Dometic HP Question. Any HVAC techs on the board?

[96 EVO]

Anything to learn from a heatpump that goes into defrost mode long before it's roofmates?

All units had the coils cleaned on the same day. The unit that defrosts early maybe get's a bit more use than the others, but I try to keep about equal hours on them.

The two that were running together on this particular 39F day, were both putting out 102F heat, even after the one unit went into defrost after about 35min running.

[Tom Cherry]

3 hours ago, 96 EVO said:

Anything to learn from a heatpump that goes into defrost mode long before it's roofmates?

All units had the coils cleaned on the same day. The unit that defrosts early maybe get's a bit more use than the others, but I try to keep about equal hours on them.

The two that were running together on this particular 39F day, were both putting out 102F heat, even after the one unit went into defrost after about 35min running.

If you are asking about the Front HP vs the Rear (Bedroom), Dometic says it is a “Monaco” thingie.  I did a lot of temp measurements in the fall in the NC Mountains.  The rear ran maybe 20 or more dF HOTTER than each of the same points or components in the front.  Great Dometic tech spent a lot of time on it and called back.  He said it was quite common for the rear unit in a Monaco to run “hotter”.  Thus, I assumed and he confirmed….it will be more efficient in THE HP mode or put out more heated BTU than the front.

The way to verify.  Pick several points inside the roof top units.  Compare the temps using a laser thermometer.  I don’t recall one point that was the same as or cooler in the rear as compared to the front.

NOW WHY.  He said that Monaco “cut off” one side.  So the duct on the rear had one full run to the front….as well as the middle if you have one,  BUT, the other side was chopped and blocked.  So it ran hotter.  The front had a longer run towards the back, on both sides.  Or the volume of the duct was greater in the front…than the rear’s were.

Now, with my elementary understanding, that made sense.  I DID talk to a RV tech that confirmed that a rear “ran hotter”.

That’s the best I can do….unless you, of course, have a control or mechanical issue…

SO….what you are seeing is probably normal….for a Monaco ducted system.

[96 EVO]

Yes, it was front and bedroom units I had running.

Front defrosted after about 35min. Bedroom unit never did defrost during the 2hrs they ran.

Temp coming from each was 102F. I use a digital meat thermometer for measuring temps. Works well (done) 🥩!

[bborc2005]

Yes I have the same problem, the front HP cycles to defrost mode more often than the rear

[96 EVO]

38 minutes ago, bborc2005 said:

Yes I have the same problem, the front HP cycles to defrost mode more often than the rear

Strange eh 🤔!

[Tom Cherry]

17 hours ago, 96 EVO said:

Yes, it was front and bedroom units I had running.

Front defrosted after about 35min. Bedroom unit never did defrost during the 2hrs they ran.

Temp coming from each was 102F. I use a digital meat thermometer for measuring temps. Works well (done) 🥩!

Ben, I don't know if I would call it "STRANGE"?  The Dometic tech was aware of WHY they were doing what they were doing....and said it was normal...for a Monaco and the ducting arrangement and was quite familiar with the question and the cause and effect....

If I wasn't clear....what I did and the Dometic Tech said it was the ONLY way to compare.  I pulled off the covers.  I used the laser temp gun.  I felt around and found out....just walking back and forth, that the front unit was way cooler than the rear.  SO, I picked a few points on the compressor and the lines and such and then recorded the temps.  I think I had 10 points or maybe 7 or so.  I went to the BACK unit.  Then did the same drill.  EVERY POINT in the rear was hotter.  IN PLAIN TERMS....the Rear HVAC, in HP, ran hotter than the front.  That made the Front Unit ice up quicker and go into defrost more frequently. The real troubleshooting, according to him, assuming the filters were CLEAN, was the internal temps of certain items or area inside the roof top and NOT measuring the output temps.

I had called and talked to a Dometic tech....and he called me back and waited on the phone while I climbed up and pulled the covers, again.  He said that BOTH units were within the expected range.  BUT, that the ducting made the difference....in that the front unit was pumping out more heat or the air flow was less restricted.

Then the rear unit could NOT kick out as much air....due to the chopped and closed off duct system on one side.  

FINALLY....NORMAL FOR A MONACO.  GET OVER IT.  The Rear HVAC, in HP mode will deliver slightly warmer air and run longer before defrosting than a front one will.

That sounded logical and he seemed to be very knowledgeable.  Told me to not worry about it....unless the Delta T of the two units, same components really went off scale.  I would KNOW this if the front did not HEAT as much and the REAR was getting hotter....or the rear was starting to cycle more.

From my perspective, having MORE heat in the Bedroom at night was good....I just kept the temp set to about 68 as we don't heat much at night....so it ran less.

That's it....

