17' Series Installing a circulating fan inside Dometic Fridge

[mikerezny]

Hi @mikerezny , your info about the "range" of operation of NC thermostats is very intriguing and valuable. I have just spent a week in the Connondale ranges at an elevation of 600 metres. The air temperature for those days varied from max 26 C during daytime and down to 4 C in the morning, both shade temperatures. I used a Technitherm metal backed thermometer to measure the heat coming off the fins of my refrigerator running on gas. Placed just where thermo would be and as close to tubing as possible. There were some surprising results!! At an ambient temp of 26 C, the fins temp was as high as 85 C but dropped down to 35 C after 2 minutes with fans on. Other times at 20 C ambient, the temp still reached more than 75 C, but dropped quickly to about 32 C with fans on for a very short time. At first thing in the morning (ambient 4 to 5 C), the temp was only about 38 C, without fan cooling!! Apparently, the colder night air movement through the back was sufficient to keep radiating fins cool. Me thinks that a 50 C thermo may be the way to go. I will do further testing next time out with varying temps to check readings. Thought you may be interested as it CAN get cool up here in winter.

Hi @Boots in Action,
these results are very interesting and very useful.

As you may remember, I have used a 50C thermostat. As it is getting colder, I have found that the fan is on more than I would have expected. But also noted that the fridge is colder then it needs to be. In the morning it is around 1C, rising to about 4C during the day.
That was with the temperature control set at 2. I set the control to 1 and the fridge now sits around 4C during the day. So I will leave it at that setting.

But, I also remember that the fan seemed to be on more after I had put extra insulation around the boiler. This was to keep the heat away from the cabinet which was conducting heat down the cabinet and eventually heating up the side of the fridge.
The other, unintended, but good side effect is that this insulation around the boiler also make the boiler more efficient and increases the temperature of the boiler. So, I think that is why the fan is on a bit more.

I can't prove it yet, but it seems possible that the gas pilot light supplies enough heat after the main jet goes out to extend the period before the boiler cools down enough for the fan to switch off. It will be good if that is the case.

My current plan is to keep the temperature setting at 1 during the colder months and around 2 during the hotter months.

I would like to find a way to get a running 24 hour plot of the duty cycle for the gas burner. I bought an Arduino kit just lately, so this might be a good project for it.

Another interesting observation regarding the fridge working on 12V. I know that it is widely reported that the fridge will, at best, just keep the fridge stable when on 12V. I have found that not to be the case with our setup. Running on 12V, the fridge actually gets colder.
The RM2350 fridge we have has a 12V 175W element and a 240V 175W element. The 12V element has no thermostatic control, but the 240V one is controlled by a thermostat. So, it would seem to me that, given the 12V and 240V elements are both 175W, the fridge should perform just as well on either 12V or 240V. In fact, since the 12V element has no thermostat, the fridge may get colder on 12V!!!

Here is my setup:
I have one heavy wire going from my cat battery +ve through a 35A circuit breaker then though a 35A relay (switched on by the ignition) and then to the 12 pin plug at the back. The 12 pin socket picks up the negative from a very good connection to the body about a meter from the socket. I have not modified the wiring on the Penguin, other then to put a small LED voltmeter directly across the 12V inlet terminals of the fridge.

I check the battery voltage of the car using the diagnostic readout on the Progidy brake controller. And check the voltage at the fridge as part of our checks when we are hooking up to leave. The car is usually around 14.6V, and we loose about 0.7 - 0.8 V through the wiring. That means we have either 13.8V or 13.9V at the fridge terminals.

If we drive for a few hours on our way home, the fridge is usually 4-6C when we leave and typically less than 1C when we get home.
We get roughly the same results even when we stop along the way (usually less than an hour).
So, I am very happy with that.

best wishes
Mike

 

[Boots in Action]

Hi @Boots in Action,
these results are very interesting and very useful.

As you may remember, I have used a 50C thermostat. As it is getting colder, I have found that the fan is on more than I would have expected. But also noted that the fridge is colder then it needs to be. In the morning it is around 1C, rising to about 4C during the day.
That was with the temperature control set at 2. I set the control to 1 and the fridge now sits around 4C during the day. So I will leave it at that setting.

But, I also remember that the fan seemed to be on more after I had put extra insulation around the boiler. This was to keep the heat away from the cabinet which was conducting heat down the cabinet and eventually heating up the side of the fridge.
The other, unintended, but good side effect is that this insulation around the boiler also make the boiler more efficient and increases the temperature of the boiler. So, I think that is why the fan is on a bit more.

