For the last few weeks, a few times a day, the thermostat reports the same "13-limit lockout" error. We just had some maintenance done, pointed this out, and had been told it should be sorted.

This had happened before, so I got a lesson then, and have looked for causes and solutions on the Internet, too. Essentially the furnace is detecting one of a few failures to reach the right state of heating, so it stops trying for a bit and retries after a little while. If it fails enough times in a row, it does this lockout. Depending on why it locked out (too many failures in a short or long window), it will lock out for a short or longer time. A number of "check airflow" and filter solutions are easy to do. Other "check gas" and mechanic solutions are outside of my toolkit, literally because I haven't got the tools, and figuratively because while I get it conceptually I don't have the right training or practice to feel comfortable tinkering with the gas combustion heat source in my home.

The HVAC people returned today to take a look. I showed the e-mailed alerts from the furnace, and the photo I had snapped yesterday when I finally made the appointment. I showed him where the furnace is, he mentioned a few of the normal causes, and I shared my Internet and previous discussion knowledge. He accepted my visual inspection of the outside of the house, and dug into the innards of the furnace. I excused myself to let him have at it without feeling scrutinized.

He poked around for just a few minutes and found a switch hadn't been set right in the furnace for the kind of humidifier we have. There are apparently in-line and external humidifiers, and we have the latter. The switch should have been set the other way on installation, so it's been wrong for five years since they installed the unit. He asked and I confirmed that the lockouts have been happening all along. He apologized for those who came before him, and cited that the filters and airflow can be the cause, and when they don't think about the other things, they get missed.

He continued his inspection and found that the gas pressure was a little higher than the furnace was spec'd for. He showed a pressure reading 4 instead of 3.8. I had to look that up, and it seams measured in "WC," which has to do with the pressure of that many inches of water. The difference is not an awful lot more gas, but it can lead to a hotter flame, and contribute to the lockouts. He turned the right screws and set things right there.

He came out and explained he made the big change of flipping that switch, which will change the airflow expectations for the sensors, and the little change of correcting the pressure to within the specs of the furnace. He recommended changing the brand of air filters I had been buying to the same as the brand of the filter "housing," which will evidently perform better for not a lot more dollars, and did agree that while the furnace and directions say "3 months," it'll probably be more like monthly or a little more often with the dogs and things we have in the air around here.

The temperature has gone up above freezing now, so it'll be hard to gauge whether the fixes he put in will lead to fewer lockouts, or that the furnace isn't blazing enough. I'm confident that he did find the tiny misses and we'll be fine again. If not, I'll call 'em up and ask for more help.

Everyone's healthy.