46 minutes ago, bborc2005 said:

Yes I have the same problem, the front HP cycles to defrost mode more often than the rear

I would NOT call it a problem.  It is a known condition that Dometic understands and explained.  Perhaps, the same "conditions" or observations.  Problems to me are when something is out of the ordinary or can't be explained or is not understood.  See the last post....this one may combine....

This is one of those..."Lets FIX something that isn't BROKE....".  You can call Dometic and probably get the same answer...and they can explain it....maybe differently or more technically that they did for me....

[96 EVO]

Tom, it's little things like this that keep me up at night 😂!

[Tom Cherry]

29 minutes ago, 96 EVO said:

Tom, it's little things like this that keep me up at night 😂!

ME TOO.  But, if you really want to put your mind at ease....call Dometic.  BUT, they will need the temps inside the Roof Top unit....and you can pick points that are different and logical....then do the front and the rear... and give them your own "observations"

I did that....and they said don't worry.  So, I wrote it off....now....on to something else.

Seriously....That is what I was told...and the tech called back after we talked....so I let it die...and neither of my two units have DIED and the HP, in the ranges they were intended for, work...  IF they die....two NEW Penguins and a new 10 Button Tstat....and the only decisision....there is another one that has a different "model" name....so is it the one or a stock Penguin.

Glad you will get a good night's sleep...  LOL...

[96 EVO]

Next time, I will open the center air dumps! 

That should take the ductwork out of the equation.

[Bill R]

2 minutes ago, 96 EVO said:

Next time, I will open the center air dumps! 

That should take the ductwork out of the equation.

Ben, that will be interesting to see what happens.  I am stating the obvious probably, the front unit because of the larger ducting arrangement has a higher volume of inside air going through the condenser in Heat mode (evaporator for cooling mode), thus more heat transfer and will in turn cause the outside evaporator to be cooler and go into defrost mode sooner.  Also is the reason for difference at register temps.   In cooling mode, my rear unit always puts out cooler air, less volume of air the reason.    But they aren't broke, units run within in spec and perform a little different because of air volume differences.

[96 EVO]

Yeah, should be interesting.

I was getting 102F heat from the closest diffusers from each unit, even after the front had gone through a defrost cycle, and the bedroom unit hadn't.

[Mroock]

Exactly.    Airflow = Heat transfer     Heat or cool

Lower head pressure = lower suction pressure , Quicker to frost over

[Tom Cherry]

23 hours ago, 96 EVO said:

Next time, I will open the center air dumps! 

That should take the ductwork out of the equation.

Nope.  The issue is that ONE side runs FRONT to REAR...totally unobstructed.  The OTHER side is cut and blocked where the shower is.  So, the rear unit has only the depth (length) of the bedroom on one side. The front on has the main galley and the center section. Bottom line, IF both ducts ran front to rear on BOTH sides, odds are, you would not see the difference in the units. The Dometic Tech said that the phenomenon of different defrost cycles or different outputs of heat from the same size (both yours and mine are 15Kw....front and rear) is unique to Monaco.  If Monaco had NOT cut the "shower side" duct, there would have been an "equilibrium" condition....so that when both units were running, the balance would have been the same.  Cut off one side.  Even though the other side's duct is still continuous, that messes up the convection air flow. 

YES, you can experiment with changing the "dumps" or the vents.  I don't know if that will equalize or balance it.  Dometic said it is NORMAL for a Monaco....and if you think yours is abnormal, then do like I did....measure each unit and call them. 

That's it....so....my take....If it ain't .....don't....it"

[96 EVO]

15 minutes ago, Tom Cherry said:

Nope.  The issue is that ONE side runs FRONT to REAR...totally unobstructed.  

Do your heat pumps have air dumps, that come right out of the bottom of the units?

[Tom Cherry]

6 minutes ago, 96 EVO said:

Do your heat pumps have air dumps, that come right out of the bottom of the units?

Yes, they are exactly like yours.  I don't have the 13.5Kw in the center like you do....but the Dometic tech said it was just the length of the ducts and the cut and blocked sections.

Now, I did NOT go that far.... as to "analyze and test" with all the air vents closed and only the "dumps" open.  I usually keep most of my vents open and directly and only use the DUMP when traveling to get heated or cooled air into the MH and not out the vents....which are covered or partially blocked by the side.

So...  maybe that will make a difference.  BUT, I don't normally use my system that way...I use the ducts and keep the DUMP closed....

Does that answer your question?  

[96 EVO]

1 minute ago, Tom Cherry said:

 

So...  maybe that will make a difference.   

That, is what I plan on finding out!

[Mroock]

You hit the nail on the head,  Different ductwork layout= different airflow= different heat exchanging.

2 exactly same units will operate differently with different ductwork.   Be careful on reducing any airflow because you will slug compressor in cooling mode.  Opening up airflow will also affect heat transfer,  Not near as damaging potential as restricting air flow.