading, with a remote compressor in a cupboard under the sink, not too hot in there, it's welled vented. It switches on roughly every two hours and runs for 10-12 minutes.

Answers on a postcard.....

Blimey, that’s the best I’ve heard of from a electrical consumption point of view.

Thanks for the comments Daverw.

 

[vas]

I have a frigo boat unit.
Chnages I have made have been to improve the door seal, reduce the opening in the back of the firdge where the thermostat wire and evap plate pipe comes in.
Added a better fan to the compressor, and an additional fan to blow cool air from the bilge into the locker where the compressor is. Added a vent to the top of the locker to allow warm air out.
Added a fan to the inside of fridge and put some vent hole in the solid acrylic fridge racks to improve circulation.
Removed original mechanical thermostat and fitted an electronic stat. This is set to stop the compressor when fridge at 2 C, and come back on at 5 C.

I bough a remote bluetooth temp sensor which can be placed in the fridge and will send the data to a smart phone/tablet.
Couple of weeks ago, I logged the temps over night, and on average, the compressor was on for 15 minutes, and off for between 40 to 60 mins.

During the day, cycles are reduced due to increased outside temps, and opening of the door.
I noted that opening the door for about 10 seconds caused fridge temp to rise about 0.5 to 0.7C

done almost identical moves as Nigel.
fridge is now useable...

Just one note, the Danfoss controllers are "sensitive" on the FAN load, so be careful when wiring larger (and older!) fans on the ducting to bring fresh air to the compressor. Had a puzzling day when fridge would look like kicking in, but wouldn't... Ended up replacing the one 150mm sq fan with a more recent one with lower Amps consumption all fine.

Further, I do think that getting a 5mm LED wired to the right terminals and placed somewhere handy (in my case next to the chinese digital controller) is vital in figuring out what's happening. A LED costs nothing the error codes, help immensly!

V.

 

[vas]

Blimey, that’s the best I’ve heard of from a electrical consumption point of view.

Thanks for the comments Daverw.

bet if Paul brings his boat down to the Med he wont see these numbers unless it's January

 

[nigel1]

Just one note, the Danfoss controllers are "sensitive" on the FAN load, so be careful when wiring larger (and older!) fans on the ducting to bring fresh air to the compressor. Had a puzzling day when fridge would look like kicking in, but wouldn't... Ended up replacing the one 150mm sq fan with a more recent one with lower Amps consumption all fine.

V.

I think the max fan load on the controller is 0.5 amp. The 3 fans I have running have a total load of 0.31 amps. If needed, the fans I got were supplied with an optional inline resistor, which will reduce fan speed/load.

 

[GHA]

bet if Paul brings his boat down to the Med he wont see these numbers unless it's January

?
Mine goes from 10/20% 2h duty cycle in winter to sometimes 100% if a load of warm stuff is added in the summer.
Timely thread though, provided some motivation to play around more with an esp32 based thermostat, messed up the circuit a bit but should just get away with it. Will be able to varispeed the compressor & alter settings based on battery SOC or voltage or anything really. Hopefully. Does leaving the lid off actually make much difference........ might know soon

 

[vas]

mind, haven't seen any significant changes by altering speed of compressor, so just left it at lowest. Higher/est just seem to consume more juice with no improvement of cycling (in my experiments at least!)

 

[GHA]

mind, haven't seen any significant changes by altering speed of compressor, so just left it at lowest. Higher/est just seem to consume more juice with no improvement of cycling (in my experiments at least!)

i've yet to wire up the full controller but watching the smartshunt current it looked like pulling 3.25A/3.85A/5A/5.85A running at the different revs. Though the current reduces a little as well as the compressor runs.

 

[vas]

GHA: easy to run the fridge compressor one day at low rpm and next on full up and see cycling (obvs with similar patern usage)
no difference,
I still use the pot (since I drilled the dash and fitted it!) to crank up speed when I fill the fridge with stuff. I think it's mainly psychological that it actually does something and things cool faster. Next day I drop it to low and leave it there till next fillup...