Dometic RM7655L - exhaust & fridge/freezer temps questions (1 Viewer)

[veevee]

We have an about 8 year old RM7655L fitted, it was replaced in the past under some form of warranty (previous owner), MH was built 12 years ago.

Last trip out the cooling became slightly erratic so decided before driving off to Le Mans in a couple of days time to take a look at the gas flame and electrical connections. Have read some of the threads here with problems others have had and found the fridge drain tube that clears moisture from under the fins in the fridge slightly blocked, just as others have mentioned. Renewed the hose and unblocked the fridge tube.

The first question is about the exhaust pipe exit. Enclosed two photos of the rising pipe where the gas exhaust gases exit, it finishes about 100mm (4") inside the inner side of the upper grill. Have only had older MH's before and thought the exhaust ended in an alloy? tube and housing which exited the gases to the outside of the body. This design (if nothing is missing) exits to the box space behind the fridge/freezer, but there is no way that the space behind the fridge is gas tight to the interior of the MH.

Is that correct or is there something missing?

Second question. It's pretty warm here today and the fridge has had the door open all day. The temp in the fridge was 29 oC when I started a test of the fridge on gas, an hour later it's 28 oC.

The fridge tube and compressor? are just faintly less than warm, not how I expected them to be after an hour. The freezer tube is too hot to touch and the fins just about touchable, about how I would think they should be. Have only one thermometer so don't know what the freezer temp is, but it feels as if it's just on the edge of frosting up.

Should there be such a discrepancy at the rate the fridge and freezer cool down or is there something wrong?

Thanks very much for any help.

 

[Richard n Ann]

Sounds like it's too hot weather for it. These absorption 'fridges' are only really chillers and struggle in temps above 25c. It's 30c here where I am and the inside of the fridge is about 10c, it does pull down to about 4c overnight.
Check the less hot side of the fins outside, if the pipe in the middle is too hot to touch, it would benefit from a fan.

 

[veevee]

In the last 45mins the fridge temperature has dropped to 21 oC . The lower fridge tubes have started to heat up too but still easily touchable. Does this fridge freezer work on the basis that it cools the freezer first and then the fridge? That would explain why the small tubes behind freezer compartment have become very hot too as they connect into the fridge tube system.

^{_{Subscribers  do not see these advertisements}}

 

[Lenny HB]

The gas fumes venting to the space behind the fridge is how they do it it on the big fridge freezers.
The freezers work quite well but the fridge is usless in hot weather without fitting additional cooling fans. The fridge needs to run for at least 12 hours, in hot weather 24 hours to get down to temperature. If you more half fill it it will never get very cold at the bottom need to add an internal fan to get the fridge working properly.

This is how I fitted my fans, going to fit an internal one before the next trip.

 

[veevee]

OK, got all that Lenny and thank you. The reason for taking a look was over 5 or 6 days about a month ago when the weather wasn't hot, the fridge which was about 3/4 full performed worse than it did the previous year on all 3 forms of power.
My biggest surprise was seeing gas fumes vent into an almost internal habitation space plus adding heat to the rear of the fridge freezer, illogical at best.

On a previous classic Hymer there was a blockage in the fluids pipes and that made sense due to age etc, expected to find something similar as this MH has had very little use but that's not the case and the heat in the pipes is relatively similar through each system.

Maybe the previous year it was cooler than this year or we had a different amount of food in them, but this year the fridge was running about 3 - 4 oC warmer than last year.

R&A, have to amend the statement about the small tubes getting hot, it's only the RH one which runs down to about the fridge cooling fins and passes inside, the other 2 are a little more than cool.
The upper row of freezer fins are hot on the RH on the top and very hot on the LH side at the top. The lower freezer fins are only warm and no more on the outside and underside. The pipes passing them are very hot.
Thanks for the points you made.

