Lennox low suction, freeze alarm
Lennox LGC180H4BL3G, 15 ton, high efficiency, 3 stage, 410A, 80*f outdoors, 73*f return, 8 VAV boxes @ about 40% open (according to control system, ceiling is too high to look at each one without major hassle) By-pass at 36%, Return pressure -.22, supply pressure 1.55 inch water, measured at the unit. Focused on circuit #1, Superheat 15* to 20*, little change if return door unit is opened. Superheat drops to nothing if supply unit door is opened. so TXV seems to work. Suction starts at 120# and drops to 75# after 20 minutes, head 325#. Distributer tubes all feeding. I checked a couple of other units on the roof of this Target and they run almost as low but not quite, and they had no VAV boxes. None seem to run like the little charging tag says.
All three circuits had one time gone off on freeze making me think it was duct work or coil. Circuits 2 & 3 run a little higher, but if #1 alarms the next kicks in. I'm not sure of the sequence of events. We had someone else the first time replace the belt and pulley. The next guy replaced the filters. I cleaned the evaporator and can see light through it. Because superheat was 15*+ I started thinking it was not air flow. I replaced drier and pulled vacuum and weighed in circuit #1 charge. Same result. I increased the blower speed, adjusting the pulley; same result. I moved and insulated the TXV bulb and brought superheat down to 10*; that brought suction up to 90# out of desperation. I don't want to flood it. What next? Are my static pressures way off? I am not experienced at that.
Try recording your suction psi with all unit panels in place, then partially open a supply panel (downstream of the evap. coil) and watch your suction psi . If it goes up, you may have to open your bypass more,or dance around in the ceiling verifying that all the vav's are right... (loose damper to actuator screws, linkages loose,etc.)
Also if your nights are getting cold, you might just orta check your fresh air damper. And did you figure maybe even an economizer that is stuck open? That'll sure do it on a cold night.
I'd leave the superheat alone both as a symptom and as a "diagnostic tool".
Also, your BMS is probably only giving you the command percentage, and not the actual status. That means you gotta get out the Genie lift or... and God forbid...the A frame ladder.
Best to you. The horse is prepared unto battle...but safety is of the LORD.
"Iron sharpeneth iron..."
Thank you very much. Alas, I now need to go there late at night with ladders. It starts on the sales floor and runs through multiple small offices. I was being lazy. I thought bad air flow would cause my superheat to drop, but I am used to refrigerated cases with touchy superheat, not AC zone systems. It may have been wishful thinking on my part.
I don't think looking for common small issues is really being lazy, Mr. McCann. If this is a basic PM Contract you were probably trying to save the customer some T&M billing. Also, I think you are trying to save some credibility among your fellow techs who appear to have tried to address this issue in the past.
It is not unusual at all to find VAV damper issues among other things even after recent installations. Sometimes it may even be the T&B contractor or even the site guys " learning on the job".
You offered too much system information to be "lazy" in my view.
Best to you.
"Iron sharpeneth iron..."
If you would like to "economize" your time on the job I would start in those offices. Customers on the sales floor don't usually complain about the HVAC system but the day to day office people would.
You might find that some dampers have been messed with to satisfy a cold/hot natured office worker. Even to the point of a fellow office worker messing with stuff to be somebody's "Hero".
Just a thought.
"Iron sharpeneth iron..."
Does the bypass damper bypass air directly from the supply duct to the return duct or does it dump into a plenum? Open bypass dampers can cause low return air temps which can cause evaps to frost.
Do the freeze alarms lockout the unit requiring manual reset ?
The best way to test a refer circuit on zoned system is to close the bypass and open the vav boxes to 100%.
What type of control system is it.
IV IV IX
use your head for something other than a hat rack.......Gerry
The by-pass VAV box dumps onto the sales floor in this Super Target store. So that's good. The return air does not get too cold. The other 8 of the VAVs go to various offices and areas. The controllers (now that I've been up there) are Automated Logic ZN341v+. All controllers are functioning.
I discovered I can call monitoring and they can tell me the CFM set point and actual. That's pretty neat I thought. Monitoring told me the CFM of all the VAVs. All are close to set-point. The CFMs added up to 4,180.
On the web, for this unit, it shows cooling ARI CFM at 5,700. Min CFM 4,200. Max CFM 7,200. Question: What is ARI CFM? Am I short 1,520 CFM, and should have them adjust their program or is the minimum CFM adequate? Target does not make changes to programs without proof of the need.
ARI (AHRI) is an HVACR product testing and performance verification entity. They test equipment according to their standards and publish them.
For instance, they tested Dave "Attaboy" Lennox's machine that he made in his garage and found out he wasn't bluffing when he said it would do 180K btuh on a hot day. Even though he appears to drive cab during the day and fit pipe at night.
ARI tested it and set the blower at 5,700 cfm to do it... and they aren't related to Dave and really don't know him from Adam. Dave's customers like that arrangement because they know Dave puts his britches on one leg at a time just like they do. Just like we all do.As the Book says "let God be true, but every man a liar". And the guy that plays Dave on Tv is kind of a "fake Shemp" out of St. Louie. Tibbits, I think is his name. So things get to looking tricky sometime. Oh well, everyone has to earn a living somehows.
And I think Dave is a fine fitter and a real American. Me and ARI.
So when Willie from Syracuse-Ah says his machine can do thus and so, Dave can say "so does mine" not because my Mom says , but because ARI says so.
Oh yeah- those min/max cfm numbers? 4,200 to 7,200? Dave says those are his machines' adjustment range.
and Good Luck.
"Iron sharpeneth iron..."
It depends on the moisture content of the air, but
in general 400 cfm per ton of air will keep you out of the woods. For dry climates 450 per ton, and for humid climates 350 per ton.
I would look at the drive shieve on the blower. If worn, this could be most of the problem.
What is the return air and supply air temperature?
Also try swapping the freezestat with another unit and see if the problem follows, or better yet if you can test the freeze stat to prove its opening at its set temperature, and not before. This is generally harder then it sounds due to most settings around 20 degrees.
The coils are separate and stacked side by side and on top of each other any reduction in air flow can cause the feeze stats to trip. I have also found that when they system goes into economizer there is no hot gas bypass and suction will fall and freeze stats will trip.
Originally Posted by mccann
I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!
Your description and your findings on the AHRI site indicate low air. That is where I would start.
ERRG! I know that number, but had never used it for anything. 400 per ton puts me at 6000. My blower is hauling buns. I will tell Target monitoring we need to change the program to open the bypass more. With the all the VAVs satisfying down to 30 to 60%, my by-pass should be opening more. Thank you HVAC-Talk gods.
Without HVAC-Talk, I'm am crust.