Where should a thermostat be located in a house?
Central to the area the system serves, in the path of the return air, on an interior wall. That location lets the thermostat read an average of the air the house is actually circulating instead of one pocket of air sequestered in a room.
Why does the thermostat need to be in the return air path?
This is the part most advice skips, and it is the whole reason the location matters.
A thermostat reads the air touching it. Nothing else. So the only question that matters is whether the air touching it represents the house.
Air moving toward a return is the air being pulled back from the rooms after it has done its job, which makes it roughly an average of the whole conditioned space. Air sitting in a dead corner is not an average of anything. It is that corner.
Note the wording: in the path, not next to the grille. In a house the return is usually down at floor level, and the thermostat is up at 48 to 60 inches — so it is never actually beside the return. It sits in the current of air travelling toward it. In commercial the return might be a plenum space or wherever the design puts it, and the principle does not change.
Here is how the cycle actually runs. The system comes on and pushes conditioned air out to the rooms. The stale air in those rooms gets pushed back toward the return first, and the conditioned air follows behind it. When the air coming back has been brought to temperature, the condition is satisfied and the system shuts off. A thermostat in the return path watches that whole process. A thermostat somewhere else watches a fraction of it.
The vent example
Picture a thermostat mounted on the wall right below a supply air vent.
It is hot outside, so the system comes on to cool the house. Cold air pours out of that vent directly onto the thermostat. The thermostat is satisfied in seconds and shuts the system off — while every room in the house is still hot.
Then the cold air dissipates, the thermostat warms back up, and the system kicks on again. That is short cycling, and it is hard on equipment and terrible at conditioning a house.
The same principle runs the other way. You want the thermostat in the current of air moving through the house, not sitting in a draft off any grille — supply or return.
How high should a thermostat be mounted?
Two numbers get quoted, and both are right for different buildings.
48 inches is the accessibility maximum. Under the ADA 2010 Code, thermostats count as operable parts, and the maximum reach height for an operable part is 48 inches. In a commercial building that number is enforced in common areas accessible from patrons and non-maintenance staff.
A thermostat in a locked box with access by maintenance staff in commercial buildings can be mount up to 60 inches. Still in an open area in the return air path. In residential houses, 48 to 60 is still the range. Most HVAC tech’s will install them at 52″ to 54″ to top of thermostat.
There is no residential code that dictates a mounting height for a thermostat in a house. If you are working commercial or anywhere accessibility applies, 48 is a ceiling and not a suggestion if they are accessible to the tenants and public.
Here are the tools you will need to get the system installed.
Where should you never put a thermostat?
Anywhere the air touching it is not the air in the house.
- Under or beside a supply vent — the short cycle described above
- On an exterior wall — it reads the wall, and the wall reads the weather
- In direct sun — it reads the sun
- On a concrete or block wall — heavy walls hold onto yesterday’s temperature and take hours to catch up to the room
- Tight into a corner — air stalls there, so stay off the corner
- In a dead-end room with no return path
Where does the thermostat go with multiple zones?
A zone system splits a house into imaginary sections that get conditioned independently.
The split usually follows how the house behaves or how it gets used. Some examples of this configuration are noted as follows but are not mandated. A building and the people who occupy it will eventually weigh in on this and a designer or owner will give feed back in new construction or complain later.
North-facing rooms on one zone and south-facing on another, because sun load is not the same on both sides. Basement on one, upstairs on another. Or the split people notice most: bedrooms on one zone and the kitchen, living and dining on another. The main living areas are occupied most of the day. Bedrooms are occupied at night. There is no reason to condition them the same way around the clock.
What makes it work is a variable speed unit paired with a damper system. The system can run at 25 percent or 100 percent depending on the load, and the dampers send that capacity where it is called for.
Each zone gets its own thermostat, placed by the same rule as anywhere else — central to what it serves, in that zone’s return path. Put a zone thermostat where it can read air from the wrong zone and you have built an expensive system that fights itself.
The dampers and the motors and the wiring have to be right or none of it works in unity. If you are going to zone a system, be sure you know how to zone a system or get an engineer involved.
