Can someone break down the 68/225 rule for me?

[newbie tech]

Can someone either explain to me why these are ideal evap and condensor pressures please. I understand that this will not always apply but I just need someone to explain how and why these numnbers?

Thanks a lot.

[jpsmith1cm]

There is a LOT more than this to checking the charge on an AC unit.

I'd suggest forgetting almost anything the person that told you this said.

Get a few more posts, get free Pro membership and we can discuss this matter a little further in the Pro's technical forum.

Suffice it to say that you are correct these pressures will not always apply. Pressure alone is not enough to determine if a system is properly charged.

[HVAC Teacher]

THROW THOSE NUMBERS OUT THE WINDOW! - They don't mean a thing!

Whoever (HACK) told you this abides by the "beer can cold method".

I normally don't respond like this but I feel I have to!

Just being honest. Hang around & see the response from the others.

[Action Air Mem]

That's going to get you in a lot of trouble if you use that. There are many things including superheat, subcooling, wet bulb, dry bulb temps to take in consideration when charging correctly. If you are doing commercial work just weigh the correct charge in if there's any question.

[wolfdog]

That one is right up there with 'just tie a jug of freon on it and leave it overnight.It will be charged right in the morning'.

[newbie tech]

Hold up guys, I think you're jumping to conclusions a little bit. I'm not saying im going to use this as the basis of knowing if a unit is charged properly and nobody told me to either. I know that ambient air conditions can affect this, the type of refigerant used can affect this.

What I really want to know is as an example why would these numbers be ideal under the right circumstances and how are they obtained.

[knewguy]

The numbers you are refering to are just that numbers.

They are Nice numbers under certian conditions BUT bad at other times.

Be careful with "rules of thumb" they can come back to bite you.

[pecmsg]

Under ideal conditions 68 / 225 is what you’re looking for at design conditions with all things perfect. You could spend a lifetime and never see these perfect conditions. Ambient air, Return and supply CFM’s, Line length, Static Pressure, voltage, and on and on can and will affect what is happening.

Pressures alone are but one tool in a long list of information and equipment needed.

[slugger1]

the rule is dude if your on a service call NOT a PM. your at the wrong address or you hooked your gauges up to the wrong unit.

[jackintheboxtec]

Beer can cold HMMMM: Maybe they can get like coors light and have the refrigerant lines turn blue when the charge is correct?

:couch:

[HVAC Teacher]

Good to ask Newbie,

These were "ballpark" numbers used by untrained hacks for R22 AC systems.

Back then: no cert, vacuum pump, nitro, micron gauge, ...........

Just Braze, Gas, & GO!

EPA & new refrigerants just about put them out of business.

[ga-hvac-tech]

Pressures are one of a multitude of tools an HVAC/Refrigeration tech uses. The key IMO is to learn to see 'the big picture' of all the items and the way they integrate with each other. This will separate a tech from a parts changer or a hack.

[airtight_tech]

That one is right up there with 'just tie a jug of freon on it and leave it overnight.It will be charged right in the morning'.

Lmao I have never heard that one before.

[hvacker]

My son was working with a service guy and he told my son that he always charges to a 225 head no matter what.
He didn't last long.