Roof A/C flow volume

[Larry Dedrick]

On our 2001 beaver marquis, we have three roof A/C units, that are ducted. The front A/C volume is good and provides good flow to not only it's area but provides some to the center trunk but it is considerably less as would be expected and in fact you can feel a slight amount of flow all the way to the rear of the coach. The rear A/C also provides good volume and good flow to it's area and a little less to the center trunk and once again you can feel a slight amount of volume all the way to the front of the coach.
Now my concern is the Center A/C, it does not provide no where near the volume that the front or rear A/C does. They all share the same trunk with the most volume coming out of it's respective section.
 My question is, should I expect more volume from the center A/C or is it's output balanced throughout the entire trunk? The volume from the center A/C ,once again, is only slight throughout the entire run of the A/C trunk (duct).
Hope I explained this adequately, any thoughts.

             Larry D

[Keith Moffett Co-Admin]

I took an air hose to our ol roof A/C's on the last coach.  Blew out the sand and salt in the fins and the fan etc.   Just a general cleaning made 10 degree drop in cooling  better than before and much improved air flow.

[Gerald Farris]

Larry,
Since your roof A/C units are an aftermarket installation (your coach originally came with basement A/C), it is impossible to say what your problem is without inspecting the coach. There are several questions that need to be answered to evaluate your system. First, are all three A/C units the same size? Next, are all of the intake filters (if equipped) and coils clean. Last, are there any ducy obstructions that were caused when the units were cut into the ducting?    

Gerald  

[Larry Dedrick]

Gerald:
            My center A/C, I am only assuming, is the same size. BCS is working on the coach and has found part of the duct at the A/C some what collapsed  and has fixed that, however I have not noticed any difference. I was hoping that someone with the same coach may have the same system and give me an idea of how much volume they get from their center A/C, in other words are they also experiencing a significant difference with their center A/C. That would give me a clue as to any other possible obstructions.
            We are not having any problem with cooling the coach so far, but I feel we should be getting more volume or force of flow from the center unit.
This past spring, I removed the cover to the unit and removed the cover to the evaporator (the coil that cools, I may have used the wrong term, I get the two mixed up) and found ice had formed on the evaporator and assumed that it may not be getting enough return air. Filter was removed during this inspection.

             Larry D

[Edward Buker]

Larry,

With all these installations the outgoing ductwork passes through the intake area and the airflow by design is limited. Unless you find an outright obstruction I am not sure that there is much you can do about it. I have just two roof units and get icing in my bathroom air unit when the humidity is high. The thermostats have fan speed controls on mine and given you are dealing with an aftermarket install you may want to verify that the wiring is correct for the fan speed control to the center air. You could try each option and see if the fan speed steps up or down as it should be. I had read one post from someone with a Beaver Marquis who got icing and had opened up the intake area somewhat by trimming metal parts to a minimum in that area.

Later Ed

[Larry Dedrick]

Thanks Ed. I will maybe do some more troubleshooting on my own and see if there is anything I can improve on.

     LarryD

[Michael Carbonaro]

Hey Larry here is thought,

When air is flowing through the duct when it gets to a vent there should be some kind of back pressure a half plat if you will that's what forces the air out the vent so if you think of it that way the air in the front and back units booth have back pressure because the air is hitting the end of the duct, your center unit if there is no back plate to force the air down you will think it is no working right example if you have a 6" duct you would want a 3" plate across the duct right after the vent to force some of the air down as it is traveling down the duct to next vent. I think that is how it works.

All so I my other motorhome I had roof air and I found that they are closed systems and you can not recharge them if you think you are not cooling enough you can have ports put in and even help the units to last longer before replacing. I know cause I had to replace one because it ran to long on low Freon so I replaced the unit and had ports put in the other.

I hope some of this helps.

Mike  

[Larry Dedrick]

Michael:

            Yes, that does make sense, and the center unit does cool adequately, just not much pressure pushing from the duct. We will live with it.