 

[Richard n Ann]

The gas fumes venting to the space behind the fridge is how they do it it on the big fridge freezers.
The freezers work quite well but the fridge is usless in hot weather without fitting additional cooling fans. The fridge needs to run for at least 12 hours, in hot weather 24 hours to get down to temperature. If you more half fill it it will never get very cold at the bottom need to add an internal fan to get the fridge working properly.

This is how I fitted my fans, going to fit an internal one before the next trip.

The problem with fitting an internal fan is that the motor gives off heat. And that's the absolute last thing these toy town 'fridges' need.
You really notice the heat given off by the evaporator fans when you get called out to one that's broken down with the internal fans still running, it's like opening an oven door. Some controllers have a 'fan stop' parameter whereby if the evaporator temperature goes above say 10c it cuts the fans out so they don't spoil the food.
My advice is leave the inside alone, that way the butter in the door is warm and spreadable and the meats etc can be under the fins where it's less warm

^{_{Subscribers  do not see these advertisements}}

 

[Reallyretired]

Bit confused by the op here, are you expecting the fridge to cool down with the door open. We have a similar fridge and today fridge was 5.5c and freezer -15c with a maximum air temp of 39C, fortunately mostly around 34c but too hot for me and the dogs so moving north. It has been on for over a month though.
The back of the fridge should be sealed from the hab area so the flue being inside the top vent should be no problem. I do have a fan in the top vent, which has been running all day in this very hot weather in south of France. Fan is set to come on at 30c and off at 28c and is on by 10am.

 

[Richard n Ann]

The problem with fitting an internal fan is that the motor gives off heat. And that's the absolute last thing these toy town 'fridges' need.
You really notice the heat given off by the evaporator fans when you get called out to one that's broken down with the internal fans still running, it's like opening an oven door. Some controllers have a 'fan stop' parameter whereby if the evaporator temperature goes above say 10c it cuts the fans out so they don't spoil the food.
My advice is leave the inside alone, that way the butter in the door is warm and spreadable and the meats etc can be under the fins where it's less warm

Just read my post again. A was talking about my experiences with commercial compressor fridges, not van fridges. Just to be clear.

 

[veevee]

Bit confused by the op here, are you expecting the fridge to cool down with the door open. We have a similar fridge and today fridge was 5.5c and freezer -15c with a maximum air temp of 39C, fortunately mostly around 34c but too hot for me and the dogs so moving north. It has been on for over a month though.
The back of the fridge should be sealed from the hab area so the flue being inside the top vent should be no problem. I do have a fan in the top vent, which has been running all day in this very hot weather in south of France. Fan is set to come on at 30c and off at 28c and is on by 10am.

It wasn't very clear but what was meant, but... I was working around the fridge while it wasn't on and the door was left open. The moment I switched on and started the test I closed the door, and at that point the thermostat was reading 29 oC

Just looked at the temperature again 21:05 and the fridge is at 6 oC.

Richard, are you advocating that van fridges with fans would perform the same way as commercial fridges with fans or that MH fridges may be helped by using fans where commercial ones wouldn't. Out another way. If your MH fridge didn't perform well enough at 25 oC or higher, would you try fans?

I have this idea to extend the exhaust tail pipe by 50mm leaving it about 50mm away from the back of the upper vent, anyone have any thoughts. The exhaust gases appear to come through the vent fairly broadly across the whole width of the vent but the upper 25 - 30% only.

^{_{Subscribers  do not see these advertisements}}

 

[Richard n Ann]

Sorry, that was as clear as mud.. again lol.
In short: Don't put fans inside van fridges they introduce heat.

 

[veevee]

Sorry, that was as clear as mud.. again lol.
In short: Don't put fans inside van fridges they introduce heat.

This is becoming persistent questioning, but what about outside? and thanks for clearing that up.

 

[Richard n Ann]

This is becoming persistent questioning, but what about outside? and thanks for clearing that up.

No bother the outside fans are good in hot weather. Same as I said earlier, if the pipe exiting the fins (heat exchanger) is hot, the 'fridge' will probably benefit from extractor fan/s

^{_{Subscribers  do not see these advertisements}}

 

[veevee]

Thanks very much Richard, very much appreciate your help

 

[Lenny HB]

Sorry, that was as clear as mud.. again lol.
In short: Don't put fans inside van fridges they introduce heat.