I can't prove it yet, but it seems possible that the gas pilot light supplies enough heat after the main jet goes out to extend the period before the boiler cools down enough for the fan to switch off. It will be good if that is the case.

My current plan is to keep the temperature setting at 1 during the colder months and around 2 during the hotter months.

I would like to find a way to get a running 24 hour plot of the duty cycle for the gas burner. I bought an Arduino kit just lately, so this might be a good project for it.

Another interesting observation regarding the fridge working on 12V. I know that it is widely reported that the fridge will, at best, just keep the fridge stable when on 12V. I have found that not to be the case with our setup. Running on 12V, the fridge actually gets colder.
The RM2350 fridge we have has a 12V 175W element and a 240V 175W element. The 12V element has no thermostatic control, but the 240V one is controlled by a thermostat. So, it would seem to me that, given the 12V and 240V elements are both 175W, the fridge should perform just as well on either 12V or 240V. In fact, since the 12V element has no thermostat, the fridge may get colder on 12V!!!

Here is my setup:
I have one heavy wire going from my cat battery +ve through a 35A circuit breaker then though a 35A relay (switched on by the ignition) and then to the 12 pin plug at the back. The 12 pin socket picks up the negative from a very good connection to the body about a meter from the socket. I have not modified the wiring on the Penguin, other then to put a small LED voltmeter directly across the 12V inlet terminals of the fridge.

I check the battery voltage of the car using the diagnostic readout on the Progidy brake controller. And check the voltage at the fridge as part of our checks when we are hooking up to leave. The car is usually around 14.6V, and we loose about 0.7 - 0.8 V through the wiring. That means we have either 13.8V or 13.9V at the fridge terminals.

If we drive for a few hours on our way home, the fridge is usually 4-6C when we leave and typically less than 1C when we get home.
We get roughly the same results even when we stop along the way (usually less than an hour).
So, I am very happy with that.

best wishes
Mike

Hi @mikerezny , I will have to sit down and study your system and other thoughts too. I believe that the fridge setting control on our Penguin only controls the temp when operating on 240 volt ac. Also, I thought that when operating on gas, there is no thermostat controlling temp and that only one gas jet operates. There is no pilot light?? The operation on 12 volts and 240 volts would be affected by voltage drop only. There probably would not be much variation on 240 v to alter heating the element, whilst there could/would be a lot of variation when connected to a nominal 12 volt supply from you tug, depending on load. Will have to think about this a bit more to give proper reply. Would appreciate more info/correction from other members please. Did not @Drover mention something about gas supply just recently??

 

[Drover]

On the couple of fridges I've done the 70 deg one worked the best, the 50 deg just ran most of the time, didn't explore the why's that much but I know the main reason for the fan is to maximise airflow not cool things down too much, just can't remember what the optimum temp of the system is. I was under the impression the heater runs at a set temp and internal fridge temp is regulated by refrigerant flow. Read about it years ago .

 

[mikerezny]

Hi @mikerezny , I will have to sit down and study your system and other thoughts too. I believe that the fridge setting control on our Penguin only controls the temp when operating on 240 volt ac. Also, I thought that when operating on gas, there is no thermostat controlling temp and that only one gas jet operates. There is no pilot light?? The operation on 12 volts and 240 volts would be affected by voltage drop only. There probably would not be much variation on 240 v to alter heating the element, whilst there could/would be a lot of variation when connected to a nominal 12 volt supply from you tug, depending on load. Will have to think about this a bit more to give proper reply. Would appreciate more info/correction from other members please. Did not @Drover mention something about gas supply just recently??

Hi @Boots in Action,
there is definitely a thermostat controlling the main gas jet and a thermostat controlling the 240V.

Here are a couple of excerpts from the RM2350 manual:

cheers
Mike

12 V DC OPERATION
Only operate your refrigerator on 12 V DC when the engine of the vehicle is running - otherwise your battery will soon be discharged.
Turn the knob A to the position marked “DC” for 12 V DC operation.
Note that there is no thermostat function on 12 V DC operation, the refrigerator works continuously.

REGULATING THE TEMPERATURE
The refrigerator is equipped with a thermostat that can be adjusted by turning the knob B to different setting to maintain the desired cabinet temperature.
Gas operation
At “OFF, the thermostat closes its main valve and the burner runs continuously at the bypass rate, just enough to keep the burner lit. At “MAX”, the thermostat allows the burner to remain on high flame continuously.

AC operation (230-240 V AC)
At “OFF, the contacts in the thermostat are open and the heating elements are off. At “MAX”, the heating element is “ON” continuously. Lowest cabinet and freezer temperatures are obtained at this setting.

