Very Cold, Very Old AC

[Jim Katen]

This 1971 PTAC unit had supply air at 67-degrees dry bulb and 56-degrees wet bulb. It produced 28-degree supply air. (I would have expected it to be in the 48-degree range.) The filter was clean. 

All I can think of is a dirty condenser coil or a problem with the metering device. 

Any other ideas? If I had left a glass of water on the unit, it would have frozen solid. 

[Bill Kibbel]

No issues with the return ducts/vents or blower?

[Marc]

I don't follow.  Are you saying it produced a 28 degree differential?

[Jim Katen]

It's a PTAC unit - through the wall. No ducts. The blower seemed fine and the filter seemed clean. 

The differential was 39 degrees. (Intake 67 degrees supply 28 degrees). 

 

 

[Bill Kibbel]

PTAC unit - you inspecting a motel?

[Marc]

Somethings wrong.  Unless the blower is on low speed, I'd start with concealed debris in the evap coil or low refrigerant charge.  If there's a TVX, and I suspect there would be one on account of the multi-speed blower, that could do that also.

Edited July 5, 2019 by Marc

[Jim Katen]

2 hours ago, Bill Kibbel said:

PTAC unit - you inspecting a motel?

Apartment converted to a condo. 

We use PTACs in lots of apartments here. 

[Jim Katen]

2 hours ago, Marc said:

Somethings wrong.  Unless the blower is on low speed, I'd start with concealed debris in the evap coil or low refrigerant charge.  If there's a TVX, and I suspect there would be one on account of the multi-speed blower, that could do that also.

Well, there's no doubt about something being wrong. I'm just wondering what because I've never seen such a great temperature differential. I had a good view of the evap coil, which looked fine, but I couldn't see the condenser coil. It it's plugged up, that might do it. The metering device might be shot, I agree. How would a low charge cause excessive cooling? 

[Marc]

The clogged condenser coil would deny liquid refrigerant to the TVX and either drop or eliminate the temp differential.  It could burn the compressor motor also as motor current goes up with head pressure, other things staying about the same.

I've fixed a lot of AC's by cleaning the condenser coil with repeated application of a solution of sodium hydroxide, lye, even thought the coil seemed clean.  These are thick coils that can hide long term debris (decades).  The coils that are actually two thin coils built together, I've never been able to clean.

A low charge drops the pressure at the TVX, or capillary tube, outlet.  The lowest possible temp that can be developed is the saturation temperature.  A properly functioning AC has saturation temperatures at the evap inlet.  Reducing the refrigerant charge first leads to reduced temperatures at the evap coil inlet.  It starts making ice once temps get below freezing.  Further reducing the refrigerant charge drops the BTU/hr of the system to the point where the available heat at the evap coil is able to maintain a temperature above the saturation point.

A low refrigerant charge does not cause excessive cooling.  Temperatures drop but the reduced airflow (cause by ice buildup) calculates to a reduced btu/hr.  If there's no ice on the evap coil and the blower CFM is normal, you will not see reduced temperatures.

Edited July 5, 2019 by Marc

[Jim Katen]

So your best guess is concealed debris in the evap coil, slowing the flow of air? 

I can certainly understand how that would make the air much colder, but, damn, this thing was blowing air like crazy. If I held a basset hound over it, his ears would be flapping backwards over his head. 

 

 

[Marc]

22 minutes ago, Jim Katen said:

So your best guess is concealed debris in the evap coil, slowing the flow of air? 

I can certainly understand how that would make the air much colder, but, damn, this thing was blowing air like crazy. If I held a basset hound over it, his ears would be flapping backwards over his head. 

 

 

I can't reconcile your observations.  It's not making sense.  Low latent load would trend towards observations like you've made but even the total lack of  latent load would not produce a 39 degree differential.

What were outside temperatures and was the fresh air intake open?

Edited July 6, 2019 by Marc

[Jim Katen]

The outdoor temperature was in the mid to high 60s and the fresh air intake was closed. 

Yes, it makes no sense. If it made sense, I wouldn't have posted it. 

[Tom Raymond]

Why bother troubleshooting? Just defer it to an HVAC tech for review, analysis, and evaluation. 

Oops. I thought this was Facebook. My bad.

[Jim Katen]

I'm not trying to troubleshoot it. I'm trying to understand it. 

[Tom Raymond]

That was a failed attempt at humor. 

And, perhaps a little venting of frustration over the modern struggle for ineptitude spilling over from personal experience. 

[Jim Katen]

Ahh. Sarcasm. 

[Erby]

Turn up your faucet, Tom.

The drip wasn't quite enough for some of us to catch it.

[mgbinspect]

On 7/8/2019 at 7:34 AM, Tom Raymond said:

That was a failed attempt at humor. 

And, perhaps a little venting of frustration over the modern struggle for ineptitude spilling over from personal experience. 

I got it, and thought it was pretty funny. But then, I just read a related recent comment,of Jim's regarding Facebook in another thread.

Edited August 1, 2019 by mgbinspect