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Boiler Technician interview questions and how to answer them

Boiler interviews are mostly troubleshooting conversations. The service manager describes a failed system and listens for the order you'd check things in, and whether you'd ever leave an unsafe boiler running. Prepare stories from real calls, with the readings and parts involved.

12 questions7 minute read

Part 1

1.Walk me through an annual cleaning and inspection on a commercial boiler.

Why they ask

It's the bread-and-butter job, and the order you do it in shows whether you've actually done one or just watched.

How to answer

  • Start with lockout and tagout, then confirm the boiler and burner model before you open anything
  • Pull and inspect the burner, clean the fireside, check refractory and gaskets
  • Test the safeties: blow down the low-water cutoff, check the relief valve, test the flame safeguard and limits
  • Fire it back up and do a combustion analysis, adjusting gas pressure and air as needed
  • Write up findings with readings and photos for the building's records

2.A steam boiler keeps shutting down on low water. How do you find the cause?

Why they ask

Low-water trips are one of the most common steam calls, and the cutoff is usually doing its job. They want to see if you look past it.

How to answer

  • Confirm the actual water level in the sight glass and check that the cutoff and gauge glass aren't clogged
  • Look at how much the feeder is adding, which points to a leak or water leaving through a bad vent
  • Check the returns, traps and wet returns for leaks, clogs or water hanging up in the system
  • Consider surging or priming from dirty water or oil after a recent repair, and skim the boiler if needed

3.How do you set up combustion on a burner, and what are you looking for?

Why they ask

Combustion setup separates techs who run an analyzer from techs who understand what the readings mean.

How to answer

  • Zero the analyzer in fresh air and sample from the right spot in the flue
  • Set gas pressure to the manufacturer's spec with a manometer before touching air
  • Adjust air for the manufacturer's target oxygen and carbon monoxide readings at high and low fire
  • Check draft and flue temperature, and look at the flame itself
  • Record the final numbers so the next tech has a baseline
4.A condensing boiler keeps locking out on ignition failure. Where do you start?

Why they ask

Modern modulating boilers fail differently from old cast iron, and many shops service a lot of them.

How to answer

  • Pull the fault history from the control before resetting anything
  • Check the condensate trap and drain, since backed-up condensate is a common culprit
  • Inspect and clean the flame sensor and igniter, and measure flame signal
  • Check intake and exhaust venting for blockage, and verify gas pressure under load
5.What's the difference between one-pipe and two-pipe steam, and what goes wrong with each?

Why they ask

A lot of older buildings still run on steam, and techs who only know hydronics struggle with it.

How to answer

  • One-pipe systems send steam and condensate through the same pipe and rely on air vents on each radiator
  • Two-pipe systems use separate returns and steam traps at each radiator
  • One-pipe problems: bad vents, pitch issues, water hammer, radiators that won't heat
  • Two-pipe problems: failed traps passing steam into the returns, banging, uneven heat

Part 2

6.When would you refuse to leave a boiler running?

Why they ask

They need to know you'll red-tag unsafe equipment even when a manager or tenant is pushing to get the heat back on.

How to answer

  • Name clear cases: a failed relief valve, a jumped safety, carbon monoxide spilling, a cracked section or signs of dry firing
  • Explain how you'd shut it down, tag it and tell the owner in writing
  • Offer the temporary options you can safely give, such as space heat or a quick part run
  • Mention calling your supervisor so the decision isn't only on you
7.How do you deal with scale and oxygen corrosion in a hydronic system?

Why they ask

Water problems destroy heat exchangers, especially on condensing boilers, and a lot of techs ignore them.

How to answer

  • Look for signs like constant makeup water, sludge in strainers and rust in the air separator
  • Find and fix the leak that's bringing in fresh water first
  • Flush and clean the system, then treat and test the water to the manufacturer's spec
  • Suggest a dirt separator or filter when the system has old cast iron radiation
8.Read this ladder diagram and tell me what happens when the thermostat calls.

Why they ask

Controls faults are where most service time goes, and wiring diagrams are how you find them.

How to answer

  • Trace the call through the limits and the low-water cutoff to the burner control
  • Explain the sequence: prepurge, ignition trial, flame proving, then main flame
  • Say where you'd put your meter to prove each step works
  • Point out the safeties that would break the circuit
9.It's freezing out and the part you need won't arrive until tomorrow. What do you do?

Why they ask

No-heat calls in winter are about judgment and communication as much as repair.

How to answer

  • Check whether a safe temporary fix exists, and never use one that bypasses a safety
  • Call supply houses and your shop for a same-day part
  • Protect the building from freezing with temporary heat or by draining vulnerable lines
  • Tell the customer exactly what's happening and when you'll be back
10.Tell me about a time a customer was angry about a repair or a delay.

Why they ask

Property managers and tenants are often upset before you walk in. They want a tech who calms the room.

How to answer

  • Set up the situation briefly: who, what failed, why they were upset
  • Explain what you said and did, especially how you kept them informed
  • Share the outcome and what you'd repeat next time

Part 3

11.How do you feel about on-call and keeping up with paperwork?

Why they ask

Missed callbacks and unfinished tickets cost the shop money and customers.

How to answer

  • Say plainly that you understand winter on-call and have done it, or are ready to
  • Describe how you close tickets the same day, with readings and photos
  • Mention how you flag quotes and follow-up work so it doesn't get lost
12.What licenses do you hold, and what are you working toward?

Why they ask

Licensing decides what you can legally work on, and it varies by state and city.

How to answer

  • List what you hold now and in which state or city
  • Say which license you're working toward next and how far along you are
  • Mention any manufacturer training on the boilers they service

Questions to ask them

Ask at least two. It shows you're picking them too.

  • How much of your work is steam, how much is hydronic, and how much is condensing equipment?
  • How does the on-call rotation work in winter, and how are after-hours calls paid?
  • Which manufacturer training or licensing costs will you cover?
  • Do new techs ride along with a senior tech first, and for how long?
  • What's your policy when a tech red-tags a boiler and the customer pushes back?
Career guideWhat a Boiler Technician does, and what it pays
Read the guide →
Skills to learnBoiler Technician skills: what to learn first
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