my father's home a/c is under or overcharged? - http://www.hvacmechanic.com/ Forums
Original message
| stevescivic
 |  "my father's home a/c is under or overcharged?"
, posted Wed 2 Jul 02:03  
Hi everyone,
My father's a/c system has always been somewhat of an issue whereby it takes forever to cool the house down and when the temps are in the 70's it does do a very good job of dehumidifying the air. Based on the information below can one someone deduce some sort of conclusion as to whether or not the a/c system is properly charged?
Thanks
Condenser: Trane TTR1036A1000AA 3 years old
Evap coil: CUBA36A4ACD 3 years old
Refrigerant type: R-22
Refrigerant metering method: FFCV/fixed orifice sized at .073
System size: 3 tons or 36, 000 BTUs
Line set length: 27FT
air handler/furnace: Old Lennox G8 series furnace - 18 years old
Furnace filter: Low restriction high velocity approved 3M filtrete filter
Home age: 18 years old
Home square footage: 1843 sq ft.
Location: Calgary, Alberta Canada
Reported problem: humidity control and insufficient cooling on extremely hot days.
Test today: July 1
outdoor temp: 75F RH 40%
indoor temp: 75F RH 45%
Low side pressure: 61 PSI with temp scale indicating approximately 33F
High side pressure: 197 PSI with temp scale indicating approximately 97F
Suction line temp at refrigerant fitting: 39F
Liquid line temp at refrigerant fitting: 75F
Superheat measured approx 5-6F (suction line measured with a laser thermometer)
Subcooling measured approx 22F (liquid line measured with a laser thermometer)
According to charging chart in condenser unit my pressures on a day like this should be (pressure corrected to match indoor coil)
Suction: 62 PSI
Liquid: 195-200 PSI
Superheat range should be about 12-13F based on the conditions that exist today at this very moment
Minimum limit of superheat is 5F according to the trane superheat chart
So I guess my question are:
1. Based on the subcooling values and from what I read and hear through the grapevine I believe the liquid line from condenser to evaporator is full of R22. This is good correct?
2. Why is it that my father has such low superheat? Based on the suction pressure of about 60-62 PSI and measured suction line temp of 39F that he is not close to the ideal superheat range? I calculated about 5F of superheat vs the ideal 12-13F. How does one get the superheat to line up with the factory design specs?
3. Compressor doesn't sweat but the suction line from the test fitting all the way to compressor is sweaty but no ice. I believe this is good b/c I looked at my sisters a/c system today and hers works perfectly and it too has the same appearance.
4. a/c system is only 3 years old but has always performed a bit sup par. Either b/c the system was low on charge (which I believe isn't the case now) or b/c of the fact that the air handler that the coil is mounted to is old and doesn't move enough air fast enough. Its an old Lennox G8 that is 18 years old but DOES have 3 blower speeds. a/c of course is set to maximum.
5. Indoor coil can be dirty but I doubt it b/c the house has always been very clean and relatively dust free and my last inspection last fall showed that it was pretty darn clean. I know dirty coils or not enough air flow can cause higher than normal suction pressures.
This is probably the most information that I can provide to you guys right now.
I want to know what my father's furnace is pushing as far as CFM and such but I don't exactly have access to all the things I need to get that information.
If really need be I'll try to get that data for you guys in a few weeks.
Let me know if there is anything else I can provide you in the meantime.
I'd call an a/c service company but the problem is that there are hardly any truly qualified techs that I would feel comfortable with them working on the system. I think you would all agree that if someone had no idea what the heck superheat and subcool and enthalphy charts were you'd probably feel a bit uneasy too esp when home a/c isn't very common here....
Thanks
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Replies:
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| sbrooks157
| "Re(1):my father's home a/c is under or overch" , posted Thu 3 Jul 18:42
A basic rule of thumb for cfm is 1 cfm per square foot.(Was told this from a hvac enginer cal poly grad Calif.) Also 400-500cfm per ton of cooling,So given the size of your dads place 1800 sq.you want 1800 cfm. You really should have 4 tons of cooling.You can test this by removeing 300-400 sq. ft of conditioned space from the load(close a door,close the damper to the room. But you still need to make sure the supply duct is large enough for the rest of the house,need to find my ductulator, it is around here some where's... you say 1/3 hp motor...Then my best guess would be 3 ton Air handler,so you have a matched system...A cheap fix would be to up size a register or 2 in the hot area's of the house, down size the cooler area's,insulate etc.. that still might not work, Or bite the bullet,lots of options could buy complete new system 4 tons, buy two 2 ton systems....coll only area's being used...
