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Head pressure and ambient temp relationship

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#21 ·
The method being talked about used to be called the "30 over" method. It came from the 'beer can cold" days. It was one of those things that got you into the ballpark for a unit you knew you would be returning to visit again soon, because a quote had to be submitted and approved for further work. Of course, proper charging for an orifice header RTU would be to use subcooling, but the method described was used, and to a limited extent, it did work.

A headmaster is a valve used to control head pressure, usually in a refrigeration condensing unit. It allows the unit to maintain temps and pressures as the system is being asked to run in low outdoor ambient temps. A change in pressure moves a pintle valve to let liquid refrigerant back up into the condenser coil, reducing its effective area, which raises pressure because it is more difficult for heat to get out of the refrigerant discharge gas coming from the compressor. You still have enough liquid for the metering device because you also have a receiver, and a correctly charged system allows for enough refrigerant to be in the coil and at the metering device in the cold weather, and to be filling the receiver in hot weather.

A Motor Master is a Carrier-branded condenser fan motor drive that controls the fan speed and operation according to condenser coil temp at a specific return bend location on the coil. This allows the device to maintain a band of temps to allow for some pressure control in the shoulder seasons of fall and spring, when you still need ac but the outdoor temps are a little low.
 
#22 ·
Tech101 said:
Well on a R22 standard efficiency unit or (10 seer) you will need to add 30 to 35 to the condenser ambient temp to get your head pressure, on a high efficiency R22 unit or 13seer you will need to add 20 to 25 to the condenser ambient temp to your head pressure. i have been taught this from a tech who has been in the field for 30 plus yours and is now a instructor at my local technical school. best of luck to you.
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W R O N G

30 years ago that might have been a starting point but doesn't cut it today. Bad advice!
 
#23 ·
Just a Q. On a R system that is installed properly and serviced properly and running properly. This system is missing any type of head press control. So on a 70*F ambient what should the head press be? What should the SCT be? How about r22.
 
#24 ·
TechmanTerry said:
Just a Q. On a R system that is installed properly and serviced properly and running properly. This system is missing any type of head press control. So on a 70*F ambient what should the head press be? What should the SCT be? How about r22.
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I know that!:angel:
 
#26 ·
The only rule of thumb for TD regarding head pressure when dealing with refrigeration is 25*F +/- 15*F. Which basically means the range is so wide there is no rule of thumb.



TechmanTerry said:
There are "rule of thumb" concerning head press. I was taught several of them and the do make sense and the do hold true. And as BBeerme said ,you need a starting point somewhere. So if ROT do not apply then how do you know what the head press is supposed to be.

I'll be back when I get back, just got a Service Call. Its about 35*F ambient right now, I wonder what my head press is supposed to be on a Med Temp WIR w/ regular TXV, and you pick whichever freon you want.
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#27 ·
hvacker said:
Thirty or forty years ago hvac guys usually didn't have instruments like today. The thirty degrees idea was based on a twenty degree delta and ten degrees heat of compression. Overall I believe it was way more accurate than the beer can method. Like charging any system the condenser needs to be moving the correct amount of air as does the evap.

Manufactures all had different methods to determine charge. Many of those methods were not available written in long lost install manuals. Remember the Carrier round with a sight glass by the compressor. Something like put something around the condenser, elevate the high side to something like 290degF for a time and charge to a clear glass. Others were even worse. It was no wonder techs looked for short cuts.
Some did charge by this method. Others by superheat. Subcooling was never mentioned. Most equipment was overcharged. Probably often still is. It's a lot more technical today but many techs aren't schooled and many employers don't know there is a problem.
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Remember those old Lennox units that had the 2 little valves on the back with Allen head stems. If memory serves you were suppose to charge till the lower one would discharge liquid and the upper gas. Those were the same systems that had the coiled up liquid line for a metering device.
 
