The $140 tune-up that prevents the $400 call on the first cold night

The first cold night in Virginia does not announce itself. It lands in mid October, somebody turns the thermostat up before bed, and in the morning the house is the same temperature it was.

What happens next is the expensive part, and it is not the breakdown.

Three prices for the same furnace

Our price guide puts these three in the same table, and side by side they say everything:

The tune-up costs a third of a repair. The repair costs a twelfth of a replacement. That is not a sales argument: it is why heating techs are fully booked the first two weeks of October and quiet in September.

What a tune-up usually covers: cleaning the burner, checking ignition, inspecting the heat exchanger, measuring draft, changing the filter, and confirming the thermostat commands what it says it commands. What it does not cover: parts. If one turns up it is quoted separately, and that is where a $140 tune-up becomes a $300 invoice.

Why it fails on the first day specifically

A gas furnace sits idle for seven months. Nothing breaks in that time, but three things build up: dust on the burner, moisture in the ignition, and residue on the flame sensor.

The flame sensor causes the most calls and is the cheapest thing on the list to fix. It is a metal rod that confirms there is fire. When it gets coated, it stops detecting it, and the unit does exactly what it should: it cuts the gas for safety. The furnace lights, shuts off after a few seconds, tries again, and locks out.

From the couch that looks like a major failure. It is almost always a dirty rod.

Before calling anyone, there are three things you can check in two minutes that explain a good share of the October calls. First the thermostat: if it runs on batteries and they have been in there a year, they can have just enough left to light the screen and not enough to send the command. Second the furnace switch, which looks like an ordinary light switch, usually sits on a basement or utility room wall, and somebody flipped it off in spring without knowing what it was. Third the filter: a clogged one starves the unit, makes it overheat and trips the limit switch, which shuts the burner and leaves the blower pushing cold air. That last one fools the most people, because the unit is running and blowing, so it looks like it works.

The burning smell on day one is a different thing and it worries a lot of people: it is dust on the heat exchanger burning off. It lasts a few minutes and goes. If it lasts more than an afternoon, or if it smells like rotten eggs, that is not dust and it does not investigate itself.

What you cannot see, and why an agency asks for this

The US Consumer Product Safety Commission recommends having your heating system inspected once a year, and not for the sake of your wallet.

More than 200 people die each year in the United States from carbon monoxide poisoning tied to consumer products, and most of those deaths happen in winter, at the peak of the heating season. An average of about 85 a year come from heating systems, ranges and water heaters.

Carbon monoxide has no smell, no color, and does not irritate. A cracked heat exchanger releases it into the house with no warning sign at all, and the only way to know is for someone to look or for an alarm to sound.

That is why the same commission gives a placement rule almost nobody knows: one detector outside every sleeping area and one on every floor, and never directly above or beside a fuel-burning appliance, because appliances release a small amount on startup and the alarm would go off for nothing.

When to call, and what to ask

The good window is now, from September through the first half of October. After that you are competing with everyone who found out the same night.

Run the heat on a cool September day for half an hour. That is the whole test. If something is wrong you find out early, while the techs still have openings.

• Does the tune-up include the filter? Some include it, some bill it. It is the most common difference between two quotes that look identical.
• Do you inspect the heat exchanger? That is the part that matters for carbon monoxide and the one a quick visit skips.
• What happens if you find a part? Ask whether they call you before replacing it, and whether the diagnostic comes off the repair.
• How long does it take? An honest tune-up on one unit runs 45 to 90 minutes. Twenty minutes is a filter change with a fancier name.
• What if my unit is over 15 years old? Ask them to tell you what shape it is in before you spend on repairs. On an old unit, two repairs in a row start closing in on the price of a new one.

If you want the tune-up before the cold gets here, tell us what you have and a verified pro in your area picks it up. It is free and takes a minute.