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| theduke03
| "Re(1):my father's home a/c is under or overch" , posted Wed 2 Jul 18:13
I couldn't agree more with your last statement. The information you provided is plentiful and answers many questions. The only other things I would like is an indoor wet bulb temp and temp difference between supply and return air. If the evap. coil is clean then I would say that the unit is slightly overcharged. This is NOT causing the problem of inadequate cooling on hot days. If you release a squirt or two from the high side that will raise the superheat to acceptable levels. Note: a dirty evap will cause low suction pressure and low superheat. I'm not sure about the climate in your area but here in CT a 3 ton system would probably not be sufficient for that size house. This really means nothing since CT and Alberta are worlds apart.
Are there any broken ducts in the attic?
"My dad was the most feared furnace fighter in Northern Indiana."
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| | stevescivic
| "Re(2):my father's home a/c is under or overch" , posted Wed 2 Jul 20:04
okay so today I got a k type thermocouple hooked up to the suction line and the liquid line and it would appear now that the superheat is actually anywhere between 7-12 F Well within the specs according to the trane charging chart.
I checked the liquid line temp and there is only probably about a 10-15F difference between the liquid line and what is registered on the high side gauge.
how does one measure subcooling (is it even important for FFCV/capillary systems)?
The liquid line measured in at 78F on a 21C day and the high side gauge is measuring in at approximately 195 PSI.
There is exactly a 20F drop in temp across the evap coil.
How can I tell if this puppy is overcharged?
Thanks
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| | theduke03
| "Re(3):my father's home a/c is under or overch" , posted Wed 2 Jul 21:07:
Subcooling is the difference between the saturation temp(what is displayed on gauge) and liquid line temp. It is important to measure subcooling when commisioning a txv system and also to troubleshoot any system.
Your saturation temp at 195 psi is 100F. Your liquid line temp was 78F so that equals a 22 degree subcooling(slightly high). This seems like a slightly overcharged unit IF the coils are clean. High subcooling + low superheat suggests it's overcharged. Trane condensers can be deceiving to the eye. Take off a side panel and look closely between each row. If not cleaned recently then it's probably getting dirty.
A 20 degree drop is pretty good.
If:
there's no broken ducts
coils are clean
chage is correct (it's close enough)
I would say it's doing as much cooling as it ever has or will. We have some Canucks that may be more familiar with your area and could say if the system may be undersized for the house. Otherwise answer me this. Will the system satisfy the stat on these hot days at all? It's normal to run all day on hot days but it should still satisfy occasionally and maintain the setpoint on the stat.
Laser/infrared thermometers are not appropriate for these measurements. The type K thermocouple is.
"My dad was the most feared furnace fighter in Northern Indiana."
[this message was edited by theduke03 on Wed 2 Jul 21:39] |
| | stevescivic
| "Re(4):my father's home a/c is under or overch" , posted Wed 2 Jul 22:25
I was just thinking that about 2 years ago my father's friend (whom was also a old school hvac guy that is long retired now) said the system was a tad bit low on freon. Now I don't recall how much hvac knowledge this guy had but my dad said he seemed to do decent work. If I recall he said the tech that installed the system back in 2004/2005 did not compensate for the extra 12-13 or so feet of extra lineset above and beyond what trane had charged their condensers from the factory. How much he put in I cannot recall but I think he said the system was probably out by almost 1 pound of R22. Its been a while since this all occurred so I could be totally wrong on this one.
I was digging around the basement of my father's home and found the original install manual for the condenser and evap coil. It clearly states that the factory R22 charge was meant for a max run of 15ft lineset and anything above and beyond that will need to be filled to accomodate the longer lines. 11ozs to be exact.
Regardless of who worked on the system prior to me looking into it with some gauges kicking around the garage I never truly felt satisfied that the system was charged properly. Only now do I feel that the superheat/subcool are within the proper ranges for this unit. I guess seeing is believing....
The house on a 32C day is more than comfortable with nice dry cool air.
Its when the ambient is 23-26C that the a/c system seems to cool but not properly dehumidify the air to make it completely comfortable.
I guess without having a TXV installed that problem will always plague my father's home.
