The short answer: before a furnace service visit, write down what you have actually noticed — sounds, smells, comfort patterns, dates — gather any model and serial numbers you can read safely from the unit or paperwork, collect prior service receipts, and list your questions. Then decide what you will not touch. This page helps you prepare a clear, factual handover; it does not diagnose your system, price work, or point you to any technician.
To assemble that written note privately in your browser, use the service-notes builder. It keeps optional model/serial and system-age fields, a description of what you observed, service history, questions and preferred visit windows, and produces a printable text note you can hand over or keep.
Start with what you observed, not what you think is wrong
A useful preparation note separates three things: what you saw or heard, when it happened, and what you did in response. "Furnace ran about ten minutes, then the burner sound stopped while the fan kept blowing; happened twice on cold mornings in the last week" is far more useful than "the furnace is broken." The first version can be checked; the second invites guessing.
Write down the ordinary details too. Which rooms feel cold, whether the thermostat setting changed, whether anyone adjusted vents or filters. If you replaced a filter, note the date and the size printed on it. These are observations, not conclusions.
Example (hypothetical, not a real case): a homeowner notices a faint musty smell near a supply register in one room, first noticed after a rainy weekend, strongest when the heat first comes on. The note records the room, the timing, and that the smell fades. That is a record. It is not a diagnosis, and this guide cannot supply one.
Collect the paperwork you already have
Model and serial numbers help a service visit begin with facts rather than a search. They are usually on a data plate on the unit or inside an access panel. Do not open panels or remove covers to find them. If the plate is not visible without disassembly, leave the field blank on the builder and note "not accessible" — that is an honest and useful entry.
Gather what you can from records instead:
- Previous service invoices or inspection reports, with dates.
- The installation or replacement date, if a receipt or sticker shows it.
- Filter sizes and the dates you changed them.
- Any warranty paperwork you were given.
If you do not know the system age, leave it blank. An approximate age guessed from memory can mislead more than an empty field.
Write your questions before the visit
Visits are short and it is easy to forget the thing you meant to ask. Keep a running list and put your most important question first. Reasonable preparation questions include: what did you check and what did you find; what maintenance did you perform; what, if anything, needs attention and why; and what records should I keep. Questions about price or scope belong in writing — see how to read a furnace service estimate.
Also decide your preferred visit windows in advance and write them down, including whether someone will be home. The builder has a field for this so the note is complete when you hand it over.
Annual inspection is the recognized baseline
The U.S. Consumer Product Safety Commission advises having a professional inspect all fuel-burning heating systems — including furnaces, boilers, fireplaces, wood stoves, water heaters, chimneys, flues and vents — each year (CPSC winter heating safety release, January 23, 2026). That is national general guidance, not a Littleton-specific rule and not a statement about any particular system.
The same release explains why the inspection matters: fuel-burning systems such as furnaces can produce carbon monoxide, an odorless, colorless gas, and CO poisoning can occur if systems are installed incorrectly, poorly maintained, have defective or blocked venting, or are misused. Those are the conditions a professional inspection is meant to look for. Your preparation note helps that inspection start from your actual observations.
Alarms are your responsibility between visits
CPSC recommends CO alarms on every level of the home and outside sleeping areas, battery-operated or with battery backup, tested monthly with batteries replaced at least yearly. It states that working smoke and CO alarms save lives and advises smoke alarms on every level and inside each bedroom (CPSC, 2026).
Follow the manufacturer's instructions for your alarms and check local requirements. A maintenance record is a good place to note alarm test dates and battery changes — see keeping a useful furnace maintenance record.
What this guide will not help you do
This is an independent homeowner preparation guide, not an HVAC contractor. It does not diagnose a system, quote prices, recommend a technician, or offer hazardous troubleshooting. Specifically:
- Do not open burner compartments, disconnect gas lines, or attempt to relight or adjust ignition components based on anything here.
- Do not treat a smell, sound or alarm as self-diagnosable from a web page.
- Do not use this page to decide whether a component is safe.
If you smell gas or a CO alarm sounds, leave the building and contact emergency services or your gas utility. Emergency guidance should come only from official sources — CPSC, your utility, and local emergency agencies.
A note for context on furnace sensing technology: a CPSC-filed standards working group log describes a proposed provision for gas-fired central furnaces that would shut down burner operation when a CO detection device reads an upper threshold of 70 ppm over at least a 10-minute time-weighted average, resuming below a 30 ppm reset threshold (September 2025 meeting log). An earlier log shows the same thresholds still listed as undetermined (April 2025 meeting log). This is a proposal under development, not an enacted requirement and not a description of any installed furnace.
If the visit is part of a replacement conversation
Permit questions come up quickly once work moves beyond routine maintenance. Littleton's building permit guide lists furnaces, air-conditioning equipment, ventilation systems, ductwork and other regulated mechanical work among common examples where a permit and inspections are generally required, while noting that routine maintenance and cosmetic work often do not require a building permit (City of Littleton permit guide). The City states that the exact answer depends on the complete scope and the property location. See permits and replacement requirements for more, and before signing any contract, check with the City about licensing and permits — the Permit Center publishes 303-795-3751 and permits@littletonco.gov.
A simple preparation checklist
- Write your observations with dates and rooms; keep them factual.
- Read model and serial numbers only if visible without opening anything; otherwise write "not accessible."
- Gather prior service records and filter dates.
- List your top three questions in priority order.
- Note preferred visit windows and who will be home.
- Test alarms if it is your scheduled month, and note the date.
- Build and print the note at /service-notes/, then read it back once to catch guesses you can turn into observations.
The about page explains what this publication is and is not.
