SevyDevy

Design a smart thermostat like nest

PremiumIoT systemEasy

  • Local schedule
  • Demand response
  • MQTT

Quick answer

This ioT system interview question asks you to design a smart thermostat like nest. Aim for a design that collects data from many devices and acts on it reliably, then explain device connections, ingestion, storage and trade-offs. Interviewers want to see how you handle unreliable networks, device scale and data volume.

The problem

Design a smart thermostat like Nest: a Wi-Fi thermostat on the wall that runs heating and cooling, follows a schedule, and can be controlled from a phone app. The hard parts are that the home must stay comfortable when the cloud is down, and that utility events across a city must not switch every air conditioner at once.

The thermostat replaces a wired one and switches the furnace or air conditioner through relays. Users set schedules and change the temperature from the app. Utilities can enrol homes in demand response: on a hot afternoon they ask homes to raise the setpoint for two hours.

What must a smart thermostat like nest do?

  • Remote control: Change the setpoint or mode from the app; the device applies it within 2 seconds on wired power, and within 10 seconds when it runs on its battery.
  • Local schedule: The device runs the schedule on its own, so heating works when Wi-Fi or the cloud is down.
3 more requirementsUnlock to see all

How well must it do it?

  • Comfort first: Heating and cooling never depend on the cloud; the device enforces safety limits on its own.
  • Equipment safety: The compressor stays off for at least 5 minutes between runs, whatever the commands say.
2 more requirementsUnlock to see all

What scale should you design for?

Use these numbers in your estimates.

The scale numbers unlock with this questionUnlock

Key concepts

  • Local schedule
  • Demand response
  • MQTT
  • Device shadow
  • Wi fi
  • OTA

How you’ll practise

6 short steps · save and come back any time
  1. Requirements
  2. Estimate
  3. Device and connectivity
  4. Ingestion and processing
  5. Device management and updates
  6. Wrap-up

Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.

What do interviewers look for?

In any IoT system design round.

  • Clarifies device count, message rate and payload size
  • Handles devices that go offline and reconnect
  • Plans ingestion that absorbs bursts
  • Separates hot real-time data from long-term storage
  • Covers device identity, updates and security
  • Explains what runs on the device and what runs in the cloud

Questions people ask about this interview question

Which design type is this question?
It is an ioT system question. You design a smart thermostat like nest and explain your choices the way you would in an interview.
Is this question asked at entry level?
It is marked Easy, so it suits engineers who are new to ioT system interviews. It is also a good warm-up before harder questions.
What should I cover first?
Start with the devices, what they send, how often and what must happen with it. Agree on scope with the interviewer before you design anything.
Can I practise it for free?
It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.

Related questions

Updated · Written by the SevyDevy team

Premium questionSign up free, then unlock it