AC/HVAC Folks

[Greg Cole]

There's no frost anywhere. Plenty of CFM's in the registers... so the a coil isn't plugged. The filter is brandy new as well. Anyway... my thought on the intermittent cooling was MAYBE the contactor was starting to flake out under heavy loads but the unit conked out at 2am last night under less than peak load so, I checked it this morning and it was a-ok.
I just ran home to check on it again & finally the condenser wasn't running when the fan was still on and the t-stat was calling for cool air. Not a revelation, but something to work with! I checked both run cap's for the condenser and one was 9 mF of 16 & the other was about 12 mf of 30. They didn't show obvious signs of failure (usually you can see them kinda dome out the end with the terminals in them but a cap meter is a wonderful toy) I guess the condenser finally found the point where it tripped the internal overload from running w.out the cap's help.
I replaced both cap's and have to wait for the internal overload to reset. If it doesn't reset, it's toast. The replacement condensor is about $1k, which is about the point where I'd rather just replace it... but here's to hoping it comes alive when I get home and try to fire it up again. I have visitors coming from Vermont for 4 days... hopefully they have a cool house to welcome them!
Thanks for the hints.

[Ken Garlock]

Being a late arrival on the is thread, I sense that we may have a problem with terminology.

First, the condenser is the set of coils that are normally wrapped around the compressor which sets outside the house. During normal operation the compressor fan will blow hot air out through the grill work at the top of the outside unit. The condenser's job it to cool the hot gas coming out of the compressor thus turning it back into a liquid.

Second the evaporator is the unit in your furnace plenum and is a coil of copper tubing and is configured often as an A. This is the only part of the AC that gets cold. It gets cold because the freon/coolant is under pressure and when the coolant flows through a very small nozzle called an expansion valve, it rapidly expands and absorbs the heat in the air that is blowing across the coils, thus turning it back into a gas.
This is real world example of the universal gas law: PV=NRT Simply said, if you change the pressure(P), or the volume (V) of a gas, the temperature(T) must also change. If we are dealing with only one gas we can toss out the NR constants that describe the nature of the gas. We can then say when we have two different conditions of the same gas: (PV/T)1 = (PV/T)2 which is Boyle's law.

So basically, the only place in an AC system that can ice over is at the evaporator. This is caused by several possible problems, bad expansion valve, or the incorrect amount of coolant in the system, or the furnace fan is doing its job.

[Scott Shepherd]

Ken, I think you may have that backwards. All instances of icing up I've ever seen are on the outside unit only. I've seen mine, and several others, frozen up solid before. However, I've never seen the inside unit iced up.

[Joe Leigh]

Ken has the terminology correct. The condenser coil is outdoors. The only time you'll notice ice on that coil is if the unit is a heat pump, operating in the heat mode, in which case that coil is referred to as the "outdoor" coil. In that case the outdoor coil becomes the "cold" coil, and given the right outdoor temp/humidity conditions, will accumulate ice.

If the system seems to cool ok during light load conditions such as night time or early morning, but struggles during the peak heat times (assuming everything else you told us was accurate) you are looking at one of two situations: First, the compressor is not running all of time that it is being told to run. This could be caused by compressor overheating due to dirty condenser coil, unit overcharged, condenser fan motor failure.
The second reason, and the one I'm leaning towards is inefficient compressor. This occurs when the internal valves fail internally in the compressor. When this happens the compressor will fail to pump, bypassing refrigerant gas internally. This can only be determined by a competent technician, an amprobe to measure compressor current draw, and a set of refrigerant gauges.

[Pat Germain]

I was wondering if I had the condensor/evaporator thing backwards. Thanks for the correction, Ken.

[Greg Cole]

The second reason, and the one I'm leaning towards is inefficient compressor. This occurs when the internal valves fail internally in the compressor. When this happens the compressor will fail to pump, bypassing refrigerant gas internally. This can only be determined by a competent technician, an amprobe to measure compressor current draw, and a set of refrigerant gauges.

Welp, me thinks Joe's got it right here. D'oh.
Seems as once a month since March I've had one large unplanned expense a month... I was hoping to get through June without.

[Jerome Stanek]

My unit will freeze up on the inside coil if my filter is plugged or there is to much humidity in my house.

[Tom Godley]

The outside units will ice up in the winter -- if its a heat pump. You normally do not notice this when it is summer because the internal ductwork covers up the evidence.

If its straight cool - Then the outside will not ice up.

If everything is running -- and the unit is properly filled -- then you do have an internal problem. A system low on refrigerant may perform like this and will often freeze up.

Some units have more sensors than others -- some of the builder grade units will run until they die. The compressors do fail with the motors still running.

I would not fix the old unit -- especially if you have already changed the evaporator. The newer units are so much more efficient and also quieter. You want a matched set.

[Joe Leigh]

If you're thinking of replacing the system you'll want to upsize a bit. 2 tons is undersized for 1100 square feet. Rule of thumb for residential application is 500 sq/ft per ton.

[Lee Schierer]

I would not fix the old unit -- especially if you have already changed the evaporator. The newer units are so much more efficient and also quieter. You want a matched set.

If you need to replace the unit, some of the new units qualify for a 15-30% tax credit, which can ease the pain of purchase considerably. In the long run a new unit will use less energy for the same cooling.

[Pat Germain]

If you need to replace the unit, some of the new units qualify for a 15-30% tax credit, which can ease the pain of purchase considerably. In the long run a new unit will use less energy for the same cooling.

That's a really good point, Lee. It's painful to pony up for a new HVAC system. But after you do, it's really nice to be more comfortable for less dollars per month.

[JohnT Fitzgerald]

Ken, I think you may have that backwards. All instances of icing up I've ever seen are on the outside unit only. I've seen mine, and several others, frozen up solid before. However, I've never seen the inside unit iced up.

The inside unit can indeed ice up, and you can still get airflow with 'icing'. Another cause of icing is low refrigerant level - sorta counter-intuitive, but it's the way refrigeration works.

[Chris Kennedy]

Ken, I think you may have that backwards. All instances of icing up I've ever seen are on the outside unit only. I've seen mine, and several others, frozen up solid before. However, I've never seen the inside unit iced up.

Not to cause trouble, but he may not have it backwards. I make no claims to having any sort of HVAC knowledge, but I have had my internal unit ice-up while my external unit is free and clear. There was some issue with the blower, and the external unit was working nicely, sending cold air in, but it wasn't being blown out and the unit iced up.

Cheers,

Chris

[Jim Finn]

In my experiance as an air conditioner serviceman I have seen the inside (evaporator) coil freeze up but not the outside (condenser) coil except in the case of a heat pump. Is this a heat pump?