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#28 ·
BBeerme said:
The only rule of thumb for TD regarding head pressure when dealing with refrigeration is 25*F +/- 15*F. Which basically means the range is so wide there is no rule of thumb.
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I'm thinking that the head press w/ fall in line,very closely in line,to the same conditions as the Alco HeadMaster. Either 95*F SCT or 98*F SCT +/- a couple *'s.
 
#29 ·
BBeerme said:
The only rule of thumb for TD regarding head pressure when dealing with refrigeration is 25*F +/- 15*F. Which basically means the range is so wide there is no rule of thumb.
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This is very true. Commercial refrigeration system components like condensing units and evaporator coils sold as matched systems as in residential HVAC where the SEER ratings will give you a pretty good idea of where to expect the saturated condensing temperature (SCT) to be.

A good example, which comes quite close to matching what BBeerme stated about the Rule of Thumb for refrigeration is with the same Tecumseh low temp R404A compressor Model AWA2460ZXD which is used on three different models of their air-cooled condensing units.

The base unit Model AWA2454ZXD is rated at 1½ hp, the capacity is 5102 Btuh at -20°F SST/90°F Ambient where head pressure would be 281 psig (112°F SCT) according to the performance chart for that unit. So the TD in this case is 22°F.

http://www.tecumseh.com/~/media/Drawing-Data/North-America/CU-Data-Sheets/2B3224-1.pdf

The mid-range Model AWA2479ZXD is rated at 2 hp at the same conditions has a capacity of 5925 Btuh and the expected head pressure would be 251 psig (104°F SCT) for a TD of 14°F.

http://www.tecumseh.com/~/media/Drawing-Data/North-America/CU-Data-Sheets/2C2114-1.pdf

At the high end we have the AWA2490ZXD 2¼ hp unit with a capacity of 6365 Btuh and a head pressure of 235 psig (99°F SCT) for a TD of only 9 °F.

http://www.tecumseh.com/~/media/Drawing-Data/North-America/CU-Data-Sheets/2C2111-9.pdf

So the same compressor, but different condenser coils and fans to provide the additional condensing capacity and as a result the lower head pressure gives you a lower TD and higher capacity.

Without the knowledge of knowing when not to use a rule of thumb, a tech who follows the rules of
"that's how I was taught" will get into trouble without knowing how that happened or why.

Learn the the "why" of what you're doing and you won't need to blindly rely on Rules of Thumb.

(The very first thing to do is learn to think in terms of temperatures and not pressures. It will make everything so much easier to analyze.)
 
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#30 ·
BNME8EZ said:
Remember those old Lennox units that had the 2 little valves on the back with Allen head stems. If memory serves you were suppose to charge till the lower one would discharge liquid and the upper gas. Those were the same systems that had the coiled up liquid line for a metering device.
Click to expand...

Yes I remember. I think it was run and elevate the head to a certain pressure for a certain time, shut down for a certain time and liquid should come out the lower tap and vapor from the upper tap. The system would work as long as line length and coil matched. It's no wonder the 30# theory developed.

The coiled liquid line was for units under 4 1//2 ton or maybe 4 ton. The line was actually a long cap tube that was supposed to create the right resistance so the refrigerant would flash when it entered the evap coil. They had a chart where the PD over the evap corresponded to the air volume. Another chart was in the cond unit where a graph used OSA and head and suction pressure to charge the unit. The system ignored indoor temp and RH. Carrier guys thought the method was wrong.
I guess back then it was cat skinning. There were lots of ways.
I entered this business when the industry was trying to get contractors to forget sight glasses. I remember a super wanting to add a sight glass to the long coiled cap tube mentioned. Some still charge by sight glass.
As long as the problems of charging with a sight glass were ignored the method seemed mistake proof. Refrigerated air in homes was new and commercial equipment used sight glasses. The knee jerk reaction to a unit not performing was add refrigerant. The two allen taps were some of the industries first attempts to stop over charging.
One contractor in Ala had an apartment contract and the units were almost all over charged. He adjusted the charge and stored the excess in a mech room. I can't remember exactly how much he collected but it was several hundred pounds.
Probably over charging is still a major problem.
 
