Furnace turns off power to the thermostat

Question

When my natural gas furnace is running for a while and it gets too hot, the limit switch turns off the burner. When it does this, the power also gets cut to my thermostat. This causes the thermostat to run on the backup battery. Is this normal, or is there a problem with the wiring?

The thermostat has been replaced more than once. Recently when the power got cut to it, it registered the house temperature at 1 degree. This caused the furnace to heat continuously until I reset the whole system. This is dangerous.

I Had a tech in, and he says it is normal for the power to be cut, and he again wants me to replace the thermostat. Why would it be normal for the furnace to cut power to the thermostat?

The furnace is a International Comfort Pro NTC6100GFA1, and the thermostat is White Rogers (Emerson) 1F85-0477.

Answer 1

Limits are only designed to open when there's a problem, so the first step here is to find out what the problem is and fix it.

The most common cause for a high limit to continually trip, is a dirty or improperly installed return air filter. Make sure you change the filter regularly, and that you're using one with a similar density to the one recommended by the manufacturer's documentation.

Restricted return and supply ducts can also reduce air flow through the system, which can lead to the furnace overheating. Make sure all return air vents are clean an unobstructed by furniture and other obstructions. Also make sure that all supply registers are open, and unobstructed.

Check the blower compartment for dust/dirt/debris. It's not common for the blower to get dirty, but it does happen from time to time. This is especially common, when a furnace is run without a filter, or with an improperly fitted filter.

A dirty blocked up evaporator coil, could also cause restrictions. If the unit has an evaporator coil fitted, you'll want to inspect it to insure it's not dirty.

If there's still trouble, you can check the blower motor and capacitor. Though this is only recommended if you have the proper tools and knowledge to do it. Otherwise you'll want to get an HVAC technician to handle it.

Unless you've recently changed the blower, there's not likely any problem with the way it's installed.

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With all that being said. Some furnaces are actually designed to remove power from the thermostat, in the case that a limit opens. Obviously this design predates fancy WiFi thermostats, which require constant power to operate. Back in the olden days, when a thermostat simply acted as a temperature switch, interrupting the signal to the thermostat was an effective solution.

Based on the schematic, it looks like the unit is controlled by the integrated furnace control module (IFC). It looks like the IFC has a separate limit circuit that it monitors, so I'm not sure that it would cut power to the thermostat if there was a problem. In any case, the thermostat should not be damaged by losing power. I suspect there's more to this, and I haven't been able to find any service bulletins that describe this behavior.

I'd likely start by contacting the manufacturer directly. If that doesn't provide any clues, I'd contact a different local HVAC company to come have a look. If there's an installers sticker on the unit (or you know who installed it), and the installation company is different than the company you've been dealing with. I'd contact the company that installed the unit, as they should be quite familiar with the equipment.

Unfortunately it's difficult to troubleshoot these types of problems through the internet, especially when I'm not familiar with the equipment.

Answer 2

Ok first and formost the limit should not be tripping so finding why the fan is not keeping the temperature of the furnace constant while the burner is running is number one priority. So make sure the fan is turning in the proper direction. If it's a belt drive fan, chances are the motor was replaced but is turning in the wrong direction. If this is not it then it could be the fan blades are clogged up and needs cleaning with a pressure washer. The other option is either the air filter is completely clogged or the return grille or grilles have been covered during reno of some sort but it's obvious air is not moving over the heat exchanger. Now to your question, this must be an older furnace with a combination fan/limit control either in the plenum or inside the furnace. When the high limit is reached it shuts off the burner only supposedly but by the sounds of it, it has been wired wrong and is shutting off the control transformer as well. You need a compitent service man to go over the furnace wiring and get it done right. Sorry for the long winded answer. Find out why not enough air is going over the heat exchanger. One more thing, with the furnce constantly doing this you need to make sure no cracks in the heat exchanger has occurred and dumping all kinds of deadly CO2 into your house. Please have this furnace completely checked out!! Pictures of the fan limit would be helpful as well for referance as well as the fan setup.

Answer 3

I had the same problem when I turned the temperature up more than 4 degrees on the thermostat. The Honeywell programmable thermostat uses no batteries, just the furnace supplied 24 volts. When the Hi limit tripped the thermostat lost all power and the blower would just run until the limit reset. The thermostat would power up again and turn the burners back on.

Later on I found out that the furnace was double the size needed for my small apartment. Simply increasing the heat blower speed has cured the problem but it also created more noise. The existing heat ducts could not transfer the heat fast enough on the slower speed.

Thermostats with batteries may not indicate the problem. The furnace needed better air flow as already indicated in previous replies.

Answer 4

Not a real wiring issue, is it possible for you to give me the make and model number of this furnace? Also if it's a direct drive motor, it could be the motor speed is on low when it should be high once the temperature is high enough. Do you leave the fan run continuously, if not you should, it saves about 10% on your gas bill and once you get used to the air moving all the time, you will love it but this has nothing whatsoever to do with your thermostat. Here's an experiment to try. Depending on it's age there could be a door switch on the fan compartment door. Take the door off and tape the swich down just for this experiment, then put the door nack on. turn the thermostat up and when the fan starts, lift the door about an inch to unhook it, then pull the bottom out towards you about three inches then while holding the top in this position, let go of the bottom. If the door get sucked back with a good force, it means your returns are not open upstairs or they are way too small for the size of the furnace. If you've been having this issue for years, there's a good chance you have heat exchanger problems but one step at a time, lets find out why this furnace keeps tripping the limit. In the end though I'll give you the answer you need. One other thing, are you using Natural Gas or Propane.