The circulator pump of a radiant floor: who enables the pump

This page was translated automatically from Italian. Read the original

Anyone replacing a radiant floor controller usually thinks about the zone valves: one relay per room, an actuator that opens, and that’s it. Then they turn everything on, the valves open, and the floor stays cold. The reason is often dozens of meters away: the pump that sends water to your apartment does not start on its own. In many apartment building systems, the circulator pump is located in the stairwell, inside the user “satellite,” and is turned on by a relay board that obeys the home controller. Once that is removed, no one asks it to run anymore.

This guide stems from the work on ORFHAn and on a Radiax system studied with rehumatism. It explains how to understand if the problem affects you, what options are available, and what rules must be followed by whoever commands a pump.

First of all: does it affect you?

You can find out with the original system still in place, in three steps:

  1. raise the setpoint of one zone, so that it calls for heat (or cooling, in summer);
  2. go to the satellite and look at the pump: many electronic circulator pumps have an indicator light that turns on when they are running;
  3. set all zones to off and see if the pump stops.

If the pump follows the requests, the pump enable comes from the controller, and this guide is for you.

If the pump runs anyway, in continuous circulation or commanded by a differential pressure switch, you don’t need to do anything. The same applies if your valve actuators have four wires instead of two: often the two extra ones are an auxiliary microswitch that closes when the valve is open, wired as a pump enable. In that case, the enable is generated by the physical opening of the valve and will continue to work with any controller.

What we found on a Radiax system

The case that gave rise to this project is instructive, because it disproves the most natural assumptions.

The last point is the most important: a fault in the house, with the system on, leaves the pump running against closed valves until someone notices.

The rules that are not up for debate

The satellite is a common area. Inside, there are almost always the heat meter, the water meter, and sealed probes: these are fiscal instruments used to split expenses, and they are not to be touched. You are working at 230 V on a system that is not yours.

The right way is to speak with the building manager and have the maintenance technician perform the modification. It is best to approach them with a precise technical request, such as “a dry contact enable on the pump branch,” rather than a generic “I would like to modify the system”: the former can be understood by any installer.

Possible paths

A dry contact, and a relay in the stairwell

It is the classic heating system diagram, and it is the recommended one. The controller in the apartment closes a contact without carrying voltage; in the stairwell, a locally powered relay detects that contact and turns on the pump. For the two signal wires, the cable from the old controller can often be reused, if it truly has free conductors.

Dry contact pump enable principle diagram

Why not power the relay from home? Because over dozens of meters of cable the voltage drops, and a low-voltage coil becomes unreliable; and because it would bring energy from your own system into a building-wide device. With a dry contact, the two systems remain isolated from each other.

Two things to clarify with the maintenance technician: whether the board currently in place only handles the pump or also participates in heat metering (in which case it remains powered and only the pump branch is interfaced), and whether the cable towards the apartment has free and identifiable conductors.

The circulator control input

Some electronic circulator pumps have a start/stop or PWM signal input. If your model has one, it is the cleanest solution: the pump is controlled according to the manufacturer’s specifications, without switching off the power every cycle. This must be verified in the manual for your specific model.

A differential pressure switch

A pressure switch on the circuit turns on the pump when the valves open and the pressure changes. No cables and no dependency on the controller, but it is a hydraulic intervention.

Responding instead of the display

On a system like Radiax, there is a fourth way that doesn’t touch anything in the shared plant room: connecting to the system bus and responding to the shared plant room board instead of the display. This is what ORFHAn does with its driver. The trade-off is that the pump behavior remains whatever is decided by that board, with its own timings.

Keep the old controller only for the pump enable

It works, but it is not recommended: it brings back exactly the dependency one wanted to escape.

The right relay

A domestic electronic circulator pump consumes little: the one in our system ranges from 3 to 45 W, with a maximum current of 0.44 A. However, inside it has a switching power supply, which absorbs a peak much higher than the nominal current upon startup. Do not choose the minimum relay: a DIN rail contactor or a relay with generous contacts lasts much longer.

Safety rules in the firmware

Whoever turns a pump on must also know how to turn it off in every situation. The rules we have written, which apply to any homemade controller:

  1. Never the pump without at least one valve open. This must be rechecked every time the program loops, even during manual testing.
  2. At startup, first of all, switch off. You don’t know what state the previous session left things in.
  3. Repeat the state occasionally. A repeated command is harmless and fixes a lost message.
  4. Before a reboot or an update, switch off and wait for confirmation.
  5. If the command comes from a remote device (a Shelly, another relay board), that device must open its relay on its own when requests stop arriving.
  6. The decision remains with the controller. Home Assistant can see the pump status, but it must not be the one commanding it: if the home server goes down, the pump must not be left hanging.

What has been proven