I just moved into a house in CA. The house is small, less than 900 sqft. It has dual pane windows and upgraded insulation. The AC cannot cool (or keep temps below) 78-80 F. The temps outside are hot, about 90-100, but I cannot cool the inside down to a normal temp like 74 F.

I have called two repair people and both have looked, taken measurements, and told me there really isn't anything wrong. My refrigerant levels I'm told are fine, I've replaced my filter, and I also close all my windows and draw my blinds.

At nights the temps drop and I can cool down to 73-74. It will then cycle on and off as a normal unit does. But once it is daytime and the outside temps start to climb, the unit will just stay on all day without shutting off. The temps will slowly climb from 74-80 throughout the day with the unit never turning off.

The second guy offered to do a whole overhaul of my system where he wanted to replace the coil and duct work...but it's a new system. I'd say less than 10 years old and the house really hasn't been lived in for a number of years. I don't want to spend the quoted $5k to just start changing things out.

Is there anything else I can check? I'm a fairly handy engineer and I'm not afraid to tackle most things. The only slight issue is that the attic is very cramped and hard to work and access things like the coil. I would really appreciate some input or perhaps other tests and things I could check.

 

A good way to check the overall operation of the A/C system is to measure the Delta-T. Delta-T is the difference in temperature between the supply air and the return air. Take a thermometer and measure the temperature of the air going into the return duct closest to the air handler. From that value, subtract the temperature of the air coming out of the supply duct closest to the air handler. Ideally, you should be somewhere between 15-20 degrees F (@50% RH). If the RH is higher, the difference will be slightly smaller.

Since your evaporator coil and ductwork are in the attic, make sure that the evaporator coil area and ductwork are adequately insulated. If they aren't insulated or have minimal insulation, much of your cooling will be lost in the hot attic because the ductwork will absorb the attic heat.

 

I took some measurements from the intake and the return duct closest to the air handler.

Return Temp: 80.0 F
Duct Temp: 67.7 F

I took these temps as the internal temperature has slowly climbed from 74 to 81 during the day with the AC on constantly.

 

Delta-T 12.3 is too low. in CA, you want it 16 at least. Also do you have strong air flow ?

 

The airflow doesn't seem to be low (maybe a little, especially in vents further away), but I've only lived here a couple of months so I'm not sure if it's decreased compared to before.

 

Hand feel the 2 pipes come out the outside condenser. One large, one small. are they ice cold and wet, just cold, cool, warm, or hot ?

 

The larger one that is insulated is ice cold and wet. The smaller one is not very cold and is not wet. Both guys checked the temps at those lines and said they were nice and cold. They hooked up the meter and took pressure readings also.

Could this possibly be due to a clogged/dirty evaporator coil? As I've been learning more about HVAC I realized neither of the guys went up into the attic and opened the panel to check the coil. It's pretty hard to access so I'm guessing that they didn't want to deal with it....the second guy was ready to sell me a new evaporator coil even (but he never physically checked my current one). I bought some evaporator coil cleaner and I'm going to see if I can open the panel and clean that thing out.

 

Yes, it's possible that you might see an improvement by cleaning the evaporator coil. If the coil is very dirty, two "bad" things happen. First, it will impede the flow of air through the coil, thus reducing the airflow. Secondly, the dirt acts as "insulation", reducing the amount of heat transfer from the air passing through the coil (less "cold" being picked up if you want to think about it that way).

While you're up there, check to see that the evaporator housing and ductwork are adequately insulated.

 

Looks like the outside condenser is OK. As you mentioned, the problem may be the inside unit, need to make sure the coil is clean, all ducts must be sealed and well insulated, even a small air leak can affect the performance. Let's hope the attic area is not too hot. Take the delta-T before the coil and after the coil will get the real Delta-T(without the duct work) which can be compared to the in-room delta-T to see if the duct work is in good shape.

 

Edit: did they check the ducts for major air leaks?

