coil freeze-up

I just had my unit serviced. Several times this season, the coils have frozen up. They checked the freon and said it was ok. They suggested it may be an airflow problem and that I needed to schedule someone to come out and perform the work. Said it would need to cut the ductwork to get into the a-frame coil. The coil is 6 yrs old and the rest of the unit is 15 yrs old. I am able to do allot of handy work, so I did the cleaning myself. The coil didn't look bad at all. I'd say maybe 10-15% clogged. I cleaned with coil cleaner from local hvac shop. Worked great. I also cleaned the motor fan. That was very dirty. Put everything back together and the same results. Still coil occasionally freezing up. Air doesn't seem to be blowing out of the vents any stronger. It was already strong to begin with. Of coarse when the coil freezes then very little airflow. I really don't think I have an airflow problem. Any help would be great.

 

I'm curious to know of you had freezing problems in previous years or is this a new phenomenon? Did it seem to start when the cooling coil was replaced but did not happen with the previous one? Whenever I clean a coil on a system like yours, I remove it and use a foaming action type of coil cleaner that will boil out dirt that is embedded deep in the coil. You would be amazed to see how much dirt comes out. Dirty coils usually require 2 to 3 applications along with copious flushing. Does the freezing seem to coincide with the weather cooling just a tad? I wouldn't be surprised to see that your indoor airflow is borderline. What size is your system and what is the measurement of your return grille? [size] When it freezes, does the larger pipe at the outdoor unit freeze over? What does this pipe feel like when the unit is running normally? Is it real cold?

 

bigjohn, thank you. The freeze ups did start at the end of last summer. Found a burnt wire connection on the compressor. I replaced the connection. Also the lead that is on the compressor was/is very rusty. I tried to clean as best I could. I have looked at that one and all the connection inside and out.

I did do 3 applications of cleaner on the coil over a full day. The product I used was made for coil cleaning and there was some foaming with this product.

The weather conditions here this spring has been very mild, so yes the weather temp may have cooled a bit durning freeze up.

The unit is a 2 1/2 ton. The return grill is 21inX21in. The airflow out of the registers seems very strong.

The large pipe outside does freeze over. I have been monitoring the coil/freeze up very closely so I have not let it freeze to a solid block. The way I tell the problem is starting is by the amount of air coming out of the registers. When the flow is real, real low (almost not at all) I check the coil and it is iced up. The large pipe both inside and out is very cold and somewhat freezen over. Both durning normal and durning freeze up the diffence between the large and small pipe is one warm, one cool/cold.

Hope I answered all the questions.
Tony

 

IF you are sure both the inside coil and the outside coil are clean.You have got to call a hvac tech and put some gauges on there to see what its doing for sure. It still sounds like low on freon.Or do you have a expansion valve here and the head gone bad ED

 

Well, I still think you need to consider the airflow. I apologize, I neglected to ask if you have a standard return grille or a filter return grille. [ IOW- is your filter in the return air grille or is it in the furnace?] A standard grille needs 2.7 cfm per sq. inch of Free Area while a filter grille needs 2.0 cfm per sq. inch of Free Area. The trick is in understanding that Free Area is the space in the grille thru which the air flows. Your grllle at 21 x 21 has 441 sq. inches of gross area and ] For louvered grilles I usually figure Free Area at .65 to .7 of the gross area. So being conservative, 441 x .65 = 287 sq inches. [rounded up] [you measured the area where air flows thru right? not the outside dimension? otherwise you have a nominal 20 x 20 grille which is 400 sq. in. of gross area and 260 sq. in. of free area] Your system requires in the neighborhood of 1000 CFM of airflow. So for a standard grille you would need 1000/2.7 = 370 sq. inches of free area. A 20 x 30 grille would give you 600 sq. in. of gross area which x .65 = 390 sq. in. It's ok to be a little larger. If you have a filter grille, then 1000/2.0 = 500 sq. in. of free area. A 25 x 30 grille is 750 sq. in. of gross area x .65 = 487.5 sq. in of free area which is close enough. Now, an inexpensive solution would be to take out the lourvered grille and replace it with an egg crate style grille of the same size. The openings in an egg crate grille are small squares and they have a greater free area then louvered grilles. I usually use a factor of .8 to .85 for egg crate grilles. So, 441 x .8 = 353 sq. in. [rounded] which is much closer to where you need to be.

