Metering Devices in Heat Pumps: TXVs, Check Valves, and Bi-Flow Devices

How Heat Pumps Meter Refrigerant

A heat pump must move heat in two directions—out of a building in cooling mode and into a building in heating mode. Because the indoor and outdoor coils swap roles when the reversing valve shifts, the refrigerant flow path reverses. That reversal creates a fundamental challenge: a metering device sized and oriented for one direction of flow will not meter correctly in the opposite direction. Understanding how the industry solves this problem is the core skill of this lesson.

The Job of a Metering Device

Every refrigeration circuit needs a metering device between the high-pressure liquid line and the low-pressure evaporator. Its job is to drop refrigerant pressure sharply and control how much liquid enters the evaporator. Too much liquid and the compressor floods; too little and the evaporator runs inefficiently. In a heat pump the metering device must do this job regardless of which coil is acting as the evaporator.

Thermostatic Expansion Valves (TXVs)

A TXV meters refrigerant by sensing the temperature and pressure at the evaporator outlet and adjusting a needle against a seat to hold a set superheat. The sensing bulb is clamped to the suction line near the evaporator outlet and is charged with a fluid that matches or is similar to the system refrigerant. When superheat rises the bulb pressure rises, pushing the needle open. When superheat falls the spring force and evaporator pressure push it closed. The external equalizer port connects to the suction line downstream of the bulb so the valve can account for pressure drop across the evaporator coil.

A standard TXV is designed for refrigerant flow in one direction only. Installed backward, the check ball inside or the valve geometry will block or mis-meter flow.

Bi-Flow TXVs

A bi-flow TXV (also called a bi-directional TXV) is designed so refrigerant can enter from either end and still receive proper metering. Internally, the valve has two inlet ports and an internal check arrangement that routes flow through the needle-and-seat assembly regardless of which end refrigerant enters. A single sensing bulb and equalizer port still control superheat. The practical advantage is that one bi-flow TXV on each coil replaces the combination of a standard TXV plus a check valve that would otherwise be needed to bypass the TXV when flow reverses.

When you sketch a bi-flow TXV, label the same parts as a standard TXV—sensing bulb, equalizer port, needle and seat, spring—and note that both inlet ports are present.

Check Valves in Heat Pump Circuits

A check valve allows refrigerant to flow freely in one direction and blocks it in the opposite direction. In a dual-TXV metering circuit, check valves are paired with each standard TXV so that in one mode refrigerant flows through the TXV and meters normally, while in the other mode refrigerant bypasses that TXV through the check valve and is metered by the TXV on the other coil instead.

Trace this logic for a system with TXV-A at the outdoor coil and TXV-B at the indoor coil:

• Cooling mode: High-pressure liquid travels from the condenser (outdoor coil) toward the indoor coil. TXV-B meters the flow; its paired check valve is closed. The check valve bypassing TXV-A is open, so liquid passes around TXV-A freely.
• Heating mode: Flow reverses. TXV-A meters the flow into the outdoor coil acting as evaporator; its paired check valve is closed. The check valve bypassing TXV-B is open, so liquid passes around TXV-B freely.

At any given moment, exactly one TXV is active and one is bypassed.

Fixed-Orifice Metering Devices

Some heat pump systems use a fixed orifice—commonly a piston-type device or a short capillary tube—instead of TXVs. These devices have no moving parts and meter in both directions without a check valve arrangement. Their simplicity reduces cost and service calls, but they cannot adjust to changing load or refrigerant charge conditions the way a TXV can. Many manufacturers use a fixed orifice on one coil and a TXV on the other, or a bi-flow TXV on each coil.

Reading a Dual-Metering Circuit Diagram

When you trace a heat pump metering circuit, use these steps:

1. Identify the mode (heating or cooling) and find which coil is the condenser and which is the evaporator.

2. Follow high-pressure liquid from the condenser outlet.

3. Determine which check valve is forward-biased (open) and which is reverse-biased (closed).

4. Identify which TXV is in the active flow path.

5. Follow low-pressure refrigerant from the active TXV outlet to the evaporator.

Practice this trace in both modes until it is automatic before the bench exercise.

Safety Notes

Before handling any refrigerant circuit components:

• PPE: Wear safety glasses and refrigerant-rated gloves. Liquid refrigerant and flashing refrigerant can cause frostbite and eye injury.
• Lockout/Tagout (LOTO): On any energized or pressurized system, follow your facility's lockout/tagout procedure before opening refrigerant circuits. Even bench components from a system may be under residual pressure—treat all valves and lines as pressurized until verified otherwise.
• Refrigerant handling: Only EPA Section 608 Universal certified technicians may purchase and handle regulated refrigerants. Do not vent refrigerants to atmosphere; use an approved recovery machine.
• Pressure gauges: Inspect gauge hoses for cracks before connecting. Stand to the side, not directly in line with fittings, when pressurizing.
• Cut-away components: Cut-away valves have sharp metal edges. Handle with care and keep fingers away from internal ports.