The thermostat can be adjusted between “MAX” and “OFF” to obtain the desired cabinet temperature. The closer the knob is to “MAX” - the colder the cabinet temperature. The closer the knob is to “OFF” - the warmer the cabinet temperature.

When the thermostat reaches the set temperature, it will cut the burner back to bypass or, in electric operation (230- 240 V AC), shut off the heating element. The setting of the thermostat is not critical, but we recommend it be adjusted to maintain a dry frost on the cooling fins. Adjust the thermostat knob closer to “MAX” when the outside temperature becomes warm.

 

[mikerezny]

Hi,
here is a link to a Dometic manual that provides a huge amount of information regarding their 3-way vapour absorption type fridges.

It has a very readable description of exactly how the fridges work, as well as a useful flow chart for determining what is wrong if the fridge is not working.

http://webspace.webring.com/people/jv/vintage_campers1/Dometic_Refr_Service_Manual.pdf

Here is a colored version of the diagram for the absorption system:

I am trying to understand the concept of 'optimal operating temperature' for the fridge. In particular, the temperature of the condensing fins.
From what I can gather, the short answer is 'as cold as possible'. Since the coolant is ammonia, water, hydrogen, and a corrosion inhibitor, the practical limit is no colder than 0C.

The task of the condenser is to condense the pressurised ammonia vapour to a liquid before it goes into the freezer, and combined with hydrogen, and in this low pressure area, the 'heat' from the freezer vaporises the ammonia / hydrogen now in a lower pressure area.

If this is the case, the colder the liquid ammonia is when it enters the freezer, the more heat it will absorb in becoming a gas.

The other bit that might be interesting is that the majority of the cooling is done in the freezer, the cooling done in the fridge is a somewhat secondary part of the process. In this case, the colder the freezer, the more cooling process is available for the fridge.
So, in the summer months, it seems that keeping the freezer filled might be useful in keeping the fridge cold during the hotter part of the day.
This would work for me on short trips when we don't ever have much in the freezer. I could freeze some milk bottles in the home freezer and put them in the van freezer.

cheers
Mike

 

[chartrock]

You guys set me thinking as one of Mrs CR's problems is frozen lettuce and other vegies, even had eggs frozen. I like the idea of fans inside the fridge to circulate the air but I feel it only needs a small movement to achieve the required result. So I ordered 2 X 40 mm fans from China for just $1.99. Neat little things, I will fit one at the top and the other at the bottom so looking forward to see how they go.

 

[Boots in Action]

You guys set me thinking as one of Mrs CR's problems is frozen lettuce and other vegies, even had eggs frozen. I like the idea of fans inside the fridge to circulate the air but I feel it only needs a small movement to achieve the required result. So I ordered 2 X 40 mm fans from China for just $1.99. Neat little things, I will fit one at the top and the other at the bottom so looking forward to see how they go.
View attachment 49492

Hi there @chartrock , that is one good thing about this forum. It gets members exchanging ideas and even though you have been fairly quiet in the background through all this, you have finally decided to commit. All it is going to cost you is a couple of dollars and a bit of your time. I look forward to hearing how successful you have been. Good luck - Mrs CR will thank you for it!! Just a little note: I have been talking about fans in my small 90 litre fridge whilst you probably have a much bigger capacity one - 130 or 150 litre???. If so, you may?? need bigger fans to move a significantly more air around in the larger fridge. Maybe??

 

[chartrock]

You're right @Boots in Action we have a 150 L fridge but I feel if these babies are running 24 /7 then air will never be still in the fridge and it will have the desired effect. I hope I am not proven wrong as Mrs CR will be somewhat miffed if I am. :moony:

 

[Drover]

When you think about the thermostat thing mentioned earlier, the gas flame must adjust as you need to set the knob on 4 to fire it up on gas, the size of the flame doesn't seem to change on the ones I've fiddled with but then they were the auto jobs and basically so as the flame stayed there I was happy...
..............My fan is still sitting in the drawer need to look at doing something about it, haven't really looked at how to utilise the light switch so that fan shuts down when door opened and light comes on......probably fit a small micro in the housing.

 

[Boots in Action]

Hi @Boots in Action,
there is definitely a thermostat controlling the main gas jet and a thermostat controlling the 240V.

Here are a couple of excerpts from the RM2350 manual:

cheers
Mike

12 V DC OPERATION
Only operate your refrigerator on 12 V DC when the engine of the vehicle is running - otherwise your battery will soon be discharged.
Turn the knob A to the position marked “DC” for 12 V DC operation.
Note that there is no thermostat function on 12 V DC operation, the refrigerator works continuously.

