By Chad Edmondson
The transition to condensing boilers as the preferred heat source for buildings has had a ripple effect on how we design and control boiler systems. There are vast differences between how these systems operate versus the old, constant flow cast iron boilers that once dominated commercial and institutional applications. One of the biggest differences is the lower water volume that condensing boiler systems contain, especially systems with water tube boilers.
What’s the big deal about water volume? It all has to do with something you probably learned about a long time ago: Specific Heat. If you can wrap your brain around that than you will begin to understand why it is so important to design and operate your condensing boiler system with sufficient volume and a minimum start-up flow.
What Is Specific Heat?
Specific heat is the amount of heat energy (in Btu’s) required to raise the temperature of one pound of a material by one degree Fahrenheit. The specific heat of water is one Btu per pound per degree Fahrenheit. In other worse, if you add 1 Btu to a pound of water, you will raise the temperature of that water by 1 degree. Since there are 8.33 pounds of water in a gallon, it takes exactly 8.33 Btus to raise a single gallon of water by one degree.
So what is happening in these new condensing boiler systems, which have significantly less water volume than their cast iron predecessors?
Lower water volumes means less fluid to absorb and transfer heat. And that means a more rapid increase in water temperature when Btus are added. That can be a problem at start-up, even with condensing boilers that have a low turndown. Remember, the lowest boiler turndown is NOT the same as start-up fire, which is typically a higher percentage of output. A boiler may have a 10:1 turndown, but that doesn’t mean it starts at 10% fire. More than likely it starts at about 30% (or higher) and then modulates down. But before it does that, it must complete a series of start-up tasks, including air purge, etc. This start-up process may take 30 to 60 seconds, which doesn’t sound like much until you look at the amount of Btus going into a low volume system.
Specific Heat and Low Water Volume Systems
Let’s say we have a 2,000,000 Btu condensing boiler. (See diagram at top.) We’ve consulted with the manufacturer and know that it starts at 30% fire. At 30% fire rate that boiler will dump exactly 600,000 Btus into the system in the span of an hour. In the 30 seconds that it takes to start the boiler up and get it ready to modulate down, that same boiler will put 5,000 Btus into the system. If there’s no load to take those Btus away, then the water will heat up rapidly.
How rapidly? Using what we already know about specific heat we can calculate the heat rise for a given GPM pretty easily.
If we have 10 GPM of flow, the boiler will see 5 gallons of water (41.5 pounds) in 30 seconds. Under these conditions, this will result in a temperature rise of over 120 degrees! All of the sudden the water temperature inside the boiler has shot up to well over 200 degrees depending on what your lowest supply temperature is, and your boiler goes off on high limit.
This is common problem and one that can be avoided by making sure that you know:
(1) The start-up fire rate for your boilers
(2) How long the start-up process lasts. This quantifies the number of Btus you’ll be putting into your system at during start-up.
With this information you can calculate what the minimum flow rate through your boiler should be at start-up (based on the Specific Heat!) so you don’t short cycle or exceed the boiler high limit.
Given the low water volume of some of the systems we see today, the number that you come up with may indicate that you need to add a buffer tank. More on that in an upcoming blog!
For more information watch this video on Condensing Boiler Startup Sequencing and Minimum Flow.