We provide useful tips and information for homeowners, facility managers and contractors looking to improve their current HVAC system. Since the minimum efficiency regulation changed to 13 SEER in January 2006, most OEM systems now incorporate a thermostatic expansion valve (TXV) style metering device as the standard for air conditioning systems. It is now extremely important for the HVAC technician to understand the design and operation of this type of valve. The thermostatic expansion valve (TXV) is a precision device, which is designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor (floodback). One of the design features of the TXV is to separate the high pressure and low pressure sides of an air conditioning system. Liquid refrigerant enters the valve under high pressure via the system’s liquid line, but its pressure is reduced when the TXV limits the amount of this liquid refrigerant entering the evaporator.

The TXV – What It Does Do The thermostatic expansion valve controls one thing only:  the rate of flow of liquid refrigerant into the evaporator. Contrary to what you may have heard, the TXV is designed to control: Trying to use the TXV to control any of these system variables will lead to poor system performance – and possible compressor failure. How the TXV Controls the System As the thermostatic expansion valve regulates the rate at which liquid refrigerant flows into the evaporator, it maintains a proper supply of refrigerant by matching this flow rate against how quickly the refrigerant evaporates (boils off) in the evaporator coil. To do this, the TXV responds to two variables: the temperature of the refrigerant vapor as it leaves the evaporator (P1) and the pressure in the evaporator itself (P2). It does this by using a movable valve pin against the spring pressure (P3) to precisely control the flow of liquid refrigerant into the evaporator (P4): TXV Pressure Balance Equation

P1 = Bulb Pressure (Opening Force) P2 = Evaporator Pressure (Closing Force) P3 = Superheat Spring Pressure (Closing Force) P4 = Liquid Pressure (Opening Force) Energy Transfer in the TXV Here is a closer view of the TXV in operation. The flow of the liquid refrigerant is restricted by the valve pin. As the flow is restricted, several things happen: The pressure on the liquid refrigerant drops A small amount of the liquid refrigerant is converted to gas, in response to the drop in pressure This “flash gas” represents a high degree of energy transfer, as the sensible heat of the refrigerant is converted to latent heat The low pressure liquid and vapor combination moves into the evaporator, where the rest of the liquid refrigerant “boils off” into its gaseous state as it absorbs heat from its surroundings. The pressure drop that occurs in the thermostatic expansion valve is critical to the operation of the refrigeration system.

As it moves through the evaporator, the low pressure liquid and gas combination continues to vaporize, absorbing heat from the system load. In order for the system to operate properly, the TXV must precisely control the flow of liquid refrigerant, in response to system conditions.do window ac units use more electricity Excessive AC Condensation Problemhvac air conditioner reviews This morning's Warranty Inspection had a situation I have not encountered before and I would like some input on it.how does an outdoor ac unit work The weather this morning was low 80s with scattered rain. It has been a particularly wet and humid summer here. When I walked in I noticed that it was pretty cold in the house. My wife likes to keep it pretty cool at home so I am used to 74 or so temps.

This was uncomfortably COLD to me. When I got to the thermostat it was holding steady at 65. When I got to the attic to look at the unit, I noticed a lot of moisture on the decking all along the primary condensate trap and line. Then I saw a pool of water on the top of the furnace housing. Looking further there was water in the secondary/emergency drain pan. Then I noticed moisture at the supply plenum. There was moisture on the top at some of the duct connections (they were pretty well installed with mastic) and some moisture on the decking under the plenum. As I bent down to look closer under the plenum I noticed that the bottom panel of the plenum was bowed out. I touched it and my finger went straight through and a stream of water flowed out. The material was completely saturated. I brought the homeowner up to the attic and explained the moisture problems to him and put a bucket under the plenum. I asked if they always kept the thermostat set at 65. The answer was yes. I would love some input on this problem/situation.

I told him that due to the low temperature they are maintaining, they are creating excessive condensation, as well as apparently creating condensation inside the plenum and perhaps even the ductwork. That he should have everything checked by a licensed HVAC guy and discuss with him whether there are any modifications they can add or incorporate that will allow them to cool the house that much, without these problems developing. I also suggested that he have a mold sampling done. As it seems that this is a perfect environment and has been present for months. Are there any methods that can be incorporated to remove moisture and allow them to run the system that low? It seems to me that even if you get the moisture out of the conditioned air, you cant do anything about the 140 degree high humidity air of the attic surrounding all the ductwork and plenum's. I have attached some pictures, but it is hard to see the moisture and problems. Re: Excessive AC Condensation Problem Tom Dietrich For Northampton County