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3.4K views 19 replies 10 participants last post by  Rusty199  
B
#1 ·
Hello group,
May I preface my post in that I am a homeowner and have a question. I use trained (I hope) people, that is, a couple local hvac businesses for any repair/maintenance, as I lack the knowledge and tools to diy. That being said, being a mechanical engineer with a basic knowledge of refrigeration cycles, besides the fact that I just like to UNDERSTAND things, I have a question I wish you to opine upon regarding my own unit so that I will at least have a clue about what said pro is advising. OK....

*American Standard 3 Ton packaged heat pump ~ 7 yrs. old
*Spring "Tune-Up" every 2 years, never needed freon or to even clean the coils, spotless inside.
*Late last summer, cooling seemed to fall off a bit
*Called hvac co..... tech said 'charge just a bit low' and added freon
*I noticed he had the bottle 'upside down' while he added freon and questioned that???? He said that would give it an extra shot of oil??
*I let it slide as the unit seemed back to normal again... then a couple weeks later the evap coil started freezing up pretty regularly as the weather got way more humid.
*Got DIFFERENT tech to come out and he said it was significantly overcharged by last tech... and he recovered freon until charge was back to normal.... but advised that upside down bottle was NOT GOOD as that fills w/ liquid vs. gas.. said watch it and let him know... unit functioned flawless rest of summer and all winter in heat mode.
*NOW... if you're still here, I bet i'm going to get a knowledgeable response:)

Problem is....
*When selecting cooling mode, unit starts and runs normally... both fans on, compressor on....
*on a moderate evening last nite (~80 deg): normal pressures according to curves supplied with literature..
Suction = 75 psi
Discharge = 220 psi
Outlet temp at nearest vent = ~56-58 deg (is that good for the weather?? I've never measured it before)

*THEN.... the longer the unit runs, the warmer the outlet temp becomes... I mean, It takes like 4-5 hours, and then the outlet temp is ambient. and it does it very gradually.... pretty much linearly with time...

*I've repeated this process over 2-3 days, letting the unit stay off at nite (with the windows open), and restarting the next day, with the SAME results... cools initially.... falls off gradually.... ambient in a few hours...

*Unit runs continuously, no overheat/overcurrent/diff_press trips... NO cycling. Starts easy, no odd noises, runs smooth

*OH. also, at first, when the unit seems to be cooling fairly, the condensor coils are warm, and the drain plumbing is dripping steadily, and the air coming out the top is quite warm (obviously if the coils are warm). As the cooling diminishes, the coils/exhaust air cools as well, and the condensation drip quits.

I called my tech, and he said maybe the resistance strips are energizing. that doesn't make sense to me because of the gradual nature... and the symptoms above.... all the sequencing of the unit seems like it's working properly. I can summon heat... aux heat if i crank up the thermostat, emergency heat... the circ fan delay circuit is working... the heat mode defrost cycle always has worked,.... anyway, dont think my aux heat is energizing while in cooling mode..... think the sequencing board is fine??

I'm wondering if MAYBE???? due to the liquid freon addition the drier/filter has become either saturated?? or clogged?? and kind of works fair, at best, for a while after the unit has sat, then clogs back up.... or also possibly, that the restriction? has reduced the coolant flow such that the unit will work at reduced efficiency for awhile until conditions finally overwhelm it??? IE... the heat balance is compromised in the HEAT direction with the unit in cool???

ANYONE??????

I guess you would REALLY need those pressures when the unit is NOT cooling anymore to help firmly diagnose..... and I don't have gauges... got those readings above from the tech last nite, who said the pressures are normal, run it tonite and let me know what happens.... and he obviously can't hang around for hours waiting for further readings....

Also, I would request info on what to look for with regard to frost on the uninsulated lines... my tech said the filter/drier line would be frozen if it was plugged up, but that doesn't make sense to me??? as it's on the 'hot' (condensor) side of the loop ???

If you've read this so far, I appreciate it and your advice. I respect your trade, and your discrection should you so choose... I'd just like to have a clue when my unit gets ...again.... diagnosed, so I don't get caught blindly replacing perfectly good parts.... AND... I want to understand what happened to it...

