Roof AC units on, Roof fans/vents open or closed?

I gotta chime in to also say...I run the rooftop A/Cs with the vents/windows/shades CLOSED except for the front windshield curtains. On a hot Southern Summer day of 95 degrees plus with humidity ranges in the high 80%-90%, the dash unit won't even keep the driver's cabin cool enough for me...much less the house. I have two rooftop units, one 13.5K and one 15K. I run both, in addition to the dash and run a small fan pointed at my driver's seat to stir up the cool air from the floor. When I get to my destination, my coach is nice and cool and there's no struggle to get it "climatized" before the camping fun begins. As for fuel mileage, on a 6 hour trip, I lose about 0.5mpg running the genset....plus it gives it a good exercising under load and I don't have to burn LP to keep my fridge cool.

JaxDad wrote:

dougrainer wrote:

JaxDad wrote:

ArchHoagland wrote:
Always close the roof vents and windows when you are running the air conditioning whether it be parked or moving down the highway.

The air conditioners bring their air in from the interior of the coach. If you leave the vents open then you are drawing in hot air.

Actually that's wrong.

Unless the dash a/c is set to 'max' or 'recirculate' the system cools outside air and vents stale air. All vehicles operate this way.

As for cooling efficiency, if you leave the rearmost roof vent open (hot air rises) and set the dash a/c to highest fan and set to blow down the back the hotest air will be drawn out the roof and the cooled air will be drawn towards the back to cool the rest of the coach.

I've been doing this for years and have proven how well it works to many non-believers over the years.

On a typical dash unit the blower fan is the weak part and it usually can't keep up with the cooling capacity of the rest of the system. This method dramatically increases air flow and really helps.

Maybe in Toronto you can use the Dash AC to cool a 30 foot plus Motorhome. BUT IT WILL NOT in the rest of the world.


Doug

No, on a 40'er it probably won't keep up, but on a 30'er, especially a C on. Ford chassis it sure will.

The Ford dash unit is far more powerful than a rooftop unit is.

It's also a LOT more fuel efficient than running the genset to run a rooftop unit is.

1. The Dash AC is NOT more powerful than a roof top unit. While the Air speed output may seem higher, the BTU capacity is slightly higher on a roof unit. Most Dash AC systems(You are talking a FORD system versus a Class A system)are about 12,000 BTU. Roof AC units are 13.5 to 15 BTU.
2. IF you have a C class, WHY are you posting on a Class A forum.
3. As to duel effieciency, There have been numerous studies and tests versus Dash and Roof top with Genset running. GASOLINE Class C is probably better on the Dash AC. Class A Gas and Diesel it is better to run the roof AC with Genset. Diesels will pull about 1/2 gallon max per hour and Gas gensets 1/2 to 3/4 gallon per hour. Doug

dougrainer wrote:

JaxDad wrote:

ArchHoagland wrote:
Always close the roof vents and windows when you are running the air conditioning whether it be parked or moving down the highway.

The air conditioners bring their air in from the interior of the coach. If you leave the vents open then you are drawing in hot air.

Actually that's wrong.

Unless the dash a/c is set to 'max' or 'recirculate' the system cools outside air and vents stale air. All vehicles operate this way.

As for cooling efficiency, if you leave the rearmost roof vent open (hot air rises) and set the dash a/c to highest fan and set to blow down the back the hotest air will be drawn out the roof and the cooled air will be drawn towards the back to cool the rest of the coach.

I've been doing this for years and have proven how well it works to many non-believers over the years.

On a typical dash unit the blower fan is the weak part and it usually can't keep up with the cooling capacity of the rest of the system. This method dramatically increases air flow and really helps.

Maybe in Toronto you can use the Dash AC to cool a 30 foot plus Motorhome. BUT IT WILL NOT in the rest of the world.


Doug

No, on a 40'er it probably won't keep up, but on a 30'er, especially a C on. Ford chassis it sure will.

The Ford dash unit is far more powerful than a rooftop unit is.

It's also a LOT more fuel efficient than running the genset to run a rooftop unit is.

Also, when I am running the RV air conditioner while driving, I turn off the dash air conditioning.

Less of a load on the engine and engine cooling system.

JaxDad wrote:

ArchHoagland wrote:
Always close the roof vents and windows when you are running the air conditioning whether it be parked or moving down the highway.

The air conditioners bring their air in from the interior of the coach. If you leave the vents open then you are drawing in hot air.

Actually that's wrong.

