We block two hours on the schedule for a furnace diagnostic. Our dispatch records hold 410 calls where a furnace had stopped making heat, June 2023 through August 2026, and 454 appointments booked against those calls — 283 of them at a full two hours. The reason is in the symptom sections below: several of the most common furnace complaints can't be confirmed without watching the system run a full heating cycle and then start another one. Of those same 410 calls, 56% went onto the schedule the same day the call came in, and 82% were scheduled within one day. (Internally we log these as "no heat" calls. That's our shop label, not something you need to say when you call.)
What should I check before I call?
Three things, and they're worth doing first, because they're what most often turns out not to be a broken furnace at all.
The thermostat. Set to HEAT, target temperature above the current room temperature, and the fan set to AUTO rather than ON. Fan set to ON is the single most common reason a furnace appears to be blowing cold air when nothing's actually wrong with it — the blower runs continuously between burns and pushes room-temperature air through the vents the whole time.
The furnace switch. There's usually a switch that looks exactly like an ordinary light switch mounted on or beside the unit, and in a garage or basement it gets bumped off by someone carrying a laundry basket more often than you'd think.
The breaker, and the filter. Check the breaker serving the furnace. Then pull the filter and hold it up to a light. If it's grey and packed, change it before you do anything else — a clogged filter shows up in the check order for four of the five symptoms below.
If all of that checks out and the furnace still won't heat, stop there. Everything past this point involves gas, combustion, or line-voltage electrical, and it needs test equipment rather than guesswork.
From here, find the symptom below that matches yours. A furnace that isn't working almost always shows up as one of these five, and each one has its own check order.
"My furnace is blowing cold air"
Almost always this means the burners aren't lighting, or they lit and then dropped back out. The blower is doing its job; the heat isn't being made.
The order we check: thermostat call and fan setting, then filter and airflow restriction, then the igniter or pilot assembly, then the flame sensor, then gas valve and gas pressure, then the limit circuit for a lockout. On any furnace with a control board we read the stored fault codes first — the board frequently records what tripped last, and that jumps the sequence straight to the answer instead of walking it. Most of this runs at the front of the visit.
"My furnace won't turn on at all"
Nothing at the vents, no sound from the unit, no response when the thermostat calls for heat.
The order we check: power at the unit, which means the service switch, the breaker, and the low-voltage fuse on the control board, which is a common and inexpensive failure; then thermostat wiring and batteries; then the blower door safety switch, because if the front panel isn't seated fully the furnace is designed not to run at all; then the transformer; then the board itself. Power-side causes are usually the quickest of the five to pin down. A failed board or transformer takes longer, because we test to prove it rather than swapping parts to see what happens.
"It starts up, then shuts off a few minutes later"
The furnace lights, you feel warm air, and a few minutes later it drops out. This one is why we block two hours — a single cycle isn't enough to be sure, so we watch one, then confirm on a second.
The order we check: watch a full cycle end to end, take a microamp reading on the flame sensor, check filter and airflow, check the high-limit for an overheat trip, and inspect the exhaust and intake for a blockage. On a high-efficiency furnace those are the white PVC pipes running out through an exterior wall, and in this valley they collect snow, ice, and the occasional bird nest. This is the longest of the five to confirm, because rushing it produces a wrong answer.
"It runs all day and the house still isn't warm"
Different problem entirely. The furnace is heating, it just isn't delivering.
The order we check: measure the actual temperature rise between the return and the supply and compare it against the rating on the data plate, which tells us quickly whether the furnace is underperforming or the duct system is losing the heat before it arrives; then filter, then blower speed and the run capacitor, then duct leakage or a disconnected run in the crawlspace or attic, then burner condition, then where the thermostat is mounted. The temperature-rise measurement itself is quick. Chasing lost heat through ductwork isn't, and it sometimes means a crawlspace trip — we'll tell you before we go under the house rather than after.
"It keeps turning on and off"
That's short cycling, and it's hard on the equipment — it wears the ignition components far faster than normal running does.
The order we check: filter, then closed or blocked supply registers, then temperature rise, then the thermostat location and cycle rate setting, then the high-limit switch. Thermostat placement is underrated as a cause: one mounted in direct sun, in a draft, or too close to a supply register reads a temperature the rest of the house never has, and shuts the furnace down early, over and over.
When is a furnace problem an emergency?
If you smell gas, leave the house first and call from outside. If a carbon monoxide alarm sounds, do the same. Soot or rust streaking around the unit, a burner flame that's yellow and lazy rather than blue and crisp, or a persistent burning-plastic smell all mean stop running it and get it looked at now. And if a pilot won't stay lit, don't keep relighting it — repeated attempts put unburned gas into the cabinet. If you're trying to work out whether what you're seeing counts as one of these, our list of furnace warning signs walks through them one at a time.
What happens when the technician gets here?
A technician diagnoses the system, then explains what's wrong before touching a repair. You get the estimate in writing, before the work, not as a number on an invoice afterward. Common failure parts — igniters, flame sensors, run capacitors, thermostats, board fuses — ride on the truck, so those repairs are usually finished on the same visit. Parts that have to be ordered mean a return trip, and we tell you that on the first visit rather than letting you find out.
What will this cost?
We don't publish a price for this, and the honest reason is that the number depends on things nobody can know over the phone: which component actually failed, whether that part is on the truck or has to be ordered, whether the furnace is gas or electric, how accessible the unit is, and whether the age and repair history of the system turn it into a repair-or-replace conversation instead. You get the estimate on site, in writing, before any repair work starts — a real number for your actual furnace rather than a number you have to hope applies to you.
When is the best time to get my furnace looked at?
Not the week it breaks, if you have any say in it. Sorting those same 410 calls by the month they came in: July produced 1, August 3, September 13 — then October 56, November 70, December 70, and January 96. Demand doesn't ramp gently. It arrives with the first hard freeze, and it arrives for everyone at once. If your furnace was marginal last winter, the easiest week to deal with it is one where you're not competing with the whole valley for the same appointment slot. The cheapest way to handle it is an annual furnace tune-up before the season starts.
Do you come to my town?
Those 410 calls came from Payette (58), Ontario (56), Fruitland (53), Nampa (34), Boise (33), Weiser (30), Nyssa (25), Vale (21), Parma (19), Caldwell (17), Meridian (16) and New Plymouth (12), plus the surrounding towns. American Rooter & Drain has served the greater Treasure Valley since 1945, and our heating work runs through American Heating & Air, our Idaho heating and cooling division, licensed and insured under HVAC Contractor License 029771 out of the shop on Commercial Way in Caldwell.
For more detail see our furnace repair service page, furnace repair in Payette, or the local pages for Boise and Payette.
If the heat is off right now, call (208) 366-5771. Tell whoever answers which of the symptoms above matches what your furnace is doing — cold air, no response at all, shutting off partway through, never reaching temperature, or cycling constantly. That one sentence decides which parts we load before the truck leaves.
