WeeRoll AC plans?

Converting Cargo Trailers into TTTs

Re: WeeRoll AC plans?

Postby McDave » Wed Aug 29, 2018 10:22 am

That is not short cycling. If the room thermostat is satisfied that is just cycling. Location and type of thermostat is as critical as compressor size in this case. Short cycling occurs when the compressor shuts off before the room thermostat is satisfied.

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Re: WeeRoll AC plans?

Postby flboy » Wed Aug 29, 2018 11:11 am

McDave wrote:That is not short cycling. If the room thermostat is satisfied that is just cycling. Location and type of thermostat is as critical as compressor size in this case. Short cycling occurs when the compressor shuts off before the room thermostat is satisfied.

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Re: WeeRoll AC plans?

Postby aggie79 » Wed Aug 29, 2018 11:36 am

Our home A/C units have variable speed compressors and blowers. Our "thermostats" are also "humidistats". We can set temperature and humidity. In the summer, with our high humidity, the thermostats/humidistats "bias" toward reducing humidity and run the compressors and blowers at lower speeds for longer cycle times. In this mode, the A/C units are acting as "smaller" units than their rated Btuh capacity.

The units run at their lower speeds until they can't keep up with the desired temperature. Then, higher speeds "kick in", When the temperature is 110 and the humidity is 70%+, the compressors and blowers are running at the highest speeds at their rated Btuh capacity.

To my knowledge, there are no variable speed window units (or RV rooftop units). So, it is a balancing act to size the unit for high humidity areas. A too small of a window unit running longer cycles will reduce humidity but may not recover temperature as quickly as desired. Too large of a unit running shorter cycles will recover temperature quickly but may not reduce humidity enough. The trick is to size a unit for the "sweet spot" and that seems to be ~8,000 Btuh for most of the CT conversions I've seen on this forum.
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Re: WeeRoll AC plans?

Postby flboy » Wed Aug 29, 2018 11:51 am

aggie79 wrote:Our home A/C units have variable speed compressors and blowers. Our "thermostats" are also "humidistats". We can set temperature and humidity. In the summer, with our high humidity, the thermostats/humidistats "bias" toward reducing humidity and run the compressors and blowers at lower speeds for longer cycle times. In this mode, the A/C units are acting as "smaller" units than their rated Btuh capacity.

The units run at their lower speeds until they can't keep up with the desired temperature. Then, higher speeds "kick in", When the temperature is 110 and the humidity is 70%+, the compressors and blowers are running at the highest speeds at their rated Btuh capacity.

To my knowledge, there are no variable speed window units (or RV rooftop units). So, it is a balancing act to size the unit for high humidity areas. A too small of a window unit running longer cycles will reduce humidity but may not recover temperature as quickly as desired. Too large of a unit running shorter cycles will recover temperature quickly but may not reduce humidity enough. The trick is to size a unit for the "sweet spot" and that seems to be ~8,000 Btuh for most of the CT conversions I've seen on this forum.


Agree! Very nice explanation. I found that out with my first run and the 6000 BTU on the current CTC. Seemed to work fine except the hottest days where it would struggle to keep lower temps at peak highs...

8,000 BTU seems to be that sweet spot on mine given size and insulation level. It is all a trade off as you know.

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Re: WeeRoll AC plans?

Postby McDave » Wed Aug 29, 2018 6:18 pm

High speed, Med speed and low speed fans will vary the cfm moving across the evaporator and the rate that the air is exchanged, Yes? So.. variable speed. 8000btu max = hi speed 13,500 at med speed or low speed =?
If you have a window unit that has Auto Fan feature such as the Crosley I use in the bedroom you will get this:
"Note: Fan starts out at high speed and adjusts to slower speeds as the room temperature dictates. For example, if the room doesn't get too warm, it will stay at slowest speed. If the room temp rises quickly, such as a door being opened, it will automatically go to the highest speed. The fan speeds will re-adjust back to the slowest speed as the room returns to the original set temp."
Also in the Energy Saver Mode:
"Note: In this mode the fan will continue to run for 1 min. after the compressor shuts off. The fan then cycles on for 20 sec. at 10 min. intervals until the room temp. is above the set temp., at which time the compressor turns back on."
Can a 13.5k unit do less work? yes. Can a 8k do less work? yes. Can your rooftop have longer run times to chase away damp cave syndrome? Yes. Medium or low speeds.

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Re: WeeRoll AC plans?

Postby flboy » Thu Aug 30, 2018 8:30 am

McDave wrote:High speed, Med speed and low speed fans will vary the cfm moving across the evaporator and the rate that the air is exchanged, Yes? So.. variable speed. 8000btu max = hi speed 13,500 at med speed or low speed =?
If you have a window unit that has Auto Fan feature such as the Crosley I use in the bedroom you will get this:
"Note: Fan starts out at high speed and adjusts to slower speeds as the room temperature dictates. For example, if the room doesn't get too warm, it will stay at slowest speed. If the room temp rises quickly, such as a door being opened, it will automatically go to the highest speed. The fan speeds will re-adjust back to the slowest speed as the room returns to the original set temp."
Also in the Energy Saver Mode:
"Note: In this mode the fan will continue to run for 1 min. after the compressor shuts off. The fan then cycles on for 20 sec. at 10 min. intervals until the room temp. is above the set temp., at which time the compressor turns back on."
Can a 13.5k unit do less work? yes. Can a 8k do less work? yes. Can your rooftop have longer run times to chase away damp cave syndrome? Yes. Medium or low speeds.

