Cost Effectively Go Green With Maximum Condensing Energy Savings Using Multiple High Recovery Boiler Hybrid Systems
Motivated by financial and environmental considerations, demand for higher efficiency in hydronic heating systems is ever increasing. Specific goals are to reduce heating bills, reduce green house gas emissions, preserve natural resources, comply with legislation, gain utility rebates, and accumulate LEED points, etc.
To achieve these efficiency goals many boiler manufacturers, including Raypak, offer condensing boilers. They are designed to condense the water vapor in the flue products to thereby capture the latent heat. Latent heat in the water vapor that is produced by burning natural gas amounts to approximately 10% of the total BTU’s available. Much of that, along with additional sensible heat, can be captured with a condensing boiler.
It is a law of physics that the flue products have to be cooled below their dew point (the temperature at which condensation will take place) in order to operate in the condensing mode associated with over 90% efficiencies. This means that the incoming water to the heat exchanger must always enter below the dew point of the flue products. With the normal levels of excess air needed to ensure complete combustion of natural gas, this is about 130°F. Thus water entering the heat exchanger must be 130°F or lower before increased efficiency can even begin (the lower the return temperature the higher the efficiency).
Most hydronic systems require water temperatures well above 130°F to adequately heat the building on cold days when the heating load is the greatest. Historically, hydronic systems were usually designed to operate at 180°F outlet and 160°F return. During most of the heating season outdoor temperatures are well above the very cold system design temperature days and the water temperature required to heat the building can be significantly reduced.By designing with lower temperature system components and utilizing aggressive outdoor reset (lowering the system water temperature as the outdoor temperature increases) the opportunity to operate boilers in the high efficiency condensing mode can be achieved for much of the heating season. This can be further enhanced by using variable volume (variable speed) pumps to achieve higher temperature drops across the system water and hence even lower return water temperatures to the boilers.
High efficiency condensing boilers cost significantly more than their non-condensing counterparts. It is generally not cost effective or energy saving to apply them to systems that are operating at elevated temperatures when no condensation can take place. Thus, for multiple boilers (2 or more) it becomes prudent to have a Hybrid system. This is a design utilizing condensing boilers as lead boilers to carry the load during mild weather or warm daytime temperatures so high condensing efficiency can be achieved. Additional non-condensing boilers can then be incorporated to meet the total load on colder days when system temperatures are too high to achieve high efficiency condensing operation.
Many engineers, performance contractors, and owners, are faced with budgetary limitations and the need for value added reductions. They have found that changing from all condensing multiple boiler systems to this Hybrid approach allows them to come within cost guidelines, while maintaining all the benefits of the condensing design.
Seasonal conditions vary somewhat geographically, but normally lower heating loads, which occur during midday and in mild weather, present the opportunity to lower the system water temperature and achieve high efficiency condensing operation during approximately 50% of the total season. Full advantage of the opportunity to operate in the high efficiency condensing mode (spring and fall, or night versus day) and to balance the operation of all the boilers can be achieved by using your condensing boilers first for the mild times and standard boilers first for the cold times. Then on milder days only the higher efficiency condensing boilers will be operating. On cold days they still can contribute to meeting the total load required. This means non condensing (up to 87%) boilers can be utilized for 50-75% of the total load. This is a very significant first cost reduction with virtually no sacrifice of total seasonal efficiency.
High-recovery low-mass boilers can rise to operating temperature within a minute without concerns about thermal shock. They are ideally suited to save maximum possible energy when utilized in a hybrid system with proper system and control design. High recovery boilers do not need to be kept hot, wasting energy idling during periods of no load when their capacity is not needed. This avoids standby losses that can be substantial with low recovery high mass boilers.
A typical Hybrid system is diagramed below. The piping /pumping as illustrated in the drawing facilitates keeping any unfired boilers out of the system until building load requires them. The primary secondary system shown optimizes seasonal energy efficiency by modulating each boiler, but does it sequentially, and only when additional capacity is needed. The typical multiple boiler system turndown becomes approximately 20:1 using this methodology. To achieve the best result controls that incorporate the capability to select seasonal water temperature requirements and switch lead boilers to achieve balanced usage while optimizing efficiency are needed. High recovery boiler controls such as the one pictured can accomplish this task with simple software modifications.
The intrinsic capability of high recovery designs to modulate, turn off during periods of non use, and sequence one boiler at a time allows an easy switchover. Their ability to accept return temperatures ranging from 105° to 120°F contributes to significantly lower temperature operation and further maximizes condensing benefits. Complex and costly control algorithms to extend the cycle are sometimes used with low recovery boilers, which allow their use in Hybrid multi-boiler systems, but can not avoid the inherent standby losses of hot unneeded boilers.
The Hybrid concept is nothing new. Raypak’s earliest installations, utilizing first generation equipment, were installed over ten years ago and saved energy from the start. A typical Hybrid system using today’s equipment and utilizing one XTHerm® condensing boiler, and two MVBs® is pictured here. In addition to state of the art condensing boilers, these systems incorporate 88% MVB non condensing boilers, for the lowest total installation savings, and maximum energy savings. The very significant challenge for earlier systems has been solved by Raypak’s Temp Tracker Mod Plus Hybrid controller.TempTracker Mod+ Hybrid read more…
This controller, specifically designed for high recovery copper finned tube boilers, is capable of switching from condensing boiler lead to non condensing lead as load demand shifts. This can be done within a minute, with no minimum individual boiler on or off time requirements.
The right combination of high recovery condensing and non condensing boilers, properly controlled, are now available in an ideal configuration for totally coordinated Hybrid systems. You can finally Go Green, reduce heating bills, and stay within budget on new and replacement commercial installations.