# Hot Gas Method for Checking Heat Pump Charge



## zootjeff (Jul 11, 2007)

So I ran across the Service Manual for my Goodman 14SEER heat pump and noticed it said that you can check refrigerant charge in heating mode by using the "Hot Gas Method" 

What are peoples thoughts about this method? It's cold out side, is there any way to do it using the gauges?

From the service manual

Hot Gas Method
System charge can be checked in the heating mode by
measuring the hot discharge gas at the compressor.
1. Allow the system to operate at least 20 minutes.
2. Attach and insulate an electronic thermometer probe to
the vapor service valve (large line) at the base valve.
3. Operate the system for 10 minutes.
4. Using an accurate electronic thermometer, measure the
temperature of the discharge gas at the probe and the
outdoor ambient temperature.
5. The temperature measured on the vapor service valve line
should be equal to the outdoor ambient temperature plus
110°F (± 4°). For example, if the outdoor ambient temperature
is 45°F, then the temperature measured by the
thermometer probe at the vapor service valve line should
be 155°F for a system that is properly charged. If the
temperature measured by the thermometer probe is
higher than the outdoor ambient plus 110°F, the system
charge should be adjusted by adding refrigerant to lower
the temperature. If the temperature measured is lower
than the outdoor ambient plus 110°F, the system charge
should be adjusted by recovering charge to raise the
temperature
NOTE: When adjusting the charge in this manner, allow
the system to operate for at least 10 minutes before
taking the next temperature reading.


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## biggles (Jan 1, 2008)

that is for a tech to perform cause every time you connect the guages you are burping freon out of the system...you can test the charge by feel..run it in the heating mode let it run for 15 minutes go into the house and see if you can grab the insulated line going into the air handler ...:wink:if the unit is charged near or above design that line should feel like when you touch hot pot on a stove.... you can't and it burns. do a temperature split on the return and supply air in that mode and write the temps on the side of the air handler note outside air also...thats a simple cheap check....as long as you have the heat on that line your charge is solid....


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## Marty S. (Oct 31, 2009)

Just checked mine. Pressures,subcool and BTU output match performance data exactly. Ambient +110 would have it way out of specs.


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## zootjeff (Jul 11, 2007)

biggles said:


> that is for a tech to perform cause every time you connect the guages you are burping freon out of the system...you can test the charge by feel..run it in the heating mode let it run for 15 minutes go into the house and see if you can grab the insulated line going into the air handler ...:wink:if the unit is charged near or above design that line should feel like when you touch hot pot on a stove.... you can't and it burns. do a temperature split on the return and supply air in that mode and write the temps on the side of the air handler note outside air also...thats a simple cheap check....as long as you have the heat on that line your charge is solid....




So in my case, after the system runs for 20 minutes (no defrost cycles in the middle), the hot line is only 115 degrees.. It is 35 degrees outside, do I have too much charge?


I have 1125 CFM with a 2.5 ton 14SEER heat pump, 3 ton indoor coil and the system only makes it to 88 degrees at the supply when it is 41 degrees outside and return of 68 degrees.


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## Hubcap626 (Feb 12, 2011)

Most manufacturers for there heat pumps include a manual with a chart that will give you a pressure and temp relationship. So you can get a idea when it's colder out what your head and suction pressure should be in heating and cooling mode.


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## Hubcap626 (Feb 12, 2011)

zootjeff said:


> So in my case, after the system runs for 20 minutes (no defrost cycles in the middle), the hot line is only 115 degrees.. It is 35 degrees outside, do I have too much charge?
> 
> I have 1125 CFM with a 2.5 ton 14SEER heat pump, 3 ton indoor coil and the system only makes it to 88 degrees at the supply when it is 40 degrees outside.


What got line? The discharge at the compressor or the liquid line at the air handler.


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## Marty S. (Oct 31, 2009)

Mine hit 110 with 35 ambient jeff.

Temp rise x 1.08xcfm=BTU

Linear interpolation shows at 40 degrees your HP should be producing 26.5K BTU and it is producing 24300. That's acceptable performance.


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## zootjeff (Jul 11, 2007)

Marty S. said:


> Mine hit 110 with 35 ambient jeff.
> 
> Temp rise x 1.08xcfm=BTU
> 
> Linear interpolation shows at 40 degrees your HP should be producing 26.5K BTU and it is producing 24300. That's acceptable performance.



Do you swap the gauges when measuring pressures in heating mode?

