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How Much Energy Can Be Saved by Reducing Thermostat Settings


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As you all know by now, the cost of heatig fuels have skyrocketed, in just the past four months. My first note of caution regards the price increases that you read about in the newspapers and hear on the radio and television. From what I can determine, these reports are looking at the price increase for the entire year. When you do that, the average price increase matches the numbers in the news. However, please consider that the vast majority of your heating energy consumption is going to take place during the 3 coldest months, when the costs are going to be the highest. In Minnesota, the largest natural gas supplier is projecting that the average cost for December, January, and February is going to be up over 12%, compared to the same three months last heating season. Therefore, be prepared for highter energy costs thatn what you're hearing in the news.

Earlier this week, I received an inquiry from a school district, asking about the savings that can be expected by reducing their thermostat settings. I thought that you might be interested in my response, as well. Keep in mind that my response was directed to someone that experiences Minnesota macho winters. If you experience more, or less, that about 8,000 heating degree days in your area, the first part of the answer will change. The remaining part of the answer is pretty universal.

The direct answer to the question of how much energy can be saved by reducing thermostat settings, is roughly 2.5% per 1 degree drop in space temperature. So, take last year's cost of heating energy X 2.12 (112% increase) X 0.025 (2.5%) X the number of degrees that you are willing to reduce the thermostat settings and you'll have the answer in dollars. But, in all likelihood, your systems are probably a little more complicated than just having that as the answer to your question.

HVAC system are supposed to provide heating, cooling and ventilation. They are not, however, supposed to provide all three at the same time. In the Upper Midwest, cooling can be provided about 7 months of the year, with outside air. We refer to that as atmospheric cooling. As important as it is to not be providing simultaneous air-conditioning and heating, it's equally important to not be providing simultaneous atmospheric cooling and heating.

Check your air-handling systems this morning. What are the mixed air temperatures? If they are all in the range of 55F to 60F, that should be telling you that your systems are probably providing simultaneous atmospheric cooling and heating. It should also be telling you that the simple act of turning down the thermostats, is probably going to cause considerable discomfort problems for your occupants.

Each occupant gives off, at least, 400 Btu's of heat per hour. When you have 30 occupants in a classroom, that's the equivalent of a 100% efficient, 12,000 Btu/hour furnace, in each classroom. If the thermostat turns off the heat at an energy saving temperature of 65F to 68F, this internal heat from the occupants, plus even more heat from the lights, has the potential to increase the space temperature to more comfortable temperatures, much of the heating season. But, if the air-handling units are blowing a constant 55F to 60F mixed air temperature into the rooms, air that cold will fight the internal heat gain, attempting to keep the space temperature at the reduced thermostat setting. By most people's standards, that's going to make them uncomfortable.

In constant volume air-handling systems, it takes approximately 16% outside air to make the mixed air temperature 55F, at -20F outside. That means that the original engineer should have designed the system to satisfy the minimum ventilation requirements with 16% outside air. The problem with a constant 55F mixed air temperature is that, as the outside air temperature increases, it takes an ever increasing amount of outside air to keep the mixed air temperature at 55F. In fact, the amount of outside air increases linearly from 16%, at -20F outside, to 100% at 55F outside. So, if 16% outside air satisfies the ventilation requirements at -20F outside, 16% will also satisfy the ventilation requirements at higher outside air temperatures. Therefore, to let the internal heat gain work in your favor, instead of having cold air blowing on the students, faculty and staff, then some temperature control system revisions, to eliminate simultaneous cooling and heating, are in order.

If you've been reading our monthly HVAC Maintenance Recovery Lessons at SchoolFacilities.com, you will recognize the elimination of simultaneous cooling and heating as step 4 of our six-step maintenance recovery outline. If your temperature control systems are still pneumatic, upgrading the sequence of operations is pretty easy and not very expensive. However, we would also encourage you to look at your HVAC systems in the long term. While Climate Makers still has servicemen proficient in pneumatic temperature controls, our industry isn't making any more service people that will carry on pneumatic temperature control skills. Everything is going to electronic controls. That is, building automation. Having the basics of a building automation system for air-handling unit control, also leads to other areas of energy conservation, such as precise boiler and circulating pump control. Energy savings of 20%, or more, are possible just by how you control your boilers and circulating pumps.

Well, that was probably much more than what you were expecting for such a simple question. We hope that you found it helpful. If you've been missing our monthly HVAC Maintenance Recovery Lessons, SchoolFacilities.com now has all of the lessons that have been published at: http://www.schoolfacilities.com/cd_314.aspx. A new lesson appears in one of the weekly SchoolFacilities.com Newsletters every month. There will be a total of over 50 of our practical lessons, so be certain to check your SchoolFacilities.com Newsletters for these and other great helpful tips, every week.

About the Author
Tom Olson, Climate Makers, Inc.

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