REGULATING THE TEMPERATURE
The refrigerator is equipped with a thermostat that can be adjusted by turning the knob B to different setting to maintain the desired cabinet temperature.
Gas operation
At “OFF, the thermostat closes its main valve and the burner runs continuously at the bypass rate, just enough to keep the burner lit. At “MAX”, the thermostat allows the burner to remain on high flame continuously.

AC operation (230-240 V AC)
At “OFF, the contacts in the thermostat are open and the heating elements are off. At “MAX”, the heating element is “ON” continuously. Lowest cabinet and freezer temperatures are obtained at this setting.

The thermostat can be adjusted between “MAX” and “OFF” to obtain the desired cabinet temperature. The closer the knob is to “MAX” - the colder the cabinet temperature. The closer the knob is to “OFF” - the warmer the cabinet temperature.

When the thermostat reaches the set temperature, it will cut the burner back to bypass or, in electric operation (230- 240 V AC), shut off the heating element. The setting of the thermostat is not critical, but we recommend it be adjusted to maintain a dry frost on the cooling fins. Adjust the thermostat knob closer to “MAX” when the outside temperature becomes warm.

Hi @mikerezny , I think I must have the "poor man's" Jayco penguin, because I do not think my fridge has any separate control on the front panel!! My fridge is a Thetford and sounds like an "el cheapo" replacement for the Dometic you seem to have. Will have a good look tomorrow and confirm. It is easy to see the confusion now. I will also check the model number and see what it says in the book again. Thanks for all the info about YOUR fridge - you lucky b......!

 

[Boots in Action]

When you think about the thermostat thing mentioned earlier, the gas flame must adjust as you need to set the knob on 4 to fire it up on gas, the size of the flame doesn't seem to change on the ones I've fiddled with but then they were the auto jobs and basically so as the flame stayed there I was happy...
..............My fan is still sitting in the drawer need to look at doing something about it, haven't really looked at how to utilise the light switch so that fan shuts down when door opened and light comes on......probably fit a small micro in the housing.

@Drover, it is hard to get yourself going when you spend so much time spreading your knowledge to help others. Might be time to make a start on your fan while it is cool so that you are ready for the warmer months, otherwise it will never get done!!!. Cheers

 

[Boots in Action]

When you think about the thermostat thing mentioned earlier, the gas flame must adjust as you need to set the knob on 4 to fire it up on gas, the size of the flame doesn't seem to change on the ones I've fiddled with but then they were the auto jobs and basically so as the flame stayed there I was happy...
..............My fan is still sitting in the drawer need to look at doing something about it, haven't really looked at how to utilise the light switch so that fan shuts down when door opened and light comes on......probably fit a small micro in the housing.

Hi @Drover and @mikerezny , it would appear that progress has overtaken my "old" Thetford N304 M . It has been upgraded since it was fitted into my van in 2013. I went on line and watched a video on the current 90 L Dometic fridge and it has incorporated a thermostat control for BOTH 240 volt AND gas operation just as @mikerezny has said. A similar upgrade has been done to the latest Thetford too. Looks like I have been "out-gunned"! Strangely enough, I have an older 3 way fridge which I use outside the van which DOES have a variable gas control which reduces the gas flow flame as you turn it down. Not really a thermostat, but I do reduce the control at night to prevent cooling going below 0 C. So there you are!!!

The other good thing that came out of my investigation was that the Thetford has an optional extra - a thermostatically controlled fan!! The important thing is that it is designed to come ON when the AMBIENT temperature goes above 38 C. I find that difficult to understand as the temp at the back of the fridge would be somewhere around 85 C or above at that rate working on my recent field tests!!! Maybe they mean 38 C ambient INSIDE the back of the fridge cooling system where a lot of us have our fans?? I tend to agree with @mikerezny that as the hot gas has to be cooled down to become a liquid, the cooler the liquid, the better the cooling (heat absorption) process. Maybe there is someone out there who can sort out this dilemma?

 

[Drover]

@Drover, it is hard to get yourself going when you spend so much time spreading your knowledge to help others. Might be time to make a start on your fan while it is cool so that you are ready for the warmer months, otherwise it will never get done!!!. Cheers

The fan isn't that important, it's really just a bit of bling as my fridge runs just fine, when someone needs some info or help I put my hand up to help if I can.............................and I have plenty of time to play on here.

 

[Boots in Action]

The fan isn't that important, it's really just a bit of bling as my fridge runs just fine, when someone needs some info or help I put my hand up to help if I can.............................and I have plenty of time to play on here.