 

it is either low airflow across the evap or low charge, if charge is in fact good, somewhere there is a piece missing...burnt wire will not cause freeze up.

 

thanks all for the replies.
Bigjohn here is some more info on the return. The filter is in the furnace. It is standard/louvered return grille. The measurement I gave was outside dimension.(sorry) The airflow dimension is 19.5X19.5 in. Useing your info that would be 380 sq. in. of gross area X .65 is 247 sq. in. of free area. You say my system needs about 370 sq. in. of free area. Looks like I'm not even close.

I wonder now why this has not been a problem for 15 yrs?

I wonder if the free area can be more....than if I take the grill completely off I should have approx 20X20=400 sq. in. of free area. Do you think that would be a good starting point?

 

Yes, that's a good idea. See if the freezing stops. Also, look inside the return air ductwork for loose insulation that may be restricting the airflow. You seem confident that the indoor coil is clean, so we have to look elsewhere. During the cooling cycle, is the indoor fan motor running on the highrst speed? Have you had the fan or motor replaced?

 

i am also very interested in finding the outcome to this problem, in case I ever see it in the field. what type of filter are you using? high flow cheapies, or the restrictive micron filters?, the reason I ask is I've seen these in older neglected equipment with dirty wheels, coils, and ducts, and it was boarderline and would freeze it up. I'm not saying yours is old and neglected, i was just wondering, thanks

 

Thanks again all for the concern and info.
I'm pretty sure that I have a 2 speed fan. It does have 3 wires running to the motor from the circuit board. Looking at the circuit board it looks like a high and a low speed wire going to the motor. I think it always run on high speed. I don't think I have ever heard a slower speed. The fan motor has never been changed. I did remove it and cleaned it real good. Along was the squirrel cage. I am useing the cheapy filters, but keep up on them to make sure they are changed when needed. I know the micro ones are much better. If I would have used them from the beginning I probably would not have had to clean the fan and coil. The duct work that I can see from the unit and from the return looks clean. It was dusty, but I cleaned while I was in there. At this point I know the fan and coil are very clean. Bigjohn, the weather here is very chilly. Can I still run the a/c and get a good test if I have the thermostat at say 65? I will let you all know the outcome later today.

 

The more I dig into this, the more interesting it gets.
I took off the grill and the cavity behind the grill is 20X20. Right after the 2X4 framing it goes into a 12X16 duct down to the unit. There is a hole cut in the 12X16 duct that is 12X16 and that is where the grill duct goes into. So what I am saying is behind the grill I am looking at a hole 12X16 going into a 12X16 duct. Before the 12X16 duct goes into the unit it expands to 14X16. So my thinking is that I have a airflow 14X16=192 sq. in. of gross area X .65 is 125 sq. in. of free area. Not looking good at all, is it? Again I wonder why it hasn't been a major problem for 15 yrs?
With running the system for the last 2 hours I have watched the coil very closely. It stated with frost on the pipes at the coil bottom. Worked it's way up to the top. Then there was frost building on the coil fins starting at the bottom and working it way up. It has increased to a heavy frost maybe 1/8 in. on the pipes. No more accumilation on the fins yet. I think on its way to a complete freeze up. (or is frost that heavy on the pipes normal?) I need to go to work so I am turning the system off.
Any help on the lastest????
Maybe another temporary check would be to open up the intake duct to get a full 16X14 in. I would have to do that right next to the unit. That would be a place I could do it without damaging the duct. And in getting to 16X14 I only get up to 145 sq. in. of free area. Could that improve it any?

 

Just an old rule of thumb

very good job 80" per ton---your 2 1/2 ton =200"


very tight job 60" per ton--your 2 1/2 ton =150"

You said you did have a 12"X16" ========192"

I still think Have some one put gauges on this unit

ED