The safety unit is a mandatory accessory as part of the water heater installation. Indeed, water increases its volume while being heated. The circuit being closed, it is necessary to provide a safety device to prevent damage to the circuit elements and to not compromise the security. Its function is to maintain a maximum pressure of 7 bar inside the water heater. It releases the pressure to prevent the explosion of the water heater.
The safety unit allows water to escape (through the drain line) when the water heater is in operation (water being heated). The consumption “lost” is about 3% of the water heater volume per heating cycle, which is not negligible.
The safety unit is a relief valve (pressure limiter – 7 bar for water heater up to 10 kW) to which a siphon is associated. It is composed of a check valve (in order not to pollute the network when coming back and to contain the pressure of the DHW device), a stop valve, a drain valve, a safety valve (calibrated to 7 bar).
The connection must comply with current standards of the country of installation (e.g. DTU 60-1 plumbing in France). The equipment needed are: an adjustable wrench, pliers, screwdriver, cutter, teflon or oakum grease, flat joints of adapted dimensions if necessary.
The upper part of the safety unit 3/4″ (20×27) is connected directly to the cold water inlet (blue) of the water heater by means of a dielectric union (3/4″ rotating / 1/2″). It is important not to intersperse anything between the safety unit and this inlet. The dielectric union is screwed on the water heater and then the safety unit on the male part of the union. The assembly is carried out with gaskets supplied (dielectric-water heater) or PTFE (teflon) or oakum for the dielectric part – safety unit. The tightening torque should not exceed 40N.m (equivalent to 8 kg with a 50 cm long wrench). Place the siphon on the safety unit and connect to its discharge towards the sewer.
Important note: installing a thermostatic control valve
The piping may be rigid (cooper) or flexible (braided hose) but in any case must be able to withstand a temperature of 100°C and a pressure of 10 bar. If it’s not the case (synthetic material or PEX) you must imperatively use a temperature limit control (or a thermostatic control valve MMV-C). As the limiter is distributing only mixed water, the water heater remains filled longer. Scaling in pipes and valves is reduced.
schéma principe raccordement groupe sécurité Source Thermor
Important note: maintenance
You can check that the safety unit is working using the drain valve, a small amount of water should flow into the siphon. Manufacturers recommend to manipulate the safety unit valve (scroll wheel) at least once a month. This operation allows to evacuate any deposits that might clog the safety unit. The replacement of the safety unit by plumber can cost you between £100 and £150.
Manufacturers recommend to replace it every 3 to 5 years. Otherwise, the guarantee of the water heater can be cancelled.
The quality of the safety unit is essential. Indeed, with time the valve becomes faulty. The unit therefore allows water to pass in the direction of the siphon in a significant way.
Steps to change a safety unit without draining the water heater:
- prepare a bucket and a mop
- disconnect the water heater power supply 24h to 48h before to ensure the water inside the water heater is not hot anymore
- close the cold water inlet of the housing (main water shut off valve)
- close the cold water inlet of the safety unit so that the water heater does not empty
- disconnect the exhaust purge pipe, loosen and remove the siphon
- briefly open hot water from any tap to “break” pressure, then close it again
- unscrew the safety unit. Screw the new unit. Open the cold water valve on the unit. Open the main water shut off valve, restore the power, open a tap to hot water position to purge air.