Thanks and Regards,

Brian MacKinnon
Guntersville, AL
 
T
#3 ·
Nothing wrong with adding liquid if it is done properly. No oil is in a freon tank, so if you were told that it was BS. Liquid needs to be added slowly so that it will vaporize as it hits the suction line, otherwise it will hit the compressor and possibly damage it.

As for your problem, hard to remote diagnose. Check your air filter. If it is clear, the inside coil could be clogged and in need of cleaning. Possibly your charge is a bit low, which could cause the inside coil to slowly ice up. If you see any frost on the inside piping, you have a problem.

Your descriptions have some conflicting data. You really need to have someone qualified look at it. If you don't trust who was there before, try another company.

paul
 
A
#4 ·
bmackinnon said:
*I noticed he had the bottle 'upside down' while he added freon and questioned that???? He said that would give it an extra shot of oil?THEN the longer the unit runs, the warmer the outlet temp becomes, I mean, It takes like 4-5 hours, and then the outlet temp is ambient. and it does it very gradually.... pretty much linearly with time.
*Unit runs continuously, no overheat/overcurrent/diff_press trips... NO cycling. Starts easy, no odd noises, runs smooth
*OH. also, at first, when the unit seems to be cooling fairly, the condensor coils are warm, and the drain plumbing is dripping steadily, and the air coming out the top is quite warm (obviously if the coils are warm). As the cooling diminishes, the coils/exhaust air cools as well, and the condensation drip quits.I called my tech, and he said maybe the resistance strips are energizing.think the sequencing board is fine? im wondering if MAYBE???? due to the liquid freon addition the drier/filter has become either saturated?? or clogged?? and kind of works fair, at best, for a while after the unit has sat, then clogs back up.... or also possibly, that the restriction? has reduced the coolant flow such that the unit will work at reduced efficiency for awhile until conditions finally overwhelm it?Also, I would request info on what to look for with regard to frost on the uninsulated lines. my tech said the filter/drier line would be frozen if it was plugged up, but that doesn't make sense to me??? as it's on the 'hot' (condensor) side of the loop ? I want to understand what happened to it Brian MacKinnon Guntersville, AL
Click to expand...
well brian from guntersville, the liquid freon if metered in properly is normal practice, but you get no extra oil by doing that!

how do you know if the compressor is overheating or not, or if the pressure switches if equipped are not tripping?

if the problem was the resistence heat coming on while cooling you would still have condensate dripping!

your info request about frosting lines is simply amswered, look for frost anywhere on the copper lineset!

if the liquid line filter/drier was resticted, under most circumstances (unless totally plugged) you will see frosting at the drier outlet!

without being there, i am thinking either the evap coil is freezing, or the compressor is overheating or maybe 1 or 2 other things!

to make a internet diagnostic, readings (presures,temps,amps,any frosting issues) need to be taken during the fault situation!



.
 
R
#5 ·
Also, I would request info on what to look for with regard to frost on the uninsulated lines... my tech said the filter/drier line would be frozen if it was plugged up, but that doesn't make sense to me??? as it's on the 'hot' (condensor) side of the loop ???
Click to expand...
Think back to your basic understanding of the refrigeration cycle.

The liquid line carries high pressure high temperature refrigerant to the metering device. The metering device is basically a restriction so the refrigerant on the other side ends up at a lower presssure and temperature. Restrict the refrigerant flow too much and the pressure and temperature drop gets below freezeing.


My best advice is to have the system somewhere towards the middle-end of it's decline when the tech gets there.
 
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S
#6 ·
If airflow does not diminish along with the performance I would not immediately suspect icing unless it's an undercharged situation and the coil is only partially frosting over (I've seen that happen).

With fixed orifice metering devices, I've seen systems that will cool in the morning but lay over and flop during the afternoon heat, without pressures looking all that different. Without obtaining superheat and subcooling readings, a tech won't really know the story with just pressures alone. In one instance I found the piston was not seating fully, which during the hot part of the day would allow the evaporator coil to be flooded with refrigerant (head pressures run higher with hotter outdoor ambients). At night and in the morning, it cooled fine due to reduced head pressure and the evap would not be overfed. I took apart the device and cleaned it, put it all back together and everything ran great day and night from that point forward.
 