McDave


Very good points. I think the lower fan speeds helps to a point with humidity for sure but there are many variables in play as you well know. In high humidity, however, and all else being equal, I have seen the Evap Coils Freeze up due to lower fan speeds... I have not studied it much other than just thinking about it, but in addition to the longer dwell time at cool the air at the evaporator, I think the head pressure changes due to the heat load reduction also cause by the decreased airflow and then evaporator drops further below dew point (good), but risk of freeze up as some ice begins to build and then airflow decreases too much and then compressor will shutdown due to protective circuitry. Probably less risk at the medium speeds.. As I understand it, it is basically the same issue you'd have if the Air filter is clogged up and etc. given the compressor speed is not changing also. It is all a trade off. I think the more complex home systems (or any more complex design) get away with this much better because they actually vary the compressor speed as well or automatically vary fan speeds as you indicate. At the end of the day... whatever works best for your needs and situation. No one size fits all, I suppose.
Last edited by flboy on Thu Aug 30, 2018 10:41 am, edited 1 time in total.
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Re: WeeRoll AC plans?

Postby McDave » Thu Aug 30, 2018 10:38 am

Also keep in mind that most rooftop and window units do not cut the fan off when the compressor cycles. This keeps the air moving and mixing to a homogenous blend and prevents stratification or the condition where the heaviest, most saturated air would fall to the lowest point like the carpet, couch, bed etc.. All the air should approx. be the same temp/relative humidity and as heat is removed via moisture, the remaining heat/moisture will rush to disperse into the dryer air in order to achieve balance. Nature abhors a vacuum.
Icing should not be an issue at lower speeds as the condenser fan is also running slower and removing less heat at a slower rate.
A clogged filter only restricts the volume of air moved across the evap. while the condenser still get full air flow and this can cause icing, unless it is full of cotton/bugs/dirt etc. as well in which case there just wont be much cooling at all, and then you will see short cycling, guaranteed.

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Re: WeeRoll AC plans?

Postby domoarigato » Fri Aug 31, 2018 8:29 am

I installed a 12k btu carrier low profile unit on my old pop up (the big, bulbous units look hideous) and it's ability to move air I thought was pretty weak. Not to mention the fact that it didnt get quite as cold as my current "through the wall" unit. Throw in the non-permanent nature of the roof seal, and the condensation line maintenance (crud clearing here and there), the height that it adds to an already tall trailer.. meh, this was the reasoning for my through the wall unit that I have now. I have no regrets, but won't dig my heels in debating one vs the other..

Just my 2c

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Re: WeeRoll AC plans?

Postby hankaye » Fri Aug 31, 2018 8:59 am

McD, Howdy;

For my roof top A/C that sits on top of my Home (RV), I can choose either to keep the
fan running or allow it to cycle with the A/C. I choose to let it run as it it makes for a
' softer' start when the A/C cuts in and also dries out any possible moisture that may
be in the ducting. Don't need any mould taking root.

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Re: WeeRoll AC plans?

Postby McDave » Fri Aug 31, 2018 9:38 am

@ Hank,
I take it your RV has a "Central Air" set up, where the A/C and furnace share duct work that is similar to residential units? Your plan to let the fan circulate the air constantly is a sound one, if the background or "white noise" of the fan is not bothersome to you. For most of the year you are in a low humidity area, but during the monsoon season it can get downright dank around there. Swamp coolers are no help then, Thank God for refrigerated air.

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Re: WeeRoll AC plans?

Postby hankaye » Fri Aug 31, 2018 7:31 pm

McD, Howdy;

2 separate systems, A/C ducting in the ceiling droppin' cool air down and furnace
ducting under the floor. acting like the Big Bad Wolf, huffin' -n- puffin' blowin' warm air up.
About the only thing I can't ignore is a 85 lb. woman that thinks she's the greatest thing since
Reba doin' Karaoke while I'm shootin' in the 9-ball final at the local VFW. Worse then fingernails
on a chalkboard. Just sent another shiver up my spine thinkin' about it. Think I'll have another
:beer:

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Re: WeeRoll AC plans?

Postby McDave » Fri Aug 31, 2018 11:13 pm

@ Hank,
"They said 'Pardon us son, but she ain't no kid, that's a cocktail waitress and a Dolly Parton wig!' I said 'I know it Dad, ain't she cool? That's kind I dig!'"
I'd join at the VFW for a game anytime, Hank.
P.S., That might be my Mom singing.... just sayin'

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