I think you have got that wrong, how much heat can a fan that only draws a couple of hundred millamps produce, not a lot.
Ask @Robert Clark about his internal fan.

 

[Robert Clark]

I think you have got that wrong, how much heat can a fan that only draws a couple of hundred millamps produce, not a lot.
Ask @Robert Clark about his internal fan.

I was going to make the same point myself.
The curt draw is negligible and is the heat generated.

^{_{Subscribers  do not see these advertisements}}

 

[peterc10]

With regards to your flue, in order for it to be safe the back should be sealed so that no fumes get in to the hab area. Both our vans had this area fully sealed. I suspect this was not done when the new fridge was put in and it should have been. Sughest you get it done ASAP.

 

[Richard n Ann]

I was going to make the same point myself.
The curt draw is negligible and is the heat generated.

Yes the current draw is negligible, but the cooling capacity is negligible too .
I'm only speaking from experience of commercial fridges, the fan motors are bigger of course but the area they are in is bigger too. I like gadgets and improving things but I wouldn't do it, they need all the help they can get. Plus when you open the door the fan will instantly take any frost off the evaporator fins.

 

[Richard n Ann]

I was going to make the same point myself.
The curt draw is negligible and is the heat generated.

You don't feel the heat while the fridge is switched on and working. Try this. When you've been home a few days leave the fridge off and switch the internal fan on, preferably with a thermometer probe inside and see what happens.

^{_{Subscribers  do not see these advertisements}}

 

[Robert Clark]

You don't feel the heat while the fridge is switched on and working. Try this. When you've been home a few days leave the fridge off and switch the internal fan on, preferably with a thermometer probe inside and see what happens.

Too many variables to consider as a proper test

 

[peterc10]

Many larger modern domestic fridges have fan based circulating systems. Our previous Whirlpool one (bought in 2000) did and our new Samsung one does. They would not work properly without it.

 

[Richard n Ann]

I'm not saying all evaporator fans are bad, bigger fridges certainly need them, and compressor fridges cope much better they would just run a tiny amount longer to compensate. It's these absorption fridges that are too smaller capacity, the stat never breaks on mine when it's hot weather and it's only on number 2, pathetic.
it's your fridge so go for it , just my opinion when all said and done.

^{_{Subscribers  do not see these advertisements}}

 

[Two on Tour]

I have just fitted a 80mm two speed (low/high) computer fan internally to our fridge because we found we had a temperature gradient in our fridge with normally 4 degrees at the bottom and 9 degrees at the top when measured in the door section. Now with the internal fan running circulating the air in the fridge and pulling air across the cooling vanes we are getting 5 degrees across the board, proof enough for me.

 

[Richard n Ann]

Oh something else, Dometic don't sell evaporator kits so they don't think it's a good idea either.

 

[Richard n Ann]

Also the thermostat if you've noticed is clipped onto the evaporator fins and will have a cut out range of say -15 to -2 so when you change to fan assisted evaporator the stat will never cut out. You need to change to a bottle cooler stat for it to be able to switch off and work properly where the stat phial hangs in the air flow and can be set to cut out at 5c. So that's 3 good reasons not to fit one. Anyway what's wrong with 9c up in the door and 4c in the bottom?

^{_{Subscribers  do not see these advertisements}}

 

[Lenny HB]

I have just fitted a 80mm two speed (low/high) computer fan internally to our fridge because we found we had a temperature gradient in our fridge with normally 4 degrees at the bottom and 9 degrees at the top when measured in the door section. Now with the internal fan running circulating the air in the fridge and pulling air across the cooling vanes we are getting 5 degrees across the board, proof enough for me.

Ours is the other way round 10 deg at the bottom 4 at the top hence the reason to fit a fan. When the fridge doesn't have much in it the cooling fins at the tfreezeeez but it is still far too warm at the bottom, crap Dometic design.

