poopshovel wrote:bravenrace wrote: In reply to Slippery: Well, I actually may have mistated that, since I don't know what car he's talking about. I was assuming it is a 134A system. Freon is Dupont's brand name for R-12 refrigerant. It doesn't apply in any way to R-134A..Well, I'm no expert or anything, but I would assume from the wording that he's not talking about a car.
Yes. My mistake.
Slippery wrote: So Freon is refrigerant then ... Just a Dupont trademark. Same as how Teflon is a Dupont trademark and the generic stuff is PTFE. I would say that judging by what Toyman wrote, we cannot state whether he knows or does not know the difference. J
Did you miss my wink? And a slight correction - Freon is Dupont's brand name for R-12 refrigerant only. It does not actually apply to any other refrigerant. So Freon is a refrigerant, but a refrigerant isn't necessarily Freon.
Curmudgeon wrote: On the subject of low refrigerant causing evaporator icing: http://www.hvacoracle.com/service/ac/evap_freeze.shtml It's been known to happen in automotive HVAC as well. Typically it's a stinking hot humid day, the charge is a little low, the car is driven at highway speeds with the system set to 'recirc'. Mercedes has an evap temp sensor installed for just this purpose, it kills the compressor if the evap temp drops too low.
Every automotive A/C system I've worked on has that. It's called a thermostat, and it's an essential part of a properly designed A/C system, like I mentioned previously.
I don't know much about stationary a/c, although I'm shocked that it wouldn't have anti-frost protection designed in, but like I said, virtually all mobile a/c systems do. So a properly designed and functioning mobile a/c system should never experience evaporator coil icing that isn't taken care of by cycling. In reality it may happen, but it shouldn't. We do extensive testing on all of our systems to insure that the coil can never ice up under any circumstance. All it takes is cycling the system for the correct amount of time on low fan to prevent it from happening.
In reply to bravenrace:
Vary seldom do you see a coil thermostat on a residential heat pump. I have seen some that used a head pressure switch to shut down the compressor if the evaporator ices up but that is usually a last resort to save the compressor and won't shut it down long enough to defrost the coil. The heat pump in my rental house, in cooling mode, uses an evaporator temperature sensor and a variable speed compressor to control coil temperature. In heat mode it uses head pressure and a timer for the defrost cycle. I think my current house uses a timer but also only in heat mode I haven't studied it enough to know for sure. Residential systems will ice the evaporator coil when low on freon/refrigerant. Capillary controlled systems are the worst offenders because they have no way to adjust the orifice to control super heat. Systems using a TVX valve will usually compensate to the point they just stop cooling. Unfortunately a lot of residential systems are the cheapest available and aren't equipped with a TVX valve.
I have seen mobile systems blow snow out of the vents, but usually if they are low on refrigerant they just cycle a lot due to the low pressure switch shutting down the compressor.
I haven't been designing A/C systems for 20, but I do read a lot.
Might want to look at this too. 
Generic trademark 
In reply to Toyman01:
Thanks for the information. Like I said, I'm not that familiar with home a/c systems, more than generalities. I did check the window a/c unit in my shop and it does in fact have a thermostat to prevent icing, but I'll take your word for it that this isn't common.
However, I stand by everything I said about mobile air conditioning. There are four major components to coil icing, air flow, moisture in the air, coil temperature, and refrigerant gas pressure. You can't do much about the moisture, it's there. And air flow is what it is, as long as there is no blockage anywhere. The thermostat is designed to cycle the compressor when the temperature of the coil gets close to freezing in order to prevent ice from building up on the coil. The low pressure switch is primarily used to cycle the compressor before the pressure gets low enough to damage the compressor, but in doing so it also aids in preventing coil icing in the case of a low charge. So there are two different mechanisms for preventing coil icing in a typical mobile a/c system, which is why low refrigerant pressure is an unlikely cause of coil icing in a mobile a/c system.
What I can't get my head around with this coil icing thing is that if everything else is correct and equal, the idea of the coil icing with a low charge implies that the coil gets colder with a low charge than it does with a proper charge. Coil temp and airflow are what dictate the energy capacity of the system, so if this is the case, then it says that if you ignore the icing factor, the performance of the system is better with a low charge than with a proper charge, which seems to defy properties of thermodynamics and physics. Can you explain how exactly this happens, because with mobile systems, maximum temperature drop across the TXV, and thus performance, is always best with a full charge.
clownkiller wrote:bravenrace wrote: I hate to have to qualify myself, but I've been designing and testing mobile a/c systems for 20 years. I'm not wrong.
Oh, and it's worth noting that any understanding I have regarding how HVAC systems work (among other things) was learned from a certain drunk canexican clownkiller
Glad the coil cleaning helped...sorry it sounds like yours is designed like mine ;) I'd still check the freon, and change your filter more often/make sure it's seated properly.
In reply to bravenrace:
It's been a while since I studied this stuff but off the top of my head, it's a pressure problem. The pressure differential across the orifice is too great when the system is low on refrigerant. Rather than having the liquid gradually gas out as it absorbs heat, it flashes to gas and creates a super cold spot at the orifice or at the beginning of the coil. That spot ices and the low temperature causes the liquid to flash to gas a little further down the coil. Gradually the coil ices completely. The system needs the base pressure to control how fast the liquid entering the evaporator turns to gas and residential systems don't have a low side pressure switch. Ideally the last of the liquid gasses out just before the end of the evaporator coil. It's a balancing act between, air flow, system pressure, coil size and temperature. Especially on a capillary system. That's why most residential systems should be filled using not only high side and low side pressures, but super heat and sub cooling. To do it right, you really need ambient temperature outside and inside, high side and low side pressure, air temperature off the evaporator coil and air temperature off the condenser and a pressure/temperature chart for the refrigerant being used.
Yeah I know, not very technical but that's the best I can do on the fly.
Quick question. Is the temperature sensor, on the window unit, on the up stream side of the evaporator coil or the down stream side? Most of the ones I've seen are on the upstream side measuring incoming air for room temperature, rather than air off the coil for unit control. I will say A/C units are getting smarter. The inverter unit in my rental house is pretty smart. It uses a pile of sensors to control fan speeds and compressor speeds. As load increases, so do the speeds. All those controls give it a 22 seer rating, but it's at the cost of reliability. I've already had to replace the compressor speed control and that unit is only 4 years old. I love the efficiency, but I'm betting that unit won't last 10 years before something major craps out.
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