Iced AC coil

[kurt]

This afternoon's inspection was a brand new home; no air coming out of the registers even though the blower was cranking. I pull the humidifier to take a look inside, & the coil is covered (literally) with 2"-3" of ice; it completely blocked the plenum.

HVAC guy says it just needs more "freon"; the fact he used the term freon makes me nervous. What makes me more nervous is how can a brand new system be this messed up? AC is not exactly my strong point; any opinions?

American Standard Model #HA03236D175B160573

Serial #6004L46701

Kurt Mitenbuler

Chicago; workin' the inner City

[Bill Kibbel]

Inadequate return air flow. If I had the time, I'd try to melt the ice by running the heat and then measure air flow at each return vent. I'll bet a big chunk of the return ain't working.

Disclaimer- One of the times I did this, the melting ice flooded the system and shorted the controls. Owners were pissed.

[Steve Knight]

I agree, inadequate return air flow is most likely the problem. Sometimes I see this when the filter is clogged. I can't see how more refrigerant would help.

[allspec33351]

The HVAC guy is probably correct. The unit will ice up if the refrigerant is low. He uses the term freon versus R-22 because either he doesn't know or he talks in terms in which most people understand.

My question to the HVAC tech is why is it low on "freon"? Leak?

If coils are blocked it will also ice up. This can happening in new constriction if they run the ac during construction without a filter.

Captain

[JEuriech]

I agree with Captain. Most likely low on refrigerant. The following is some other things that may be causing the problem:

Anything that prevents proper heat transfer. Low air flow from:

1. Bad indoor fan motor - not running or running slow

2. Clogged or blocked air filter

3. Supply and or return vents closed

4. Kinked ducts

5. Blockage in ducts

6. Extremely dirty or damaged indoor coil

7. Bad indoor fan relay

8. Clogged blower wheel. Insulation will sometimes come loose inside the

air handler and cause this problem.

9. Improperly sized ducts

Anything that changes where "change of state" occurs in the indoor coil. Such as:

1. Partial blocked capillary tube

2. Low refrigerant charge (Why is it low? Leak in system?)

3. Faulty expansion valve

4. Blocked orifice

Others things that may cause icing:

1. Faulty thermostat

2. Setting the thermostat too low (running the A/C under 70 degrees)

3. Running air conditioning with windows open

If I can recall anything else, I will post it later. In a new house I would suspect a refrigerant leak somewhere and the unit is low on freon. On an older house I would start with reduced air flow first and low freon second.

Jeff Euriech

Peoria Arizona

[Scottpat]

I find that most new systems are low on refrigerant. When they install them they just put a half charge in the systems for some reason.

[BlackJack]

Most new systems come pre-charged. This way they can be installed on a cool day and have a reasonable chance of working, since they work on temp and pressure, testing them on a cold day has almost no chance of getting accurate readings. You have to stress the system to get it really good. So if your new condenser is pre-charged for a 25-50 run to evaporator, and evaporator coil is 25-50 foot away, when you open the valve you should be good. The Install Tech should take a superheat reading (on a hot day) to test it properly. In addition, iced coil could be overcharged condition, if someone added too much. 25% of the units I see are overcharged. Tech will come out and add a couple of pounds when not needed to make a few extra bucks. If it is in a high humidity area, hot humid air from the attic could be getting into the air intake and cause the icing. All the things said here could be true, and need to be eliminated one by one. Why did you say using the "term Freon makes me nervous". It’s a term used at the EPA certification school required for Freon Recovery Cert. in all 50 states. Refrigerant and Freon are mostly used interchangeably.

[kurt]

Thanks folks. I couldn't tell much of anything about air flow, because when I say this thing was ice up, I mean iced; the duct & coil were literally full. It was freaky. I couldn't see any ducting due to vaulted ceilings, inaccessible crawlspace attics, etc.

I discussed the "run during construction" scenario; there was drywall dust in the blower, but not excessive. Still, that could put the baby out of warranty & I advised my customer. Told him to get the mfg's. rep out for verification of warranty. I'm going back tomorrow after the 150 pound block of ice is melted to see what the heck is going on.

I was nervous about the reference to freon because the guys I work w/never use the term; no one I know does. Maybe it's a local thing. The HVAC contractor is a one man chain smoking maniac; that might have put me off my feed a little.

As usual, lot's of good advice here @ TIJ.

[Chad Fabry]

Someone will have to explain to me how a low charge can cause icing at the evaporator.

Less refrigerant equals less system capacity by reducing the heat exchange medium, low operating pressures and a smaller pressure differential. None of those factors equals icing at the evaporator. An overcharge may cause icing, but the most prevalent cause would be low air flow.

[Garcha]

Sounds like a Paul Harvey filter to me.

[Jerry Simon]

Originally posted by Chad Fabry

Someone will have to explain to me how a low charge can cause icing at the evaporator.

Less refrigerant equals less system capacity by reducing the heat exchange medium, low operating pressures and a smaller pressure differential. None of those factors equals icing at the evaporator. An overcharge may cause icing, but the most prevalent cause would be low air flow.

According to my heating guru, a low charge will cause "part" of the coil to freeze-up. Low air flow will cause the "entire" coil to freeze up. He didn't know the science, though...he has thawed-out hundreds of coils in his time however.

[mcramer]

Originally posted by Chad Fabry

Someone will have to explain to me how a low charge can cause icing at the evaporator.

Less refrigerant equals less system capacity by reducing the heat exchange medium, low operating pressures and a smaller pressure differential. None of those factors equals icing at the evaporator. An overcharge may cause icing, but the most prevalent cause would be low air flow.

At atmospheric pressure, Freon boils at -41 degree F. We operate the system under pressure to get the freon to boil at a temperature of around 38 degrees, so the water that forms on the evaporator coil doesn't freeze.

When you have a small leak, the pressure drops, hence the boiling point of the Freon drops, causing the water on the coil to freeze.

This might happen only in the AM. As the temperature rises (and therefore the pressure) the boiling point goes up and the ice melts.

Eventually, so much leaks out that you have no cooling capacity and nothing happens.

Anything that restricts air flow (which is the heat source that causes the boiling) will lower the coil temperature. Dirty coil, filters, restricted air flow, etc. Usually, if you have ice, it's a leak or it's undercharged.