Checking subcooling in low ambient conditions

[dave1234]

What do you guys recommend for checking subcooling to verify charge in low ambient conditions. Obviously head pressure is lower. I was just thinking about a Trane A/C class (I choose to post here instead of comm HVAC) and the instructor said they look for 10* of subcooling for refrigerant charge in their equipment. Lets say there is no means of head pressure control on a split system. What kind of number do you guys look for in low OAT's ?

[icemeister]

You probably won't get much action on this question here in the Refrig & Icemaking Forum. You should have posted it up in Pro Technical.

Us Fridgies typically don't care much about subcooling because our systems usually have receivers and Headmaster type head pressure controls. The subcooling off one of our units just "is what it is". You can't "adjust" it by adding or removing refrigerant.

As for the A/C side, as far as I know, most manufacturers' charging charts only go down to about 60ºF or so. If you're charging below that it's probably best to go by weight and go back when it's warmer to dial it in.

[mtech]

What I've generaly done is cover the condencer with my coat. Watch the guages and adjust the amount you cover acording to the head preasure. What you are doing is limiting the amount of air going across the coil simulating a warmer outside temp. You can then watch the suction preasure and super heat and subcooling. Some will not like this method but it is effective. You should however keep in mind if the unit is running in low ambiant temps. it should be modified to do so. If it is not warm enough to charge the unit it most likely should not be running.

[-frozen-ocean-]

Low oat #'s = open the freckin windows free cooling man

[local 832s]

Low oat #'s = open the freckin windows free cooling man

I'm not a residential guy but what would you do if your doing a new install and its 60* outside and you have to adjust the charge?

I think the above is what the OP is getting at. Blocking the condenser is the easiest way to simulate.

[thefonz1]

you'll need some kind of head pressure control in low ambient if you wanna run that split in cold temperatures all the time. condenser fan speed controller is probably the easiest.

[Andy Schoen]

I'm not a residential guy but what would you do if your doing a new install and its 60* outside and you have to adjust the charge?

I think the above is what the OP is getting at. Blocking the condenser is the easiest way to simulate.

mtech is correct here. Block condenser air flow until head pressures are at design operating conditions. Then check to make sure superheat and subcooling are correct. Then take the coat off the condenser and make sure the system is operating adequately. ;)

[pecmsg]

mtech is correct here. Block condenser air flow until head pressures are at design operating conditions. Then check to make sure superheat and subcooling are correct. Then take the coat off the condenser and make sure the system is operating adequately. ;)

Andy

I thought you gave up all this technical BS and you are now just concerned with pumping refrigerant?

[Andy Schoen]

Andy

I thought you gave up all this technical BS and you are now just concerned with pumping refrigerant?

I'm finding pumping refrigerant to be a bit more technical than I first thought. :eek:

[MikeySq]

I'm finding pumping refrigerant to be a bit more technical than I first thought. :eek:


sweet!

[dave1234]

I'm not a residential guy but what would you do if your doing a new install and its 60* outside and you have to adjust the charge?

I think the above is what the OP is getting at. Blocking the condenser is the easiest way to simulate.

Thats exactly what I was trying to get at. I went to a call the other day to follow up after someone else who admitted to adding gas to "get the suction pressure up". :beat: I suspected a frozen evap- sure thing when I showed up the next day. I know it hadnt been looked at because the case coil panels were un-disturbed. Let it defrost with the fan overnight to clear it. I was working out of town the following day when a co-worker went out there. Given the info at hand I would have recovered and weighed in the charge to be certain but ... ... I wasnt there. Anyway the question came up how much subcooling should we have given the fact it was only like 45-50* OAT and how accurate of a method is that just to block airflow to check subcooling? Granted its realy the only method given the low OAT. 5 ton split- comm install pretty steady heat load. I told everyone involved right off we need to put in fan cycling. I also suspect someone will be back this summer recovering some gas.

[tjc76]

I told everyone involved right off we need to put in fan cycling. I also suspect someone will be back this summer recovering some gas.

Dave- for what it's worth, you posted this in refrigeration and icemaking, and fan cycling is forbidden here. You need to rig that thing up with a receiver and headmaster.:LOL: