Question on Charging TWO SPEED HVAC systems

[w2jo]

I thought I knew how to charge HVAC and HP equipment but I am slightly confused by measurements made on my own new system that I just put in. The system is a two speed 3ton/2ton carrier AC (only) with TXV and the 58mvc furnace. Nominally, it all works fine, but I am not understanding the sight glass not being full of liquid when the system is in LOW speed mode and operating steady state. The measurements are as follows:

For all experiments:
OAT = 75F,
Furnace Incoming Air data: WebBulb =56, RH= 50, Drybulb=70F

High Mode (3 ton) Discharge Air data: Temp= 50
Coil Exit R410a temp = 51
Coil Exit R410a press= 145psi (50F equiv)
Air Flow = 900cfm (measured)
Suction Line temp at OUTDOOR unit = 56F
Sight Glass is full liquid
Above looks normal.
Then:
Low cool Mode (2tons)
Discharge Air Temp = 57
Coil Exit R410a suction Temp= 58
Coil Exit R410a suct press = 162psi (57F equiv)
Air Flow = 500cfm (measured)
Suction Line temp at outdoor unit = 62F
Sight glass has "foamy" look.
Note: The "coil exit pressure and coil exit Temp" are
measured at a point on the suction line inches from
the inside coil housing.

It appears that in both cases, the superheat at the compressor is >5F and so is "safe". Note that this test was run with outside air temp = 75F which is not where the unit is rated to operate. This may be part of the problem.

So.. My question is: Do the above low cool mode pressures/temps all look OK? They seems to be in the right range, but I am wondering why, at lower load, the sight glass is not full?

I think perhaps one of you ME types can compute BTU cooling from the above data which would be an aid in determining if the system was putting out rated capacity. Thanks!

If someone can help me understand this, I will appreciate it!
Thanks
Joe

 

[w2jo]

Someone please comment. Did I not include enough data? What am I missing here. Thanks. Joe

 

[KiwiMace]

Hey Joe,
Most of us are engineers rather than technicians. This is too specific for me to answer and likely many others are in the same boat. It probably also requires the charging chart of the equipment in question. Someone might chime in, but you are more likely to get recommended over to another more hands-on forum.

The BTU calc is difficult with data taken from the refrig cycle, much easier to calc from the temp and airflow of the incoming and supply air.

 

[ccjersey]

OK, look at this logically......

Your compressor can handle a certain amount of refrigerant per hour while on high speed. You charge the system and clear up the sight glass. The sight glass gets clear because the compressor has accumulated enough liquid refrigerant in the condenser and line through the sight glass to the TXV. The TXV only meters enough refrigerant into the evaporator to meet the subcooling setting.

Then you switch to a slower speed, roughly 2/3 of the high speed and the compressor can no longer produce the same amount of liquid refrigerant. The load is the same as it was when you had it on high, so the TXV would meter close to the same amount of liquid into the evaporator if it were available. The evaporator pressure and temperature will be higher so compressor efficiency might be slightly better, but the net effect is there is now a "shortage" of liquid refrigerant at the sight glass, so it looks milky instead of perfectly clear.

I expect the instructions for your system say to do all charging while running on high speed.

 

[w2jo]

Thanks ccjersey.
Your explanation sounds quite logical. I would expect that the expansion valve would try to "flood" the evap coil at low speed and thus could run out of liquid since the pump is running slower. I was considering that PERHAPS, the suction line pressure would rise so that less refirgerant was boiling away in the evaporator and that PERHAPS I should add enough refrigerant until the sight glass was "full" of liquid at the lower speed. However, I guess that is not correct as no one has suggested it.

"The book" does not refer to using a sight glass at all. Rather, it speakes to either a) weighing in the refrigerant (good on a brand new system where you know what is already in the outdoor unit or b) using pressure/superheat measurements in HIGH mode. It does not mention what to expect in low speed mode with respect to the pressure/superheat.

So.. Many thanks for your answer. It sounds like a good one!
Joe

 

[imok2]

At 900 CFM at 70 DB and 56 WB the equivalent is equal to 540 CFM or 1.7 tons so your load in very low. Then at the same indoor conditions but at 500 CFM and 57 *F leaving your tonnage is 0.6tons and equilivent air flow of 840 CFM so the sight glass head pressure in very low and the TXV is barely feeding the evaporator Looks to me like its doing what it’s supposed to be doing but the existing conditions are not where you should be. To get the best results will come when your OSA conditions go to 95 *F then you will be able to determine what the system can really do.

 

[w2jo]

Thanks imok2!
I think waiting until I get a really hot day to check the charge is a good idea. The high side pressure on a 75F day is > 215psi while the low side is <170 so the 10psi minimum pressure across the expansion valve "should" be satisfied. However, I will recheck on a 90F+ day and see what the readings are. Thanks for your suggestions.
Joe

 

[w2jo]

OK.. Here is the real answer on how to charge a TWO SPEED HVAC Residential sized unit. To charge in COOLING mode Proceed as follows.

1) Assuming a good TXV, the Superheat in cooling should be from about 5F to 10F and is taken care of you automatically.
2) So how do you make sure you have the right charge in the TWO SPEED UNIT. The proper range is about 5F to 15F of supercooling.
a) Go to LOW COOL and add/remove refrigerant until the supercooling on the outside liquid line is in the neighborhood of 5F after maybe 10 minutes running time.
b) Then go to HIGH COOL mode for 10 minutes and measure the SuperCooling again. It should again be between 5F and 15F but may be different in the two cases.
c) If the inside coil is not a good match to the outside condensing unit, you may find that one or the other reading is ooutside of the desired 5F to 15F supercooling range. You can add/remove refrigerant to try and balance up both readings between the 5F and 15F boundaries.

But what if (as in my case) the supercooling in LOW is 4F and the supercooling in HIGH is about 25F? Well.. IF the superheat is right in both cases, This likely means the indoor coil and outdoor condensing unit are not "matched". In my case, the indoor coil is a replacement "high efficiency" coil (ie: lots more tubes and fins than the unit that it replaced) and so it holds more refrigerant than the old coil. So. To make this mismatched coil operate in "normal parameters" I should add about a 3 pound liquid receiver in the liquid line for optimum results.

However, a supercooling of 27F in this case is not "the end of the world". The compressor amps in HIGH mode with the supercool at 15F is about 9.8A and with the supercool at 27F it is about 10.4A. This efficiency reduction is not all that important as this particular unit only rarely runs in HIGH mode.

So: QED. I left the supercool in LOW mode at 4F for maximum efficiency in that mode and had a full sight glass and in HIGH mode the supercool is 27F. I will recheck when the weather gets hot and see if there are any significant differences.

Thanks to all for your thoughts!