What about a house with two furnaces?
Usually the split is obvious from the house: upstairs and downstairs, basement and main floor, or a finished attic space that runs hot and needs its own system. Big houses may take two or three.
Here is the difference between residential and commercial that explains the whole approach. Nobody (there may be a handful in very large mansion type houses) puts a twenty-five thousand dollar rooftop unit on a house. Residential buys several smaller, affordable furnaces and ties the house together with zones instead. It is the cheaper way to the same result.
Once that is in place, the zones across all the furnaces can be run together or separately, and a main thermostat plus remote thermostats can control multiple zones.
A 1,200 square foot apartment and a 7,000 square foot house do not need the same system, and they do not get the same answer to this question.
Do mini splits change where the thermostat goes?
Mostly it stops being a question. A mini split usually conditions one room and comes with its own thermostat and remote, so the same wall location rule applies. Just a much smaller zone.
Where it gets more interesting is the ducted version. I have seen mini split systems feeding ceiling-mounted units with short duct runs into an open space or a couple offices, zone controlled across several heads, all piped back to one condenser outside.
Why is a new thermostat always set to 70 and 78?
Because the residential energy code says so, and most people never find out.
The programmable thermostat section of the 2021 residential energy code requires that a thermostat be shipped programmed by the manufacturer with a heating set point no greater than 70°F and a cooling set point no less than 78°F. The same section requires it to hold a schedule across different times of day and different days of the week, and to be able to run a zone anywhere from 55°F to 85°F.
Code editions get adopted state by state, so check what your jurisdiction is on if it matters to you. The numbers above are what the 2021 edition says.
So that is a factory default, not a rule you have to live with. Change it to whatever keeps you comfortable — the code is telling the manufacturer where to start, not telling you where to keep it.
Keep resources in mind when adjusting temperatures. On a hot day a thermostat set to 68 to cool to will most likely run the majority of the day and drive up power bills.
Before you blame the thermostat
Every article about thermostat placement implies your comfort problem is the thermostat. Usually it is not.
In four years in the trade I never once had to move a thermostat.
I did get called back on a room that would not get comfortable. The customer was certain we had built the system wrong. We went out and proved the airflow matched the drawings — the CFM was exactly what it was supposed to be.
The design was the problem, not the install. We pulled the six inch take-off and put in an eight inch. That moved the room from 240 CFM to 300, and the person was comfortable.
Nothing to do with the thermostat.
So if one room is never right while the rest of the house is fine, the thermostat is almost certainly not your problem — a thermostat is a whole-house device and a single cold room is an airflow question. Some rooms are just harder than others. A room with big south-facing windows takes more capacity than a system built for the average of the house wants to give it. That is the reason zoning exists.
Start with what is feeding the room before you start moving a thermostat.
Common Questions
Should a thermostat be near a return vent?
It should be in the return air path, which is not the same thing. In a house the return sits at floor level and the thermostat is at 48 to 60 inches, so it is never beside the return — it is in the current of air moving toward it. That air is an average of what the house is circulating, which is exactly what you want it reading.
Can a thermostat go on an exterior wall?
It is recommended to never install a thermostat on an exterior wall. An exterior wall tracks the outdoor temperature closer than the interior temperature, so the thermostat ends up reporting the weather instead of the room.
What happens if a thermostat is under a supply vent?
The conditioned air hits it directly and satisfies it in seconds, so the system shuts down while the rest of the house is untouched. Then it warms back up and starts again. Short cycling, poor comfort, and unnecessary wear on the equipment.
Who decides where the thermostat goes?
On commercial work the engineer marks it on the drawings, the electrician roughs in the box, and the mechanical contractor mounts the thermostat and runs the low voltage cable. In a house it is usually the mechanical installer picking the spot — standard height, central to the system, in the return air path.
Does one thermostat work for a whole house?
For a small or simple house, yes. Once you have a sun-exposed side, a second floor, or rooms used at completely different times of day, one thermostat is averaging conditions that are not alike, and zoning or a second system is the answer.
Need some more info:
If you have a twisted run of duct, check out this info on bad fabrication.