                  Thanks            Larry D

[Gary Wolfer]

I am in the process of coating my roof with Dicor Fiberglass roof coating and yesterday took both air conditioners off and changed the bottom gaskets. The exit boxes that were mounted on top of the ducting were taped down to the duct with tinfoil tape. They were both loose so I had a roll of tinfoil tape and reinstalled the exit boxes to the ducting. Then my son and I tried to center the air conditioners over the holes and line up the air conditioners with the boxes. It was tough and could not be sure from observation from the top and from the bottom I had to feel boxes and air conditioner and guess I had a good seal because the boxes are above the ducting and impossible to see. Finally I tried by lining up the 4 long attachment bolts best I could then tightening them down firm but not too tight.  The main problem was the gaskets were twice as thick as the old ones and the air conditioner sat over the boxes with a gap and was sucked down when the bolts were tightened. I did get good air flow on most of the flow covers in the ceiling however some were stronger flow than others. I found out if I only ran for instance the back air conditioner air would flow thru all the vents both front and back and I could get more air flow to the whole coach by turning the fan in the bedroom to the high position then turning the fan in the front of the coach to high and the on auto switch to on. It blew cold air to all the vents. I am a happy camper after figuring that out because I am plugged into a 30 amp shore power and cannot use both air conditioners at the same time unless I am using my genset.

[John Fearnow]

The last post mentions running the rear a/c with the front unit on fan only.  When you run the fan only does it draw outside air or recirculate inside air?  I thought the fan brought in outside air and therefore avoid using it without the a/c when it's hot.  But I'm not sure my assumption is correct.

John F
2002 Pat-thunder

[Joel Ashley]

Wrong assumption John.  Both the fan-only and the AC functions recirculate interior air, the same fan for both operations.  That's why there is a foam filter over each AC intake on the bottom of each unit.  You should occasionally pop open the grill and remove, clean/wash, dry, and replace the filter (or buy replacements).  The fan draws interior air in that grill, exposes it to any cooled AC heat-exhanger surface, and pushes it back into the living space, just like your furnace or AC in a house.  Though they aren't exactly high-tech, keeping clean filters in place helps cut down larger air contaminants.  

Like others, we also maximize 30 amp connections by using one unit on AC and the other fan-only in order to spread the cooler air through the coach more effectively.

Joel

[Edward Buker]

John,

I believe that the air units only recirculate the inside air. It would not be beneficial to pull humid/hot air into the coach from the outside as part of the mix. In cleaning the condensers and checking drains with the air conditioner cover off, I have not seen what looks like an air inlet on the unit.

Later Ed

[Gary Wolfer]

I was thinking some ceiling vents blow more air than others I wonder if It would help to add a computer fan above the weak vents to increase flow. They are cheap and could be set above each ceiling vent and wired to come on with either or both air conditioners. The ductwork runs from front to back the only thing that keeps air from flowing seems to be a path of least resistence.

[Joel Ashley]

When ours was new BCS discovered some sort of blockage, misdirected tubing, or collapsed tubing was the cause of low flow in our bathroom ceiling port.  As I recall it was at or very near the port itself.  You might remove the grills from low-flow ports and check with a flashlight for anything obvious obstructing things.

I'm not sure wiring in mini-fans that get power from the thermostat would be any small job, Gary.  Closing off the high-flow ones a bit might leave more for the weaker ones, though, and balance things out.

Joel

[Lee Welbanks]

When I bought the Thunder some of the vents didn't seem like they were blowing enough air. So I dropped the center decoration panel in the front and bedroom, also the center air. Took down all the vents, looking into them with a light and mirror you could see where some of the tap was coming loose in the ductwork, also where the vents were cut into the ducts they didn't tap off the edges, air doesn't like sharp edges, make them as rounded as possible. Also found a place in the duct where the air went into the attic through a crack. Went a got a couple of rolls of the aluminum reinforced FSK tape and sealed up and re-taped the whole duct system as much as I could get to. Put it all back together and fired up all 3 units and man what a difference.