The Painter Who Taped Over the Return Grilles
Why this matters
Painters cover grilles for a good reason: overspray on a register is ugly and paint inside a return duct is worse. The problem is that a forced-air furnace is designed around the air those grilles move, and covering all of them with plastic while the heat is running starves it. The furnace does not break in the obvious sense. Its safety controls do exactly what they are built to do, all day, until the control board gives up and the house goes cold. This case was reconstructed from three records that already existed by the time the HVAC tech arrived, and it shows how a tech traced a failure that had already stopped happening. How the duct system works for other trades is covered in Ductwork and Return Air for Electricians, Insulators and Painters.
Safety first
- A carbon monoxide alarm sounding, or anyone with headache, nausea or dizziness: everyone out to fresh air, call 911 from outside, and nobody goes back in to check the furnace.
- A gas smell: everyone out, no switches, no lights, no phone used inside; call the gas utility or 911 from outside.
- Do not reproduce a limit trip to prove a point. Covering the grilles again and running the furnace to watch it overheat would deliberately drive a combustion appliance to its safety limit. The record is enough.
The job, as the painters saw it
A three-person crew was spraying the walls and ceilings of the upstairs and main floor of an occupied house in January. At 8:10 in the morning they sheeted plastic over every grille in the work area so the overspray would not land on them: 11 supply registers and 2 return grilles, the central return in the hall and a bedroom return upstairs. 11 plus 2 is all 13 grilles in the house. The thermostat stayed on heat at 68 F, because the homeowner was working from a basement office and the outside temperature was around 30 F.
The crew noticed the furnace running often and the house not warming, put it down to the cold, and left at 4:30 with the plastic still up, planning to finish in the morning.
At 5:00 the homeowner noticed the house was 63 F and falling and the furnace had gone quiet. By the time the HVAC tech arrived at 6:30, it was 61 F inside.
What the tech found on arrival
Nothing was running. The furnace's control board was showing a flashing code for lockout. The burner would not light on a call for heat. All 13 grilles were still under plastic.
A tech walking in cold to that scene could reasonably start on the board, the igniter, the flame sensor or the gas supply, because a furnace in lockout that will not fire looks like an ignition failure. The plastic on the grilles changed the order of the work: before touching the furnace, the tech read three records.
Reading the record backwards
The board's fault history. The control board stores its recent faults. Read back through its diagnostic display, it showed a long run of "limit circuit open" events, then the lockout. The limit is a temperature switch on the heat exchanger compartment that opens when the air there gets too hot, shutting the burner off while the blower keeps running to cool the exchanger. It recloses when it cools. Many boards count limit trips within one call for heat and, after the number their manufacturer sets, lock the burner out for a period, also manufacturer-set, which is what had happened at about 4:40.
The thermostat's runtime log. The homeowner's smart thermostat kept a history of its calls for heat. The day before, at similar outdoor temperatures, it called for heat two or three times an hour and each call was satisfied in 10 to 15 minutes. On the day of the job, the call that began at 8:12 am never satisfied at all: one continuous call for 8.5 hours, until the board locked out. A thermostat only sees its own call, not the burner, so the log cannot count limit trips. The homeowner, working beside the furnace room, supplied that part: the burner lit "about every ten minutes" and ran only a few minutes each time.
The painters' daily log. The crew foreman's note: "8:10 sheeted all registers and returns, both floors."
Three independent records, written for three different purposes, line up within two minutes: plastic at 8:10, an unsatisfied call from 8:12, limit trips until the lockout. That is a reconstruction a tech can stand behind without running the failure again.
What the records say together. With every grille covered, the blower could barely move air. With little air flowing over the heat exchanger, its temperature climbed within minutes of each ignition until the limit opened. The burner shut off, the blower cooled the exchanger, the limit reclosed, and the burner relit, about every ten minutes, all day, delivering only a few minutes of heat each time through grilles that could not let it out. The house cooled because almost no heat reached it. The furnace was not failing; its limit was working on every one of those cycles. That matters, because replacing a limit that opened correctly would have left the real cause in place.
