Kids These Days are Talking Turn – Down: Is Higher Really Better?

PART 3

As we learned in Part 1, the biggest advantages of a High Efficiency Condensing Boiler (HECB) are their ability to withstand condensation in exhaust flue and in the heat exchanger. This single trait allows for several design flexibilities, for instance, system designs can withstand lower return water temperatures, and in fact the colder the return water temperature, the more efficient the boiler will run. This ability to run at lower return water temperatures allow for larger temperature drops from the supply and return water temperatures. Lastly, this low return water temperature allows for these boilers to run at much lower demand so they can run longer, but at a lower fire rate (partial load). The partial load level on these boilers is called the “Turn-Down rate”. Example: If a boiler can run at 20% partial load rate, then that would give it a 5:1 turn down. High efficiency boiler turndown ranges from 4:1 to as high as 25:1.

With all this talk about turn-down, one question you might task is, “How much turn-down does one need in system designs?”. A follow-up question could possibly be this, “How much do you trust your heat load calculations?”. Some think that this high turn-down is an extra layer of protection to overdesigning your system or protecting against short cycling during the shoulder seasons. Short cycling occurs during times of low demand for heat. When a boiler is called for heat during low heat demands in the system, the loop heats up very quickly and the boiler shuts off after a very short period. The goal in is for the boiler to run continuously for 10-minutes, or not fire more than 6 times per hour. Can turn-down help this? The answer is “yes”! But how much turndown is really needed? Better to investigate the actual minimum heating load of the system and the total firing rate turndown of the entire boiler system, not just the turndown of a single boiler.

As we learned in Part 1 efficiency is directly related to return water temperature and is not related to turn-down. The table below show a comparison between input BTUs at varying turndown rates. As you can see, boiler that has a turndown of 25:1 sounds much better than a boiler that has only 8:1 turndown, but as seen in the diagram below, there is only net difference 9% between boiler inputs of these two boilers at low fire.

When designing hydronic heating systems, consider the number of boilers being used.  A single boiler should be able to operate at the system’s minimum heat load while firing at it’s low-to-mid firing rate.  If it cannot, a buffer tank should be applied to the system to prolong the firing time and prevent short-cycling. 

Almost there.  Look for “PART 4 – So How Do You Maintain These Things?” coming soon!