I got this from my heat people, probably explains it:

Tonight into tomorrow, we all will experience temperatures and wind chills that our homes' heating systems are NOT designed for. The state building code dictates that a home heating system must be sized to a 5-degree night with a 10 mph wind. We will be experiencing a -10 degree night with 20 mph wind.   If your heating system is properly sized, it will NOT maintain temperature. The temperature WILL drop while the heating system is working at full capacity.    WHAT TO DO 1 Please raise the temperature in your home 2 to 4 degrees above your normal setting.  2 Take all programmable thermostats out of setback mode and set on a permanent HOLD.   3 If you have a furnace; replace the filter or remove it completely until the weather event is over.  4 Keep garage doors closed.  5 Limit opening exterior doors.  6 Make sure air vents and radiators are not blocked or obstructed.If you have a 90%+ furnace and boiler, you must keep the intake and exhaust clear of ice and snow. During these cold temperatures, ice can build up. A 90%+ furnace and boiler have 2 white PVC pipes; an exhaust pipe, and an intake pipe that are generally on the side or back of your home.  7 Refrain from using pellet stoves or supplemental heat forms if you have a water-based heating system. The pipes WILL freeze.If the temperature in your home is dropping and your radiators are HOT with boiler systems, or you have HOT air coming out of your vents with furnaces, DO NOT PANIC. Please make sure that your heating system continues to operate. 8 If the temperatures drop in your home, it will not be able to recover until temperatures rise and the wind chill diminishes. Heating systems can not overcome temperatures below 0 degrees. Set your thermostat on hold at 70 degrees or higher. 9 To help minimize temperature loss, you can boil water, make soup or stews, or bake something - it helps introduce humidity and warmer temperatures into your home.  10 Please check on neighbors and elderly residents during these extreme temperatures. Stay warm and safe this week.  11 The key is to NOT panic with falling temps inside your home if you have heat coming out of your registers. It is doing all it can. If it had feelings, it would be crying!  12 Stay safe and bring your pets inside!    13 Run a small stream of cold water in every faucet to help prevent a water freeze.   If your water stops flowing or a how water heating loop stops heating – Immediately shut off the water feed to your home!  This would indicate a frozen pipe that may have burst.  Once the thaw hits, the pipes could leak and cost tens of thousands of dollars worth of damage.