Mini splits, argh! Charging ..

[Things]

I've got a tiny 2nd hand Fujitsu mini split, which I have installed temporarily in my bedroom through the window. The unit was pumped down when it was removed, however I don't know what it's original installation was, lineset length etc. It's only a tiny unit at just under 2kW cooling capacity, but I figured it should be OK for the room. The lineset is probably only a metre and a half long. It seems to work OK and the air leaving the evaporator is nice and cool - though I can't help but feel it might not be running optimally. As usual I've only got a service port on the suction side to deal with, and I'm getting just under 60PSI. However, I notice the discharge line seems to be quite hot, and the compressor also. I had a feel of the loops in the condenser and noticed that the top few are quite hot, but as you work your way down the coil, below about half way, they're just ambient temp. Almost like it goes from warm to nothing in a single row. I was told that the condenser should be about equal temperature over it's entire surface, so I found this a bit odd.

Also looking at the evaporator, the incoming lines are frosted up for a couple of loops, then gradually tapers off across the coil (seems to have dual inputs). Towards the "end" of the coil it's not all that cool, however feeling the return line out at the compressor, it is cool enough to sweat.

The unit is missing it's dataplate, and I can't find anything online for it, so I've emailed Fujitsu hoping to get a manual and at least some specs. It does work though, so it can't be far off, or maybe I'm just thinking into it too much

[jonjon]

What refrigerant is it on ? Normally there's a minimum pipe length

[Things]

It's R22. It has a sticker on the outdoor unit that says "10m maximum pipe length, refrigerant must be added for lengths over 5m", but nothing about shorter, or even how much to add per metre over 5m. The spec sheet says it's charged with 700g of R22, but I have no idea what's in it. If I completely evacuate the system and charge with 700g exactly, it should be a fairly safe bet? My lineset is probably 1.5-2m at most, so if 5m is the max before having to add charge, I'd think that would be within spec for the factory 700g charge?

[chemi-cool]

Just connect it, vacuum the pipes and indoor unit, open the valves and let it run.

If you will be short of refrigerant you will have to replace the whole charge with 22 substitute.

[Tayters]

Hi Dan,

College told us about the central 80% of a condensor is where it's changing state. Can't tell you how true that is but a little nugget anyway and proves I do listen sometimes.

If the suction is sweating then good chance charge is fine.
Think you can get away with a very short pipe run on smaller units. Have seen newer Fujitsu inverter models making a pulsing sound through the pipework but this is only on the larger units (10kW region). A longer run (coil of pipe) sorted it out so my guess is that the added pipe volume dampened the compressor pulse each time it cracked out a discharge stroke (oo-er missus!).
Not foolproof but a good way to check if you are short of charge short of reclaiming is to check compressor discharge temperature which shouldn't be over 100*C unless you have an undersized unit working its genitals off.
Any indoor model numbers at all?

Cheers,
Andy.

[Things]

Thanks guys.

I think the unit is probably a bit undersized for the room I have it in, though it's only a temporary install so if it gives me some relief in summer, I can look at beefing it up later on. Waiting to get my thermometer back but I can touch the discharge line for probably half a second or so, it should be under 100C. Compressor is also fairly hot but I was told rotary compressors do tend to run hot.

It's quite an old unit and just has a capillary for metering. Model numbers are AST7ASA-W/AOT7ASA for the indoor and outdoor units respectively (oddly, very little information online). Suction line is sweating, and the evaporator seems to be almost equally cold along it's length. That said, it doesn't seem to be overly cold.

One thing I found interesting was that the evaporator seems to have 2 input/output points. One side of the evaporator could still be barely cold, yet the suction line sweating. Wonder if they have some sort of intentional bypass to ensure the compressor is always getting a bit of cool vapour back?
Should the liquid line from the condenser be sweating too, or actually frozen?

[hyperion]

Some of these older Fujitsu units were very sensitive to having the correct gas charge and if any has been lost during the disconnection/re-install then this will reduce the cooling efficiency of the system.