3-way valves: how to choose and wire them

In the guide to underfloor heating control we saw that the three-way mixing valve is the first link in the chain: it takes hot water from the heat source, blends it with water returning from the floor, and sends the right temperature to the loops. A 3-way mixing valve for underfloor heating is a simple component, yet it is often chosen or wired wrongly, and the consequences show up for years.

This article is for anyone who has to specify one on a project or fit one on site, whatever the brand.

Mixing or diverting

Three-way valves are used in two ways that look the same but do opposite jobs.

Mixing. Two inlets and one outlet. Hot water (port A) and return water (port B) come in, mixed water (port AB) goes out. Opening port A more warms the flow; opening port B more cools it. This is the typical case for an underfloor circuit fed by a boiler.

Diverting. One inlet and two outlets. Water enters at AB and leaves through A or B, or partly through both. It is used to send water to one of two loads, for example the heating circuit or the hot water cylinder, or as a bypass for a terminal unit.

Many valves can be fitted either way, but not all: the manufacturer states which configuration is allowed. Fitting a mixing valve as a diverter, or vice versa, can cause noise, leakage across the seat and unstable control.

Kvs in brief

The figure that decides the valve size is Kvs: the flow of water, in cubic metres per hour, that passes through the fully open valve with a pressure drop of 1 bar. The higher the Kvs, the bigger the valve.

Flow, Kvs and pressure drop are related by:

Δp [bar] = (Q / Kvs)²

where Q is the flow in m³/h. Pipe diameter says nothing about the right Kvs: a valve with connections matching the pipes is almost always too big.

A sizing example

An underfloor circuit has to deliver 10 kW with a 5 °C difference between flow and return.

1. Flow rate. Q = P / (1.163 × ΔT) = 10 / (1.163 × 5) ≈ 1.7 m³/h.
2. Target pressure drop. For a mixing circuit, the usual target is a pressure drop across the open valve of about 10 kPa, or 0.1 bar. A valve with too little pressure drop does not “take control” of the flow and regulates poorly.
3. Kvs. Kvs = Q / √Δp = 1.7 / √0.1 ≈ 5.4 m³/h.
4. Selection. Commercial ranges come in steps: 2.5, 4, 6.3, 10. Pick the nearest, here 6.3, and recalculate the actual drop: (1.7 / 6.3)² ≈ 0.07 bar, or 7 kPa. That’s fine.

A valve with Kvs 10 or 16, chosen “because the connections fit”, would nearly always work barely open: a small movement of the actuator would change the temperature a lot, and the flow temperature would hunt.

The actuator: 3-point or 0–10 V

The valve is moved by an actuator. There are two common types.

3-point. It has two commands: open and close. The controller drives it with pulses of varying length. It is simple and cheap, but the controller never knows the valve position for sure, unless it runs the full stroke again.

Modulating 0–10 V. It receives a continuous signal: 0 V valve closed, 10 V valve open, values in between for intermediate positions. Many models also return a position signal. It is the best choice for weather compensation, because the position is always known and the controller can use proportional-integral control.

For both types, stroke time matters: the time the actuator takes to go from fully closed to fully open. For underfloor circuits, long stroke times of one to two minutes are fine, because the system is slow. Times that are too short make control jumpy.

Wiring it to Brick

In the panel, a modulating valve needs only a few conductors:

• actuator power, often 24 V AC, protected and separate from other loads;
• the 0–10 V control signal, from one of the controller’s analogue outputs;
• the signal common, connected to the controller’s reference;
• position feedback, if available, to an analogue input.

And one sensor: the flow sensor, downstream of the valve and pump, a few pipe diameters away from the valve so the water is already well mixed. Strapped to the pipe with thermal paste and insulated, or better still, in an immersion pocket.

Brick works with Siemens SAX, SAS and SSB modulating 0–10 V actuators and Siemens VXG three-way mixing valves, but the principle applies to any brand.

Mistakes seen on site

• Ports swapped. Port A connected to the return and B to the boiler flow: the logic is reversed and control becomes impossible. Ports are marked on the body: check them before tightening.
• Actuator fitted the wrong way round, with its opening direction reversed relative to the signal. Many models have a switch to reverse the direction.
• Flow sensor too close to the valve, out of the flow, or uninsulated. It reads temperatures that are not what goes into the floor.
• An oversized valve. The most common mistake, and the most visible on charts: a flow temperature that keeps rising and falling around its set point.
• No high-limit thermostat. Valve, actuator and controller can all fail. The hard-wired high-limit thermostat on the pump is still mandatory to protect the screed.

Before you switch on

Two simple tests save a lot of phone calls. First: with the system off, drive the valve to 0 V, 5 V and 10 V and check that it travels its full stroke in the right direction. Second: with the system running, watch the flow temperature chart for an hour. If it swings by more than a degree around the set point, lengthen the integral time or check the Kvs. Set the heating curve afterwards, once the flow temperature is stable and predictable.