A thermal switch that shuts the boiler off early and cuts gas bills

A thermal switch that shuts the boiler off before it should

The paradox is this: the boiler shuts off when the radiator is already at 70 degrees, and that leftover heat keeps raising the living room temperature after the thermostat has already cut out. A homeowner in northwest Madrid decided to cut things off earlier. He fitted a thermal switch costing a few euros —a normally closed contact that opens at 40 degrees— in series with the home's thermostat, attached to the bottom of the nearest radiator. When the return water reaches that temperature, the boiler stops. The radiator finishes heating the room with what it already holds inside. And the gas that isn't burned doesn't get paid for.

The result reported by the author: the boiler idle more than 30% of the time compared with the previous day, and a gas bill of €156 for two months (November 9 to January 9) in a three-bedroom flat with children, hot water included. With the thermostat set at 21.5 degrees by day and 19.5 at night.

What radiator hysteresis is and why it matters

The physics argument is simple. A radiator is not a tap: when the boiler stops pumping, the iron stays hot and keeps emitting heat. In a conventional system, the room thermostat cuts out with the radiators at flow temperature —the author mentions 70 degrees at the top—, so that surplus is dissipated after the cut-off and pushes the house temperature above the setpoint. Energy you paid for that wasn't needed.

The gadget reverses the order. The cut-off is triggered by the return water, not the room air. If, while the radiator cools from 40 degrees downward, the setpoint is reached, the cycle has closed without spending a single extra gram. If not, the contact closes again and the boiler starts up on its own. The full setup: the thermal switch, 4 metres of 2x0,75 cable and two self-tapping screws.

The numbers measured at home

With 4 degrees outside, the recorded cycles kept getting shorter as the house approached the setpoint: runs of 8 minutes with 10 off at 19.1 degrees; 3 minutes with 12 off at 20.8. In total, 40 minutes of boiler per 104 of rest while keeping the radiators hot. The house reached 21.5 and took three hours to lose two tenths, with the heating off and 3 degrees outside.

The longer-term tracking is summed up by the author himself: from January to March, 22% fewer kWh of gas than the previous year, in the same flat with the same habits. Another user who replicated the system reported 20% less consumption and a bill 10% lower than the previous year's, though with one honest caveat: he ran the heating 33% more hours, from six to eight a day.

The criticism: controlling water instead of air

Not everyone bought it. The most serious technical objection was that the system adds the radiator's thermal inertia to the control loop, and that if the boiler cuts out earlier, the room temperature will be lower. In other words: comfort is traded for savings. The author's response was to admit that his home has high hysteresis and that he doesn't mind taking a little longer to reach the setpoint.

There were also those who pointed out the electrical risk. The warning forced him to edit the original post in capital letters: some thermostat contacts operate at 220 volts, so insulation must be extreme. The original setup was encapsulated in plastic and used the relay contact of a boiler that didn't handle mains voltage. The warning was pinned in the text: if you don't know how to do it safely, don't do it.

Where it fits and where it doesn't

The gadget shines in radiator installations with a fixed-flow boiler and a room thermostat with generous hysteresis. It fits less well with underfloor heating, where the inertia of the screed —10 centimetres thick in some cases— overwhelms any return-temperature control, and where the flow temperature should never exceed 38 degrees. For those systems there are thermostats with dual probes, room and floor, that already handle the problem another way.

The open question is not whether the gadget works in one particular house. It's how many Spanish boilers are burning gas to heat radiators that no longer need more heat. With these numbers, the answer invites you to look at the bottom of the radiator.