A/C Size?

[jwh92020]

I just discovered that something in my 12k btu portable unit sounds like metal shredding, so i to replace the unit. I'm going with a window unit this time (I used the portable because I had it and it worked). The trailer is 7 x 14 with a 6'6" ceiling. The is 1.5" of foam board in the ceiling and 1" in the walls. The a/c will be built into the cabinet that the portable was in. The top of it will be 34" above the floor. I know higher is better, but I don't want to rebuild all the cabinets if I don't have to. Is 6000 btu big enough or should I go to 8000? I'm not an a/c guy, but my understanding is that if the unit is too big, it will not cool correctly. I used a 6k unit in an old canned ham that I remodeled and it worked great, so that is the direction I'm leaning. But if 8k would be better, then 8k it is. Any thoughts will be appreciated.

[flboy]

I just discovered that something in my 12k btu portable unit sounds like metal shredding, so i to replace the unit. I'm going with a window unit this time (I used the portable because I had it and it worked). The trailer is 7 x 14 with a 6'6" ceiling. The is 1.5" of foam board in the ceiling and 1" in the walls. The a/c will be built into the cabinet that the portable was in. The top of it will be 34" above the floor. I know higher is better, but I don't want to rebuild all the cabinets if I don't have to. Is 6000 btu big enough or should I go to 8000? I'm not an a/c guy, but my understanding is that if the unit is too big, it will not cool correctly. I used a 6k unit in an old canned ham that I remodeled and it worked great, so that is the direction I'm leaning. But if 8k would be better, then 8k it is. Any thoughts will be appreciated.

I have a 7*18 Vnose which is well insulated and a 6,000 BTU does it, but when I replace it someday, I'll probably go with the 8,000 BTU. 8000 BTU is still small enough to run off the Honda 2000 generator, but would have some margin. It does take all the 6000 BTU has to keep up in the middle of a real hot day. The good part of that however, is it does not short cycle and removes all the humidity... which his good. I imagine the 8000 BTU will not short cycle either, but will pull it down quicker.

Hope that helps..

[jwh92020]

Thanks Don. In talking with some other folks, it's about 50/50 on 6k vs 8k. From the "V" back towards the rear, the first 48" inches is actually a bathroom with a full wall across the trailer, so I'm actually only cooling 98 sqft. I crack the door of the bathroom and run a vent fan to draw some of the cool air in there. Don't spend a lot of time in there, so it being 70 degrees all the time wasn't a priority.

[McDave]

Hey jwh92020,
Did you ever hear the saying "old wives tale" ? I think you know where I am going...Anyways, I don't think anybody with a Cargo Conversion will EVER have to worry about having too much BTU and any type of short cycling issues. It's just not gonna happen. You may have short cycling and/or humidity issues, but it wont be from having too much available BTU.
Here's a hint, water, ie moisture is the vehicle by which heat is contained and travels, and is removed.
As far as the height of AC mounting, Don is correct, higher is better but it's not hyper critical. PTAC's, like those used in motel rooms are generally mounted just above the floor.
I know you are not gonna want to hear this so I'll preface by saying IMHO, There is a reason that true RV AC units are sized and located where they are. Wanna' take a guess? Somebody did the math, and then the competition did the math, and surprise...RV AC's are still located and sized in the same manner, Who'da thunk it? .....just sayin'...

McDave

[jwh92020]

McDave,

I totally agree with everything you said, and in a perfect world, there would be 15k of btus on the roof of the trailer. However, my world is far from perfect, so I made do with what I had available to me. The portable was here, paid for and sitting in the garage gathering dust, so in it went. I built the living area around it (sort of) with the knowledge that if (or when) it died, I could replace it with a window unit in the same cabinet. My options - another dual hose portable (not gonna happen), tear out 1/2 of my ceiling, reinforce said ceiling, run 110v to the location, reinstall ceiling and mount a $600 - $700 roof top up there (again, my world isn't perfect, so that isn't gonna happen either), which leaves a window unit. A simple frame to mount it in, inside the cabinet, an exhaust plenum straight through the floor (I really don't want to cut the exterior of the trailer if I can avoid it), 2 boxes to cover the intakes with 5" hose fittings on them, use the current intake/exhaust hoses as dual intake hoses (with 4" 110v computer fans in the lines to aid in air draw) a/c and fans plugged into an automated vacuum switch so that when the a/c fires up, the fans come on as well. Shut off a/c, fans shut off. I wanted to use the current exhaust from the portable to exhaust the window unit, but I don't think that channeling all that hot air into a 5" exhaust hose will let a window unit perform well. All of that is why I'm using a window unit. Now, if you think there can never be enough BTUs, I can get an 8000 for a pretty good price, so I should go with that? Here is the automated switch I was talking about. I have 3 of these, one has never been used. BTW - the next build (yes, there is going to be a next build - even my wife is on board for it) will have 1 15k on the roof and 2 fantastic fans - 1 in the far rear and on in the front where the bathroom will be.
https://www.amazon.com/BCTINT-10031-010 ... B0035YGLZG

[lrrowe]

I more then adequately cool my 6x12x6 ft trailer with a 5000 BTU window unit. Will my next one be bigger? I am not sure yet.

