Fluctuating Head Pressure

[freshair]

I recently installed a CARRIER 50JS042 heat pump package unit for a Dairy Queen store. I returned a service call from them saying they weren't satisfied with the current temperature. When I got to the site, supply air was 55F and return was about 76. It still is getting a good t-split of 21F. When I checked the unit, I was getting a fluctuating head pressure of 210 -> 275 PSIG. Suction fluctuating pressure was 70 - 74 PSIG. Fluctuation is immediately following start up. Superheat was low about 7F. Subcooling was high like 35F. My first impression was maybe it was overcharged so I recovered about 1 and 1/2 pounds. It brought the head pressure down to a fluctuation of 195 -> 225. Suction pressure to 69 -> 72. Superheat was the same. I left and followed up the next morning on the situation, and the manager told me that the unit was on for over an hour and half but fixture temperature was only 75F, set temperature was 72F. I'm thinking maybe early restriction...

I'm fairly new in the field. A year experience, after 2 years of technical schooling and a cert. Can someone please advise me on a solution to this?

[R12rules]

Welcome Air.
This is a wonderful place to hang out.

Many of us in fact spend our entire social exhistance right here on HVAC-TALK!

Most of us share stories and jokes and news with our wives about people we know right here...


(isnt that sick?):)


Anyway ... about your question..... when you measured those temps and pressures ... you need to measure them AGAIN when you return and give us a little more data.
Like what is the ambient and space temps when you took all these other temps and pressure readings.
And where exactly did you take these reading at?
Where did you place your probes?


Before I would remove a pound and a half of refrigerant from a factory sealed system, which is still under warranty .... I would take some readings ... document my discovery and then ask for advice right here.

Then I would proceed as directed.

I would first trust people here before I would trust a co-worker in the field.
Not unless I knew the co-worker to be top notch. There is a lot of politics in the field. Getting a straight answer requires knowing where a person is coming from before they give you an answer.


Here's a sugestion .... go pull the charge, change the drier and evacuate. Weigh in the correct namelplate charge and see what happens.
Document all discovery you obtain.
I carry a PDA. I sugest you do the same. ebay is great for deals on such.

Do a look up on which PDA to buy, right here.

Also, go visit the FYI forum and read up on charging systems by superheat and subcooling.

A lot depends on your metering device, as you well know.

[moe-air]

Sounds like air in the system/non-condensibles. Can't be to sure though without more info. Evac. would be good but check with your warranty before you open up the system.

[cool frog]

if you had a restriction your superheat would be high.
also,when troubleshooting never take anything for granted.
you might have to look at the duct system for restrictions.

youcould have a pefectly good unit out of the box but a bad install job cause alot of problems......good luck,get back to us with more info

[i b cool]

If it has tx valves make sure bulb is attached and insulated properly.Ive seen new package units come through with rubbing lines, loose tx valve bulbs,low on refrigerant ect...

[airworx]

overcharged. residential systems as far is i know will never be over 20 degrees subcooling and thats on high seer equipment with a txv. if you have a orfice then superheat should be check and adjusted according to the charging chart.

[rubobornot]

I had a problem like this once with a carrier package heat pump and it turned out to be the check valve on the out door coil. The check valve wasn't flowing freely causing the head pressure to fluctuate. The Check valve was built into the coil and would be hard to replace,This customer replace the whole outdoor air coil.

[chiller mekanik]

Wish you had left the charge alone. Its easy to correct though. No one here [myself included] is trying to blast you. Usually the factory charge is right. If the m# is correct that sounds like a 3-1/2 ton. Is it a small D.Q. or do they have more than 1 unit? Is it reg. ductwork or is it concentric? Is a curb adapter involved? My first impression is under ton but, the symptons you gave resemble non-condensibles. It really wouldn't take alot of time to pull the charge, change the drier, pull a good vacuum [atleast 500 microns] & weigh in the charge. this step would rule out several things. Keep us informed & I'm sure evryone here will be willing to help,[you have the right attitude] good luck!

[freshair]

Thank you everyone for your replies.

05/03/05 I'm going to pull the charge, change the filter, put it into a vacuum (500 microns) and weigh-in the charge. That was my first thought of action. here are my original notes:

Ambient Temp: 76F (taken 1/2" from c-coil)
Fixture Temp: 75F (taken inside store by t-stat)
Head: 210 -> 275 PSIG (105F-124F)
Suction Pressure: 72->75 PSIG (42-44F)

LL Temp: 70 F (Taken about 2" from bi-flow filter drier)

SL Temp: 49 F (Taken approx 12" from the compressor)

Supply Air - 56F

TRC - 30+
TSE - 34 F
SC - 35 F
SH - 6-7F

The unit is side discharge with exposed concentric ducts. The runs are pretty short. Unit was installed in November. Warranty issues with my CARRIER dealer (USACD) usually means we provide the labor anyways.

The DQ actually has two units. One CARRIER 50ZH0485 208/230V 3PH (CORRECTED MODEL#) package unit 4 TON for the front and a LENNOX 4 or 5 TON for the back section.

