effiency of generator and house A/C vs dash A/C

[jls]

When picking up our new Class C the mech said using the onboard Genny and Roof A/C is more efficient than the dash. Does anyone have any experience with this issue? Also I am wondering about heat building and taxing the chassis engine. Thanks in advance.

[pnichols]

The cab of a Class C is not the same environment as the cab of a Class A or B. The cab of a Class C has way less heat gain because it's completely shaded at all times by the cabover. What this means is that a large amount of the cab A/C's BTU capacity can be used for cooling of the coach area of - at least - Class C motorhomes as large as our 24 footer.

With our E450's excellent Ford designed doghouse insulation, heavy floor insulation as installed by Winne, our cab vent fan on it's highest setting, it's air circulation mode set to Max A/C for complete recirculation/reprocessing of the interior air, and it's fan outlet vanes aimed over our shoulders right back towards the coach area - the coach DOES remain comfortable in scorching outside temperatures.

As I've mentioned in other posts, I think a key factor may be using the cab air of a Class C in it's Max A/C configuration to stop having to constantly cool new outside air. That's how the rooftop A/C handles the air and how home air conditioning systems usually handle the air.

In hot summer temperatures we know the above procedures work because we set things up this way all the time when we have passengers back in the coach area when traveling, and even when we ourselves sit at the dinette in hot blacktop parking lots eating lunch with the V10 idling and it's cab air set up as I describe above so as to not have to run the rooftop A/C and Onan generator in noise-sensitive parking situations.

[JaxDad]

MrWizard wrote:
really doesn't make any difference,if the dash A/C is rated 2500 more BTUs
that ability only extends into the cab area

the dash A/C does not move/cool the same amount of air as a roof top or basement A/C
it is not centrally located, it can not force cool air to the rear half of class A MH, it will cool the driving area and Maybe the sofa 'maybe'

a class C has a smaller cockpit area to cool, allowing for more cooling for the area behind the cockpit

simple 'BTU capacity' numbers do not tell the whole story
and a short class C, does not have nearly the cubic volume of space as a larger class A

There’s many posts, here and elsewhere that all state basically the same thing.

Open the rearmost roof vent wide open and vacuum moves far more air than you’d believe, AND vacuums out the hot air.

[MrWizard]

really doesn't make any difference,if the dash A/C is rated 2500 more BTUs
that ability only extends into the cab area

the dash A/C does not move/cool the same amount of air as a roof top or basement A/C
it is not centrally located, it can not force cool air to the rear half of class A MH, it will cool the driving area and Maybe the sofa 'maybe'

a class C has a smaller cockpit area to cool, allowing for more cooling for the area behind the cockpit

simple 'BTU capacity' numbers do not tell the whole story
and a short class C, does not have nearly the cubic volume of space as a larger class A

[pnichols]

wa8yxm wrote:
That said. Dash air is not big enough to cool off a 38' Motor home or for that matter a 15' motor home. when it's 90 out in the shade you got to run the roof air's no choice.

I don't know about a large Class A motorhome, BUT ... the above is certainly not true for our 24 foot Class C and it's E450 chassis ... at least with outside temperatures up to what we have driven in so far ~ 108 degrees F. I've learned that an important key to getting the most effective cooling out of a vehicle's cab air system is to run it in it's recirculation mode - as I mention below. That's also how our stick house heat pump runs all the time, whether in heating mode or cooling mode.

This discussion is similar to another related running discussion, so I've lifted my post from that thread since it also applies here:
------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Below are just four (4) links talking about the BTU cooling capacity of vehicle cab air conditioning systems ... and a search yields many more articles that reinforce what these links are saying.

The information in the links confirms what I have experienced ... our motorhome's (and our sedan's and our pickup truck's) cab air conditioning systems have terrific BTU capacities (well beyond 13.5K BTUs at typical traveling engine RPMs). Our motorhome's E450 cab air conditioning system is easily capable of cooling our entire Class C when traveling at highway speeds in scorching temperatures - especially using it's MAX AC setting such that the interior air is only recirculated - instead of the cab air conditioning system having to constantly cool new hot air coming in from the outside.

https://www.google.com/search?lr=&hl=en&as_qdr=all&q=cooling+capacity+of+car+ac&sa=X&ved=0ahUKEwi6p_...

https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3023729/Average_auto_A/C_BTU_rating?

https://autoacforum.com/messageview.cfm?catid=2&threadid=20990

https://www.quora.com/What-is-the-typical-nominal-cooling-capacity-kw-or-BTU-h-of-a-car-air-conditio...

[wa8yxm]

I have heard that myth many many times.. I do not believe it for a second.

Thomas Edison optimized the effiency of a generator at around 90% (Now that is just the generator portion we are not including the engine here) and likewise an electric motor.

So once the power comes off the Engine Drive Shaft.. you lose what 19%.

The DASH A/C the power comes straight off the engine, NO LOSS period.

Now we have the fuel needed JUST TO RUN THE ENGINE.. No-load.. so you loose perhaps even more when you run the Roof air.

That said. Dash air is not big enough to cool off a 38' Motor home or for that matter a 15' motor home. when it's 90 out in the shade you got to run the roof air's no choice.

[JaxDad]

Duplicate thread.

[Grey_Mountain]

We routinely use the generator and house air while traveling. Dash air will do very little to cool a 39 foot coach.

GM

[dougrainer]

1. On a Class C, the ONLY way you will get cold air IN THE CAB area is with the Dash AC
2. NO MOTORHOME DASH AC WILL COOL THE REST OF THE RV. IT WILL ONLY COOL THE FRONT CAB AREA AT THE DASH.
3. If you want the rest of the motorhome to be cool in transit you will have to run the Genset and roof AC systems
4. The Dash AC will pull anywhere from .5 to .7 gallon extra gas per hour to run the Dash AC compressor depending on the chassis
5. Depending on the Load a Genset will pull .7 to 1.1 gallons per hour of fuel. A Diesel Genset will use less than .5 gallon per hour. Doug

[Ranger_Smith]

Kennyg wrote:
If you run the generator while driving what damage if any, occurs when the house A/C cycles off resulting in no-load on the generator??

none

[ksg5000]

I have read a few threads where individuals have stated that the fuel consumption of Generator is about push with chassis air conditioner. I myself use the Generator on hot day as dash air won't provide enough cooling.

[Kennyg]

If you run the generator while driving what damage if any, occurs when the house A/C cycles off resulting in no-load on the generator??

[hypoxia]

I don't agree, the engine A/C is fairly efficient.

As posted above, the box will be cooler if you run the house A/C. I will sometimes start the generator a while before stopping to get the house cooled down and if it is really hot will run the generator all day when needed.

[gbopp]

Are you talking about overheating the engine while using he engine/cab air conditioner? That shouldn't be an issue.
But, it's a good reason to run the generator and house A/C if it is a concern.

I don't think it's any more or less efficient running the generator/house air. The difference would be minimal either way.
Don't worry about the fuel use. Fire up the generator and be comfortable.

You will not hurt the generator by running it when you are driving.

[Ranger_Smith]

I don't know about more efficient but it certainly keeps the box cooler with the genny and roof top ac running. I will use both when it is real hot out. It's nice pulling into a park for the evening and the coach is already cool.