Confirming it without recreating it
The tech asked the homeowner for permission and removed the plastic from all 13 grilles, rolling it to keep the overspray off the floor. Then, with the furnace off, the tech replaced the filter, which had a fine coating of dried paint mist on its entering face; the blower had pulled overspray through the gaps around the plastic all day.
Then the tech reset the board by cycling power at the furnace switch and called for heat. That is the return-to-service step, and it was done with the cause removed: grilles clear, filter new, panel on, a personal CO monitor clipped to the tech's collar.
The furnace lit and ran. After the reading settled, about 10 minutes in, the tech measured the temperature rise, the air temperature leaving the furnace minus the air temperature returning to it: 108 F supply minus 62 F return is a 46 F rise. The nameplate on this furnace gave a rise range of 35 to 65 F, so 46 F sits inside it. The limit stayed closed. The house was back to 68 F by about 9:15.
Before leaving, the tech looked at the heat exchanger as far as it could be seen through the inspection openings and found no scorch or discoloration, and read 0 ppm CO at the supply registers in the two nearest rooms. A single day of limit cycling is unlikely to damage an exchanger. Repeated over weeks, it can, which is part of why the painters' fix below matters.
What would have pointed somewhere else
The records ruled out the alternatives a tech would otherwise chase. A cracked igniter or a dirty flame sensor causes failed-ignition faults, not limit faults. A dirty filter on its own, without the plastic, would have caused limit trips that grew over weeks, not a pattern that switched on at 8:10 one morning. A blower motor failure would have left the limit tripping after the plastic came off, and the 46 F rise said the blower was moving air normally.
Supplies, returns, or both. The crew had covered both, but covering either set alone does the same thing in kind. Covered supplies leave the air nowhere to go; covered returns leave the blower nothing to pull, so it pulls what it can through leaks in the return duct and the furnace cabinet, which is how a filter and a duct fill with overspray and attic or basement dust. The less open grille area there is, the higher the rise climbs. A crew that leaves "just the returns" open has not solved the problem; it has reduced it by an amount nobody measured.
One thing in this house kept the case from being worse. The furnace was a sealed-combustion condensing unit, drawing its air from outside through a pipe. In a house with an atmospheric water heater or furnace near the return, a starved, leaky return can pull air out of that room and pull flue gas back down the vent. See related: Which Trade Jobs Change House Pressure and Moisture.
How the shop explained it to the customer
The painting shop called the homeowner the next morning, with the HVAC tech's findings in hand.
OWNER: The furnace company says it was the plastic
over the vents.
PAINTER: It was. We covered all thirteen grilles and
left the heat on. The furnace could not move
air, so it kept overheating and shutting
itself off, and by late afternoon it locked
itself out.
OWNER: Is it damaged?
PAINTER: The tech checked it and says no. The filter
was full of overspray, so he replaced it.
We are covering the service call and the
filter.
OWNER: And today?
PAINTER: Today we take the grilles off and paint them
on the bench, and nothing gets covered while
the heat can run.
The shop paid for the service call and the filter, and said so plainly. It did not describe the furnace as faulty, because it was not.
What the painters do now
- Grilles come off, not under plastic. Supply registers and return grilles are removed, labeled by room and painted or protected on the bench. The openings are left open, or covered loosely with a single layer of paper only while that room is actually being sprayed.
- Nothing is covered while the system can run. If an opening has to be masked, the homeowner turns the system off at the thermostat for that time, and the masking comes off before it goes back on. In winter that means short spraying windows planned with the homeowner, not a whole day.
- The filter is on the closeout list. After any interior spray job, the crew tells the homeowner to have the filter checked or changed and notes it on the invoice, because overspray gets past any masking.
- The foreman writes the time. The 8:10 note is what let the tech reconstruct the day without recreating it.
References
- The furnace's installation manual and nameplate for temperature rise range and limit behavior
- See related: Ductwork and Return Air for Electricians, Insulators and Painters
- See related: Which Trade Jobs Change House Pressure and Moisture
- See related: Describing a Problem in Another Trade's Own Words