Is there such a thing as too much A/C?

[rowiebowie]

My 13,500 btu a/c in my 20' class B Avenue Suite needs replacement.

With 13,500 btu's from the factory, it almost seems like overkill since Roadtrek 190's ship with 9,000 btu units, and most short Sprinters shipping with 11,000 btu units. So, I ordered a similarly sized 13,500 btu Coleman Mach 8 "Plus" as a replacement because I require a low-profile unit and wanted to stay with the same brand for ease of wiring blocks and installation.

When the a/c arrived by freight delivery today, inside was the 15,000 btu model. This is 11% larger. Not that much, but I figure more than necessary. I know an over-sized house a/c can cause comfort problems by not running long enough to de-humidify. But I've never heard anyone with an rv (or car for that matter) complain about too much a/c cooling.

My question is this: Should I send it back and make them send me a 13.5K unit like I ordered?

Pluses of the 15K btu:

- I received a unit that normally costs about $100 more at no extra cost.
- The size is the same as the 13.5K unit, so no install problems.
- The new inside grill I ordered fits both units.

Minuses of the 15K btu:

- It uses .3 more amps for the compressor compared to my old 13.5K unit. (Not much difference and due to more efficient fans, the new unit will actually use 1 amp less when running at high speed)
- Possible less humidity control.

Ok, your opinions are appreciated. Let the voting begin!

[OTRA15]

The concerns you raise are legitimate IMO. The larger unit will cycle more [on/off/on/off] but you can minimize that if you have the option of using a Low fan speed [only] once the room is at temp. Using an Economy mode, or Auto fan, can result in more annoying on/off issues IMO.

How well did your old 13.5k unit cool the place when it was really hot out? If it was marginal, maybe the 15k unit is a good upgrade IMO. Does it have more fan speed options maybe, like Low/Medium/High?

Do you have any decibel ratings for the two units? Sometimes the larger units are a tad quieter. Worth checking out.

You are in Texas, so if there is any fence riding to do on this, I would go for the larger unit.

Good luck,

Peter

[Alex AVI]

ROWIEBOWIE - I think you should now seriously consider getting the EXT model to utilize the extra oomph.

Seriously, though, OTRA15 makes good points. My first thought was if you just use LOW setting, would that not effectively run the bigger A/C at longer cycle? Most of us know a larger A/C does not translate to better as you have already noted all the valid reasons. But I doubt any rv manufacturer had taken into account all variables when they sized A/C units. They probably only took sq.ft. as one variable and maybe an "extra surcharge" for the fact that it is a van and not a properly insulated stick house, and maybe used a "mean temperature" as their starting point not considering some less fortunate folks (me and those in the sweltering SW) go for months above 105°F. My unit in this weather is on 100% duty cycle until the sun goes down. It does an adequate job when under full LV sun provided I have all windows shades down. A larger unit would give me benefit of having windows shades up and still freeze my a** off in the AI. I also agree with OTRA15 the annoying AUTO. If this were mine (personal opinion only with no techie info to back up my choice) and the retrofit is not too big of a hassle, I would choose the bigger one and run it LOW and be able to have the entire campground in Death Valley all to myself in the middle of summer

EDIT, again just personal opinion on this one - if expunging more humidity is an issue with this unit, we got 2 vents in the AI, more than adequate to purge high volumes of indoor air. And the AI vent only closes when A/C is in AUTO, so should be able to vent out in LOW ?

[rowiebowie]

Well, you both raise a good question about using the low speed fan setting.

My old unit had a single motor for both fans and two speeds. There was really no low speed, hardly any difference between them at all. Specs confirm the fan amp draw was 3.65 on low and 3.8 on high.

The new Mach 8 Plus has dual fan motors and specs show a big improvement in efficiency and a big difference in amp draw between the two settings. I don't have the specs in front of me, but the blower amps were something like half on low speed. Crossing my fingers it is quieter than old unit, but it wasn't too bad itself.

I wonder if I run the new a/c on low fan speed, will there be enough air flow to keep the unit from freezing up? This concern I had even when ordering what I thought was an exact replacement 13.5K btu model and would be the same for the 15K unit.

I must say old 13.5K unit didn't freeze us out and was working properly before the cage fan started to fall and rub. And this is based on the only time we camped in it was in late May in temps no higher than 90 in the day, not the heat we are experiencing now. Nights got much cooler and the old unit cycled regularly yet seemed to still run about 50% during the night. Of course, we were away from the coast so humidity was low except for what we generated inside the van.

[Alex AVI]

Originally Posted by rowiebowie

I wonder if I run the new a/c on low fan speed, will there be enough air flow to keep the unit from freezing up? This concern I had even when ordering what I thought was an exact replacement 13.5K btu model and would be the same for the 15K unit.