You need to do some simple testing:
All you need is a good air-temp probe thermometer (digital reading in tenths preferable) an HVAC Supply might sell you one; or use your mercury TH & estimate the tenths; if you don't have a humidity gauge, you need to get one ASAP; a low cost 'INDOOR Humidity Monitor' - ACU-RITE Digital only $8.94 at Wall-Mart or over the Internet; it shows you if %RH is OK; plus records high & low Humidity & Temps during a 24 hour period.
Check to see if it registers the temperature correctly; if not, take it back for a full refund.

No Kids; do not lay anything on top of the outdoor condenser, & hang onto the thermometer so it doesn't drop into the fan!

If we find problems, you will need to contact & qualify a good HVAC Contractor to fix those performance problems.

1) Outdoor condenser’s discharge-air-temperature ___°F
Subtract Outdoor air temperature: __°F

Outdoor Condenser Air-Temp-Split ___°F

2) Need the ‘Indoor’ percent of relative humidity - in the middle of the rooms or, at Return-Air inlet grilles ___%RH

3)Indoor Return Air Temperature __°F Indoor Supply-Air Temperature __°F
Subtract Indoor Supply-Air Temp from Return Air Temp __°F
Indoor temperature-split = __°F

We ought to have superheat & subcooling temps.
TXV or piston metering device?

Copy & paste in your Reply; Fill the #numbers in where the blanks are.”
Easy Safe testing of your A/C or heat pump cooling performance for all forum visitors to use.

 

Update:

I crawled through the attic and was able to open and access the panel for the evaporator coil. It did not seem very dirty but I cleaned it with the cleaning foam anyways. It was very hard to access and took me a good 3 hours to open and clean. I added some extra foil tape to the joints.

I redid the test and now my temp differential is still the same (worse actually) at a delta T of 10.7 F.

No the two different AC repair guys who came out didn't really check anything for leaks, they poked their heads into the attic and opened the panel to look at the blower, but that was about it. The first output duct that comes out is very close to the evaporator box too, I can't see any leaks that are apparent.

I'll have to see if I can go get a humidity gauge today as I don't have one on hand.

Also I mentioned previously that the unit is less than 10 years old, well I saw an installation date sticker and it was installed in 2013, so this is barely a two year old system.

 

While you were up in the attic, did you check to see if the evaporator coil assembly and the ductwork was well insulated?

 

At this point, I think you should take a 'coil Delta-T' to eliminate the duct issues. Take temperatures 1 foot before the coil and 1 foot after the coil and see the difference between them. You may need a needle thermometer (so you can insert it into the duct) to do this test. from 12.3 to 10.7, something is changing the AC performance.

 

I think I'll have to save that for another day. The coil box is very hard to access and get the bolts and cover off.

What I can say is this, there is a very short run of duct from the inlet (where the filter is) to the blower. Also, the closest vent to the coil box is less than 5 ft away from the box, the duct is a direct line from the coil box to the vent with no other offshoots. I could be wrong, but I think the temp readings I've taken at the vent and the intake are probably going to be pretty close to what's inside the coil box.

Bob14525, as far as insulation goes, visually it looks okay to me but I'm no expert. The box itself seems very insulated with the panels having some sort of fiberglass insulation on them. I checked for kinks in the ducts and didn't see anything either. I can upload some photos of the ducts coming out of the coil box soon if that would help.

 

If the evaporator coil box and ducts appear to be well insulated, that's good. I didn't know if there was any insulation on the box & ducts. Obviously, since the unit is located in a very hot attic, you want to minimize the heat absorption, and insulation is the only way to minimize the heat absorption.

 

Outside condenser OK, inside coil & duct OK, but Delta-t is not OK, that does not make sense, something has to be wrong. While the return air temperature is at 80 one foot before the coil, you need to find out what is the output coil air temperature (one foot after the coil). it should be around 61 or 62. another word, 18 or 19 degrees lower. If not, you may have coil or metering device problem. We can not discuss refrigerant pressure at this forum, but you should get them from the Tech for future references.

