A/C Charging Specs

I have one of my 3 split system A/C units (R22) that will only obtain ~14 degrees TD....The other 2 units are doing 18-20 degrees TD. Good condenser flow through all units and evaporator coil/filter clean.

I checked the pressures and suction line temp and came up with ~4 degrees of superheat...seems a little low (too much charge) to me as most non-TXV units call for 15-20 degrees of superheat, if memory serves. I called the manufacturer's "tech line" who referred me to several authorized dealers for charging info. I called 3 of them, and got 3 different answers as to target superheat. One of them even used the old shade tree mechanic adage "beer can sweat" on the suction line, so I discounted him right away!

I intend to play with the pressures a little and watch the evaporator TD, but would like to know if there is a spec for target superheat before I start. Any Marine Airrrrr pros out there?
Thanks in advance!

My understanding is that it is very difficult to get the charge right in these units by looking at temps and pressures. The right way is to remove all of the refrigerant and put the right amount back in by weight.


David

For cap tube systems, the most accurate method is to use a https://www.google.com/search?q=sup...sAQINg&biw=1280&bih=632#imgrc=Gs-lzvowB5lDqM:chart based on outdoor dry bulb and indoor wet bulb. Since the condenser is water cooled, you can approximate the temp based on water temps. Granted, it's a fudge, but the wet bulb factor is the key to getting the SH close, and it gives you more accuracy than a SWAG. Make sure your system is cap tube! Some of the CruiseAire systems have a TXV, and if it does, you charge by subcooling, and that can be 10-15F and it will work, the TXV will compensate. If you don't have a sling psychrometer, improvise with a t-couple probe and a wet paper towel and fan. Take the refrigerant temps on the suction line, optimal location is downstream of the evap coil, and best to use a thermocouple probe that's taped to the tube and insulated or with a clamp-on tube thermocouple purpose-built for that function. Shooting with an IR reads too much background, won't be nearly accurate enough.

I think I just woke up in a foreign country.

Thanks Maerin.....Yes I'm sure that the unit is not a TXV, and I am using a psychrometer and clamp thermocouple for temps. I'm just trying to find the manufacturer's superheat curves for a Marine Airrrr 10k BTU unit. I'll use the generic R22 curves as a last resort, but wanted to be as precise as I can.

djmarchand said:

My understanding is that it is very difficult to get the charge right in these units by looking at temps and pressures. The right way is to remove all of the refrigerant and put the right amount back in by weight.


David

Click to expand...

+!

Rob

Weight may be the optimal methodology for a self-contained unit, but split units can have varying lengths on linesets, which affects the required amount of charge. I have one unit with 7' lines, another with 35'.

greysailor said:

Weight may be the optimal methodology for a self-contained unit, but split units can have varying lengths on linesets, which affects the required amount of charge. I have one unit with 7' lines, another with 35'.

Click to expand...

Manufacturers provide specs for added line lengths.

Rob

Unless you are a certified refrigeration technician, as a past HVAC technician for the F.A.A., I wouldn't want to get caught with a set of manifold gauges, a can of refrigerant, (especially R-22), and no recovery equipment with you, and "oh, my recovery equipment is in the garage" won't hack it. The EPA is super serious about refrigerants these days and will pay informants who snitch on you. I know most of the refrigerant laws are an overkill, but the risk of serious fines just isn't worth it in my opinion. The days of DIY refrigeration repairs are long over.

Hire a professional........

Don

CaptDon01 said:

Unless you are a certified refrigeration technician, as a past HVAC technician for the F.A.A., I wouldn't want to get caught with a set of manifold gauges, a can of refrigerant, (especially R-22), and no recovery equipment with you, and "oh, my recovery equipment is in the garage" won't hack it. The EPA is super serious about refrigerants these days and will pay informants who snitch on you. I know most of the refrigerant laws are an overkill, but the risk of serious fines just isn't worth it in my opinion. The days of DIY refrigeration repairs are long over.



Hire a professional........



Don

Click to expand...

With all respect, the EPA threat is a paper tiger. With all the regulations and red tape created with the Montreal Protocol and the resultant scare tactics that followed, in my 30 yrs in the HVAC business, I've NEVER met in person, nor have I encountered another tech who has met, an EPA cop. The certification is a farcical test of successful indoctrination and has little to do with qualification of a service tech yet EVERYTHING to do with bureaucratic intrusion.

I would not argue that technical service work should remain in the realm of an experienced tech, there is simply too much technical knowledge involved in effecting a repair, to wit: the "foreign country" comment. But EPA regulations would not discourage me from working on a system, nor would it dissuade me from offering technical advice to a knowledgable individual who asks. I DO have a well-worn EPA card; I can legally buy refrigerant. Woo-hoo! That's about the extent of the value of that certification. It does not represent much else.

greysailor said:

Thanks Maerin.....Yes I'm sure that the unit is not a TXV, and I am using a psychrometer and clamp thermocouple for temps. I'm just trying to find the manufacturer's superheat curves for a Marine Airrrr 10k BTU unit. I'll use the generic R22 curves as a last resort, but wanted to be as precise as I can.

Click to expand...

The wet bulb entering is about as accurate as you can get, even with having to judge the variance in condenser air vs water temps. You can also check with frostback on evap, useful for small unitary stuff, but my go-to would be wet bulb.

By TD I assume you are talking about the temperature difference of the air entering the remote air handler and the temperature of the air leaving the air handler (evaporator).
I am not an expert on your particular unit. After reading your posting I think you understand the refrigeration cycle reasonably well. I offer this advice. Do not overlook the possibility that the inside of the air handler has rusted apart and is pulling in hot air from behind the bulkhead. Also take a look at the inline filter dryer, if it is restricted it will be very cold. By the way that old guy with his hand on the suction line is not as dumb as you might think he is. Slow down and look at your system in its entirety, I am sure you will find the answer. By the way I am certified Universal.

Unless you have the gauges and correct tools and work on reversible A/C on a regular bases....
My best advice, employ a certified technician with lots of experience. That way, if it is not right, you can bitch at someone other than yourself.