ROWIEBOWIE - I almost always run mine on LOW pretty much all day & night and had not frozen on me, of course smaller A/C, bigger volume.

Here's a thought - the 2019 AI Shorty 19' uses the 13k btu unit

[rowiebowie]

[QUOTE=Alex AVI;2140771]ROWIEBOWIE - I think you should now seriously consider getting the EXT model to utilize the extra oomph.

If this were mine (personal opinion only with no techie info to back up my choice) and the retrofit is not too big of a hassle, I would choose the bigger one and run it LOW and be able to have the entire campground in Death Valley all to myself in the middle of summer

I'm thinking I can pump out some of the excess cold air and sell it to my fellow campers sweltering in their EXT models.

Originally Posted by Alex AVI

ROWIEBOWIE - I almost always run mine on LOW pretty much all day & night and had not frozen on me, of course smaller A/C, bigger volume.


Good to know. I didn't know there were forum members out there who actually had 2 speed fans on their a/c's. Mostly I've read about the dreaded a/c noise and figured everybody's low speed was like my old unit (not really a low speed at all).


Here's a thought - the 2019 AI Shorty 19' uses the 13k btu unit

And I bet they'll never hear complaints of not enough a/c cooling. Fridge ventilation, inverters, etc. are another matter.[emoji14]

[LB_3]

The big A/C will be nice when you’re trying to cool down a van that has been baking in the sun all day. My only concern would be whether the generator can start it. If it can great, if not, you can always install an easy start. For comparison, the roof top model you got is 1.25 tons and the van’s AC up front is likely between 4 and 7 tons.

Humidity control is the general concern with oversized A/Cs. I’m not sure that will be too much of a problem with how we run our roof top ACs

[rowiebowie]

Originally Posted by OTRA15

Do you have any decibel ratings for the two units? Sometimes the larger units are a tad quieter. Worth checking out.


Don't have a meter for noise levels, but even though I really couldn't complain about the noise of my old Polar Mach, quieter is always better and the new unit definitely will have a true "low" speed instead if two high speeds (high & higher).


You are in Texas, so if there is any fence riding to do on this, I would go for the larger unit.


Yeah, to avoid the hassle of returning the 15K unit I might just take a chance on the low speed fan, small increase in btu's (11% bigger) , and not being "frozen out" buy the old unit as reason to keep the one they shipped.[emoji6]


Good luck,

Peter

Maybe someday I'll regret it and be the first forum member to ever curse his luck at being too cold in his Airstream.

[OTRA15]

Originally Posted by rowiebowie

. . .
I'm thinking I can pump out some of the excess cold air and sell it to my fellow campers sweltering in their EXT models.
. . .

Careful, as you may be remembered as "blowing a lot of cold air" or something like that!



You could store a bunch of those soft blue-ice medical PT packs inside, using them as heat/cool sinks, to reduce the rate at which the vehicle cools, then go around the CG and rent them out as "cool packs" . . . ?

[emoji41]

PS -- agree with going for the 15k unit FWIW.

[Alex AVI]

Originally Posted by rowiebowie

Maybe someday I'll regret it and be the first forum member to ever curse his luck at being too cold in his Airstream.

ROWIEBOWIE - I doubt it, unless you move to MN or Canada. FYI - pulled up Mach 8 data sheet and it has a freeze protect thermistor on compressor relay ckt w/ 26° cutout to prevent evap freezeup AND a freeze-switch on condenser that opens at 18° coil temp, not ambient. Would this alleviate your concerns of freezing unit to the point of destruction?

[OTRA15]

Thanks. Curious whether you see any decibel level comparisons between the two units?

[rowiebowie]

Originally Posted by LB_3

The big A/C will be nice when you’re trying to cool down a van that has been baking in the sun all day. My only concern would be whether the generator can start it. If it can great, if not, you can always install an easy start.

Compressor running amps for the new Mach 8 Plus 15K unit is 12.1 -vs- 11.8 for the old Polar Mach 13.5. I have the gas 2800QG gasoline generation so not much concern there. I'm prepared to install a start kit since there seem to be no negatives even on a new model a/c.

For comparison, the roof top model you got is 1.25 tons and the van’s AC up front is likely between 4 and 7 tons.

So . . . you're saying I need to go bigger?

Actually, that's pretty amazing. And I've never been known to complain that my cab a/c was over-kill.

Humidity control is the general concern with oversized A/Cs. I’m not sure that will be too much of a problem with how we run our roof top ACs

I've wondered about why over-sized a/c units are more a factor in homes than cars and rv's and my conclusion is that the cycle times in our home is much longer (at least relatively speaking). So we feel the humid and still air proportionally longer.

