Why does one radiator not have a thermostat?

Question

We recently exchanged on a new house, but we've just noticed that one of the radiators doesn't have a thermostat. Why is that?

Scott · Answer

Most likely the bypass radiator - also a radiator in the same room as the room thermostat shouldn’t have a TRV as they can work against each other

Dylan · Answer

This is due to being called the bypass which allows the expansion of heat to release from the central heating system when is turned off or reached temperature to the ambient temperature required. This radiator will also feel the hottest when the others are cooling down due to releasing heat from the system. 
They are normally fitted in a bathroom or hall way.

Pawel · Answer

It is normal—and usually intentional—for one radiator in a heating system to **not have a thermostatic radiator valve (TRV)**. The most common reasons are the following:

### 1. System Reference / Bypass Radiator

Traditionally, one radiator (often in a hallway or landing) is left without a TRV so it can act as a **constant heat emitter**. This ensures:

* Continuous water circulation when other TRVs close
* Protection of the boiler and pump from dead-heading
* Stable system operation

In older systems, this radiator effectively acts as a **bypass**.

### 2. Room Thermostat Control

If the property uses a **single wall-mounted room thermostat**, the radiator in that room should **not** have a TRV. Otherwise:

* The TRV could shut off the radiator before the room thermostat is satisfied
* The boiler would continue running unnecessarily, reducing efficiency

This radiator provides accurate feedback to the main thermostat.

### 3. Boiler or System Design (Older Installations)

Many older heating systems were installed **before TRVs were standard** or before modern automatic bypass valves (ABVs) were required. At the time:

* One permanently open radiator was considered best practice
* Retrofitting a TRV there could cause pressure and flow issues unless an ABV is installed

### 4. Safety and Reliability

Leaving one radiator without a TRV ensures:

* Minimum flow through the system at all times
* Reduced risk of boiler short-cycling or pump stress
* Better overall system reliability

### When This Can Be Changed

You *can* fit a TRV to every radiator **if**:

* The system has an **automatic bypass valve (ABV)** fitted, **and**
* The radiator is **not** in the same room as the main thermostat

A heating engineer can confirm this quickly during a service.

### Summary

One radiator usually has no thermostat because it:

* Acts as a bypass or reference radiator
* Prevents system flow problems
* Works correctly with a central room thermostat

If you want, you can tell me:

* Boiler type (combi, system, or heat-only)
* Whether you have a wall thermostat
* Approximate age of the system

I can then advise whether adding a TRV would be safe and worthwhile.

Russ · Answer

Typically this is the rad in the hall and is designed to be a heat sink incase of pump failure. Therefore its important that this rad has full flow and has two lockshield

Clive · Answer

The system needs a bypass radiator with out any control on it,

Ian · Answer

The radiator without a thermostat should be the room where thermostat is situated. The thermostat will control that room to set temperature, having no thermostat also allows a minimum boiler flow rate and an open circuit for pump to operate.

Why does one radiator not have a thermostat?

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