Hi @Drover , can't help but admire your attitude. Keep it up as I have lots more information to get out of you yet!!

 

[Dobbie]

Be warned....be not afraid....be prepared....bees knees.

:kev:

but @Drover is, as always, a helpful guy.

Shame about the fish!

 

[Boots in Action]

Be warned....be not afraid....be prepared....bees knees.

:kev:

but @Drover is, as always, a helpful guy.

Shame about the fish!

Hi @Dobbie , thanks for the warning, but I am a glutton for punishment. And I like the challenge too!!! Can't give @Drover too easy a life now that he is no longer fishing!!! Cheers

 

[Boots in Action]

Hi @mikerezny , your info about the "range" of operation of NC thermostats is very intriguing and valuable. I have just spent a week in the Connondale ranges at an elevation of 600 metres. The air temperature for those days varied from max 26 C during daytime and down to 4 C in the morning, both shade temperatures. I used a Technitherm metal backed thermometer to measure the heat coming off the fins of my refrigerator running on gas. Placed just where thermo would be and as close to tubing as possible. There were some surprising results!! At an ambient temp of 26 C, the fins temp was as high as 85 C but dropped down to 35 C after 2 minutes with fans on. Other times at 20 C ambient, the temp still reached more than 75 C, but dropped quickly to about 32 C with fans on for a very short time. At first thing in the morning (ambient 4 to 5 C), the temp was only about 38 C, without fan cooling!! Apparently, the colder night air movement through the back was sufficient to keep radiating fins cool. Me thinks that a 50 C thermo may be the way to go. I will do further testing next time out with varying temps to check readings. Thought you may be interested as it CAN get cool up here in winter.

Hi @mikerezny and @Drover , I need a bit of help please. I have received thermostats of various ranges and have also bought metal clips from Bunnings, and now wish to attach same to tubing as per setup in another recent thread, but not with plastic zip ties. I think @Drover said he used a rivet to secure thermo to clip. It would be great if someone can explain how this was done and even better send a picture of finished product. If I can, I will probably use a couple of thermos on different clips to check out which one works best for me. Expert and experienced opinion would be really appreciated. I am sure there is plenty out there!!!

 

[Drover]

I cut a small bit of alloy, 2mm thick, and riveted it to the spring clip then riveted the thermo to it..........worked a treat and a pic of it is somewhere in this link, just can't find it...............http://expandasdownunder.com/threads/drovers-14-44-3.4502/page-4 so easy to swap around.

 

[mikerezny]

Hi @mikerezny and @Drover , I need a bit of help please. I have received thermostats of various ranges and have also bought metal clips from Bunnings, and now wish to attach same to tubing as per setup in another recent thread, but not with plastic zip ties. I think @Drover said he used a rivet to secure thermo to clip. It would be great if someone can explain how this was done and even better send a picture of finished product. If I can, I will probably use a couple of thermos on different clips to check out which one works best for me. Expert and experienced opinion would be really appreciated. I am sure there is plenty out there!!!

Hi @Boots in Action,
I used only two cable ties to secure the thermostat to the pipe. It was only later that I found out about the good idea from @Drover who used the metal clips. I am fairly sure he rivets the thermostat to the clips.

Given that you are still in the experimenting stage of deciding which thermostat to use, would it be easier to use cable ties until you have found the correct thermostat for you and to then remount it more permanently using the tool clips?

I have not had any problems with using two cable ties. I bent the ends of the thermostat mounting plate up a little to stop the cable ties slipping off. You can see what I have done in the photos here:
http://expandasdownunder.com/threads/dometic-3-way-fridge.10180/page-2#post-175505

I hope this helps.

cheers
Mike

 

[Boots in Action]

Hi @Boots in Action,
I used only two cable ties to secure the thermostat to the pipe. It was only later that I found out about the good idea from @Drover who used the metal clips. I am fairly sure he rivets the thermostat to the clips.

Given that you are still in the experimenting stage of deciding which thermostat to use, would it be easier to use cable ties until you have found the correct thermostat for you and to then remount it more permanently using the tool clips?

I have not had any problems with using two cable ties. I bent the ends of the ends of the thermostat mounting plate up a little to stop the cable ties slipping off. You can see what I have done in the photos here:
http://expandasdownunder.com/threads/dometic-3-way-fridge.10180/page-2#post-175505

I hope this helps.

cheers
Mike

Hi @mikerezny , thanks for your advice. I will try it out with zip ties firstly as you said, easier to practise on fitting different ranges. I will let you know how I go when testing at home. The real test is out in the scrub!! Cheers