It's the Coldest Day of the Year: Is Your Boiler System at Full Capacity?

I’ve been in the commercial HVAC rep business for over 15 years now and have helped many people design and select boiler systems for their buildings. Yet, the sizing of these systems is most always derived from a load calculated by a mechanical engineer (which I am as well, by the way – see my other blog “Is Your Commercial Boiler System Oversized”). As we run into bitter-cold days here in the Carolinas, (what we in the engineering field call “Design Days”) I cannot help but wonder, how close were we in sizing these systems for actual conditions?

If you are a building owner or have designed systems for owners, on these really cold days, have you ever checked the boiler plant to see how it is running? On Design Days, which present the worst-case conditions your building should see throughout the year, is your boiler plant running at 100% capacity? If you have multiple boilers, are all of them firing at 100%? Are your variable speed pumps running at 60-Hz (100% speed)? And, if the answer is “Yes" or the answer is “No," is that a good thing? The point is to learn what we can about a system from this answer.

• If it is the coldest day of the year and your system is running at 100%, it could mean that it was, indeed, sized correctly. The system should be able to maintain comfort in the building, however that may be defined (Occupied vs. Unoccupied, for example). If the building maintains comfort, congratulations, you have a really well designed system!
  
  
• What if it is running at 100% and it cannot keep up the temperature setpoints throughout the building? Then, the system may be undersized (rarely the case – again see “Oversized” blog), there may be an equipment problem in the mechanical room, or there may be an issue elsewhere in the HVAC system on the water or air-side. Use this opportunity to do some in-depth investigations. Get help from your engineer or equipment rep if needed. These extreme conditions really put your system to the test and there is no better time to find problems then when it meant to be firing on all cylinders.
  
  
• What if your boiler plant is not running at 100% on the coldest Design Day? It could mean there is a problem, as stated above. Perhaps there is a faulty sensor? Maybe the boiler plant was oversized, and this may be intentional if equipment redundancy (having standby equipment) was part of the original design intent. So, what does this mean for you? It should give some indication as to how the system might perform should a boiler go down. It may also give you some insight as to what you might need when the time comes to replace your equipment. If you have never seen the system fire at 100%, you may elect to purchase smaller equipment during the next retrofit project.

Again, the point is, on the coldest days of operation take a few minutes to observe what your boiler system is doing. What you learn may surprise you and will certainly educate you for decisions you will have to make in the future.