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B
#7 ·
Wow! Thank you all for the excellent info.
The airflow does not dimish along with performance, so I don't think the evap coils are freezing (and I've had that happen in the past and have been able to recognize it was happening). I now understand why the drier outlet would freeze if it was restricted as it then effectively becomes an orifice resulting in press/temp drop across it. DOH! Thanks for the info on the liquid refrigerant.. that makes me feel better, cause it ran fine all winter.

Couple more basic questions?:
Another tech is (hopefully) coming today. I guess I can't verify the compressor isn't overheating, but it hasn't cycled off or gone out on overload (to my knowledge). I'll make sure the guy 'amps' the compressor. Is it correct to assume normal amps would indicate a non-overheating compressor?? (at least in most cases?)

The tech indicated over the phone that he had recently encountered a unit where the 'second-stage cooling' wasn't working and it was tough to figure out. I read my manual and it talks about the 2nd stage cooling initiating after approx. 1 deg temp rise. Could someone briefly tell me basically what this means. It looks like (to me, DOH) that nothing really much happens until that 2nd stage initiates. Before that the thermostat switch closes, energizing the switchover valve sol. coil and "nothing else happens". (it says exactly that in the 'sequence of operation' section of the manual.) Then it talks about the 2nd stage sequence (switches, contactors, relays, motor starters, etc) and everything lighting off. So does any cooling actually occur in what I'll call the 1st stage cooling, or is it just getting ready (anticipating) a run? (or is that part just me moving the thermostat switch from 'off' to 'cool'?). If so, it would seem not too difficult to figure out it wasnt working... ie. no fans, no compressor, no 2nd stage??

Lastly.... :), could someone just give me a brief, basic idea what your talking about when you evaluate/measure superheating and subcooling. I know the basic definition of the terms, but not how it relates to your diagnosis along with the pressures/amps/volts/frosting etc., and how can you measure such values?? calculations using pressures, temps, rel. humidity, etc??

PS - the 'refrigerant control type' is listed 'TXV-NB' - does that stand for thermal expansion valve??? it looks like there are 2 of them, does that affect any of your advice?? Do they give trouble??

I greatly appreciate your generous responses. Again thanks!!

Brian MacKinnon
Guntersville, AL (and man it's HOT and HUMID today here in 'bama!!)
 
S
#8 ·
Two stage...somehow I missed that in the first read-through. By your latest info it sounds like when you turn system on after leaving windows open all night, second stage cooling is energized due to deadband between first and second stage being exceeded (space temperature is warmer than one degree spread between first and second stage). After the space cools off, second stage deenergizes, but first stage is either not energizing, or since you mention there are two TXV's there may be two circuits in the evaporator coil, one of which is either not feeding properly or has a liquid line solenoid valve not opening.

Here's something to try until your technician arrives. Read the existing temperature at the thermostat in the room, then set the thermostat (in cooling) one degree below that. Next, go outside and see if anything outside is running, and if it seems to be dumping heat. Come back inside and then lower the temperature setting on the thermostat two more degrees. Go back outside and see, if nothing was running before, or the heat coming out of it was weak, it is now running or putting out a lot more heat.

If so, you may have a problem with the staging sequence. Since the system has been installed for seven years I would not initially suspect a miswiring of staging, as often occurs with new installations. I would rather, as your technician, focus on observing the staging process and if it was not staging correctly, find out why.

Let us know what the technician finds.
 
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S
#9 ·
Rereading your latest post again, you may be reading in your instruction manual regarding how the reversing valve is energized first, then the compressor. If your compressor is not a two speed compressor, or if it does not have any other form of capacity control, then please ignore my last reply.
 
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B
#10 ·
hello shophound,

I believe your last post to be correct. The manual describes the cooling mode as : (summarizing)

1) with the disconnect 'ON', current is applied to sump heater and control transformer.

