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AIR CONDITIONING SYSTEMS
Gil and I poke fun at how our coaches are set up and we talk about and use terms that might be confusing new owners and portential buyers of coaches. So I will try to sort through the opinions and blather and try to objectively discuss what is out there in heat and air systems and list performance, strengths and weaknesses of all systems currently offered on most coaches.
OTR. This has been generally applied as the name of the Prevost installed full coach heat and air conditioning system. The heart of the system is a 6 cylinder air conditioning compressor that is engine powered. The condensing unit occupies (on pre X3 coaches) about 20″ of the second bay ahead of the drive axle, passenger side and the evaporator unit occupies about 20″ of the second bay ahead of the drive axle, driver’s side. The OTR system takes up the equivalent of about 1/2 of a bay. Over the years Prevost has made changes to the OTR system going from the original R12 freon to 134A, and the system has gone from about 84,000 BTU to in excess of 105,000 BTU.
It provides heat and air conditioning to both the driver’s area and the balance of the coach. It is typically ducted into the coach at the vents in the front and along the base of the windows along the sides of the coach. The return air flow is usually at floor level in the center of the coach on the driver’s side.
The advantages are the capacity is so great it has the ability to quickly cool a heat soaked coach. Because it is a whole coach system the entire coach is heated or cooled. With OTR it is not necessary to supplement it with any other heat or AC sources while the coach engine is running.
The disadvantages as cited by owners is it uses up bay space that is important to some owners (such as full timers).
Claims it saps the engine of valuable horsepower are true in the context of when it is working to pull the temperatures down on a heat soaked coach, but when it is maintaining a coach temperature the energy it uses is comparable to any other system maintaining temperatures in the entire coach.
Converter built OTR. This is a name to describe whole coach systems for AC only that are built and installed by a converter in a Prevost chassis. There are variations of these systems, some using a single compressor that provides for AC at the driver’s area as well as for one or more heat exchangers (evaporator units) in other areas of the coach. Some systems have a second compressor in addition to the drivers AC compressor. The second compressor is usually serving two evaporator units located centrally and to the rear of the coach. There does not appear to be any stated values for output but it can be assumed the performance of converter installed OTR systems have performance that is a fraction of the Prevost OTR. The systems are also AC only.
The advantage of converter installed AC systems are they supplement the required driver’s AC system that is installed by Prevost. Under certain conditions they are able to keep a coach comfortable and in that respect they are beneficial because of the lower weight and the fact they do not require bay space.
The disadvantage is they are not both heat and air conditioning. Some installations have compromised the ability to easily service the chassis of the coach, specifically the air dryer. The output is minimal so on hot days the system may have to be supplemented with house AC run from inverters.
I expect the above to be a start. While we are hacking away at what is written above I will attempt to add to it a discussion on the house systems as well as correcting the above as necessary. I can expect Gil to be all over this, but input from everyone will make it more accurate. When we finally have a concensus on all the descriptions this can be turned into an article for future reference.
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13 Replies
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Excellent explanation of system, Jon. Now would come down to texas with your gauge set and check mine??
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Jon,
That is interesting. My Prevost OTR takes up the complete bay all the way through. Or at least I think that is all bus AC and heat in there.
It works great though. I can not imagine driving in the summer without it.
DKO
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Your OTR does NOT go all the way through. Sandwiched between the evaporator side and the condenser side is your bus fuel tank. So even if the OTR was removed you would only gain two shallow bays on each side.
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Cruiseairs. Cruiseair units are split systems with the condensing units located in the lower portion of the coach and the evaporator units located in the house portion of the coach. They have ratings of around 13,000 BTU. Usually 40 foot coaches will have three units and 45 foot coaches will have 4 units. Generally there will be an evaporator in the front of the coach, typically behind the front upper cap supplying cooled air to the front portion of the coach. There will likely be one or two units mounted midships and the cool air flow will usually be directed to the bath area as well as forward. The unit in the rear will be used to cool the bedroom and will likely be located near the rear cap either in the center or to one side depending on the configuration of the bedroom.
The condensing units are usually installed behind the front drop down bumper, and depending on the converter there may be two or three condensing units in that location. If needed one or two condensing units will be located in the center of the coach immediately aft of the first bay. The refrigerant lines from the condensing units will run to their respective evaporators. Except on older coaches the Cruiseair units will be operated by a control often referred to as an SMX control. The control is programmable for humidity control, automatic or manual operation and in almost all cases except on older coaches the units can function as AC units, heat pumps or both. Cruiseairs will generally not function as heat pumps once the outside temperatures reach about 35 or below. They are not available with electric heat strips which have no temperature limitations.
The advantages of Cruiseair units begins with their construction. They were never intended to compete on price so while expensive they are also relatively durable. They are meant to be serviced to a considerable degree and are not considered as “throwaway” units. Because of the way the controls are set up the Cruiseairs can be very quiet during operation. They can be programmed to ramp the fan speed up and down making their operation non-intrusive.