[working on it]

• Just "supposing", based on my personal experience with window units: perhaps using a combination of two small units, placed to cool "zones" in the trailer, might work for you. Install a 8k BTU unit in the area of greatest need (cooking area, near a door being constantly opened, or near a large window -not insulated- that allows a lot of sunlight in). The smaller 5-6k BTU unit could be at the other end, and each would cycle as dictated by the settings you assign each one, in overlapping coverage. You could run either, separately, to match the conditions you face, and not force one unit to meet all needs.
• When my house's central air/heat failed, years ago, I talked my wife into not replacing it, and we started using A/C window units, arranged in "zoned" areas, and electric space heaters, arranged the same way. We experimented for awhile, and found the right settings for our A/C units (8k, 10k, and two 15.1k BTU units) that cooled the house better than before, and for 1/3 the bill. The space heaters get moved around with changing furniture arrangements a bit, but the principle is the same as with the zoned A/C units; put the conditioned air where it does the most good, for less.
• I admit, I am biased toward A/C window units, as I have always liked them for many (nostalgic) reasons. But, I did gain real efficiency in my home's HVAC situation, when I "zoned out", using separate units, all around the house. This scheme would work in a large trailer, too, if scaled to fit.

[McDave]

@ workingonit,

BINGO!!!

McDave

[jwhite]

I had portable waste of money I bought a 5000 btu and it would take hours to cool in the summer I went with a 8000 and it's perfect, I like that with in 30 minutes you have to turn it down living it the south you almost can't have enough cool.

[jwh92020]

@ working on it - I agree with the idea of "zoning out" the HVAC in house. I never understood the idea of heating/cooling rooms I don't use. My trailer is a 7 x 14 bull nose, with the living area being 7 x 14 and the bathroom in the front. I'm going with an 8k unit, which should be more than enough to cool the trailer. With it, I can leave the bath door open while I'm gone to allow cool air to flow in there naturally.

jwhite - I've had no issues with the portables I've used, other than the fan on this one giving up. As I said, when I started this build, I had a perfectly good 12k portable that was just taking up space in the garage. I used it's twin brother in another build and the woman who bought that trailer lives somewhere southwest of Dallas and she lives in it full time. Says it stays plenty cool still. Now I just need to figure out how I'm going to vent the exhaust. I wanted to build a "plenum" to capture the exhaust and send out of the trailer via the existing 5" duct for the portable, but I don't think a 5" vent is sufficient.

[flboy]

Hey jwh92020,
Did you ever hear the saying "old wives tale" ? I think you know where I am going...Anyways, I don't think anybody with a Cargo Conversion will EVER have to worry about having too much BTU and any type of short cycling issues. It's just not gonna happen. You may have short cycling and/or humidity issues, but it wont be from having too much available BTU.
Here's a hint, water, ie moisture is the vehicle by which heat is contained and travels, and is removed.
As far as the height of AC mounting, Don is correct, higher is better but it's not hyper critical. PTAC's, like those used in motel rooms are generally mounted just above the floor.
I know you are not gonna want to hear this so I'll preface by saying IMHO, There is a reason that true RV AC units are sized and located where they are. Wanna' take a guess? Somebody did the math, and then the competition did the math, and surprise...RV AC's are still located and sized in the same manner, Who'da thunk it? .....just sayin'...

McDave

McDave, short cycling (defined here as an air conditioner that cools an area too quick and then turns off) is a real issue in a high humidity environment like Florida or most of the Southeast USA. An air conditioner too large for the area being cooled will leave the space like a cool damp cave as opposed to a dry cool (conditioned air). It simply doesn't run long enough to remove enough humidity which takes time . I am sure there is some science/math (temp, airflow vs % humidity, and etc.) behind it, but it does happen in Florida in practice and is what the local AC contractor guys I know shared with me. In a low humidity environment, it probably doesn't matter so much.