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#32 ·
There is still nothing wrong using "Rules of Thumb" as a barometer just like you might touch the lines the see if the feel close to what you'd expect. It's just that the information isn't conclusive.
Resist the temptation to throw the gauges on the machine. Collect information first so you have a direction to go.
I remember a movie. Electra Glide in Blue where the young cop asked the older detective at a crime scene "What are we looking for?" The answer was " What ever doesn't belong" Really good trouble shooting advice.
 
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#33 ·
I wouldn't say they are ballparks if the condenser on some is meant to be 30 degrees warmer than the air passing in over it.

Sent from my SM-G930T using Tapatalk
 
#34 ·
hvacker said:
There is still nothing wrong using "Rules of Thumb" as a barometer just like you might touch the lines the see if the feel close to what you'd expect. It's just that the information isn't conclusive.
Resist the temptation to throw the gauges on the machine. Collect information first so you have a direction to go.
I remember a movie. Electra Glide in Blue where the young cop asked the older detective at a crime scene "What are we looking for?" The answer was " What ever doesn't belong" Really good trouble shooting advice.
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I remember the ending to that movie. Robert Blake is the older cop and he get wasted by a hippie in the final scene.
 
#35 ·
This is an interesting thread for me. In class we are currently doing the plus 30 for low seer machines and plus 20 for the higher seer machines. It is all new to me, well everything is new to me since I am just a student.
 
#36 ·
psehunter said:
I wouldn't say they are ballparks if the condenser on some is meant to be 30 degrees warmer than the air passing in over it.

Sent from my SM-G930T using Tapatalk
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That ballpark might come close if the indoor load is near a normal 75degF. and both coils are clean and the unit has correct air flow, no restrictions etc. But I wouldn't want to charge a unit and swear the charge is within 1% as that's the % that is achievable.
It's not that a unit is supposed to function in that range it's more like under "normal conditions" it might fall there.
It still beats beer can.
I wonder how many goofy ways some have used to charge systems. One guy said he charges to 225 (R22) head regardless of anything else.
 
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#37 ·
It is important to note that if the condenser coil is anything less than just cleaned, the heat transfer will be less and the head pressure will be higher...so you can think you are in the ballpark when the condenser is dirty, yet still have an undercharge. You double check this by looking at delta T.
 
#38 ·
Fender60 said:
This is an interesting thread for me. In class we are currently doing the plus 30 for low seer machines and plus 20 for the higher seer machines. It is all new to me, well everything is new to me since I am just a student.
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I'm surprised your being taught a method that has too many ways of being wrong. Especially today with the instruments available for the measuring of subcooling, superheat and pressures and OSA is way more accurate because of more information.
Just my view but I can understand a class mentioning the old ways just for the history, but the new ways are much more accurate. Remember the objective is to get within 1% of correct charge. You get that by weighing in the complete charge with a digital scale or using accurate instruments to charge the system.
It's like getting a blood test vs a blood panel. One just has a lot more information.
 
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#39 ·
timebuilder said:
It is important to note that if the condenser coil is anything less than just cleaned, the heat transfer will be less and the head pressure will be higher...so you can think you are in the ballpark when the condenser is dirty, yet still have an undercharge. You double check this by looking at delta T.
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The same holds true for SH or SC.. if the evap is plugged, SH is effected... condenser plugged same holds true.. SC will be effected..
 
#40 ·
I am always looking at the TD on both coils. You see, I don't really pay a whole lot of attention to pressure; I'm looking at the temp scale more than anything. It's a lot easier that way. In fact, unless you're setting or checking a pressure control, the pressure in and of itself is pretty useless.

It's always the temp of saturation we are more concerned about. Which means I am always looking at the TD of the coil. How much over ambient. For the indoor, how much below the return temp. I'm not even aware that there is another way to "check pressures".
 
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