The disadvantages of Cruiseairs begins with the cost. They are relatively expensive to purchase, and their location makes access difficult so servicing them can also be expensive. While improvements have been made over the years Cruiseair units will not work well when the coach is traveling because the condensers require cool air for efficiency and heat radiating from a sun baked highway may be too much for the units causing them to shut down. Consequently having an OTR coach is almost a mandate if Cruiseairs are to be installed.
If a Cruiseair coach is parked in tall grass (such as when dry camping) it cannot be lowered without blocking air flow to the condensing units so there are limits to where and how a Cruiseair coach can park.
Since I have no personal experience on a coach with roof airs the information below needs to be verified and corrected where needed.
Roof Mounted AC units. The roof mounted AC units were originally designed for the RV industry so their relatively compact, low profile configuration is ideal for the application. They typically are rated between 13,000 and 15,000 BTU. The converter when using roof mounted units can opt to use the units in an unducted manner, with each having their own cool air outlets and controls, or ducted such that two or more units can supply cool air to an entire coach with as many units running as needed to maintain the temperatures.
Roof mounted units are produced in significant numbers and are found in everything from pop-top campers to high end RVs. As a result of production quantities they are relatively inexpensive. Despite the easy access for servicing, or maybe because of that generally units are replaced rather than repaired. That might also be decided on the basis of the replacement cost compared to the hourly cost of service.
The advantages of roof mounted AC units is they are up in clean, relatively cool air so there are no impediments to their use when the coach is on the road. They are inexpensive so it is often more economical to replace one rather than trying to repair one.
The disadvantages of roof mounted units is they are inexpensive. The RV industry continually pushes for the least expensive costs and manufacturers of roof mounted AC units have responded. The low cost is seen in the life of the product. That is not to say they are junk, but they are not built to the same degree of quality as the Prevost conversion to which they are attached. A frequently mentioned disadvantage is the noise level, although converters now forced to use roof air units due to circumstances are making a serious effort to duct the air flows in such a way as to mask the fan noise and to isolate and eliminate the low frequency “drumming” from the roof mounted units.
The fact that roof air units require roof penetrations is another disadvantage.
Again, this is intended to be amended and criticized so when we are all done tweaking them for accuracy we can convert them to an aritcle for future reference.
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I saw a new H-45 in Phoenix last week that had 3-24,000 btu 11 amp Webasto packaged units with heat pump and was amazed how cool the H-45 was in 112 degree heat with 2 running, as most here know a H-45 is a beast to cool
Check those out they were not much larger than the Crusieair I have but sure put out some cold air,the guy told me they making a 30,000 btu that was smaller in size but was 240 volts and drew less amps
Clifford
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I saw a new H-45 in Phoenix last week that had 3-24,000 btu 11 amp Webasto packaged units with heat pump and was amazed how cool the H-45 was in 112 degree heat with 2 running, as most here know a H-45 is a beast to cool
Check those out they were not much larger than the Crusieair I have but sure put out some cold air,the guy told me they making a 30,000 btu that was smaller in size but was 240 volts and drew less amps
Clifford
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I wasn’t finshed but this board will not let you edit for the AC guys how can you make the Webastos work in a bus aren’t they a chilller type that uses water ? it was a nice setup but I am at a lost how one could work without water maybe a tank and pump ?
Clifford
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Clifford,
I wasn’t finshed but this board will not let you edit for the AC guys
I don’t understand your comment, were you trying to edit your post? If so, that’s easy, just click on “edit.”
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Clifford, I just edited this post by clicking on the “Edit” button at the bottom. You do have to be logged in, if your a Guest, it will give you the non access message. If you’re logged in and can’t edit this message, let me know and I get our web guy involved.
If you leave the computer for a time and come back to it, it may have timed out and even though you think your logged in, your not. Then you may be trying to edit your message as a Guest and it won’t let that happen. You have to login again. The security for the site is set that way in case you were using a public computer and just got up and left it on the PC site. It times out to keep someone from picking up where you left off and starts advertising blow up sheep!
Tom
Tom, it tells me “You are trying to access a area which is not yours” it has always done that when I try to edit is it me ?
I have a bad habit if something needs done like for the wife,grandkids or friends come in I hit the post button so I don’t loose it which I do anyway most of the time then I go back and edit
thanks for the reply
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I tried unsuccessfully to cut and paste another perspective on air conditioning systems. Instead I submitted it as an article. I would have simply attached it, but this forum doesn’t appear pdf attachments to a thread.
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Sorry, no PDF as an attachment or cut and paste, but you could submit a link if it was accessible on the net.
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Tom, you should have it as a submitted article. The “Add Link” button is not available, but here’s the ducument on the web:
At some point I’ll include a couple of charts to help visualize the differences.
Is that what you wanted to do? Tom
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Tom what wrong with the insert link button?
Thanks,
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