Topics
What is Low Delta T Syndrome?
The Solution Starts at the Coil
Low Delta T and Heating
Condensing Boiler Technologies
Controlling Variable Speed Pumping Systems
Free Cooling Plate Heat Exchangers
What is Low Delta T Syndrome?
Low delta t syndrome occurs when a hydronic system is operating below design delta t (the difference between supply and return water temperature at the coil) at part load. The lower the system water delta t, the lower overall system efficiency. The vast majority of hot and chilled water heating and cooling systems typically operate at 1/3 to 1/2 of design t delta at part load. The main contributors to low delta t syndrome:
- Poor control valve performance and range
- Inaccurate control set points
- Overflowing the system heating and cooling coils
- Inefficiently piped hydronic systems
Many of today’s most efficient hydronic systems are still operating at 25-30% higher energy usage than is necessary. Rising energy costs will continue to increase the impact that low delta t has at the owner’s bottom line. Fortunately, there are methods and technologies available that directly address these system inefficiencies. They are simple and typically require very little system modification.
A system owner can, at no cost, quickly determine if he is a candidate for these system improvements. These improvements may not only reduce operating cost, but also improve comfort and overall system control. Contact us if you want to learn more about improving system efficiency, reducing energy costs, and limiting maintenance and hot/cold calls.
Back to Topics
The Solution Starts at the Coil
Low Delta T is a problem that has been widely recognized for some time. Unfortunately, most of the “solutions” that have been offered in the past do not address the root of the problem. Primary / secondary piping, controlling a tertiary (building) crossover, and turning on additional chillers before full loading has been reached are all examples of ways to counter the effects of low delta t. None of these approaches address the real issue, which is poor control valve performance. Since low delta t occurs when we flow more water than is necessary through the coil, the only real solution is to better control this. Delta P pressure independent control valves are the only product guaranteed to heat or cool with the absolute minimum amount of flow. This means more simplified piping systems, less pumping, fan, and chiller energy consumption, better control, and more comfortable spaces. Blackmore & Glunt offers free consultation to owners who are concerned about the effects of rising energy costs and want to make their hydronic systems more effective and efficient. Since 1936, we have been assisting engineers and contractors in the proper design and installation of hydronic systems. Let us guide you, from identifying issues to forming a team of professionals to get your hydronic HVAC system working for you.
Back to Topics
Low Delta T and Heating
Low delta t affects heating and chilled water systems alike. For systems with conventional heating water boilers (scotch marine, flexible tube, cast iron sectional, copper finned-tube), low delta t has served to protect the boiler from dangerous condensing conditional (when boiler return water falls below 140F, causing condensing of the flue gases and damage to the boiler). However, new boiler technology, specifically designed for condensing operation, allows us to take advantage of lower return temperatures, greatly improving efficiency and saving on gas usage. Combining pressure independent valve technology with condensing boilers guarantees the highest possible heating system efficiency and reliability available today. Many systems, even those using steam, may be a candidate for such a conversion.
Back to Topics
Condensing Boiler Technologies
There are many choices in condensing boilers today. Designs differ greatly from one another, and it is critical to understand how and what features are important to realize true "high efficiency" operation. Materials include stainless steel, steel, copper, cast aluminum, and cast iron. The important point to consider here is that flue gas condensate is acidic and corrosive, so selecting a non-ferrous material such as stainless or cast aluminum is critical. The second point to evaluate is burner turndown. Most condensing boilers modulate, but significantly higher savings can be achieved by limiting cycling with a boiler capable of a minimum of 10:1 turndown. Finally, minimum and maximum flow limitations should be considered. Using a boiler with low or no minimum flow will allow for more flexibility in installation and operation.
Back to Topics
Controlling Variable Speed Pumping Systems
Most of today’s larger hydronic systems employ variable speed pumping. This can save pumping energy during low load conditions, when less flow is required to meet heating and cooling requirements. Unfortunately, conventional control valves do not reduce flow rates to the minimum flow required, resulting in pumps running at higher speeds than necessary during many hours of part-load operation. Compounding this problem are the many improperly controlled and installed variable speed pumping systems. If your energy management system provides staging and speed control of your pumps, it is not likely that critical de-staging capabilities exist to turn off additional pumps at low load. Many of these systems also fail to look at differential pressure sensor signals frequently enough to provide sound control (2x/second recommended). Finally, many other systems do not have the sensor properly located to maximize savings. Making the appropriate improvements to your variable speed pumping system, combined with flow reductions realized with the use of pressure independent control valves will guarantee the maximum savings and best control possible today. If your pumps do not stage and de-stage regularly, or do not vary in speed much, contact us for a free site visit to show you how some simple improvements can save you energy and improve system reliability.
Back to Topics
Free Cooling Plate Heat Exchangers
More and more large chilled water systems are installing free (refrigerationless) cooling heat exchangers to meet their year round chilled water needs. These are typically plate heat exchangers installed between the cooling towers and the chilled water loop. When cooling tower water temperature can cool the chilled water loop to its desired setpoint, no chillers have to run and system efficiency is increased tremendously. This typically occurs during the spring and fall of the year when outdoor air temperatures are cool. Typically, most free cooling plate heat exchangers experience low delta t when conventional control valves are used. When pressure independent control valves are applied in this application, system delta T across the heat exchanger is maximized and the required cooling tower flow rate is minimized. By maximizing delta t, fewer cooling towers may have to run to provide the necessary year round cooling. If your system has plate heat exchangers operating below design delta t, or if you are interested in free cooling applications, please contact us to arrange a free onsite visit and consultation.
Back to Topics
