Silly A/C question

[Urlauber]

Nothing serious, just out of curiosity... lets say you are in the Summer desert somewhere and want to hang out for an hour or two in the van, so you need to run A/C... which would be cheaper?

1) Running the 2800i Onan Propane Generator to power the roof A/C
2) Idling the 3.6l Ram engine while blasting the dashboard A/C

Propane being more expensive and less energy-dense than gas?

(I do not know how much propane is used /hr to run the genny under load... nor do I know how much an hour of idling takes out in terms of gas. Hey... that's why the post.)

[avanti]

Dunno.
However, I once did a quantitative analysis of diesel vs propane For heating a van. It may contain some of the parameters you need:
https://www.classbforum.com/forums/f...tml#post122374

[folivier]

I've found that dash AC in any vehicle will work much better if the vehicle is moving. If stationary not so good. So the roof air may work better if stationary.

[Urlauber]

Originally Posted by avanti

Dunno.
However, I once did a quantitative analysis of diesel vs propane For heating a van. It may contain some of the parameters you need:
https://www.classbforum.com/forums/f...tml#post122374

Thank you, yes, maybe... a bit different when heating I think.
Difficult to say without having exact usage of propane vs gas (vs diesel) to run different things (engine vs generator) to run the different types of A/C.

[Urlauber]

Originally Posted by folivier

I've found that dash AC in any vehicle will work much better if the vehicle is moving. If stationary not so good. So the roof air may work better if stationary.

Yeah, that's a very good point. Not sure if vans without roof A/C are more suited to that, or maybe they just have to do without.
Hoping to get hook-ups at the very hot places (Death Valley ) but there are some like Page AZ where I can see us doing the dispersed thing so nothing to plug in.

[jakegw2]

Well let's see:

Promaster Chassis
From this post I am going to use the value of 0.54 Gal/Hr @ idle for the Promaster. This gets you the dash air conditioning with probably in excess of 18k BTU at idle (at cruising speed this rises up to something like 60k BTU). Since these are estimates I will go ahead and estimate that using the AC increases idle fuel consumption by 10% bringing it to 0.59 Gal / Hr for 18k BTU of cooling.

Roof AC
The roof AC draws 13.6A at full load.

The Onan produces about 23A at full power, or 11.5A at half-load. At half-load it consumes 0.31 Gal / Hr for the propane model. By interpolation we get about 0.35 Gal / Hr to support a continuous 13.6A load for 13.5k BTU of cooling.

Comparison
I'm on shaky ground here as I'm not really sure how to do comparison adjustments for different BTU ratings, but I'll take a stab at using a linear comparison. In a very hot environment the roof AC will be under-powered and cannot be increased to 18K. Most of the gas consumption from the dash AC is from the engine idle, so it can't be decreased. That makes this comparison entirely theoretical, but here it goes:

scaling 13.5k to 18k is a 30% increase in cooling, so using a linear approach we would get to a power consumption increase to 18.1A. Using that same increase and interpolating between half-load and full load consumption we get to 0.45 Gal / Hr of propane for equivalent cooling.

I'll skip the energy density comparison and go right to the price comparison:

I am using data for Rapid City, SD in the link, choose your own adventure here if you wish.
Propane: $4.10 / Gal
Gas: $4.70 / Gal

Conclusion:
Propane $1.44 / hr of operation ($1.85 / hr of operation for equivalent cooling performance)
Gas $2.77 / hr of operation

[dicktill]

Originally Posted by Urlauber

Nothing serious, just out of curiosity... lets say you are in the Summer desert somewhere and want to hang out for an hour or two in the van, so you need to run A/C... which would be cheaper?

1) Running the 2800i Onan Propane Generator to power the roof A/C
2) Idling the 3.6l Ram engine while blasting the dashboard A/C

Propane being more expensive and less energy-dense than gas?

(I do not know how much propane is used /hr to run the genny under load... nor do I know how much an hour of idling takes out in terms of gas. Hey... that's why the post.)

One point I have not seen brought up yet is the massive amount of heat from the engine and exhaust system that soaks up into the camper, and this lasts long after you have turned the engine off. This is very unpleasant on our Chevy based Roadtrek, but don't know how it is on the Promaster. And if the engine is cold when you start this "cycle", you essentially "waste" all the energy needed to heat the engine/water/radiator to running temperature. I'm not saying that running the Onan is a walk in the park though ...

[Urlauber]

Originally Posted by jakegw2

Well let's see:

Promaster Chassis
From this post I am going to use the value of 0.54 Gal/Hr @ idle for the Promaster. This gets you the dash air conditioning with probably in excess of 18k BTU at idle (at cruising speed this rises up to something like 60k BTU). Since these are estimates I will go ahead and estimate that using the AC increases idle fuel consumption by 10% bringing it to 0.59 Gal / Hr for 18k BTU of cooling.

Roof AC
The roof AC draws 13.6A at full load.

The Onan produces about 23A at full power, or 11.5A at half-load. At half-load it consumes 0.31 Gal / Hr for the propane model. By interpolation we get about 0.35 Gal / Hr to support a continuous 13.6A load for 13.5k BTU of cooling.

Comparison
I'm on shaky ground here as I'm not really sure how to do comparison adjustments for different BTU ratings, but I'll take a stab at using a linear comparison. In a very hot environment the roof AC will be under-powered and cannot be increased to 18K. Most of the gas consumption from the dash AC is from the engine idle, so it can't be decreased. That makes this comparison entirely theoretical, but here it goes:

scaling 13.5k to 18k is a 30% increase in cooling, so using a linear approach we would get to a power consumption increase to 18.1A. Using that same increase and interpolating between half-load and full load consumption we get to 0.45 Gal / Hr of propane for equivalent cooling.

