[skizot]

Is this a new unit? If so, shouldn't the installing company make these adjustments for free? I know sometimess that easier said than done when dealing with contractors.

The unit is 2 years old, and I'm outside the builder's 1 year warranty. I never noticed any problems the first year I lived here because I had the t-stat set at 79 all throughout Summer.

 

[skizot]

I got a hold of the company and they are coming back out after it gets warmer here. They have been very pleasant to deal with. My question is the tech that comes back out is probably going to be the one that was out last, and he said that I should just keep it slightly overcharged because it will give more capacity on the really hot days. Is there any merit to his statement?

 

[beenthere]

No. Or we would all overcharge systems.

 

[Shophound]

I don't agree with the tech's statement regarding "really hot days" scenario. Allowing that it might be true, it also means your system underperforms on any day less than a "really hot day", since it is overcharged.

Fixed metering systems are critical charge systems, meaning they perform best when the charge is as spot on as possible. A few ounces either way makes a difference; I've proven it to myself on systems using fixed restrictors by adjusting the charge back and forth courtesy of a recovery bottle and a fresh bottle of refrigerant. The system always did best when it was very close to the manufacturuer's charging chart specs.

Overcharged = reduced capacity throughout the normal operating range of the system.

 

[skizot]

Thank you very much for the replies, guys. I will make sure he gets the charge spot on.

 

[Lorne]

I had posted in a previous thread about the superheat reading taken on my system being about 11 *F too low (possible overcharge). Today another tech came out and measured everything again. This tech measured the subcooling as well this time. Here are the measurements he took:

OAT DB: 68 *F
Low side: 68 psi (39 *F)
High side: 140 psi (78 *F)
High side line temp: 71.7 *F
Subcooling: 7 *F (charging chart says subcooling should be 13 - 15 *F)
IAT DB: 72 *F (taken at AHU return)
IAT WB: 62 *F (taken at AHU return)
IAT DB: 53.4 *F (taken at AHU supply)
Delta T: 19 *F
Low side line temp: 49 *F
Superheat: 10 *F

He said the charging chart for superheat says it should be at 19 *F. The chart I looked at says 19.5 *F for the parameters given. So, he said he thought it was really weird that subcooling revealed the system was undercharged, while the superheat showed the system was overcharged. He said the pressure chart said at 65 *F OAT DB LS should be 70 and HS should be 147. So, he said the pressures were almost right on. I asked him why the subcooling number contradicted the superheat number, and vice versa, and he said he didn't know what would cause that.

He said it was up to me on whether I wanted him to remove some refrigerant to get the superheat up. However, he said if he does that, the Delta T across the coil will go down. I was going to say go ahead and get the superheat where it should be, but I told him not to remove any since the subcooling wasn't right. I don't think I'm in a position to answer that question, and feel that there's still something going on with the system since the two numbers were contradictory. He said he would leave the system the way it was since it was getting a good split across the coil, and the pressures were good.

Any ideas what's going on here? How can the subcooling say that the system needs more refrigerant, but the superheat says that the system is overcharged?

Sub-cool low super heat low= fixed orifice not seating or to large, if airflow has been verified by dip switches for variable speed furnace and/ or static pressure PSC motor but even variable speed furnace air flow can be wrong if static is to high.
I would verify airflow by blower performance before pulling the orifice.
Our coils can handle multiple tonnage and orifices sometime don't get changed.
13 degrees sub- cool is a good number with fixed orifice. I see by the number you posted he is charging by WB.

 

[Little NATE]

Is your system having problems cooling your are techs just telling you this while doing preventive maintanance you didnt tell us your original issue. superheat subcooling will never be perfect unless ideal condition

 

[Daltex]

First of all, you have the best of the best helping you with this. I'm just a homeowner.

It seems that you are concerned with the subcooling when it's a fixed metering system. SH is low so it's overcharged for the current conditions. As it heats up outside isn't this an ideal situation for slugging back refregerant? The chart can't account for all the variables in your system.

The purpose of the TXV is to keep the numbers good regardless of the conditions. Maybe you should invest in one and set it and forget it. I'd wait until the conditions heat up so you can get a better result as Darrell said though.

Just my thoughts.

 

[Shophound]

TXVs rock.

 

[skizot]

Tech was back out today. He measured the following (not sure why he didn't take superheat initially, and only subcooling):

OAT DB: 86 *F
Low side: 75 psi (44 *F)
High side: 190 psi (98 *F)
High side line temp: 90 *F
Subcooling: 8 *F

The unit's chart reads subcooling should be 11 *F, 198 psi (high side), 78 psi (low side), and superheat of 14 *F.

