ambient + 30° versus subcooling,superheat

[newinthefield]

Many old school guys told me that when checking or charging sistem I can use ambient+30° rule to charge unit, they even said just look suction line and when it starts sweting it is good. What do you think?????

[stonewallred]

I understand, from a thread I starting asking about the subject, that 30+ years ago, it may have been pretty close to being right.
Today's equipment, not so good.

[Mac222]

those are old school rules of thumb, best not to use those until you get really familiar with systems then still check/use proper super heat and sub cooling, and when available weigh in the refrigerant according to the unit data tag when it has the charge listed then still check the super heat and sub cooling and log the unit..use your tools the old beer can method was never a good thing to do and is or should be a thing of the past, nothing wrong with being professional and taking the job seriously...

[newinthefield]

Yea I understand that,with txv and other controls,beer can can't be used. Ambient + 30° gets you somewhere but cant relay on that. I'm just wondering what other techs think about that and getting my posts to a number for pro memberahip.

[jpsmith1cm]

Couple points....

1. This post should be in tech-to-tech chat. Maybe a friendly mod will come by and move it for us.

2. 'Old School' rules of thumb like that need to GO AWAY! They do not assist techs in charging systems properly nor do they do home owners any favors in getting what they paid for out of their unit.

Best practice is to follow the manufacturer's guidelines.

[Makinhole]

To be as professional as I can, I use superheat/subcooling method depending on the metering device, in conjunction with the manufacturers charging charts. I just do not believe that I can outsmart the engineers with an old rule of thumb trick. Do it once and do it right.

[BaldLoonie]

We can't move this or the OP can't see it. "new" needs to apply for pro membership so we can get even more technical with this with him.

[newinthefield]

I all ready did, all i need is post number count . I am allready approved. Cant waith

[flange]

using those rules of thumb dont tell you much. you really need to use superheat/subcooling as your friends. we do a lot of startup work for manufacturers, and two very specific brands show refrigerant charge in weight on their nametags. one is consistently thirty or more percent off, the other is a dice roll. when I questioned one specifically, I was told " we used to be good at calculating charge when we used r22, but we havent figured out these new gases yet". it is commonplce for my guys to "adjust" factory charges at startup.

[Senator]

When I used to do installs on 410a systems we would find the unit "overcharged " also...sometimes after cracking the valves or after weighing the charge in...a quick way to do this is use tstat wire that has feet printed on it-no tape measure needed. feet x 0.6 oz for 3/8 liquidline. Anyways its airflow.undersizes supply and undersized returns. Salseman got his comission and were stuck dealing with the callbacks.

[warm]

units will have a charging chart /pressure chart inside the lids - if not then rundawg or someone else with lead you to the manual for that unit - keep a notebook with manuals /or printouts to check back to when charging

[knewguy]

I have been in this trade going on 30 years was taught from the beginning
to use subcooling and superheat when checking units refrigerant charge.
It is best to use the equipment makers method if you can find it and with the
access to information on the internet it is much easier to find it the ever.

[Cool_Air]

Many old school guys told me that when checking or charging sistem I can use ambient+30° rule to charge unit, they even said just look suction line and when it starts sweting it is good. What do you think?????:whistle:Today the outside units a hi eff. and may only need a 20* diff. from the ambient

[Shophound]

Many old school guys told me that when checking or charging sistem I can use ambient+30° rule to charge unit, they even said just look suction line and when it starts sweting it is good. What do you think?????

In about all cases with equipment less than thirty years old, following the old "ambient + 30" rule of thumb will result in a system being mildly to grossly overcharged.

Don't do it. Get accurate test instruments that can give you target superheat for charging systems with pistons and good subcooling readings for charging systems with TXVs. ALWAYS check both superheat and subcooling whether you are charging a system or not.

I agree with CM, the old rule of thumbs MUST DIE. Refrigeration is a technical field. It is an applied science. Enough "fly by the pants" stuff already.

[Capz]

I work with a 12 year tech who charges everything he works on by gauge pressure using the 30+. He doesn't even own a thermometer. And he frequently overcharges units. I am embarrassed to work with him but as the new guy, I have to keep quiet and let him lead. I have my Fluke combo, my pocket Fieldpiece and I always use it when I am leading and/or working alone. It's a real problem for me working with this guy. I don't know what to do about it while trying to keep the peace and my job.

Don't do it. It's not pretty!

[Thomasg]

That sucks. Lol been there.

[ChaseAir]

Get one of those fieldpiece sman3 digital gauges that shows superheat and subcool simultaneously. He will think it is so cool he will want to learn how to charge and troubleshoot a system properly. If not, you have to learn yourself.

Seriously, having a digital set that shows everything will help you put theory into field practice and charge a system with precision.

[bruced]

BEFORE CHECKING CHARGE- you should know the static pressures across your evap coil and furnace/air handler, filter.You cannot charge a system correctly if these are not acceptable. Improper air flow will not allow you to determine correct charge. You must also know "latent load". You should ALWAYS charge or check a system w/sub cooling whether it be a txv or fixed orifice, you also check superheat on fixed orifice but after sub-cooling. There are no other ways to charge a system any longer. Even calculating the amount of refrigerant will not get you there because of system quirks. It is actually very simple and will give you much more confidence and help you understand what the refrigerant is doing to remove or gather heat, what "state" the refrigerant is in, vapor liquid, etc. You should be able to determine how much heat the condenser is handling etc. Find and read all you can on the subject from your factory/distributor. You will never be able to charge a unit with precision, or know if a system is operating correctly without this knowledge. We all did it the "wrong way" for years. I was taught to do it the "right way" and am no longer in the dark. It will open your eyes! When checking you must know the temps before evap, after evap, before condenser, after condenser and before and after every component (txv, RV, service valves,driers, anything in the circuit). This will let you determine what changes in the refrigerant are taking place.

[MichaelPaladin]

Well the only reason I still use the Ambient +20 to +30 (depending on brand and age) is to see if a condenser cleaning is going to be needed. Other than that its not a good indicator of total system charge, but still a good indicator on a dirty condenser coil or non-condensibles in the system.

[rdholder]

With the SEER being over 8 you will over charge a system if you do a ambient +30 degrees. Staring with the 10 SEER systems it was more like ambient + 22 to 25 degrees and today wit the SEER being 13 and 14 it is more like ambient + 15 to 20 degrees. With the higher SEER the best way to charge follow manufacture charging chart and use superheat for a fixed orifice and subcooling for TXV. that way you know how much refrigerant is in the evaporator and the condenser.

All way check the air flow and the Wet Bulb "latent load" on the evaporator and the temperature difference of the coil the TEET on a fixed orifice system.

[rdholder]

Any time superheat on a fix orifice is charged to the technician need check the evaporator TEET of the air differential air temperature. Temperature difference of an evaporator coil will vary with the total heat of the air entering the evaporator and the load on the condenser. This temperature will vary from 10º F to 30º F depending on total heat of the air or the volume of the air entering the evaporator. If the air going into the evaporator is 80 degrees dry bulb and 79 degrees wet bulb the temperature of the supply air is 72 degrees, If the air is 80 dry bulb and a 55 degrees wet bulb the supply air temperature is 56 degrees. Both need to be check. With air flow at normal operating conditions the temperature difference should be 16ºF to 22ºF depends on the design of the system.