How many rooftop a/c's?

[robertglines1]

Welcome..we all have different needs ....intrest...financial limits...so you have alot to consider..40 ft coach with all the glass still in =3 or 4 ac..take half glass out drop to 2to3...must insulate insulate...insulate...consider mpg also if your going a lot of miles...also roof tops come up to 15000 and heat pumps are available also to take care of your heating and ac with one unit..jut a few thoughts  Bob

[MikeH]

...also roof tops come up to 15000 and heat pumps are available also to take care of your heating and ac with one unit..jut a few thoughts  Bob

Bob, thanks for the input. "roof tops come up to 15000" what? Dollars, KW?

I think I am trying to keep out of the basement so we have more storage space down there. I am guessing that a heat pump will end up down there?

Thanks,
Mike

[Ednj]

Us the Over The Road HVAC when traveling.
When parked 1 Roof Air can keep up, 2 is all you will need.

[Sean]

... "roof tops come up to 15000" what? Dollars, KW?

BTU's per hour (BTU/h).  The other common RV size is 13,500 BTU/h.

BTW, unless they are doing calisthenics, each additional human only adds about 300 BTU/h of heat load.  So if you are getting advice about what works for a couple, you only need worry about another 1,200 BTU/h of heat removal for your family of six, or less than a tenth the capacity of a standard roof unit.

The vast majority of the heat will come from the external environment, as has been noted.  Glass is the big killer, followed by inadequate insulation and dark external colors.  Paint the roof frost white, and the lighter you go on the sides the better off you will be.

I think I am trying to keep out of the basement so we have more storage space down there. I am guessing that a heat pump will end up down there?

Heat pumps are also available in rooftop package units.

Rooftops are more efficient and generally easier to service and replace than basements.  Also, basement units are getting hard to find -- see the extensive thread on this subject elsewhere.  The principle reason to stay off the roof would be aesthetics.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

Us the Over The Road HVAC when traveling.

That's some very subjective advice.  Unless you spend many more hours traveling than parked, which would describe very few of us, the OTR system is a huge maintenance expense, space hog, and weight burden that very, very few RVers can justify economically.  If it floats your boat, go for it, but I suspect that with six souls aboard, you will need every cubic foot of space and pound of weight for other things.

When parked 1 Roof Air can keep up, 2 is all you will need.

Again, very subjective and probably true for very few people.  I can tell you unequivocally that we need three units running full-tilt when the outside temps are over 110°, and we have an extremely well-insulated and efficient bus with only two humans and three ~15lb pets on board.  That's in a 40' coach, which I would also guess would be the minimum length you would want for six folks.

I would frame and wire for three units.  You can always put them in one at a time when you better understand your needs.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[John316]

Bill B Bus is spot on.

I agree about the floor insulation. We have 1.5" foam under our half inch plywood. That has helped alot.

We have the Oasis heating system (like Aqua hot) and it is great.

And everything else that Bill mentioned hits the nail right on the head.

God bless,

John

[robertglines1]

see we all have opinions..I was speaking of the 15000btu heat pump roof air...I don't use anymore because I like the Basement air...I have found it to be more efficient..again a point of discussion...comercial units are getting hard to find..home engineered units take a little thought,but can be done..I recently posted pictures of how I did mine.  how I did homemade basement air...planning locating new system in old condensor compartment....Roof air is very simple...the coach I am doing now had 4 roof airs on it before I got it,was a 6 bunk entertainment coach...

[Sean]

see we all have opinions... I like the Basement air...I have found it to be more efficient..again a point of discussion...

It may be a "point of discussion," but there is no debate.  This is a simple matter of physics and it can be proven beyond doubt -- the thermal efficiency of a package unit on the roof will always be higher than the thermal efficiency of an identical system below the floor.

Different systems from different manufacturers with different designs, will, of course, vary.  So it is possible to have poorly designed (or engineered) roof units whose performance can be surpassed by a well designed and engineered basement unit.  But that's true of anything.  The general case is not a matter of opinion, but fact: rooftop package units are, generally speaking, more efficient than basement units of the same effective capacity.

