Subcooling and superheat both too low

[skizot]

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?

[amd]

If the system has a fixed orfice, it should be charged by superheat; if it has a txv, it should be charged by subcooling.

The following can adversely affect pressure/temp readings...

- Dirty coils
- Incorrect blower speed
- Incorrect metering device used, or not adjusted properly if txv
- mismatched condenser and coil

[skizot]

If the system has a fixed orfice, it should be charged by superheat; if it has a txv, it should be charged by subcooling.

The following can adversely affect pressure/temp readings...

- Dirty coils
- Incorrect blower speed
- Incorrect metering device used, or not adjusted properly if txv
- mismatched condenser and coil

Both coils are clean. System is fixed orifice. Both condenser and coil are 3.5 ton. Airflow is approx. 425 CFM / ton.

So what should I be doing at this point? He said that leaving it overcharged like it was increased its capacity. I don't think that's correct. Should I call them back out and request that they remove refrigerant until the superheat comes up to what it should be? He told me that removing that refrigerant would reduce the temp split across the coil. Wouldn't it actually increase it as the evap. coil would be colder after the excess refrigerant was evacuated?

[CincyHVAC]

If the system has a fixed orfice, it should be charged by superheat; if it has a txv, it should be charged by subcooling.

The following can adversely affect pressure/temp readings...

- Dirty coils
- Incorrect blower speed
- Incorrect metering device used, or not adjusted properly if txv
- mismatched condenser and coil

Both coils are clean. System is fixed orifice. Both condenser and coil are 3.5 ton. Airflow is approx. 425 CFM / ton.

So what should I be doing at this point? He said that leaving it overcharged like it was increased its capacity. I don't think that's correct. Should I call them back out and request that they remove refrigerant until the superheat comes up to what it should be? He told me that removing that refrigerant would reduce the temp split across the coil. Wouldn't it actually increase it as the evap. coil would be colder after the excess refrigerant was evacuated?

Hmmmmnn.....................

[skizot]

Hmmmmnn.....................

Hmmm what? I thought 350 CFM - 450 CFM was acceptable. Every tech that's come out to my home (4 of them now) has tried setting the blower speed even higher than that.

[beenthere]

Probably should have ask him to remove some.
Thats a heavey indoor load, for the outdoor temp.
Your ID RH was 57%. The DP of the air was 56degrees. So a 19degree discharge air temp wasn't really ideal.

Low OD temp is helping to condense liquid too quickly, so your head pressure is low and so is your SC.

[CincyHVAC]

Hmmm what? I thought 350 CFM - 450 CFM was acceptable. Every tech that's come out to my home (4 of them now) has tried setting the blower speed even higher than that.

I don't want to get my shorthairs clipped by the mods, but maybe the techs have been going the wrong way with the blower speed.........

[skizot]

Probably should have ask him to remove some.
Thats a heavey indoor load, for the outdoor temp.
Your ID RH was 57%. The DP of the air was 56degrees. So a 19degree discharge air temp wasn't really ideal.

Low OD temp is helping to condense liquid too quickly, so your head pressure is low and so is your SC.

The A/C hadn't run all night or morning since it's been cool and raining here. Head pressure was low, but within spec given the low OD temp. The condenser has a panel that shows what the ideal pressures should be given the outside temps. The charging chart (on the panel) also accounted for the cool outside temp when it said subcooling should be 13 - 15 *F) - I'm assuming anyways, because the outdoor temp was on one of the axises. So should I call this company back out and ask them to get the superheat back up to what it should be (by removing refrigerant)?

[skizot]

So a 19degree discharge air temp wasn't really ideal.

Sorry, but what do you mean by this?

[beenthere]

Sorry, but what do you mean by this?

Too cold for the load it was working against. SH charts are based on 400CFM.

That charge doesn't allow for an overcharge.

[amd]

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[skizot]

Too cold for the load it was working against. SH charts are based on 400CFM.

That charge doesn't allow for an overcharge.

How many non-variable speed systems out there move exactly 400 CFM though? I would guess not many. That being said, is that why they say being within 3 - 5 *F target superheat is what you shoot for (the range accounts for the variance in things like blower speed)? I would think being off 9 - 11 *F is too much.