It never seems to be a problem in cars maybe because of heat gain which causes the a/c to run almost constantly. As I posted above, when we used the a/c overnight it was May and not nearly as hot as conditions are now. Yet our body heat (me, wife, & 70 lb. dog), residual rv heat, and road heat kept the a/c cycling regularly overnight. It ran quite a bit more than I expected based on outside air temperature.

Leaning toward keeping the unit I was sent.

[rowiebowie]

I'm actually stoked about the new inside a/c grill. I got the one with two extra round vents that can open and swivel just like the dash vents on a car.

They are just below the fan plenum. When I think of the hard 90 degree turn the air must do from the cage fan to the grill outlets, it seems these will provide additional quick chill capability by letting more air pass straight out.

An added bonus will be if they provide directionality (I thought it was already a word, but I guess I made it up) that allows more individual comfort that just the air coming out the two ends. My wife hates direct air blowing on her head, while the dog and I don't mind at all.

I plan to post my review of it after I get everything installed.

[Alex AVI]

Originally Posted by rowiebowie

I've wondered about why over-sized a/c units are more a factor in homes than cars and rv's and my conclusion is that the cycle times in our home is much longer (at least relatively speaking). So we feel the humid and still air proportionally longer.

It never seems to be a problem in cars maybe because of heat gain which causes the a/c to run almost constantly. As I posted above, when we used the a/c overnight it was May and not nearly as hot as conditions are now. Yet our body heat (me, wife, & 70 lb. dog), residual rv heat, and road heat kept the a/c cycling regularly overnight. It ran quite a bit more than I expected based on outside air temperature.

Leaning toward keeping the unit I was sent.

ROWIEBOWIE - Not going to take credit for this next statement, coz it came from my local HVAC guy when I asked about "supersizing" our units. His response - <quote> it takes over 15mins. runtime before unit starts to do effective dehumidification. Time how long the AC runs on a hot afternoon. 10mins. or less, and unit is oversized. 20mins is OK. 30mins. at a time or longer, and your humidity levels will be low. <end quote> He says this very basic formula/test works on any size room/home. With AI being a small studio apartment, maybe same practices apply. I can not imagine going from 13.5k to 15k is going to dramatically decrease it's run time that even approaches levels that compromises dehumidification, especially based on the performance of your previous unit last May.

[ITSNO60]

Things are different in an RV, it is not kept temperature controlled all the time which means it will need to bring the temperature down from a higher degree than if it were your home. You also have a pretty big heat gain in the RV with the low R value and the windows. Add Texas into the equation and I would say keep the 15K unit. I think you will love it.

[GMFL]

I swapped my 13.5 to a 15K in our 23D and would not go back if you paid me. The 15K works great and cools much better than the 13.5 did. It is dryer in the trailer and the condensation line pours water. Not short cycles noted and with the Micro Air easy start the amp draw difference is not noticeable.

[Bgibbs]

Have you told the supplier that you got the bigger unit? You say it is only $100 more, so it might be a good idea to let them know in case of a warranty issue.

[rowiebowie]

Originally Posted by GMFL

I swapped my 13.5 to a 15K in our 23D and would not go back if you paid me. The 15K works great and cools much better than the 13.5 did. It is dryer in the trailer and the condensation line pours water. Not short cycles noted and with the Micro Air easy start the amp draw difference is not noticeable.

I'm surprised my original a/c was sized so large and that it didn't feel oversized even when run under less that Summer heat conditions.

The other funny thing is that my interior cubic feet volume is most likely only 1/2 of yours (see pic).

But then, it is only 11% increase and no greater amps than the 13.5K I'm replacing. I was hoping to go down in amps though, due to the greater efficiency of the new units.

[rowiebowie]

Originally Posted by Bgibbs

Have you told the supplier that you got the bigger unit? You say it is only $100 more, so it might be a good idea to let them know in case of a warranty issue.

Yes, I called them this afternoon. Told them their website for the 13.5K listed the number for the 15K and how was I to know. Their ad was still wrong this morning before the a/c was delivered and I printed the page on Monday when I ordered. Their order confirmation email even showed it was a 13.5K I had ordered. Funny thing, they corrected the listing this afternoon some time, before I called them.

Ron in PPLmotorhomes.com Parts Dept. was very nice and is going to see if they can get the 13.5K and also verify the heater and inside grill I received are correct for either size Mach 8 Plus. When he calls back in the morning, I guess I'll have to make my final decision. If the other parts are good, I'll probably keep the larger unit so I can get it installed this weekend.

[OTRA15]

New thread FYI:

http://www.airforums.com/forums/f37/...cs-185460.html