Want Green? Want an Energy-Efficient system?

Written on: November 1, 2012 by ICM

We have all heard the comment, “Don’t throw the baby out with the bath water.” This comment can also apply to Large Mass (Oil or Gas) Boilers that are in workable condition and still have a number of years of life left in their operation. Your customer has done his research and has made up his mind that he wants to convert from oil to gas. So we immediately focus on replacing his present boiler or boilers with new energy efficient gas boilers and neglect to value engineer the total system needs and options available to the client. We could be throwing away a golden opportunity to retain the workable oil fired boiler by marrying his present large mass conventional boiler with a low mass condensing gas boiler.
Why should we consider this approach?
Simply put, a hybrid multiple boiler system will maximize heating system efficiency.
Use Low Mass on warmer days
Low mass condensing boilers operate when the outdoor reset temperature is below the dew point (135°F or lower). Condensing boilers operate with a 5:1 turn down ratio, which reduces boiler cycling. Low mass condensing gas boilers will operate during low load periods during the shoulder season (Spring, Fall and mild weather periods), maximizing boiler efficiency. In our 2011/2012 heating season, the entire winter became the “Shoulder Season.”
Large Mass works best on coldest days
Conventional large mass cast iron boilers, on the other hand, operate when the outdoor reset temperature is above 140°F, to protect the cast iron from corrosion and flue gas condensation. These boilers have a 2:1 turn down ratio when firing the burner in a Low-High-Low burner operation, increasing burner on time to increase steady state efficiency of the boiler. These boilers reach their peak efficiency when operated near maximum capacity, which becomes a short period of time during the heating season.
The bar chart, Figure A, represents a typical Heating Load Distribution for a region with a 5,600 degree day heat demand. As can be seen, only eight days of the 193-day heating season actually average a load between 80% and 100% of the peak heating load.

Figure A

Now let’s look at “Part Load Efficiency.”Because of this, conventional large mass boilers don’t provide the burner turn down flexibility to match boiler output to system reduced loads, boiler and wasted energy through jacket and off cycle flue losses.
In Figure B, the curve is a typical representation of overall efficiency vs. heating load. At 50% heating load, the conventional boiler is operating at best at 75% efficiency. This reduction in efficiency is caused by frequent burner cycling along with high standby losses. It becomes evident that the way to avoid wasted energy, which results in high heating costs, is to provide high turn-down ratios and higher operating efficiency boilers.

Figure B

The chart represented by Figure C shows the results of using a conventional boiler in a heating system with the typical heat load distribution as discussed in the previous graphs.

Figure C

The overall efficiency is shown along with the days spent at each load range. The seasonal efficiency is the weighted average of these overall efficiencies averaged over the entire season.
Figure D illustrates the combined efficiencies of the conventional boiler and condensing gas high efficiency boiler in a hybrid system. The condensing gas boilers are sized at 60% of the design load. At that load point and below, the condensing gas boilers take over and thermal efficiency jumps up to 93%, raising overall seasonal efficiency to 88% vs. 71% for the conventional boiler system.

Figure D

This article presents suggestions that can significantly increase heating system efficiency by retaining the existing high mass (oil or gas) conventional boiler and installing a low mass gas high efficiency boiler in a hybrid system, saving the owner precious up-front replacement costs.
Our industry has struggled with the ability to use “Consultative Selling” and “Solution Selling” ideas to translate value and a respectable return on investment back to the client.
This hybrid system approach can keep both the conventional high mass oil-fired boiler and a new low mass condensing high efficiency gas boiler working together as “one.”

This hybrid system at the Gettysburg, PA YWCA was installed by contractor Carl E. Frantz, Inc. It consists of two condensing gas boilers mated with a large mass boiler. The Gettysburg Y is a 22 year old, 50,000 sq. ft. facility with an outdoor design temperature of +13°F. The building has a calculated heat loss of 3,250 MBH.
Two cast iron boilers, which provided both hot water and space heating, were originally in the building, but one cracked and needed to be replaced. The contractor proposed a hybrid replacement in which two two Weil-McLain Ultra Commercial 750 Condensing Boilers were arranged in a multiple boiler system design, providing a 10:1 turndown ratio (each has a 5:1 turndown). The existing large mass cast iron unit was retained.
The condensing boilers are used for the shoulder seasons and the large mass unit is used during high load periods. The entire system is controlled by a Mini-Mod CNC Control Panel (supplied to W-M by Heat-Timer Corporation). The CNC control panel automatically switches between the condensing boilers and the large mass cast iron boiler based on return water temperature to achieve optimum system efficiency. The hot water system is fitted with pneumatic controls on multiple zones with circulators.