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Almost everything you ever wanted to know about stack temperatures.

Astute property managers use stack temperature a sign of the status of their heating system.

If the stack temperature is low, the assumption is that the HVAC system is running fine; however if it is too low, condensation will damage the flue. If the stack temperature is high, there is likely inefficiency in the system.

A high stack temp implies that some heat is not being transferred to the boiler water and is being wasted by going out the chimney.

However, stack temperature measurements are complicated and property managers and superintendents need to fully understand how they work in order to run their buildings efficiently.

First, what is stack temperature? Stack temperature refers to the temperature of the gases that exhaust through the chimney. The combustion process heats the boiler water to make steam, hot water heating, and/or domestic hot water.

The "stack" refers to the flue, and a high stack temperature can indicate a burner that needs adjustment and a boiler that needs cleaning. The burner has two primary functions: making sure the fuel and air mix in the correct proportions and figuring the direction and the shape of the flame.

There are two primary reasons why the stack temperature might be high--either the burner is producing a greater flame than necessary and/or the heat is not being transferred properly to the water in the vessel.

Many property managers wonder how high they should let the stack temperature get before having the tubes cleaned. Is there a difference between if you are using oil or gas? What other factors should be considered? Since cleaning the tubes involves an expense, it is important this process is well-timed. The answer to this question is complicated.

The following is a rule of thumb for evaluating stack temperature.

6 Steps to Stack Temperature Evaluation:

1. The most important factor in evaluating performance is the "actual input." If the input is above the manufacturer's ratings for the boiler, the stack temperature will be higher than expected; and if the input is lower, then the stack temperature will be lower.

2. A low stack temperature does not mean the boiler is running efficiently. You can only know this if you are sure the input is correct.

3. Make sure that the draft control (whether barometric or sequential) is properly adjusted and functioning properly. The draft control determines how fast the hot gases are moving through the boiler and therefore how much time the boiler has to absorb the heat released during combustion.

4. How to calibrate the input of the burner: Start with an absolutely clean boiler; carefully calculate the input of the burner (can be done with gas or oil); adjust the draft control; record the 'steady state' outlet temperature of the boiler with the equipment running on high-fire and use this as the base-line stack temperature.

5. When the outlet temperature rises 10-15% above this baseline temperature, it's time to have the boiler cleaned.

6. Have a qualified burner company set the controls when the boiler is clean and know the optimum stack temperature.

The majority of superintendents and property managers are not monitoring the stack temperature and simply have their boilers cleaned on a regular schedule.

Energy Management Systems (EMS) and Energy Information Systems (EIS) can send out alerts as to specifically when cleaning is required. In many cases, this can lengthen the time between cleanings and save money. Alerts can also identify when the systems need maintenance and identify boiler malfunction.

For example, the USE Manager[TM] provides daily alerts and informs property managers if the stack temperature is too high. The system also helps with tracking changes in stack temperature over time and comparing buildings throughout a portfolio.

Newer boilers with new burners might have some features which enable better maintenance of stack temperature. For example, the flue gasses might be recirculated to facilitate efficient combustion and the turndown ratio (between max and min firing states) might be very precise.

How much fuel do you waste because of heat escaping out the stack?

It is insidious--you can't see it happening, but it is costing you and our environment.

Stack temperature is an important indicator of how your HVAC system is operating, and it is important for property managers to monitor and manage it for complete building control and optimal efficiency.

Like everything else, you can't manage it if you don't monitor it--and that is just one of the ways an EMS can help you save money and help the environment.

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Comment:Almost everything you ever wanted to know about stack temperatures.
Author:Pindus, Jerry
Publication:Real Estate Weekly
Date:Jan 26, 2011
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