2) The thermostat temp switch TSC-1 closes, completing 24V from Term. 'R' to Term. 'O', energizing the switchover valve solenoid coil. "(Nothing else occurs)"
--I assume this means when you move the switch from off to cool, the reversing (switchover?) valve is energized to the cooling mode position???--- and nothing else happens unless the temp differential exceeds the 'turn on' point at the thermostat

3) "Second Stage Cooling (after 0.7 - 1.5 deg temp rise):" thermostat temp sw. TSC-2 closes completing 24V from Term 'R' to Term 'Y' energizing compressor contactor (MS), and to Term 'G' to energize fan relay (FDR).
MS-1 and MS-2 contacts close simultaneously enegizing compressor and OD fan motor. FDR-1 closes and energizes ID fan motor. When TSC-2 closes, the cooling anticipator is bypassed.

Sound like what you said??? ie. reversing valve energized by switch moving to 'cool', everything lights when temp exceeds thermostat setpoint?

I can't find anything to make me believe the compressor is 2-speed, or anything else fancy about the system. It's just a (what should have been economy... arggg!!) basic, non-high-end American Standard WCH series 3-Ton over/under packaged heat pump.

Thanks so much for the help... the tech should be here soon.... any last tips for things to look for while he's here???

BTW, it looks like there's one TXV (with bulb) on each coil (OD and ID). I would assume one is for heating direction, one for cooling direction?? or am I stupid (don't answer that)....

And the low level dissertation on subcooling/superheating???? :eek:

Thanks
Brian
 
S
#11 ·
Resolved: single stage heat pump with standard control wiring and strategy.

Established: airflow does not diminish alongside cooling performance (as best as homeowner can determine without using an airflow measuring device). Indoor and outdoor coils have TXV metering devices.

Well...just said all that for clarification for myself and anyone reading this thread. As for what to do, as someone else mentioned it would be ideal if the technician is present when the system does not perform well. That being said, even when it is up and running he can measure various points on the system to see if there are potential trouble spots.

Superheat: amount of sensible heat vapor refrigerant gains beyond the change of state from a subcooled liquid to a saturated vapor in the evaporator. Superheating begins late in the evaporator circuit and continues up to the compressor.

Subcooling: amount of sensible heat rejected from refrigerant beyond the change of state from a superheated vapor to a saturated liquid in the condenser. Subcooling begins late in the condenser coil circuit and continues up to the metering device at the evaporator.
 
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M
#13 ·
Brian I hate to hijack your thread, but I'd like to ask you a question if it is O.K. What do mechanical engineers do? What does your job entail? You said in an earlier post that you have a basic understanding of the refrigeration cycle, I always thought that ME's were the pinnacle of the HVAC field. Is it that ME's focus more on theory and system design than on the nuts and bolts?

Again, I apologize for jacking your thread.


-JB
 
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R
#15 ·
I have always liquid charged. I've heard it over & over to not do it, but: 1) I don't have that kinda time, and 2) I'm a stubborn ass.:p

I have even opened the tank full throttle just to see if I can get it to slug, but it just never happens. No accumulator, just right to the compressor. It flashes by the time it gets through the service valve & into the sump.

I guess it could happen, but..........
 
P
#16 ·
My nephew has an ME degree, but when I started to talk air conditioning with him he said "400 sqft per ton" and I decided to change the subject<g>. Somehow he graduated without learning the modern basics.

Best wishes -- Pstu
 
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J
#18 ·
This discussion is exactly why I don't talk HP/HT, LP/LT, blah, blah, blah.

Superheated, Subcooled, Saturated.

Really helps the novice techs (and not so novice) learn to troubleshoot the refrigerant circuit.
 
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R
#20 ·
Um, not bragging, just stating a point. And the point is, since it eluded you, is that it is actually very difficult to slug a compressor by liquid charging.

If you actually vapor charge a system that needs say 2-3 lbs of refrigerant, how long does that take you, and how many times do you have to wait for the tank to warm up so it will boil off & build up enough pressure?

Oh, BTW. Don't think you can tell me what I should "brag about". K?:)
 
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