Remote Temperature Sensor: The Room the Thermostat Ignores

A thermostat runs the whole house from one point on one wall, and a second reading moves that point into a room somebody is actually sitting in.

The Thermostat Only Knows One Room

A conventional thermostat measures the temperature at exactly one point on
one wall, and the whole system runs to satisfy that point. Everywhere else in
the house is inferred, and the inference is often wrong.

A remote temperature sensor is a small device placed somewhere else that
reports back to the thermostat, so the control decision can be made on a
reading from a room that matters rather than only from the hallway the
thermostat happens to be in. That is the entire idea, and it fixes a specific
and common complaint at a very low cost.

What it fixes, and what it cannot

The line between the two halves of that table is the important one. A remote
sensor changes which room the system optimises for. It does not change how much
heat reaches any room, so where the real fault is delivery, moving the sensor
just moves the discomfort somewhere else.

Averaging, or switching?

Systems handle multiple sensors in one of two ways, and they behave very
differently.

• Averaging. The thermostat takes the mean of the sensors
  included at that moment. This smooths a house with mild variation and is a poor
  answer where one room is genuinely an outlier, because averaging a cold room
  with three comfortable ones produces a number that describes nobody’s
  experience.
• Selecting. One sensor is in control at a time, chosen by
  schedule or by occupancy. This is the more useful behaviour for the common
  case: living space during the day, bedroom at night.

Before buying, find out which of these the equipment does, and whether the
schedule is per-sensor or global. That single question determines whether the
purchase will solve your complaint.

Where the sensor should actually go

A badly placed remote sensor is worse than none, because it drives the whole
system from a misleading reading. The same rules that apply to a thermostat
apply to it:

• On an interior wall, not an exterior one, and not on a wall with a chase or
  duct behind it.
• Out of direct sun at every time of day, including winter sun that reaches
  further into a room.
• Away from supply registers, from a mini split head’s discharge, and from
  the draught of a door.
• Away from heat sources — a lamp, a television, a computer, a kitchen.
• At roughly the height people occupy, not at floor level or on a high shelf,
  because a room stratifies and the reading from the ceiling is not the reading
  from a chair.

Where it belongs in the order of fixes

A cold or hot room has a cause, and a sensor is the right answer only for
some of them. Working through the list in order avoids buying the wrong
thing:

• Is the room getting air at all? A blocked register, a
  crushed flexible duct or a closed damper is a delivery fault. The whole-system
  version of this is
  airflow per ton, and the room-level version is usually a
  five-minute inspection.
• Can air get back out of the room? A closed door with no
  return path pressurises the room and stops the supply entering. That is a
  transfer path problem, and it is astonishingly common
  in bedrooms.
• Is the room’s own load higher than the design assumed?
  Three exterior walls, a large window, a room over a garage. That is an
  insulation and load question rather than a control one.
• Is the room genuinely on a different schedule from the rest of the
  house? Now a remote sensor is the correct and cheap answer.
• Does the room need its own equipment? Where the load or
  the schedule is far enough from the rest of the house, the honest answer is a
  zone, which is a design decision covered in
  adding a zone later.

Ducted, ductless, and what “remote sensor” means in each

On a ducted system the sensor is what it sounds like: a device reporting
room temperature to a central thermostat that runs one air handler.

On a ductless system the picture is different, because each head has its own
sensor built into it, in the return air path at the top of the unit — which is
near the ceiling, where the air is warmest in heating and coolest in cooling.
That is why a mini split can hold a room a couple of degrees off what the
occupants feel, and why many systems offer a setting that shifts control to the
handheld controller’s own sensor instead, or to a wall-mounted one. Turning
that on is frequently the single most effective adjustment available on a
ductless system, and it costs nothing.

It also interacts with where the head was mounted in the first place, which
is a decision made at install time and expensive to revisit — see
wall mounted head placement and
how many heads a house actually needs.

What it does to running cost

Usually very little in either direction, and claims otherwise deserve
scepticism. Controlling from a more representative room can reduce overheating
elsewhere, which saves a little. It can equally mean the system runs longer to
satisfy a genuinely harder room, which costs a little.

What it reliably buys is comfort, and comfort is what stops people
intervening — the setpoint changes, the space heater, the closed vents — which
are the things that actually cost money on a heat pump. The behaviour worth
avoiding in cold weather is covered in
what to change and what to leave alone, and if the whole
house struggles at once, the question is capacity rather than control, which is
where the balance point sits.

Practical notes before buying

• Compatibility is specific. Sensors are generally tied to
  their own manufacturer’s thermostats, and often to particular models within
  that range. Check the exact model rather than the brand.
• Battery life and what happens when it runs out. Some
  systems fall back to the thermostat’s own sensor silently, which is a comfort
  problem that appears months later with no obvious cause.
• Range and interference. A wireless sensor two floors down
  through masonry is not a certainty. Confirm the connection before deciding
  where it lives permanently.
• How many the system supports, and whether that limit
  counts a sensor built into the thermostat itself.

Frequently Asked Questions

What does a remote temperature sensor do?

It measures temperature somewhere other than the thermostat’s own location and reports it back, so the system can make its control decision using a reading from a room that matters rather than only from wherever the thermostat happens to be. It changes which room the system optimises for, and nothing else.

Will it fix a cold room?

Only if the room is cold because the thermostat is being satisfied elsewhere. If the room is cold because it is not receiving enough air — a blocked register, a crushed duct, a closed door with no return path, or a load higher than the design assumed — a sensor reports the problem without solving it, and moving control there just moves the discomfort somewhere else.

Should sensors average or take turns?

It depends on the house, and it is worth checking which your equipment does before buying. Averaging suits a house with mild variation and handles an outlier badly, because averaging one cold room with three comfortable ones produces a number describing nobody’s experience. Selecting one sensor at a time by schedule suits the common case of living space by day and bedroom at night.

Where should I put it?

The same rules as a thermostat. On an interior wall with no chase or duct behind it, out of direct sun at every time of day, away from supply registers and door draughts, away from lamps, televisions and kitchens, and at roughly the height people occupy. A room stratifies, so a reading from a high shelf is not the temperature anyone is sitting in.

Do mini splits have this problem?

In a specific form. Each head senses return air at the top of the unit, near the ceiling, where air is warmest in heating and coolest in cooling, so the room can sit a couple of degrees off what occupants feel. Many systems offer a setting that shifts control to the handheld controller’s own sensor or to a wall-mounted one, and enabling it is often the most effective free adjustment available.

Will it save money?

Usually very little either way, and claims otherwise deserve scepticism. Controlling from a more representative room can reduce overheating elsewhere, and it can equally mean longer run times to satisfy a harder room. What it reliably buys is comfort, which matters because discomfort is what leads to setpoint changes, space heaters and closed vents — and those do cost money.

Will any sensor work with my thermostat?

No. Sensors are generally tied to their own manufacturer’s thermostats and often to particular models within that range, so check the exact model rather than the brand. Also check how many the system supports, whether that count includes the thermostat’s own sensor, and what the system does when a sensor’s battery dies.

What happens if the battery runs out?

Some systems fall back to the thermostat’s own sensor silently, which produces a comfort complaint months later with no obvious cause and no alert to connect it to. Find out whether yours warns you, and treat an unexplained return of an old comfort problem as a reason to check the sensor before anything else.