[beenthere]

Lot of VS blowers are set up at 400CFM, and then reset to what ever CFM afterwards.

Over 400CFM is generally used in areas that have low humidity.

The plus or minus allows for CFM, lin e set length and other minor variations of the system.
And manifold and temp probe inaccuracies.

[skizot]

Lot of VS blowers are set up at 400CFM, and then reset to what ever CFM afterwards.

I was referring to non-VS blowers. I would assume ESP is what dictates the exact CFM / ton delivered at a certain speed tap, and that varies with each person's ductwork. So, I guess what I'm trying to say, is that you are never going to deliver exactly 400 CFM / ton or 350 CFM / ton with a non-VS blower, even if your ESP is within the nameplate. Another shortfall of the fixed speed blowers I guess.

So is it worth calling them back out to remove some refrigerant? Should I have them slow the blower down as well? They all seem to want to set it as high as it'll go.

[beenthere]

I'd want the CFM verified, and the SH be set up to proper specs.

[skizot]

I don't want to get my shorthairs clipped by the mods, but maybe the techs have been going the wrong way with the blower speed.........

Unless what I've read is wrong, one of the causes (in regards to airflow) of low superheat is low airflow (across evap coil). One of the causes of excessive superheat is excessive airflow (across evap coil). Given that, I wouldn't think the extra 25 CFM / ton is causing the low superheat (should be the opposite). If anything, the superheat should be higher (if the system was charged correctly).

I will have them come back out and set the blower speed tap to medium (393 CFM / ton <-- Medium = 1376 CFM @ 0.8" w.c. ESP), and charge based on the target superheat.

reference: Residential construction academy: HVAC

[udarrell]

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?

First, those are very light load conditions, I would wait until there are some real heat-loads to put on the system & see what data you get.

The Subcooling is only 3-F above the OAT, therefore, I don't see that as a problem with all the other light load conditions. Could actually be somewhat over-charged with those light conditions.

Usually, that is an insufficient OAT temp to deliver liquid to the E-Coil which also has a light heatload being absorbed by the evaporator coil.

I repeat what I stated in the prior thread: "Evaporators are shipped with the orifice that matches its tonnage, but especially if the condenser is a lower tonnage it always requires the condenser's fixed orifice size."

The 72-F IDB minus 62-F IWB is 10-F drop for 57% RH.

If the air entering the evaporator puts a heavy heatload on some of the circuits & a light load on others, this unbalanced load can also cause low SH & higher than normal for conditions SC.

Check the fixed orifice number first; then if it comports with the condenser's number, I'd take the temps of the evaporator circuits at the manifold & see if there is too much variation.

An evaporator orifice could be over feeding, which can result in high SH & low er than normal SC.

There isn't something obstructing airflow through some of the circuits, or is there an internal circuit restriction.

I would not try to balance a fixed orifice charge under those light load conditions.

Wait for some hot weather conditions, then take the test data! - Darrell

[Shophound]

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?

68 outdoor temp is a bit cool for a fixed orifice system to render trusty data. I'd want to see a good load on the system before I'd wonder about the superheat/subcooling being one way or the other. Sure, there's a charging chart saying "x" for the given conditions, but in my experience, the more heat load the better for testing system performance.

I got your email and apologize for not answering it immediately. I honestly don't know of good techs in your area, but I'm sure there's one or more out there in your city who can get a handle on your system.

[skizot]

Thanks, Darrell and shophound. I'm just wondering what the downsides/negatives are if the system is slightly overcharged. I know that being too overcharged is going to cause liquid refrigerant to flow back to the compressor under certain loads, but what about a slight overcharge? If it's not enough to ever cause slugging, is it doing anything to energy consumption or system capacity? Too high of a superheat is what affects capacity, right? I'm just asking because this whole thing is starting to get rather expensive with all of the service calls I've had already. I would like the system to have the perfect charge, but if it's running the best it can, and there's no risk of liquid flowing back to the compressor due to the slight overcharge, then maybe I should just call that good?

[john_ertw]

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.