The day I was there, they had a fundraiser, like 30 people and the door wide open, that's why it was hard for me work with. I have followed up, and the manager says it doesn't get cooler than 75F after a couple hours of cooling. I'm going to try to get the system running at the correct capacity first, than see if there is a load issue.

Again thank you for your replies, I will document my corrections and post when done. Regards.

[Edited by freshair on 05-03-2005 at 11:37 AM]

[key]

how about checking the reversing valve for an internal leak

[hungry]

I'd do what i b cool said first before you do anything else and see if the bulb is where is's supposed to be.

[freshair]

I'll check out the txv bulb placement when I'm there, thanks.

Reversing Valve:

How do you check for an internal leak? I know during fluctuation in cooling mode sounds like the reversing valve makes a pretty nasty noise.

One of the first things I did was I applied 24V to the RVS to make sure it was working (without compressor on) and it did. Then I turned the unit on heat mode. The fluctuation still existed, noise was still there, just not as loud as during cooling mode.

So I'm dont think its a RVS problem because in both modes fluctuations exist and noise is prevalent. Still how do you check for an internal leak?

[jim reynolds]

Do you have and ambient temperature contol on the condensor to cycle fan on and off. Maybe your fluctuations are from this control.

[ac/dc]

Welcme Freshair, nice to meet you. R12 1s right about this
site.
You said the pres. fluctuates immediatly after start,does
it ever stabelize more after running a while? I agree the
factory charge is rarely off and I cant imagine noncondensables but stranger things have happened. I agree
the subcooling is high. Where was your high side hose
connected when measuring subcool.?
If problem still exists after weighing in charge... I would suspect checkvalve, reversing valve, or metering
device. Are you sure it has a txv?
I check the Rev. valve by checking temps of hot gas in & out of valve and suct. gas in and out. They wont be the
same but they should be close. within 5-10 deg. I think.
The supply air temp and delta T is normal. It should cool
the space unless the load is to much. Doors open and close
alot? Is this a digital t-stat?
What you have is not a concentric duct system. Where does
the return pull from?

[freshair]

Today, I pulled the charged, triple-vac, and wieghed in the design charge. I did not change the filter drier because I thought it was most likely overcharged according to the original numbers and probably noncondensables. If it was a restriction I would have considered changing out the filter drier but that was not likely according to the unit's behavior. *I do see the point in changing the drier* There were two on this unit. I dont think I saw a TXV.

Anyways, here's the results:

CARRIER 50ZH0485 packaged heat pump 230/3PH

Design System Charge: 8.6 #
ACTUAL RECOVERED: 20 # (I didn't know a unit could work with over 200% charge!)

Ambient Temp - 66F
Fixture Temp - 69F
Suction Pres/Temp - 50#/26F
Head Pres/Temp - 185#/96F
Suction Line Temp - 44F
Liquid Line Temp - 80F
Supply Temp - 48F

TRC - 30F
TSE - 43F
SC - 16F
SH - 18F
T-Split - 21F

I started like 3PM finished about 6PM Pacific so ambient was a little lower. I think the unit is rocking now. I'm going to follow up during the week to see if it meets set temperature for DQ, but a 10F - TD in supply temp, I think it will be alright. Got a free rootbeer float for this job. Cool deal.

I'm going to write CARRIER about this. I did like the panel design for this unit, changing out the driers, e-coil, or compressor looked a bit easier if needed.

Its was nice to hear the unit running smoothly, it was screaming before!

I like the PDA idea, I'll probably get one soon. I think it sounds very useful, especially for maintening a personal technical log book.

Appreciate the replies, they were useful.





[Edited by freshair on 05-04-2005 at 01:03 AM]

[freshair]

Originally posted by ac/dc

I check the Rev. valve by checking temps of hot gas in & out of valve and suct. gas in and out. They wont be the
same but they should be close. within 5-10 deg. I think.
The supply air temp and delta T is normal. It should cool
the space unless the load is to much. Doors open and close
alot? Is this a digital t-stat?
What you have is not a concentric duct system. Where does
the return pull from?

Thanks AC for responding to this question. I will def. keep this in mind now.

Just the first time I was there, they had a fundraiser and there was people everywhere with the door constantly open. I dont think they keep it open all the time though.

When I first got there, they had a mechanical t-stat. The temperature indicator was at 80F. When I checked the fix. temp with my thermometer it was 75. So I changed it to a prog. dig. t-stat for them. Just so it could be monitered easier.

What is concentric. I look it up on the dictionary and it said having a common center. So i figured circle. anyways I dont know the technical name, but from the plenum the short run ducts were round metal ducts with ninties into the roof/boot and 2x2 supply diffusers. The return pulled storefront where the supplies were too.

Thanks again for the great tip on the reversing valve, I know it will be useful.

[ac/dc]

WOW I cant believe it was that much over charged from the
factory...but the numbers dont lie,(35 deg subcooling and
7 deg superheat) Good Call.
A concentric duct sys is return and supply all in one
Return in center with supply's around the return. You've
probably seen them at dept stores, Home dopot and Lowes
have them. Like a concentric vent for a high effeciency
furnace will have the exhaust in the center with the intake around its outer edge.