Unlike traffic laws and even my coveted codes, the laws of physics can not be violated.

-Sean
http://OurOdyssey.BlogSpot.com

[Brassman]

Sean, why are the roof top units inherently more efficient?

[TomC]

Give you a cost difference- a 13,500btu/hr Coleman with heat strip (they do work) about $500.00.  A 15,000btu/hr Coleman heat pump about $800.00.  Good Luck, TomC

[JohnEd]

Dometic calls the "heat strip" a "chill breaker".  It is not adequate for heating....not at least in my coach.  As I understand the instructions it is to be used when warming things up after a cold soak and in concert with the furnace.  I would skip the expense but Tom is known to be correct with frightening regularity and that gives me pause.  Get the heat pump AND the wabasto system so you can get hot water and warm the engine.  There is only one instance where flexibility is a real detractor and HVAC isn't it..

I urge duplication in systems.  Wabasto and heat-pump and your OTR.  3 roof air so you can normally run two and keep the whole coach comfy. Three on to pull the temp down quickly after a heat soak.  One breaks and you still have little to worry about.  OTR that cools the whole coach using an efficient modern auto compressor and the OTR is a back-up for the entire roof AC system that can be taken out by one electrical problem.  Lose AC in 90 plus and you cannot stand the heat in the coach...literally.  Seems like over kill in a lot of ways but in the planning stages it isn't all that much of a chalange or expense......doing it yourself, of course.

I think you are off to an excellent start.  Good luck to you and your family and your mission.

John

[Sean]

Sean, why are the roof top units inherently more efficient?

The short and simple answer is because heat rises.

The longer version:

Air conditioners are inherently inefficient machines.  They take heat from one place, and move it to someplace else, but they also use a lot of energy in the process, much of which is dissipated as waste heat.  Lots of the waste heat is discharged through the condenser, but not all of it.

Take as an example a pair of idealized systems.  We will use package units for our example but the same reasoning applies to split systems (most RV units, whether roof or basement, are package units).  One system will be a roof system and the other will be basement.  They will use identical parts, IOTW the same evaporators, condensers, fans, compressors, and other items.

Let's further assume that either of these systems would have ducted supply and return vents located in exactly the same spot in the coach, mid-way between the floor and the roof, so the duct lengths would be the same in each case.  In addition, we will also duct the supply and return condenser air from an identical location midway up the outside coach wall, again same duct length.

So to be clear, the only difference between these two systems is one has all the working parts on the roof, and one has all the working parts under the floor.

At a bare minimum, whatever internal heat is generated by the working parts that is not discharged through the condenser rises into the coach in the case of the basement unit, and rises away from the coach in the case of a roof unit.  So at a bare minimum, you have this additional heat load that will cause the basement unit to have to work harder than the roof unit to achieve the same result.

Of course, our "idealized" system is far from practical.  We theoretically located the evaporator and condenser supply and return in an impractical place, just for the sake of proving that, no matter what you do with them, there is always a penalty for units under the floor versus units on the roof.  As a practical matter, things are actually even more lopsided.  Nobody ducts the condenser air this way, and in practice, hot condenser discharge from a basement unit ends up under the coach, where it ends up heating the coach even more, whereas hot condenser air from a roof unit is discharged above the coach where it will rise away from the coach.

Likewise the most efficient arrangement for evaporator discharge (cool air into the coach) is near the ceiling, and it requires less ducting and therefore less fan horsepower to supply air to ceiling registers from roof units than from basement units, which must first be ducted all the way from the basement to the ceiling to achieve the same effect.

So for units of identical cooling capacity, roof units are more efficient than basement units.

Now where this breaks down is when you start getting into larger capacities.  The largest roof units are about 15,000 BTU/hr, whereas basement units could theoretically be built in any size, and in practice come in capacities up to 30,000 BTU/hr.

