Metering water flow by zone with the Shelly Plus Uni: splitting the heat-pump cost

Here's a very hands-on question from a customer: «I have a heat pump heating two floors, and I need to measure how much water each one uses so I can split the electricity cost by usage. I know I need two Hall-type flow sensors, 3/4". Can I connect both to the same Shelly Plus Uni? And where do I wire them?». Great question — and the answer has a catch worth knowing before you buy: one Plus Uni can meter one flow sensor, not two. For two circuits you need two Plus Uni. Here's why, and how it all wires up.

One Plus Uni meters one flow sensor

A Hall flow sensor doesn't give a continuous value: it emits a train of pulses whose frequency is proportional to the water passing through. To measure flow you have to count those pulses, and the Shelly Plus Uni has a single pulse-counter input: the COUNT IN terminal (purple wire), rated up to 1 kHz. Its other two inputs (IN1 and IN2) are digital status inputs —they detect a contact open or closed— but they don't count pulses or measure frequency, so they can't read a second flow meter.

The takeaway is direct: one flow sensor per Plus Uni. For your two floors, the cleanest setup is two units —two water-meter kits—, each metering its own circuit independently. We confirmed this against the official Plus Uni documentation before answering, because it's exactly the kind of detail that leads to a return if bought blind.

Where each wire goes

The Hall sensor has three wires and the connection is simple. Following the kit's own wiring:

• Yellow wire (signal) → the COUNT IN terminal of the Plus Uni. This is the one carrying the pulses.
• Red wire (+) → positive of the 12 V supply.
• Black wire (−) → negative of the 12 V supply.

The Plus Uni itself runs off that same 12 V supply: its VAC1 terminal to +12 V and VAC2 to −12 V. In other words, the red and black wires of the sensor and the Uni join at the supply; the yellow one is the only wire that goes on its own, to COUNT IN. The sensor works between 5 and 18 V, so it shares the Uni's 12 V supply with no problem (the Uni accepts 9 to 28 V DC).

Once wired, the Shelly app shows the Uni's pulse counter. To turn pulses into liters you apply the sensor's factor (pulses per liter from its datasheet) or calibrate it by passing a known volume and recording the pulses. From there you have that circuit's water usage.

The «cut the connectors» part: how to splice

You've read it right: the sensor comes with its connector and the Plus Uni has bare wires, so you have to join them. The correct way:

• Sensor: cut off the connector and expose the three wires (red, black and yellow). Join them to the Uni's with a terminal block or Wago connectors —fast and reversible— or by soldering with heat-shrink. In a damp area, keep it inside a junction box.
• Power supply: if it's an adapter with a round barrel jack, you have two options: cut it off and strip the two leads, or —cleaner, cutting nothing— use a DC jack to screw-terminal adapter. Either way, measure the polarity with a meter before wiring; don't trust the wire color.

Split by liters or by energy?

Here it's worth being honest. Measuring liters per floor is a good indicator of usage, but it isn't exactly the energy each circuit receives: the heat delivered depends on the flow and on the temperature difference between supply and return. If both floors run at similar temperatures, volume splits the cost reasonably. But if you want a fairer split, each Plus Uni can also read DS18B20 temperature probes on its 1-Wire bus (DATA terminal, blue wire): with one probe on the supply and one on the return per floor, you compute the real energy (flow × temperature rise) and the split of the electricity bill is far more accurate.

Since you need one Uni per circuit anyway, adding the probes doesn't add control units, just the probes themselves (each Uni takes up to 5). It's the difference between splitting «by water» and splitting «by heat actually delivered».

Before you buy, keep in mind

• One flow sensor per Plus Uni. There's a single COUNT IN input. Two floors → two Plus Uni (or two kits).
• The sensor emits pulses, not liters. You'll apply the sensor's pulses-per-liter factor or calibrate it once with a known volume.
• The Plus Uni is low-voltage. It runs on 12 V DC (9–28 V range); don't connect it to mains. The sensor shares that same supply.
• Liters ≠ energy. For an exact cost split, add the DS18B20 supply and return probes and measure the temperature rise.
• Clean splices. Terminal block or Wago, and polarity checked with a meter: swapping + and − can damage the sensor.

Frequently asked questions

Can I connect two flow sensors to a single Shelly Plus Uni?

No. The Plus Uni has a single pulse-counter input (COUNT IN), and a Hall sensor needs that input to count its pulses. The other two inputs are digital status inputs and don't count pulses. For two sensors you need two Plus Uni.

Where do I connect the flow sensor on the Plus Uni?

The yellow wire (signal) goes to the COUNT IN terminal. The red (+) and black (−) go to the 12 V supply, together with the Uni's own power (VAC1 and VAC2).

The sensor has a connector and the Uni has bare wires — what do I do?

Cut off the sensor's connector and, if needed, the supply's jack, and join the wires with a terminal block or Wago connectors (or solder). Before wiring, check the supply polarity with a meter.

How do I go from pulses to liters?

In the Shelly app the Uni counts the pulses. To convert them to liters you apply the pulses-per-liter factor from the sensor's datasheet, or calibrate it yourself by passing a known volume and recording the pulses.

Is measuring just the liters enough to split the cost?

As an approximation, yes, especially if both floors run at similar temperatures. For an exact split it's better to also measure the temperature difference (supply and return) with DS18B20 probes and split by energy, not just by volume.

Does the same Plus Uni measure flow and temperature at once?

Yes. Each Plus Uni reads flow on COUNT IN and, at the same time, up to five DS18B20 probes on its 1-Wire bus. So a single unit per floor gives you liters and temperatures to compute the energy.

What power supply do I need?

A 12 V DC supply powers both the Plus Uni (9–28 V) and the sensor (5–18 V) at once. The kit already includes one. If you use a supply with a barrel jack, cut it off or use a screw-terminal adapter.

Summary and products

• Water meter kit 3/4" — kit with Hall sensor, Plus Uni and 12 V supply. For two floors you need two: one per circuit.
• Shelly Plus Uni — the module that counts the flow meter's pulses (COUNT IN) and reads temperature probes over 1-Wire.
• Hall water flow sensor 3/4" — pulse flow meter rated 1 to 30 L/min, in case you need a spare or standalone sensor.
• DS18B20 temperature probe — to measure supply and return and split the cost by real energy, not just by liters.

Tell us how your heat pump's two circuits are laid out and we'll confirm the setup and, if you like, help you get the split calculated automatically. Technical support in Spanish and shipping from Spain.