I know my sister's house has a new a/c system in it with matching air handler and with TXV and even on the mildest days the dehumidification seems to be top notch.
I guess old air handlers and new a/c systems can never really work well together.
Anyone here know what the CFM rating of a lennox g8 3 speed furnace is?
Thanks
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| | sbrooks157
| "Re(5):my father's home a/c is under or overch" , posted Thu 3 Jul 10:46
The HP rating of the motor will give you a pretty good idea of the cfm out put, how ever if the duct is under sized you may not achieve the cfm your unit is capable of.
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| | stevescivic
| "Re(6):my father's home a/c is under or overch" , posted Thu 3 Jul 17:32
its a 1/3HP G8 Lennox G8 on max speed. Ducts should be sized right b/c heat was NEVER an issue out of this beast.
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| | stevescivic
| "rechecked the system pressures and superheat" , posted Thu 3 Jul 21:18
Today was a hot day here (84F - that is hot for us) and the a/c system was running and I figured what the heck... why not check the gauges to see how well the system is performing.
R-22 refrigerant
63.5 PSI low side approx 36F boiling temp
Suction line temp: 43F
220 PSI high side approx 106F saturation temp
liquid line temp: 93F
84 Ambient outside
Indoor 75F
WB 61-67F
Fixed orifice at 0.073 on a 3 ton unit with matching indoor coil
Based on trane charging papers in unit 5F superheat minimum to a max of about 13F.
Calculated superheat of ~7F
Calculated subcool of ~13F
So I guess I'm a bit on the low end for superheat but I'm not sure if the subcool is within range. The strange thing is that there isn't a subcool table on the charging papers that came with the unit.
I'm not using TXV we're using FFCV.
What is considered acceptable subcool for a FFCV 3 ton system that is about 3-4 years old?
If these figures are right then I will consider this matter of the a/c system working perfectly and I won't bother messing with it any further.
Thanks
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| | stevescivic
| "Re(1):SAT and RAT Wb" , posted Fri 4 Jul 18:16
Can you clarify what RA, RAT, SA, SAT, and T split are?
You also put a question mark on the suction line pressure at condenser unit. It is 63-64PSI
Precise superheat requirement based on charging chart shows a minimum of 5F to a max of about 12-13F superheat based on that day that I measured the system.
CFM air flow? I can't possibly tell other than to tell you that its a Lennox G8 installed in the home in 1990 that is running on the highest of the 3 speeds the motor has to offer. 1/3 HP motor so my best guess is 1100 CFM?
Temp drop across coil at time of testing: 21F
WB temps are hard to measure b/c when I measure it at the grill of the return air inlet I basically wrapped a paper towel soaked in water and place it in the relative area of the return vent and got 61F WB temperature.
Temp at unit 84
RA Wb 61~67?
RA Db 75
SA Db ?
T split from chart (need SAT Db)
REFRIGERANT CHARGE TEST(s)
Fixed orifice (Superheat)
P Suction Line at unit 73?
T Saturated line 36
T Suction Line 43
SH ~7
Desired superheat 8.2~17.8 need precise RAT Wb
A higher superheat will take some load off compressor.
TXV (Sub-cooling) check sub cooling anyway even with FOMD
P high side 220
T saturated line 108
T liquid line 93
SC 15
The RH range on your RAT Wb range is between 44.7~66.4 RH.
Humidity Ratio 57.91~86.68 grains water/pound of air.
With this wide range, hard to tune system and calculate air flow using sensible heat equation:
BTU cooling= 1.085 X Delta T (air temperature difference across coil) X CFM
Example: 3 Ton cooling, 1200 CFM Fan: 36000 BTU/(1.085 X 1200 CFM)= 27.6F Temperature different across coil.
Tighten up your RAT Wb and measure SAT Db, post results or recalculate. Take the RAT Wb at the return grill. If you have more than one RA grill calculate the mixed air RAT Wb based on RA duct sizes and lengths to get the mixed RAT Wb in the RA plenum.
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| | stevescivic
| "Re(3):SAT and RAT Wb" , posted Sat 5 Jul 04:06
Thanks for explaining everything to me guys. Aside from measuring pressures it would be crazy to try to screw with the a/c system esp. when I'm not 100% positive on certain measurements.
I'll leave the system be as it does cool sufficiently. Perhaps when more reputable guys can be found then I'll call them in for some annual maintenance.
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