If you need a full 30,000 BTU/h of cooling, it will almost always be more efficient to get it from a single 30,000 BTU/h unit that from two 15,000 BTU/h units, and so in this case, a 30kBTU/h basement unit might edge out a pair of 15kBTU/h roof units.  However, this equation flips around completely if you only need half that amount -- running a single 15kBTU/h unit at full capacity will be more efficient than running a 30kBTU/h unit at half capacity.

Over the full range of most usage profiles, a mix of capacities is required, and slight efficiency differences due to this size/efficiency disparity tend to even out.  If your usage profile is such that you mostly need only 15k or so of cooling most of the time, perhaps moving that cooling from the salon to the bedroom and back as needed, then two 15k units, irrespective of whether those are basement or roof, will be a more efficient solution for you, whereas if your usage profile is such that whenever you need air conditioning, you need 30k of it, then a larger unit may be a better choice.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[WEC4104]

Welcome MikeH!   I'd like to mention one other factor in the AC selection that I don't think has been stressed thus far....   A big factor is how you plan to operate the cooling system. Let me explain:

On our family's 4104 (a 35 foot bus), we have only a single 13.5K BTU roof unit.  With 15+ years useage on this Coleman, I'm sure it has lost some efficiency.  Nevertheless, we still find it adequate for our needs. We have been on multiple week long trips to places like Alabama and Florida in the heat of summer, with 5 occupants.  The key is we use the AC "smartly".  We try to stay ahead of the heat curve by turning it on before the bus gets too warm (or we run it 24/7).  When parked, we take full advantage of window shades, awnings, and tree shade.  Overall, it works well for us, and my desire for a second rooftop unit has been steadily dropping on my priority list.

However, our usage does not match everyone else.  Some folks need AC systems that can handle extreme situations.  Take for example a bus that is left sitting in the afternoon sun on a 95 degree day, with everything shut down. The interior could easily reach 105. At 4 pm, everybody piles onto the bus and gets readily to leave.  You want the bus cooled down immediately!  That is going to take serious BTUs.

My point?  .... How you plan to use the bus is as much a factor as the insulation, paint color, window glass, etc.

[robertglines1]

so as you see it turned into a debate..ser factor never came up though...so to finish my coment it comes to cost efficiency...over the road air is a major expense and headache to maintain...would be idea but could cost you more in the long run.ck to see what 30 lb of refrigerant installed would cost.? My basement air units discharge hot air out the side where it rises to the sky not the bus..I do use a 20 inch fan to bring heat off the ceiling to mix with air and also my ac vents are discharging cold air up at a 45 degree angle..have experience with monthly elect bills at RV park (apples to apples) compared with sticks and staples 'same time same month..my bill was 91 dollars a SS friend was 142 and another 240..they both had roof top air....my coach is a 40 ft with 2 slides with 37000 BTU ac capacity..total initial cost under 900 dollars...haven't lost a basement unit to bridge or tree yet...so will agree to disagree...I'm sure my ideal is not as efficient but feels good inside our coach.   Bob

[muddog16]

Mike, welcome!  A couple of answers here.  Entertainer buses usually use many more roof airs than a basic family conversion.  Most buses are broken down into three main area's, living room/kitchen, bathroom, and the bedroom!  Duotherm makes air conditioners, and heat pumps for roof mounting!  Roof tops, are definately more efficient, easy to work on and replace!  Duotherm systems can be zoned together allowing each area the benefit of the need in that area,  with remote sensors in each area they can operate independantly of each other depending on the zone or owners need!  Another feature of Duotherm is it allows the addition of a full heating system remotely located and capable of running on the same thermostat, all of these units can be operated in a zoned environment, as the owner wishes.

Like Sean said, basement units are becoming rare, which was in my opinion caused by a downturn in the economy!   A basement unit will take up a lot of storage area in a bus usually an entire compartment.

Roof airs by many are considered an eye sore.  It depends on you the owner and your pocketbook.  I'm not a fan of window air conditioners in a bus but many use them and seem to be satisfied!  Remember this, one day you will sell the bus.........and what you build into the bus today will determine the price of the bus when the day to sell comes!

Again good luck and ask many questions before starting or buying............