 

Is that attic vented? Either by gable or ridge vents?you say it cools at night but in the heat of the day is when you have the problem,is that condenser sitting in the sun most of the day?what is the Delta T of the air going into the condenser and the air leaving the top?
Geo

 

The attic is passively vented with some openings, but there are no attic fans or anything like that. The attic is very small and cramped. It gets very hot in the attic, I have not taken any temp readings in there but it gets very hot. I can take a reading later today when I'm home but my guess is that it gets to well over 100 in there since the outside air is already mid to high 90s (I'm in Southern CA).

Like you I thought the cooling difference between night and day was useful evidence. But then, even at night when it is able to cool the Delta T at between the vent and the intake is still less than 15 degrees. It cools, sure but the air is cool already at night so it's not really doing much. On top of that my thermostat set point is 75 at night so it doesn't have to do much to cool, I suspect that if I tried to cool it to 70 or below at night it would not work too well.

I had suspected an issue with the attic being too hot, but I'm unsure about the effect this might have since the flexible ducts and coil box seem well insulated.

 

I have some additional info that I'm hoping solves this mess.

So the second guy that came out said that the superheat was slightly fluctuating. He used this to tell me I needed a new evaporator coil. He said the expansion valve was "hunting" or searching and that this means I need a new coil...

Well after doing some searching and reading the manual for the coil and the expansion valve, I can clearly see one issue. The sensing bulb of the valve is NOT secured to the return pipe. It is just dangling in the attic air. I'm no expert, but this is wrong correct? From everything I read the bulb has to both be secured to the line and also insulated. This to me seems like it would cause major issues right?

Now the question I have is, can I reposition the sensing bulb and affix it to the return pipe? I don't have to do anything special if I want to reposition it right?

 

How many b.t.u's is the unit and what height are your ceilings?

 

That bulb must be attached to the suction line and wrapped with special insulation tape. You can find videos on Youtube to see how it was done.

 

This is how the bulb was installed. It was positioned like this then wrapped with some foil tape and insulation.

 

I would cut back some insulation unravel the the tiny tubing and mount tight and close to the evap.before the cabinet. Mount at the 12 o'clock position on pipe then insulate it well.

 

So I ended up cutting back the insulation and reoriented the bulb flat against the pipe. The manuals I found said that the prongs have to be at the top when installing vertically. I found the best vertical section I could since I don't have anything horizontal without opening the cabinet again. I steel wooled both to clean them up a bit, and then clamped them together with copper clamps I was able to make. I bought some rubber insulation and insulated the pipe and bulb and sealed it all up with foil tape.

Firing it back up again I am getting a delta T of 17-19. By the time I finished it was becoming afternoon so I'll have to wait until tomorrow to truly see if it can keep up with the hot day, but I can immediately tell that the temps at the register have dropped. Yesterday the temps at the nearest vent were from the upper 60F to mid 70, but now I'm getting temps in the mid 50 to 60.

I don't want to celebrate just yet but it looks like the issue might have been solved.

 

Good work and good luck, got to be better then what was there.Sounds like the installer and techs didn't care or wanted to sell you a bill of goods, or didn't like the heat,not that it's fun.

 

Just curious, why didn't the tech pick that up? Better find a different contractor.
Geo

 

No idea. I hired two different people. They charged me $60 and $125 each and basically just hooked up some gauges to and said my system was fine. One had me replace the filter and the other wanted to redo the majority of the system.

Both tried to say that on a 100 degree day it's normal to only be able to cool to 80.... Neither actually climbed into the attic to inspect the evaporator coil box, it was actually very obvious, I had noticed it before but prior to this week I had zero HVAC knowledge so the bulb didn't strike me as something out of place.

Btw, the system is working great now. On a 100 degree day it's having no problems maintaining at 72.

 

It is unbelievable that something so obvious was totally overlooked!

It is getting real bad when a novice has to fix what so-called experts either wanted to sell you a lot of things that weren't needed or, told U it was working okay; which what a lot of dummies say.

Fortunately U didn't follow their advice.