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I have a Carrier 38Tkb030 320 =2.5 ton unit that I'm having…

I have a Carrier...

I have a Carrier 38Tkb030 320 =2.5 ton unit that I'm having trouble getting more that 10 degrees of cooling between the return and the vent. I've adjusted the charge to the spec in the manual for a piston metering device (I checked at the evaporator - there is no txv. but I see the piston device and capillaries. Ive also adjusted the SH from the spec (22 degrees based on current conditions) to 1 degree in increments - letting the system stabilize each time. No different results. Good airflow inside, clean condenser coils outside. I'm stumped.

Contractor's Assistant: Did you double-check the thermostat? And what about the unit's filters?

Units filters are new. Thermostat is set at 74, but it can't get the room below below 77 all day.

Contractor's Assistant: Do you plan on doing the work yourself?

Yes

Contractor's Assistant: Anything else we should know to help you best?

Unit is on second floor. of 3 story house. other units are working fine.

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Customer reply replied 7 months ago
Outside temperature here is 90 degrees
Answered in 35 minutes by:
7/15/2017
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,199
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello. tell me the suction pressure and the head pressures, and the *exact inside and outside air temperatures when those pressures were taken.

Tell me if the suction line is sweating at that outside unit or not/

If the compressor inside the condensing has any part of it sweating, then the system is over charged with refrigerant and that is the problem.

We can go from there.

.

I am a retired mechanical engineer and HVAC/ electrical/ refrigeration and boiler contractor ... with 56 years in this business. The last 20 of those years consulting for major firms nationally..

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All I ask is please select the ***RATING LINK***at the top of your page, that is the only way I am credited through the site..

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Rating does not affect your charges nor ability to refund if not satisfied..If you need to be coached through a full diagnosis/repair, and wiring etc. for up to two days, and wish to be trained to use test instruments...... that goes well beyond' Just a simple Answer' of course.. ...

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If you rate my simple answer so far positively..and I will deliver that training and coaching to you All on the same dime so to speak..

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If you can agree to that, rate now, lets continue by all means.

Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,199
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified
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Customer reply replied 7 months ago
Thanks Phil, the outside temperature is 90 on the dot. suction pressure=77.6 Vsat=46.0 SLT=62.3 LLP=344psi lsat=141.6 LLT=128.2 Inside temp=77 temp of air coming out of vent 67deg
Customer reply replied 7 months ago
the suction line is slightly sweaty coming into the condenser unit (past the insulation and for the first few inches inside the unit. No sweat by the time it reaches the compressor and none on the compressor.

Thanks.

after 50+ years in the business, I would have to guess at what all those abbreviations mean... you can skip most of them such as Vsat (saturated vapor pressure I presume).. those are hair splits that do nothing but add confusion to the core issues.

So far it sounds like you have an Refrigerant 22 system, IF that is the case... that is about 20% over charged with refrigerant...OR the condenser coil needs to be cleaned.

Let me know what refrigerant you are using, we can go from there

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Customer reply replied 7 months ago
It is R-22. SLT was suction line temp. LLP was liquid line pressure and LLT was liquid line temperature. Sorry about that. The condenser coil "looks clean - although I haven't washed it down this year. By the way, I tried removing refrigerant - the suction line temp went up to about 70 )pressure change, but I didn't record it. Inside the air temp didn't get any better (or much worse for that matter). I added back refrigerant to bring the suction line temp back down to the low 60's. Still not any measurable difference inside. The manual said at the current temperatures, the superheat should be about 20-22. That;s where I started from before making the adjustments adding, then removing refrigerant. I took the superheat value up to 30- no effect. Now its at about 16 with still not much difference inside. I had removed enough at one point where the superheat on the suction line was a 1.0 still not much difference. I am using a Fieldpiece sMAN digital gauge set.
Customer reply replied 7 months ago
I did let the system stabilize when removing and adding refrigerant. and only added, removed in small incremental amounts along the way. Added from stock R22 canister. Removed into recovery cylinder.

The electronic service gauge mfgrs like to sell those things, for $900 or more.. and that has confused the living daylights out of novice techs... those super heat and sub cooling reads are grossly skewed... the gauge programming assumes a clean condenser and 450 cfm per ton across the cooling coil... those conditions *never exist perfectly... it also assumes a cooling coil of one type or the other but not your specific coil.

That gauge software barely differentiates variable speed blowers from fixed speed, and does not consider zoned system configurations at all. and I am being polite about it.

If I had a set of those gauges I would use them to impress customers but only go by the head and suction pressure readings. and the sweating of the suction line. The head and suction pressure readings are absolutely critical.

With the house at 72F suction pressure should be 60 to 61 psig , no higher regardless of what the manual says.

head pressure is determined as follows for a SEER 15 system, add 25F to the outside air temperature, then look across from that for the head pressure, it should not be any higher.... your 300+ psig is way high.

If the suction pressure falls below 59 psig with the house at 72F inside then the piston/ orifice is too small.

(the manual instructions are designed to get the highest test efficiency NOT the most effective dehumidification and cooling, that is necessary so the mfgr can advertise such efficiency)

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Customer reply replied 7 months ago
Phil, I think that is the best advice I've ever heard. Thank you for cutting to the chase on instruments and theoretical manuals. I am very much the novice, but determined to learn, none the less. My background is chemical engineering - so I get the theory, but practical experience is closer to the zero mark:-).
The inside temp is right at 77. Does that change the suction pressure to a slightly higher target? Also the system is an older 11 seer unit.

Suction pressure should be in the 63 to 64 psig range if the inside of the house is at 77F... however the thermostats are always off, often by 2 deg F too high or too low, so that can be misleading.

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Customer reply replied 7 months ago
Thanks Phil. I'll go recover to get the pressures in line. My Thermostat and Klein meter are both right at 76-77 inside , so hopefully I'm close there. Is there a chart for pressure values that I should be looking at going forward?

click here please

the head pressure runs a pressure equivalent of 25F hotter than the outside air on a SEER 15 unit... 30 degrees hotter on an older SEER 10 unit.

Suction pressure needs to be 34 to 36F when the house is at 72F, or 59 ot 60 psig for R22, and 101 psig for R410a

it will pay to practice with that chart until you are very good at it.

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Customer reply replied 7 months ago
Here is the latest. After cleaning the coils and the outside temp down to 85, I have the suction line pressure down and stable at about 62. The head pressure is at 228 though versus 115 range.

That sounds pretty good, watch it for a few days, get back to me as needed.

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Customer reply replied 7 months ago
Thank you Phil, So I shouldn't worry about the head pressure being so high?

Head pressure is only about 18 psig high, if the suction tube is longer than 25 feet, remove refrigerant until the suction pressure is 59psig at the unit (it wll be 2 lbs higher at the cooling coil due to pressure drop in the suction line)... at 85F outdoor air head pressure should be 210psig on a perfectly clean coil... its likely not perfectly clean however.

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Customer reply replied 7 months ago
The suction tube is probably in the 25 ft range, so sounds like I'm in the ball park then. Can't thank you enough for the advice Phil - and thanks for sticking with me through the changes. Have a great rest of your evening.

You are welcome.

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Customer reply replied 7 months ago
Phíl,
Temps in Richmond Va are approaching 100 and the AC is still working great! Thank you again for the guidance and for teaching me a few things along the way. I've told at least 20 people about justanswer and particularly your followup and patience with getting me through the issues.
Best regards,
Greg
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Phil
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Category: HVAC
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