Building a Micro-Split AC System

[luthj]

The workmanship skill and thought process as always is impressive. I just don't get how one drive on a silty or muddy road doesn't ruin this thing though.

So, the AC unit itself is going to be mostly behind my underfloor battery box. Some protection there. Big mudflaps help too. With the condenser facing rearwards, It should stay somewhat clean.

But your right, it could last one year and need reworked. Such is the way with prototypes. On a previous underbody install, what ended up being the problem wasn't mud or dust. It was submersion in water during high crossings. Most automotive fans are fine with getting wet, however they have small gaps. During a water crossing, especially if you are moving quicker than you should, the axle and wheels stir up rocks and sand (pea sized or maybe marble sized) in their wake. The water carries them into the fan housings. Hence why I spent some time sealing the fan casing and bearing caps.

The final protection step, is going to be a vinyl cover. I plan to make one with 3/8" snaps. So I can slide it up and over when needed, and snap into place. Say for the winter, or if I plan to go mud slogging.

Since I used the low loss disconnects, I can drop the unit in about 20 minutes. If the fan motor or condenser kicks the bucket, I can drop and replace with a spare. My plan is too pick up one of these units used for spare parts later this year. They typically start showing up on craigslist at the end of September.


I know lots of folks like to go all-in on reliability, and for safety stuff, I tend to do the same. For comfort items, I tend to lean towards run it till it breaks, then repair and upgrade as needed. Perfect can be the enemy of good. Or in this case, the cool.

 

[luthj]

Quick update while I am waiting for the unit to stabilize. I charged about 11oz of R410A. Though I vapor charged (intentionally) to get more of the R32 out of the mix. R410 is a 50/50 mix of R32 and R125. I wish I could have run the unit previously to see what the superheat was. Anyways, I am seeing about 125psi on the low side, and around 460psi on the high side. Adding any more refrigerant pushed the power consumption up over 600W. The superheat is something like 18-22 degrees, which seems a bit excessive. This capillary tube was designed for R32, so it may be a little small for 410A? I would need to compare the viscosity to know for sure.

Anyways, It seems to be working fine now. Evap inlet is 85 degrees and outlet is around 61 degrees, which seems like a reasonable detla. I lost a bit of refrigerant when I removed the highs side, as the shrader valve didn't seal. Crap valve...

 

[luthj]

Started the AC unit at 8:30 this morning. Its been a bit over 8 hours. Total energy consumption is around 3.8kw-hr. The van has been in full sun since 9:30. Its currently 90F and 35% humidity. Its not cold in the van, but its stayed below 85F, and the humidity is low. So its comfortable inside, which is all I really need.

 

[IdaSHO]

Looks good man. I have the same AC

I considered such an in stall in my camper, but simply dont trust the AC unit that much.
So I opted to keep the AC "stock" to make swapping it out quick and easy.

 

[skirunman]

Great project. I was thinking of doing something similar and I think the 8k BTU Midea U "semi-mini-split" would be a great starting point and pretty efficient with variable speed compressor and fans as others pointed out. This guy takes one apart.

 

[bgflyguy]

I know zero about ac service, but if I took something like the midea u and cut it inhalf and installed it, would an ac guy be able to add the right charge? What if I lengthened the lines?

 

[luthj]

Assuming you installed the lines properly, and added a service port, yes. You would need to add extra refrigerant for the lines. It's just a matter of line length and diameter. All these units have a sticker with the charge weight on them.

 

[straypoet]

Its been a bit over 8 hours. Total energy consumption is around 3.8kw-hr.

Do you have a rough idea of how often the unit was cycling? Was that constant run?

Your modification of this wall-unit is quite impressive.

 

[luthj]

Do you have a rough idea of how often the unit was cycling? Was that constant run?

Your modification of this wall-unit is quite impressive.

I wasn't around enough to know. The input power was pretty consistent, so you can estimate the duty cycle from that.

 

[luthj]

De-winterized the unit last week when someone burned off a huge field upwind of our campsite. Closed the windows and started the unit up off the inverter. Worked just fine.

At some point I will recover the refrigerant and do a couple small changes.

• shorten the capillary tube, as its too long for R410A (higher viscosity compared to R32)
• swap the speed wires for the lowest speed. The "low" fan setting is actually the medium speed wire on the fan motor. The plan is to make that the low speed wire. Its just a bit too loud in a small van.
• replace the valve cores on the service valves. The ones they came with aren't rated for R410A pressures (or are crap). So they leak a bit with the caps off.
• I may also remove the directional control louver on the indoor unit. It isn't really needed in such a small space, and it makes some extra noise.

 

[luthj]

I modified the unit recently. Swapped to about 110" of 0.052 capillary tube, and changed the fan wiring. After a recharge the unit leaked out after a few days. I spent a long time trying to find a leak, and best I could do was a failed valve core, and a cap that had lost its O ring. Since the unit ran a while without refrigerant I decided to charge it with some commodity propane as a test. Its currently running well, though the cap tube is too long for propane. Output is around 3500BTU or so? Power consumption is down to 400W.





After hunting around a bunch, it appears Danfoss makes some TXVs which can be sized for 5,000BTU and R410A. The TU and similar series specifically. I haven't had a close look, but it appears that you would just select the orifice size based on the expected cooling load, and the desired temperature. If I ever modify the unit, I may swap to one of these.

https://assets.danfoss.com/documents/67716/AI029986431933en-US0402.pdf

 

[broncobowsher]

Doing an even smaller version. Taking one of these https://www.rigidhvac.com/micro-dc-aircon-12v
Splitting the condenser off for a remote location. The twin computer cooling fans as pushers are swapped out for a motorcycle cooling fan as a puller. Didn't look like the fans had any weather protection to them.
Hard parts installed, friend is going to run the lines for me and charge it (does A/C work for a living, I'm learning from him)
Going into a van. That is not currently insulated but will be. Work in progress.
It is matched to the limited solar, I hope. Bench testing before we took it apart showed power consumption of 100-180W depending on load. I am not expecting total comfort. Take a little edge off in an evening, keep the parked van so it is only hot inside and not blazing hot.
At the rate I am going, it should be usable by winter.

 

[luthj]

1535BTU/450W is just enough for two people in a curtained bed area. Assuming reasonable insulation. Depending on the blower head pressure, you may consider just routing the air under a blanket (or cooling shroud/blanket like hospitals use). That would make it feasible for sleeping. Remember a resting person produces around 90-120W of heat on their own.

As for cooling a vehicle with windows and exposed sheetmetal? In direct sun you would be better off just ventilating at high flow with outside air (with only 450W of cooling). It would probably be cooler by a significant margin. In the shade, you could cool a bed area with some success. An entire van would probably barely keep up with dehumidification and/or cooking for two people.