High Performance HVAC Air Conditioning & Heating Answers: Honeywell Chronotherm Plus 8624D Thermostat and the Trane XV-90 Gas Furnace

Trane XV-90 and Honeywell Chronotherm Plus 8624D Thermostat Problem and Troubleshooting Answer

I have a Honeywell Chronotherm Plus 8624D running a Trane XL19i with a Trane XV90 - all installed new in new construction in 2003. Every winter, during the morning warmup from 66 to 70, the burners stays on for approx 1.5 hours at 70, then rises quickly to 73 before finally shutting off. The only way to interrupt the process is to turn the system to OFF at the theromostat. The system is on AUTO with the cooling set at 75 for the winter. I have swapped the thermostats (there are two sets in the house, this is the one in the attic for the second floor) with the same results. I have not swapped the thermostat circut boards, but I can not see a reason to. The main floor is on the same schedule, but has never done this three degree burst. I have had three different contractors out, with correponding charges for thermostat troubleshooting with no luck. Have you ever seen this? Any thoughts? Thanks!

I get questions like this all the time which people email to me and it is difficult to give them a good answer simply because I'm not there observing this strange equipement or thermostat behavior. It sounds to me that you've eliminated the problem being the thermostat when you changed them out. You know the one in the other zone doesn't exhibit the same behavior and the one in the zone giving you the problem didn't exhibit the bad behavior (or three degree burst as you describe it) when you changed it to the other zone. At least that is how I'm reading it. You eliminated the Honeywell Chronotherm Plus thermostat as being the problem.

I would go back to basics on this one and check the installation location of the Honeywell Chronotherm Plus. A thermostat should be located in a central located on an inside wall preferably as close to the return as possible. It should never be mounted where direct sunlight can hit it, on an outside wall, or near a window or door which leads outside. If that is okay then check the wall where the thermostat is mounted. The hole behind the Honeywell Chronotherm Plus thermosat? Is this a drafty hole where air comes in through crawl space or attic somehow? If that is the case stuff some insulation in the hole to prevent the drafty air from hitting the thermostat. Seal it off as best as you can to prevent this drafty air from hitting the Honeywell Chronotherm Plus thermostat.

The next thing to do if this doesn't solve the thermostat problem is to have an HVAC professional check the furnace out completely. I would schedule an appointment with you for morning furnace warm-up so I could observe this behavior myself. I would eliminate thermostat and control the furnace at the furnace to see if it still did this. The Trane XV-90 is a high efficiency model so you will an HVAC specialist to look at it. Preferably someone who has had some Trane factory training. These guys are not hard to find as Trane does a pretty good job of offering contractors classes for their HVAC Technicians. I believe it is requirement for Trane dealers (HVAC Contractors) to send their Technicians to so many classes every year so call a Trane dealer and ask for their best technician to look at this problem.

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