The heating data table says high pressure is measured at the suction service valve, that's the red valve right?

It also says low pressure is measured at the gauge port connection. Is that the shrader valve that isn't the liquid line, but on the back of the unit up a few inches from the main ports?


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## Marty S. (Oct 31, 2009)

I leave the high side(red hose) on the liquid line, it's liquid in heat or cool mode. Discharge gas can be hot enough to be rough on hose seals so liquid line is a better option IMO. That's correct on the suction location.


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## biggles (Jan 1, 2008)

so what does the insulated line feel like in the heating mode at the air handler inside:wink:..you should not be able to touch it if you can your short on charge no matter what's going on outside...the more the evaporator(outside in the heating mode) drops away from that 40F typical inside evap during the summer run you are getting closer to defrosting cycle where the resistance heat cycles on to compensate for the switching of that compressor gas now going out to defrost the condenser


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## zootjeff (Jul 11, 2007)

OK,

I'm still confused,

The Table says This:

IN HEATING MODE:

"High pressure is measured at the suction service valve (the larger valve). 
Low pressure is measured at the gauge port connection."

There is the small 3/8 line liquid port at the valve which should have High pressure liquid, There is the 3/4 line vapor/suction port at the valve that will have High pressure vapor.

Then there is the third "service port" which is tied right off the compressor where low pressure vapor enters the compressor.


So if I want to match the values in the table, I connect the blue low side gauge to the low pressure "service port" and I can connect the red high pressure port to either port to get an accurate high pressure reading even if the table calls specifically the larger port valve?


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## Doc Holliday (Mar 12, 2011)

Red is always red, liquid. Blue to compressor port.


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## biggles (Jan 1, 2008)

any shradder fitting right at the compressor Hi or Lo to take a guage stay the same either in heating or cooling.the suction line changes from in coming low pressure (cooling) freon to hi pressure (heating) super heated freon going out to the air handler.the liquid line also changes direction but is the same with hi pressure readings..if you put a hi side RED hosed guage the shradder outside the condenser on that brass locking nut you can read the hi side in heating and the suction in cooling...as the 3-way slides from heating to cooling


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## zootjeff (Jul 11, 2007)

biggles said:


> any shradder fitting right at the compressor Hi or Lo to take a guage stay the same either in heating or cooling.the suction line changes from in coming low pressure (cooling) freon to hi pressure (heating) super heated freon going out to the air handler.the liquid line also changes direction but is the same with hi pressure readings..if you put a hi side RED hosed guage the shradder outside the condenser on that brass locking nut you can read the hi side in heating and the suction in cooling...as the 3-way slides from heating to cooling


Cool, So I guess my only question would be, how much pressure drop will you see across the indoor coil in heating mode between the suction and liquid. Ultimately if the datasheet tells me to measure the high pressure of the vapor (suction) port, how much pressure if any will drop if I measure the liquid side?

It is still going to be high pressure, but is it going to be the same pressure? Does the indoor coil just release heat by the phase change of the gas to a liquid or does it also drop a little bit of pressure in the process.. Seems like for a heat pump, I might want to do what the datasheet says if those are the numbers I check, unless you tell me there is no pressure drop across the indoor coil, or if you say it is always a fixed drop.. 

It makes sense that you'd rather release the liquid line from the hose than the vapor from a usability aspect, because the vapor has so much energy and you'd rather not get burned..


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## biggles (Jan 1, 2008)

sure you might get burned but it is hi pressure gas and it is close to the liquid pressure just a different stat....and you loose more with a liquid line disconnect then a hi side disconnect.some brands want the unit chargd in the heating modes or the cooling modes. but there should be no pressure drop from the hot freon into the air handler and the liquid line coming out...same s the summer run when you connect the hi side guage then on the Liquid line should be close....still haven't said how that line into the air handler is hot or warm or what:huh:.keep in mind when you defrost in the heating mode the unit goes from a 100F plus discharge air into the house to a 40F discharged tempered by the electric heat till the defrost is over....


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## zootjeff (Jul 11, 2007)

biggles said:


> ...still haven't said how that line into the air handler is hot or warm or what:huh:.keep in mind when you defrost in the heating mode the unit goes from a 100F plus discharge air into the house to a 40F discharged tempered by the electric heat till the defrost is over....


In post 4 I said it was 115 degrees.. not too hot to touch.. the manual says to remove 410a in this case, not add..


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