I'll skip the energy density comparison and go right to the price comparison:

I am using data for Rapid City, SD in the link, choose your own adventure here if you wish.
Propane: $4.10 / Gal
Gas: $4.70 / Gal

Conclusion:
Propane $1.44 / hr of operation ($1.85 / hr of operation for equivalent cooling performance)
Gas $2.77 / hr of operation

Excellent, thank you so much for spec'ing this out! This is what I was looking for.

Not sure about the prices (gas reaching $7 now over here). Looks like the scales tip towards using the roof A/C, which makes sense, since that is what it is up there for anyway.

The downside is the capacity. The Thor Rize holds 5.7 gal of Propane, so enough for ~17 hours using your numbers above.

I will know more after our July trip through AZ/UT.

[MsNomer]

If you have decent insulation, cover the outside of glass areas, and run the fan, you can at least keep the van interior at ambient.

[Urlauber]

Originally Posted by dicktill

One point I have not seen brought up yet is the massive amount of heat from the engine and exhaust system that soaks up into the camper, and this lasts long after you have turned the engine off. This is very unpleasant on our Chevy based Roadtrek, but don't know how it is on the Promaster. And if the engine is cold when you start this "cycle", you essentially "waste" all the energy needed to heat the engine/water/radiator to running temperature. I'm not saying that running the Onan is a walk in the park though ...

Good point, another strike against idling & Dashboard A/C. Of course, the generator creates noise and exhaust too.
My scenario is more like arriving somewhere and going for a walk, or shopping, while the angry teen stays in the car and doesn't want to be cooked. Worth firing up the generator? Sounds like it.

[Urlauber]

Originally Posted by MsNomer

If you have decent insulation, cover the outside of glass areas, and run the fan, you can at least keep the van interior at ambient.

Sure. I cover my front glass from the outside and it makes a difference over just the inside cover. But, the insulation is what it is (not great) so some A/C will be needed when it's past 90 outside.

I run the ceiling fan most of the time while the camper is stationary. Need to figure out how to get some vents that pull in air from below the car, where it is coolest.

[SteveJ]

When we park in warmer temperatures we pop open the hood. This releases a lot of engine/transmission/exhaust heat that would otherwise be trapped in the dog house area and will heat up the interior. Doing this makes a pretty significant difference, though not officially measured.

[RAA357]

i have a fuel flow meter on both my Onan 2.8 and the Rt 210V, 6.0 liter engine.
at idle with the Coach AC running on Max cool i am burning. .67 gallons per hour, with the Onan 2.8 running with the Cool Cat AC running i am burning .64 gallons per hour.
When we arrive at our Camping spot i open the hood and let the hot air out and leave the
AC on to cool down the inside, after about 15 minutes i switch over to the Cool Cat.

[Urlauber]

Thank you for recommending popping the hood. The tiny hood on the snub-nosed Promaster is something I overlooked... good idea to pop that after a hot drive through a hot place.

[jakegw2]

Just a point of clarity, the specs for the generator linked to above is the QG 2800i. The specs for the Microlite 2800 are slightly different (and a bit lower in fuel consumption).

[ra2jim]

Minimizing solar heat gain is always a good idea before solving the cooling issue. I once camped outside of Las Vegas in 110 F temperature. The rv park site I got was long, where a small part had a 6' h privacy screen. I noticed quite a bit of temperature difference parked next to it versus direct exposure to sun. In your case, it may help to get shade cloth mounted at roof edge then secured with stakes to the ground. I remember reading a study once that claimed reduction of heat gain thru walls/windows by 15 to 20% when shaded.

[Rlum]

Originally Posted by ra2jim

Minimizing solar heat gain is always a good idea before solving the cooling issue. I once camped outside of Las Vegas in 110 F temperature. The rv park site I got was long, where a small part had a 6' h privacy screen. I noticed quite a bit of temperature difference parked next to it versus direct exposure to sun. In your case, it may help to get shade cloth mounted at roof edge then secured with stakes to the ground. I remember reading a study once that claimed reduction of heat gain thru walls/windows by 15 to 20% when shaded.

Shade and insulation makes a whole of difference for vans. Embassy RV is not a big fan of solar because who want to park in sun to get electricity? Terry wants his clients to spend money for better batteries, AC units, 7 layers of airplane insulation and dedicated alternators. Park in shade and enjoy comfort of AC.

[jakegw2]

After experiencing solar I have to agree. It is mostly useful for keeping the batteries trickle-charged when not in use and compensating for the refrigerator power usage during use. Neither of these uses requires a large solar array or getting direct sunlight for more than an hour or two a day (combined with the contribution of indirect sunlight). Anything else requires an impractically large array to achieve.

Running the generator or stock alternator for 30 minutes provides as much or more power than a relatively large array in direct sun all day long. If you take even a short trip from your campsite every day while dry-camping you end up with far more charge. We almost never find ourselves stationary for multiple days without hookups. Even with hookups we almost never manage to stay put for more than 2-3 days at a time (too many fun opportunities to take advantage of). A bigger battery bank gives far more bang for the buck.

[mloganusda]

Did your MH come standard with propane for the generator? My Trend, based on the Pro Master, has the 2.8K generator using gas off of the gas tank, not propane. There is less power from the gen (2.5K) using propane instead of gas (2.8K).

[avanti]

After careful consideration, I have decided that my new rig is going to have a single 200W solar panel. Its purpose is not to extend my boon docking (it will be a drop in the bucket), but to power certain systems during storage. Such a setup now costs very little and in the center of the roof is visually unobtrusive. Also, don't forget that a panel is "portable shade" -- keeps a lot of solar heat away from the roof.