Thursday, 11 February 2016

Forced ventilation for the fridge

On the boat the other day I made a bit more progress in my quest to make the fridge more efficient. I had already cut a hole in the floor under the fridge's radiator; and cut a ventilation grill in the panel immediately behind the fridge. Now I have added a small 12V fan in an attempt to blow the warm air from the radiator.

At the moment this relies on me connecting the fan to 12V and switching it on and off manually. A quick look at the back of the fridge shows me that a direct connection to sense when the compressor cuts in won't be easy, so I'm considering either a noise sensor or a heat sensor to tell the fan to come on.

The fridge is by far the biggest drain on our domestic batteries. If this doesn't work I might have to spend a small fortune on buying a 12V fridge, surely more efficient than running the 3kVA inverter and the mains fridge.


Simon. said...

Hi Halfie,

When I did this job, I used one of these. Strap the sensor to the radiator on the back of the fridge and the fan comes on less than a minute after the fridge starts up and goes off again ditto.

I used two 12v fans in series to reduce the speed and noise. I've never tried to quantify the power savings, but it certainly works as intended.



ditchcrawler said...

I have 3 fans in series on 24 volt. I leave them on all the time, we don't notice them running but think we would if they kept starting and stopping.

ditchcrawler said...

These are some readings I took on mine

I have been running another set of tests on our 240 volt fridge.
The fridge runs via a Mastervolt Mass Sine 24/1500 and all current
readings are take from a Sterling Battery Management unit.

The inverter + Fridge + fans =3.5 amps
The Inverter + Fridge = 3.4 amps
The Inverter = 0.4 amps

I have connected a RS hour meter across the refrigerator compressor to
record compressor run hours.
The refrigerator is a LEC Elan and sits under a work top with 25mm air
gap each side and 50mm between the top of the refrigerator and the
underside of the work top. The air space behind the refrigerator is
well over the recommended space as the hull side slopes from the floor to
the gunwale.
There are ventilation holes drilled in the floor behind and below the
fridge. Also there are 4 computer fans mounted in the floor connected in
series parallel that when running blow cool air from under the floor up
the condenser. All readings were taken with the fridge in normal use.

The fridge thermostat was set at 3 and had been running for a couple of
days to allow things to stabilise before readings were taken at 1700 hrs
each day. The thermometer inside the fridge was reading at the top of
the acceptable range.

At the end of the first day with the fans off and the fridge running as
normal the hour meter recorded 7.6 hrs in 24 hrs.
On the second day the fans were switch on and the reading at the end of
24 hrs was 7 hrs.
The next day the fans were switched off and the hours run were 7.1
This indicates to me that the fridge was adequately ventilated as
installed. That night I insulated the fridge on both sides and the top
with sheets of 25mm close cell insulation foam covered on both sides
with aluminium foil as used in the building trade. At this point things
got interesting which I can't explain. The running hours were slightly
less but the internal temperature of the fridge fell. Over the next 2
days I adjusted the thermostat setting to bring the internal temperature
back to its original level. This resulted on it now being set at 1.5 as
opposed to the original 3. I don't know why this happened as in my book
the thermostat should keep the inside of the fridge at a constant
temperature. I suspect it is a timer and not a stat.

Once this had settled down I started recording readings again which run
between 6.3 and 5.7 hours per day
11/8/06 1700hrs 1170.2
12/8/06 1700 hrs 1177.8 7.6
Fan on
13/8/06 1700hrs 1184.7 6.9
Fan off
14/8/06 1700hrs 1191.8 7.1
Insulation added and fans on
15/8/06 1700hrs 1198.5 6.7
reduced stat setting
16/8/06 1700hrs 1205.0 6.5
17/8/06 1700 hrs 1211.3 6.7
18/8/06 1700hrs 1217.0 5.7
19/8/06 1700hrs 1223.6 6.6
20/8/06 1700hrs 1230.0 6.4
21/8/06 1700hrs 1235.5 5.5
22/8/06 1700hrs 1241.2 5.7

Halfie said...

Simon, thanks for the link. I've ordered one! If I find that my tiny fan really doesn't make much difference I'll have another go at cutting more floor away (ouch!) in order to use two larger fans and connect them in series as you have done. I found it difficult to cut the floor, even using a hole saw on an electric drill. The standard ply is covered with 3/4" oak T&G. Perhaps I could separate the boards and cut them with an ordinary handsaw ... One thing I'm concerned about is removing floor where a possible replacement fridge would be wanting to put its feet. But I'm jumping the gun; I'll try the sensor/fan arrangement first (when the sensor arrives...)

Brian, that's very useful and encouraging info. I'm fairly sure my fridge has far too little space around it, but that's just how the boat was built. My fridge is not against the side of the boat and sits with its back very close to a bulkhead; there's probably no more than an inch behind the radiator. There's also only an inch air gap above the top of the fridge so I wouldn't be able to add extra insulation here. I could insulate the sides, though, which might help.

Halfie said...

[This somehow didn't appear here so I've copied it from my e-mails]

Dave Ward has left a new comment on your post "Forced ventilation for the fridge":

Many of the better quality inverters have a low power setting, where they effectively look for a load on the output. If there is, and it's above a certain threshold, the unit switches on to provide full output. If the load drops below the threshold they revert to low power mode. This typically reduces the idle DC battery demand from around 8-10 watts to as little as 2. This mode may not work with newer fridges with any sort of electronic controls. Victron is one company that includes this facility, but it would doubtless cost more to replace your existing inverter, than to buy a 12volt fridge! Unfortunately even these can have problems - particularly voltage drop. Both subjects are discussed in this post:

I can think of ways to adapt some existing installations, but with the number of "ambulance chasing" legal firms out there, I'm not prepared to elaborate here...

Halfie said...

Thanks Dave, and I don't know why your comments sometimes don't appear even though they come to my e-mails. My inverter (also a Sterling device) doesn't have a low power mode. The boat is wired up for a 12V fridge; I don't know whether the cable is fat enough for the current draw - I wonder if there is an easy way to measure the cross-sectional area of a cable. Alternatively I could put a known load across it and take measurements of current and voltage. I'll have a look at the CanalWorld link.

Andrea Lidey said...

This looks good and as others said, there are ways to improve it too though.