 

[Two on Tour]

Oh something else, Dometic don't sell evaporator kits so they don't think it's a good idea either.

So Dometic have stated that have they ??

Also the thermostat if you've noticed is clipped onto the evaporator fins and will have a cut out range of say -15 to -2 so when you change to fan assisted evaporator the stat will never cut out. You need to change to a bottle cooler stat for it to be able to switch off and work properly where the stat phial hangs in the air flow and can be set to cut out at 5c. So that's 3 good reasons not to fit one. Anyway what's wrong with 9c up in the door and 4c in the bottom?

So if it's ok to keep perishable food at 9 degrees why are domestic fridges recommended to be set at 5 degrees. ??

As I said it's all working fine for me, so where am I going wrong ??

 

[Richard n Ann]

Who said anything about perishable foods at 9c?.. no as I said earlier butter and eggs in the door where it's a bit warmer and meats low where it's cooler.
You can't set a fridge with an evaporator thermostat to 5 degrees, all the thermostat senses it evaporator temperature, that's why they have numbers on the dial and not degrees C.
If it works for you then fine, but be aware that the heating element on your fridge will be on 24hrs a day.. Actually that proves my point in how rubbish these fridges are, any real fridge where the compressor was running continuously would freeze the food.
Anyway I'm pleased you're happy with it

^{_{Subscribers  do not see these advertisements}}

 

[Two on Tour]

Who said anything about perishable foods at 9c?.. no as I said earlier butter and eggs in the door where it's a bit warmer and meats low where it's cooler.

Where did you say that earlier ???

I think we all agree that pretty well all motorhome and caravan absorption fridges are not really fit for purpose and over the top on price but as we are stuck with them then some of us try to better their performance rightly or wrongly depending on the individuals view.

 

[gus-lopez]

In the last 45mins the fridge temperature has dropped to 21 oC . The lower fridge tubes have started to heat up too but still easily touchable. Does this fridge freezer work on the basis that it cools the freezer first and then the fridge? That would explain why the small tubes behind freezer compartment have become very hot too as they connect into the fridge tube system.

Yes

I have this idea to extend the exhaust tail pipe by 50mm leaving it about 50mm away from the back of the upper vent, anyone have any thoughts. The exhaust gases appear to come through the vent fairly broadly across the whole width of the vent but the upper 25 - 30% only.

I would the older types had an extendable aluminium tube connected to a standalone metal outlet.
& seal the fridge to the inside as well .

I think you have got that wrong, how much heat can a fan that only draws a couple of hundred millamps produce, not a lot.
Ask @Robert Clark about his internal fan.

You'd be amazed.

Yes the current draw is negligible, but the cooling capacity is negligible too .
I'm only speaking from experience of commercial fridges, the fan motors are bigger of course but the area they are in is bigger too. I like gadgets and improving things but I wouldn't do it, they need all the help they can get. Plus when you open the door the fan will instantly take any frost off the evaporator fins.

I've modded commercial no fan assisted evaporators as well & they never work. Ice disappears off the fins. Trying to do something that they weren't designed for.

Too many variables to consider as a proper test

Turn off with digital probe inside % meter outside. Leave x days until temperature is steady. Turn on fan. Temperature will eventually rise exceedingly high.

Many larger modern domestic fridges have fan based circulating systems. Our previous Whirlpool one (bought in 2000) did and our new Samsung one does. They would not work properly without it.

& they are designed around working with fan assistance. Turn off/disconnect fan & see how much difference it makes.Also many are temperature controlled ,as well as stopping when door is opened.
& those designed without fans are going to do exactly the same when a fan is added in the majority of cases.
As @Richard and Ann said the stats will be calling for refrigeration 24/7 .

 

[Robert Clark]

@gus-lopez
Fluctuations in temperatures inside the van would make keeping a constant temperature inside the fridge impossible.

My own experience is that an internal fan has kept the contents cooler than before - that's good enough for me

^{_{Subscribers  do not see these advertisements}}