How TXV thermostats work

If you are smart and want to save money, you have a programmable thermostat; a thermostat that monitors air temperature and controls heating and cooling digitally. Most Americans however, have an old fashioned thermostat with a wire thermostatic expansion valve. If your thermostat is round or square and says "Honeywell" you are in the same boat with most Americans with their wire thermostatic heating controls.

These thermostats work by monitoring the size of a coiled length of wire. As the temperature increases, the wire expands, and as it decreases, it contracts. Although not the most accurate, these thermostats have been a staple of Denver HVAC for over 50 years. Inside the actual cooling apparatus, there is another kind of thermostat that regulates the equipment. This is a thermostatic expansion valve, or TXV.

Instead of measuring the ambient air temperature, the expansion valve controls the refrigerant as it flows into, and boils out of the evaporator. This split separates the high and low pressure sides of the system. Although still mechanical, the TXV is a little more complex than the Honeywell on the wall, because it must balance three different forces: the bulb pressure trying to open the valve, the spring pressure which tries to keep it closed, and the evaporator pressure, which also forces the valve closed.

All certified Denver air conditioning services technicians learn these forces early on in their HVAC training. With the advent of central air conditioning and heat pumps, many thermostats ore more complex. However the simple thermostatic expansion valve will close if evaporator pressure increases and bulb pressure is stable, and will open if the bulb pressure increases.

Superheat is a Denver air conditioning term that refers to gaseous refrigerant that is still maintaining excess heat. This increases pressure on the bulb, which opens the valve to allow more refrigerant into the evaporator. A problem with your TXV will send your air conditioning out of line because the pressure differential essential to the cooling cycle cannot be maintained if the valve is stuck open or closed. A Denver HVAC technician can identify problems like this relatively quickly. The thermostats come from the factory already calibrated, but the settings may not match your local air conditioner use profile very well.