He saw the low subcooling, and decided to add 1 lb. of freon. He took readings again, but didn't give pressures or line temps, just subcooling and superheat. They were now:

Subcooling: 6 *F (subcooling went down after he added 1 lb. freon?)
Superheat: 6 *F

I didn't know what to do at this point, and he said those numbers were good. However, since they didn't match the charts, I wanted him to get the superheat correct. So, he removed the 1 lb, and an additional 8 ounces. This brought the superheat up to what it should be, but brought the subcooling back down. After he removed the 1 lb. 8 ounces, the numbers were:

Subcooling: 3 *F
Superheat: 14 *F

So, the results are pretty close to the same as they were with my original post; the subcooling and superheat were both low. Now that the superheat is up to where the chart specifies, why is the subcooling so low? I know that the fixed orifice system is charged according to superheat, but shouldn't the superheat and subcooling numbers agree on the state of the charge, and not contradict it like they're doing here? Is there something else wrong with the system here?

 

[beenthere]

What was the indoor wetbulb temp.

 

[udarrell]

The subcooling was only 3-F off 11 @ 8-F.

Most mfg'ers allow +/- 2-F for subcooling.
Could have let it ride...

Could be other things causing a problem.
Could be a refrigerant system problem.

Also, the entire home & duct system are important parts of the HVAC system. op: - Darrell

 

[Shophound]

Yup. This is why analysis/troubleshooting over the internet ranges from frustrating to impossible, due to incomplete information. And even when thorough info is available, there is yet one more hurdle that cannot be overcome online...being onsite in person to take in ALL factors affecting the system's performance.

Bottom line from this point...since the latest adjustments, is your system keeping you comfortable? Just now skimming over this thread I may have missed it...but do you have an ongoing comfort complaint with your system? Doens't keep you cool enough, or doesn't dehumidify like it should?

 

[skizot]

Sorry, forgot to add the IAT WB. It was 65.7 *F. Also, the blower was set up to move 1376 CFM across the coil (verified with static pressure reading of 0.8" w.c. and blower chart, giving 393 CFM / ton).


udarrel, you say that the subcooling was within 3 *F, but the superheat was off. I've read as much as I can of the information on your site, and you always say to look at both subcooling and superheat. What I gather from what I've read there, and other places, the two numbers should agree with each other regarding the charge. Not like what's going on in my case where the subcooling is saying that the system is undercharged, and the superheat says it's overcharged.

shophound, the house cools fine with it's < 80 *F outside. But when it's above 85 *F, it runs for hours trying to move the temperature down 2 degrees. With the system running the way it is, the only way to keep it comfortable in the house is to set it on permanent hold at the temperature I want. Surely this is going to raise my bills.

I just wanted to get the refrigerant charge dead on, which shophound has stressed in a previous post about fixed orifice system. I just don't understand why the system cannot be charged to achieve the superheat and subcooling values the manufacturers states on the cover. I guess maybe I think it will do a better job being charged 100% correct, but maybe I'm wrong. I also assumed there was something wrong with the system that the superheat and subcooling were always off, regardless of how the charge was adjusted. Getting the subcooling right threw the superheat way off from what it should be, and getting the superheat right on threw the subcooling way off. That's why I just had him go ahead and charge it to the targeted superheat, even though that caused the subcooling to go down.

 

[beenthere]

Mine will run for hours to lower the temp 2 degrees, if I let set my temp up when its near or above 90 oiutside.

And I can maintain 72 when its 98 outside.

 

[skizot]

Mine will run for hours to lower the temp 2 degrees, if I let set my temp up when its near or above 90 oiutside.

And I can maintain 72 when its 98 outside.

What kind of set-back are you using to keep it 72 in the house? I assume nothing more than 75? Also, does it cycle when it's in the mid-80s? Mine rarely does, keeping it at 75.

 

[beenthere]

Depends on the day.

Sometimes, 78, sometimes 76.

 

[skizot]

Depends on the day.

Sometimes, 78, sometimes 76.

If your system runs for hours to drop it 2 degrees, and your set temp is 72, with a setback of 78 (or even 76), how are you getting it to 72 before late at night?

 

[beenthere]

I don't sometimes.

But, as long as the humidity is under 50%, it doesn't feel to bad, waiting for the temp to get to 72.

Last year, when it was 98 out, it would run for 6 hours straight to maintain 72.
I did NOT set my temp back those days. It wouldn't have been able to recover until after midnight.

 

[skizot]

So is expecting the system to drop it more than 2 degrees in 3.5 hours when it's 86 out asking too much? 86 is not that hot for this area, but that was the OAT and behavior of the system yesterday (and every day when it's > 85 *F outside). If it is, maybe I'm expecting too much out of the system.