Nest Learning Thermostat: Why Room Temperatures Overshoot and How TRV Balancing Fixes It

The wall sensor sees only one room

A Nest Learning Thermostat samples air temperature through the sensor behind its display and compares that reading with the target set on the device. When the measured value falls below target, it closes the relay that fires the Worcester Bosch Greenstar boiler or opens the demand signal to the heat source. The relay remains closed until the sensor reaches target plus a small hysteresis band, typically around 0.5 degrees Celsius.

That single measurement point governs every radiator on the circuit. It has no direct knowledge of how much heat the bedroom, front room or far landing radiator is delivering at the same moment.

Overshoot shows up when the room containing the thermostat warms more slowly than rooms outside its view. A hallway radiator under the Nest may be undersized or short of flow. While that wall sensor continues calling for heat, a bedroom radiator two metres of pipe away may already have driven its room to 23 or 24 degrees Celsius.

The boiler is then running to satisfy the coldest measured point. Every radiator receiving a generous share of the flow keeps adding heat until the thermostat room finally reaches its target. The Nest is following its control logic; the uneven water distribution across the emitters is the fault that produces the hot rooms.

What unbalanced flow does to radiators

Water leaving the boiler at 70 degrees Celsius follows the path of least resistance. The radiator closest to the pump, especially one with a short pipe run and a wide-open lockshield valve, receives the highest flow rate and reaches full output within minutes. A radiator at the far end of the circuit has longer pipework and more fittings in the way, so its flow rate is lower and its return is cooler.

That is how one house can feel too hot in one room and too cold in another during the same heating cycle. Near radiators may run close to flow temperature across almost the whole panel. Far radiators can be hot at the inlet and much cooler at the outlet, sometimes showing a 15 degree drop from the top-left area to the bottom-right area of the panel.

The design target is a consistent temperature drop of around 11 degrees between flow and return on every emitter. Without balancing, the first radiator might show a 6 degree drop while the last shows a 20 degree drop. The last room then struggles to reach setpoint, while the first room becomes stifling before the thermostat is satisfied.

A thermostatic radiator valve deals only with the room temperature at its own radiator. Once the air around the valve head reaches the chosen setting, the head throttles flow through that radiator. During the warm-up period, though, it cannot correct the share of water each radiator receives from the circuit. Two radiators with identical TRV settings can heat their rooms at very different rates when their flow rates are different. The lockshield setting is what proportions that flow during the call for heat.

Balancing with lockshield valves

Every radiator has two valves. One end carries the TRV or a manual wheelhead. The other end carries the lockshield, usually hidden under a plastic cap and adjusted with a small adjustable spanner or a purpose-made lockshield key. Balancing means setting those lockshields so every radiator works with roughly the same flow-to-return temperature drop.

The usual method needs two clip-on pipe thermometers, or a single infrared thermometer used quickly on both pipes. Begin with a cold system. Open every lockshield fully. Fire the boiler with all TRVs wide open, so the system is calling everywhere while the readings are taken.

The radiators closest to the pump will warm first. On the first radiator, close the lockshield down to roughly a quarter turn open. Measure the flow pipe and return pipe temperatures, then adjust the lockshield until the difference is about 11 degrees Celsius.

Move out through the circuit radiator by radiator. Each successive lockshield is usually left slightly more open than the previous one, because the farther units need less restriction to reach the same temperature drop. The furthest radiator from the pump usually ends up fully open.

If the furthest radiator still cannot reach an 11 degree drop with maximum flow available at its lockshield, the limitation is usually circulator pump speed or pipe sizing. A Grundfos or Wilo pump set to its lowest speed on a large property can starve the index radiator even when the lockshields have been opened in the correct sequence. On a system with a variable-speed pump, balancing is done at a fixed proportional-pressure setting so the readings remain stable while each radiator is adjusted.

Allow several minutes after each adjustment before trusting the thermometer readings. Pipe temperature lags behind valve movement, and an early reading can send the adjustment in the wrong direction. On an eight-radiator house, those settling delays are why a full balance can take an afternoon.

When the circuit is balanced, the Nest room receives heat at a rate closer to the rest of the house. The relay opens nearer the true whole-house setpoint, and the fast rooms no longer run two or three degrees beyond comfort before the thermostat is satisfied. The remaining overshoot should shrink to a fraction of a degree when the emitters and room heat losses are otherwise suitable.

Weather compensation changes the flow temperature

Weather compensation control changes the way the boiler delivers heat. A weather-compensated Greenstar with an outdoor sensor lowers the flow temperature as the outside air warms, instead of running a fixed 70 degree flow and cycling on and off. At 5 degrees outside it might run 60 degree flow; at 12 degrees outside, it might run 45 degrees.

Lower flow temperatures give gentler heat delivery and reduce the tendency to overshoot. A radiator is no longer being driven with 70 degree water when the room only needs a slow input of heat to hold setpoint.

A Nest operates by default as an on-off call for heat and does not natively drive the modulating weather compensation curve on most boiler models. When both are used, the Nest opens the heat demand while the boiler sets its own flow temperature from the outdoor sensor. Balancing still matters in that arrangement, arguably more at low flow temperatures. At a 45 degree flow, the smaller temperature difference across each radiator makes maldistribution show up as rooms that never quite reach target. A poorly balanced system running low flow can leave the far end short of heat altogether.

Check circulation before valve adjustments

If several radiators are cold at the bottom or slow to warm, dose the system with Fernox F1 and confirm circulation before spending the afternoon on lockshield settings. Sludge and magnetite settled in the lower panel can imitate a flow imbalance, and valve adjustment will not clear that deposit from the radiator body.

Insulation can change the apparent overshoot

A room that overshoots and then holds heat for hours after the radiator cools is behaving differently from a room that overshoots and then cools quickly. The first pattern points to a room retaining the added heat well, so a balancing correction is more likely to hold. The second pattern shows heat escaping through the building fabric fast enough to bring frequent boiler cycles, with each new cycle giving the faster radiators another chance to run ahead of the thermostat room.

Raising loft insulation from a typical 100mm to the current recommended 270mm of mineral wool changes the heat demand of the rooms below. Cavity wall insulation changes demand at the external walls in the same way. After that work, the old lockshield settings are often wrong, because the rooms need far less heat input to maintain temperature.

Radiators that were properly proportioned for a leakier building fabric can become oversized for the insulated rooms they serve. After insulation work, the circuit needs rebalancing if the overshoot correction is expected to last. The same logic applies to an air source heat pump retrofit, where the system runs at 35 to 45 degree flow and radiators are often upsized to compensate.

On those lower-temperature systems, the balance tolerances are tighter because the driving temperature difference between water and room is only a fraction of what a gas boiler provides. Pipe thermometer readings can show whether the water side is behaving evenly, yet they do not say how long each room will coast after the radiator cools.