AC: turn it off/down while you’re gone?

It’s an age old question:

Is it better to leave the AC ON when you’re not in the building, -or- turn it OFF/warmer while you are gone, then turn it back on/down/cooler when needed ? ? ? 

The conventional wisdom is that IF you turn it OFF or “UP” (a lot warmer), then it will take a longer time and more energy to get it cool again. Therefore, they say, “leave it on, and maybe just move the thermostat’s set point to 78 when you’re gone.” Don’t let it get too hot, they say. And it makes some sense.

However… If you want to really BE green and $AVE green, conventional wisdom needs to be challenged with a more holistic understanding of the problem, and better solutions.

THE PROBLEM with “conventional wisdom” regarding AC is that it assumes all facilities, temperatures, AC units and kilowatt-costs are the same. And they’re not. What works in one church (or home, or office), is just wishful thinking in another.

Moderating the thermostat is really the LAST THING you need to do.

Why? Because if your interior space is getting SO WARM that it’s “too warm” when your AC is off, or takes too long to get cool when your AC is on, then you have more important problems to address than the set points on your thermostat.

1. Building interiors HEAT UP more than they should due to poor insulationold-technology windowspoor roof venting, and lack of shade and green space around the exterior walls.  If it’s 90 degrees outside, and 80 inside without the AC running, you need to address these other problems. Why? Because running the air conditioner to keep a “bad building” cool is throwing money out the window (or roof, or walls). Moderating the thermostat doesn’t address the issue.

Case in point: A church I know recently upgraded their AC and thermostats, but not their uninsulated metal air ducts which ran through an overheated roof so that the “cool” air could come out of the ceiling vents in the Sanctuary. By the time the air got to the vents, it wasn’t that cool anymore. So…they had to run their new units longer than needed. They had a roof problem that needed addressed. And they also needed to insulate their airducts to reduce energy loss.


2. AC Units over the past 10 to 15 years have become 30-35% more efficient. Therefore if you have an older system, or a poorly installed and maintained one, then just keeping the room ‘somewhat warm’ while you’re gone -to save 10 to 20% costs, is an actual loss of 15 to 25% due to the inefficiency of your AC.

3. Uninsulated ductwork, poorly designed ductwork, or ductwork without routing controls can also waste a lot of money. Ductwork is NOT something most heating and cooling companies pay that much attention to. Turning ON (or “down”) the AC when you get to the church …and then letting it blow through the seams in the ductwork and into every unused room (during the summer months when parts of the facility are sometimes closed), is wasteful. 

Most air ducts leak like seives because they are hastily built. Sealing seams with special tape (not duct tape which will eventually rot) will improve the air pressure in the system. And insulating air vent puts more cold air where you want it, rather than leaking it into walls and attics.

4. Clean heat exchangers are CRICITAL to saving green and truly being green.

A quick refresher: The liquid coolant that moves the heat from your building must pass through two exchangers in the typical forced air system. There’s one outside surrounding the fan (aka what most people call “the AC unit”), and there’s a second heat exchanger in your furnace (yes, in the furnace).   These exchangers are nothing more than masses of copper tubing with heat fins and a fan that blows air over them to exchange hot for cold.

The same FAN that blows your hot furnace air through your ductwork, also blows the cool air through the ductwork. Actually, it sucks the air FROM the hot room, and blows it over the heat exchanger in the furnace where the coolant filled copper coils SUCK THE HEAT out of the air as it is being forced over the coils and keeps blowing it through the ducts -sending the cooled air into the room. 

IF your heat exchanger is DIRTY, or UNDERSIZED, or if your FAN is undersized, then your efficiencies can become GREATLY reduced. Less efficient heat-cool exchange = longer time to cool the air = more $$$. 

This is why your church needs to annually clean it’s heat exchangers, both inside the furnace, and outside in the unit (those ‘fins’ of metal that surround the fan and coils can get full of grime). 

If you have an industrial strength ‘water mist’ system, it still needs cleaned from build-up.

Furnace FILTERS are an important way to maintain the heat exchange efficiency in your system. They trap airborne dust before it can accumulate on the heat exchanger fins -which it can quite easily because water condenses around the fins -trapping the dirt. A clogged furnace filter is the quickest way to throw God’s money at the electric company.  Many large units use electronic filters and self-cleaning filters. They are worth the investment, as is semi-annual maintenance.

5. Zone controls. REALLY begin Green these days means adding some smart technology. PREMISE: Not every room needs the same amount of heat and air, yet in many churches the builder put a bunch of rooms on the same thermostat. And they have too few thermostat zones. This is often the result of “old thinking” system design when energy costs were low.  Upgrading your system should include upgrading the number of zones, and the means to direct air WITHIN the zone as needed. (It really depends on your layout and a number of other factors. This post is simply encouraging the idea!)

Advanced heating and air companies can provide you with POWERED LOUVERS inside your ductwork to control the flow of air based on temperature readings in all your rooms. It sounds expensive, but really isn’t. Now a thermostat located in the nursery and CONNECTED to the louvers can automatically direct more cold air or heat AS IT NEEDS IT, without blasting the other rooms/zones. Powered booster fans. The tech is cheap, simple to retrofit, and saves money and energy.

With a little bit of thinking and technology, in the summer you can have the office set to 73, the hallway at 76, and the bathrooms at 78. You just need thermostatically controlled vents inside your ductwork. 

While we’re on the subject of “zones” … installing doors at specific locations to keep cool or heated air in the right place is another way to create and manage zones in your facility.  In one former church, the single door to the outside near the fellowship hall SUCKED the expensive and nicely cool air out of Fellowship Hall every time the outside door was opened, which was frequently as kids came in and out.  They had the same problem during the winter… this old-tech single metal door sucked the heat out of the hallway in the winter, -which triggered all the thermostats to call for more heat. That single door was literally sucking money out of the church.

5. Other Tips   Metal roofs reflect more heat than shingle roofs, and last longer. Add a thermostatically controlled attic fan to the church’s attic. Lighter shade church building materials reflect more sunlight/heat. Ceiling fans actually do work because they improve the distribution of cool/hot air in the room if you have the fan blades circulating in the right direction. And if you have a standard forced air system, don’t put your AC units up on a flat roof or next to the parking lot. The radiant heat from the tar roof and asphalt will kill your efficiency and equipment life expectancy.

Going the extra “audit” mile…

Most churches are not designed to be green. Most were built in an era of cheap energy. Thus, congregations today have to be smart and holistic in going green and saving money.  Most could benefit from an energy audit. Some heating and cooling companies provide such expert service. All the others will claim to, though the “audit” may be little more than a sales checklist. If the energy auditor doesn’t record ACTUAL temperatures, and address the REASONS why a building doesn’t stay cool, it isn’t much of an audit. 

You also want to see some MATH… The auditor needs to calculate the total amount of air mass that needs moved over the heat exchanger (measured in “tons”) in order to properly SIZE the capacity of the units that need to be installed, AND to size the ductwork. Big units with improperly sized ductwork or exchangers are just another way to waste money. There IS an art and science to all this.

Yes, it will cost more to have a trained energy expert to do the study, but it will pay for itself, and allow you to keep your principles in truth, rather than just lipservice.  

An energy expert will also be able to share with you many other things particular to your building that will help you get green-er, and keep more of your green.

Speaking of “green” …don’t forget the greenspace around your building. Shade trees are God’s own air conditioners (see my post about this in this Green Jesus blog thread).

…..So yes, turn “up” or “down” the thermostat when your not in the building, but make it the LAST energy efficient thing you do, not the “only.”

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