Unless the dash a/c is set to 'max' or 'recirculate' the system cools outside air and vents stale air. All vehicles operate this way.

As for cooling efficiency, if you leave the rearmost roof vent open (hot air rises) and set the dash a/c to highest fan and set to blow down the back the hotest air will be drawn out the roof and the cooled air will be drawn towards the back to cool the rest of the coach.

I've been doing this for years and have proven how well it works to many non-believers over the years.

On a typical dash unit the blower fan is the weak part and it usually can't keep up with the cooling capacity of the rest of the system. This method dramatically increases air flow and really helps.

Maybe in Toronto you can use the Dash AC to cool a 30 foot plus Motorhome. BUT IT WILL NOT in the rest of the world. The Dash AC on "Normal" brings in hot outside air and then attempts to cool it. BEST case on this mode is 56 to 60 degree output air AT THE VENT closest to the Evaporator. We are talking 90 degree ambient outside heat. Now, if you put it on MAX(recirc) that will recycle the Inside air at the DASH area only and will put out 5 to 10 degrees colder air. But to think by opening a rear roof vent that will allow the RV DASH AC to PULL 60 degree area rearward and COOL the rest of the RV, well, you are mistaken big time. Besides that would actually pull needed cold air from the Dash seat area. The Dash AC systems in Motorhomes are designed to do one thing. COOL the front Cockpit only. RV roof AC units are sealed systems for the inside of the RV. They have no way to draw in outside air. The recirculate the inside air only. Motorhomes have a natural vacumm in transit. They will suck in hot air even with all doors and windows/roof vents closed. This is more of a problem due to Slide rooms and how many slide rooms you have. Now, I have 38 years as a RV technician and still working at it. I have worked on ALL brands and types of RV motorhomes(under $750k) for almost 40 years, the Dash AC and the Roof and basement systems. YOU CANNOT COOL A LARGE MOTORHOME WITH THE DASH AC SYSTEM. PERIOD. Regardless of any thing you try to do. Doug

JaxDad wrote:

lanerd wrote:
Obviously many of you do not understand how an AC system works.

Ron

It's equally obvious some people some people don't read the question being asking before they answer, incorrectly..

The OP said;

RJL wrote:
We plan on leaving this upcoming Friday to head back to NJ. With both units running as well as the DASH AIR should the roof fans/vents be open or closed while travelling with the AC's cranking?

So yes, there IS vents open drawing in outside air.

Sorry, but I think you need to read the question again. He's asking should the vents be open or closed!! Of course like many others above responded.... the answer is the vents should be closed.

I was responding. not to the OP's question, but to other erroneous replies.

Ron

lanerd wrote:
Obviously many of you do not understand how an AC system works.

Ron

It's equally obvious some people some people don't read the question being asking before they answer, incorrectly..

The OP said;

RJL wrote:
We plan on leaving this upcoming Friday to head back to NJ. With both units running as well as the DASH AIR should the roof fans/vents be open or closed while travelling with the AC's cranking?

So yes, there IS vents open drawing in outside air.

JaxDad wrote:

ArchHoagland wrote:
Always close the roof vents and windows when you are running the air conditioning whether it be parked or moving down the highway.

The air conditioners bring their air in from the interior of the coach. If you leave the vents open then you are drawing in hot air.

Actually that's wrong.

Unless the dash a/c is set to 'max' or 'recirculate' the system cools outside air and vents stale air. All vehicles operate this way.

As for cooling efficiency, if you leave the rearmost roof vent open (hot air rises) and set the dash a/c to highest fan and set to blow down the back the hotest air will be drawn out the roof and the cooled air will be drawn towards the back to cool the rest of the coach.

I've been doing this for years and have proven how well it works to many non-believers over the years.

On a typical dash unit the blower fan is the weak part and it usually can't keep up with the cooling capacity of the rest of the system. This method dramatically increases air flow and really helps.

That's a good point- the dash AC should be set to recirculate or MAX to have it work in concert with the roof ACs. That way all the conditioned air is just recirculated within the coach. All vents and windows would be closed for that configuration.

Obviously many of you do not understand how an AC system works.

It is a closed system. The AC unit does not draw out side air in or vent inside air out. There is no AC vent to draw outside air in or to expel out to the outside...it only cools the interior air. It draws inside air into the unit, cools it, and then sends it back out into the area it's cooling. This is a simplified explanation as I won't go into details on condensers, compressors or evaporators.

When using your AC (house, car, rv) keep all vents, windows, and doors closed.

Ron