A word about Design Day and not-so-warm homes.

In the world of Heating and Cooling, there is a temperature that gets thrown into the mix of both longhand calculations and computer models that establish the BTU outputs of all our equipment. It’s called a “design temperature” and it varies as we follow typical climate variations going north and south, and up and down in elevation. In our part of the world in 2026, the design temperature is 13°F.
That means that heating systems are designed to maintain an indoor temperature of 70°F when the outdoor temperature is 13°F.
Efficiency is defined many ways by the different industries that use the term but in fuel-burning boilerspeak, peak efficiency means the boiler runs at 100% of its output at design temperature, and never shuts off while hovering around that outdoor temperature, and combustion is steady with minimal standby or flue losses at the boiler which factor heavily into manufacturers' published efficiency ratings. In other words, the two most *efficient* boiler heating cycles will have the boiler never come on, or have the boiler maintain the 70° indoor temperature by never shutting off. We call that a "Design Day" heating condition. 
So, what happens when the outdoor temperature drops below design temperature? Like lots of days these past two weeks? Well for us professionals, the first thing that happens is all our online forums and message boards start blowing up with warnings to each other about trouble coming, and stories of engineers on current projects nervously considering supplemental heat sources in their designs.
But the bottom line is design temperature is often an average of the ten coldest days of the year and it is well known and expected that there will be be a handful of days colder than the median or average cold day. And so it is accepted that the internal ambient temperature of some of those structures operating on design have the potential drop below 70°. Does that sound like a failure? Maybe. But what you have to consider is that aside from the heating system, there are plenty of other things in the home or building that give off heat to the space. Cooking a pot of stew adds heat energy to a space for hours. Lights, TVs, computers, cable boxes, solar gain, hair dryers… all these things being in place or running add BTUs to the home and take some of the onus off the boiler or furnace. So, this high-design-temperature-vs-low-outdoor-temperature thing is rarely ever an actual problem. To design a home heating system with significantly higher heating capacity, or to change the design temperature to 5° or less will often change the design greatly, increasing installation costs substantially and operating costs permanently.
If you feel you are getting close a to problem caused by low outdoor temperatures versus your system’s design, you would be well advised to do what you can to close that gap. Literally. A tight building envelope will outperform even the most highly efficient of boilers and furnaces. You heat your home only with the BTUS you keep inside. The rest are gone instantly and forever.A lot of words for a Sunday morning but I hope this helps someone. 

(In the photo below, two boilers provide redundancy to make heat and hot water. Multiple smaller boilers tend to burn fuel at higher efficiency than a single larger one due to something called "turn-down-ratio" limitations so each of these units is sized for 75% of the home's peak demand. If the primary boiler can't meet the target temperature, the secondary unit kicks on at 10% intervals. The full capacity of both boilers being 150% of the demand means the home's heating zones are always satisfied in the most efficient way possibly.)