For sure, there are other reasons for an AC to turn on and off too soon like low freon and etc. (also called short cycling) that have nothing to do with being over sized for the job .. but that is another problem. If the CTC is not insulated well or very large, a 13,500 BTU may run long enough to cool and remove the humidity. These days, alot of the small RV's less than 18' in Florida, I am seeing smaller adapted window units.

[m.colley]

I just discovered that something in my 12k btu portable unit sounds like metal shredding, so i to replace the unit. I'm going with a window unit this time (I used the portable because I had it and it worked). The trailer is 7 x 14 with a 6'6" ceiling. The is 1.5" of foam board in the ceiling and 1" in the walls. The a/c will be built into the cabinet that the portable was in. The top of it will be 34" above the floor. I know higher is better, but I don't want to rebuild all the cabinets if I don't have to. Is 6000 btu big enough or should I go to 8000? I'm not an a/c guy, but my understanding is that if the unit is too big, it will not cool correctly. I used a 6k unit in an old canned ham that I remodeled and it worked great, so that is the direction I'm leaning. But if 8k would be better, then 8k it is. Any thoughts will be appreciated.

jwh,
My CTC 7x16x6'9" isn't much bigger than yours and has the same basic insulation as yours too. I'm cooling mine with a 8000BTU window unit and it works great. As many on the forum know I bounced about what AC unit to use and I sell HVAC for a living. I chose the 8000 for comfort and power usage reasons and as Don mentioned I felt that the roof top unit would be overkill for a trailer our size. Being from Georgia humidity is a big issue and I just didn't think the rooftop would keep it comfortable, yes it would keep the temps down but would it keep the air dry?

BTW,Most 6000 & 8000BTU window units share the same size "cabinet". Did I mention the fact that my window unit pulls a whooping 672 watts and will run off of most any small generator? Yup thats the OTHER reason I bought it..


Martin

[working on it]

• Part of the reason my home A/C works so well for me is that we use air circulators at (various heights) to move the cooled air into areas that would otherwise have hot spots, or stagnant pools of air. Ceiling fans, pedestal fans, and floor fans all contribute to the overall scheme, since each type can reach areas the others may not be able to influence, due to hidden corners, furniture in the way, or just out-of-the-way spaces where the A/C fans can't directly affect circulation. Using an independent fan in an air conditioned trailer may make even a smaller unit more efficient, and short-cycle less (if at all), due to being able to handle more air volume that needs cooling, than if working alone.
• I use a similar scheme in my small 4x8 TTT, to circulate the warm and cold strata of air, to effectively induce the cooler air to first cool the sun-heated ceiling, and return by deflection off of the 45 degree sloped front panel, and cool me at the rear wall. Helped by a 10" air circulator, aimed towards the rear, the cooled air sinks downward, displacing warmer air, which rises up, and eventually is either sucked into the A/C intake, or out of the open vents near the ceiling (warm air rises, cool air falls, etc...). My TTT is only 88 cubic feet inside, and though the 5k BTU A/C unit is nominally too large for the area being cooled, using air circulation and open ceiling vents makes my system cool really well, without short-cycling. I've never turned it to high, and keep the thermostat at halfway or less, and it has worked great for me, and never failed to dehumidify while cooling. Perhaps the complete air circulation is a key; I don't know for sure, but it's worth mentioning.

• 

  warm air rising up and out.jpg (154.32 KiB) Viewed 4213 times

[jwh92020]

Martin,

Thanks for the input. I'm pretty much settled on an 8k unit. Since you are in the HVAC biz, you may have a knowledgeable answer to this. My first thought was to have a "plenum" made for the heat exhaust that would connect to my existing 5"exhaust line for the portable. The plenum would be about 6" deep and there would b a 5" collar on the side for the existing flex hose to connect to. Would doing this restrict the heat flow too much and cause the unit to run hot?
Thanks,
John

[McDave]

One last word on removing humidity via refrigeration, I have been in hvac close to 40 yrs, Texas, Oklahoma, New Mexico, Utah, Arizona, Wyoming, Montana, Idaho, Washington, Oregon, North and South Dakotas...probably seen more differing and varied environments than I'd like to remember. But for the ultimate in high humidity issues I will use my own case as a study. I have a 25,000 gal. 18x36 10ft deep, heated indoor swimming pool, in Montana, In the winter....In Montana,. I'm vaguely familiar with what it takes to remove humidity from a room. In this case a 250,000btu Dehumidifier,...ya, that's fun. I'm only speaking from experience though, not the words of some guys I know in the AC biz....just sayin'..