GUEST POST: The Case Against High Thermostat Setbacks
This article in the Energy Vanguard Blog is a guest post by Ted Kidd, whom I know through LinkedIn and Home Energy Pros. If you've interacted with Ted, you know that he's nothing if not passionate, especially on the topic of setbacks and heating and cooling equipment that can change its capacity to match the loads. I don't agree with what he says below and have posted my response in our first Point/Counterpoint series here. He makes a couple of good points, though, and his article is worth the read.
The Case Against High Thermostat Setbacks
Particularly in Hi Mass (i) Situations
By Ted Kidd, Energy Efficiency Specialists ©2011
Setback Strategy is an energy conservation strategy of programming your thermostat to drop 8 or more degrees for 8 or more hours a day. Often people would setback during the day when they were at work then at night when they slept.
Setback strategy developed in the 70’s as a result of the energy crisis, and technological advances in HVAC controls. The benefits of this now aged strategy are assumed to be a prescription for energy savings in a broad range of heating applications. In order to understand why this strategy may not be effective today it is important to understand why this energy saving “trick” was effective 40 years ago.
The rationale behind programmable or setback thermostats was essentially that if you turned the thermostat down, you used less energy. While this may make sense if you go away for a few days, for shorter periods (which vary depending on the house) the cost of reheating may negate any savings. Without measuring before and after there is no way of know what, if anything, setback is saving, so people simply implemented these strategies and hoped they were saving energy.
Setback on a daily basis made more sense 40 years ago due to a combination of factors that are not applicable in modern homes. The average home of 40 years ago had a tremendously leaky shell (ii). Leaky ductwork magnifies these shell air losses. A nearly complete lack of insulation was the norm, so any heat created quickly left the home. These poor building conditions all contributed to savings from setback.
There are 2 sides to this energy thing. One is the heat lost side; the other is replacing what is lost. The common assumption was that setback savings was from reduced furnace run time. But surprisingly the primary savings from setback probably came from the ability of oversized, inefficient equipment to dramatically improve delivery efficiency during 2 long daily recovery periods. In other words, savings came from much more efficient creation of the energy delivered to the house, not from miniscule reduction in losses from the house. Let’s explore this idea.
Every time a large, heavy furnace comes on from a cold state it operates really inefficiently until it reaches design-operating temperatures, which may take 5-10 minutes. Add to this that the ductwork needs to be warmed up uniformly before it can effectively deliver heat evenly to rooms. If this equipment only runs for a few minutes before the thermostat is satisfied, the equipment never reaches efficient operating range and rooms often experience temperature imbalance.
Before setback, equipment would run full speed for a short period, then shut down, run, then shut down again, and so on. Setback allowed two significantly increased run-times which provided a huge portion of the household’s daily heat generation requirement. Delivering this heat in large blocks dramatically reduced daily short cycling and allowed an increase in the percentage of heat actually reaching living spaces versus going up the flue. Coming off setback the furnace would run continuously to warm the house up. When going into setback it would be off for a significant time rather than short cycling (iii). This left intermittent cycling for only a short time in between.
Imagine driving an automobile 100 miles a day. This old automobile gets 5-mpg city and 20-mpg highway. Setback is akin to planning your route so that 80 daily miles are on the highway instead of none. In other words, the savings from setback was due to much more effective route planning (delivery of heat) rather than from reduction in losses due to a miniscule temperature reduction.
There has been a paradigm shift in all the components of comfort and the old “rules” should be thrown out the window.
Instead of only driving 110 mph or shut off, our new automobile has a throttle and can drive at varying speeds that meet conditions. This is sometime referred to as load matching, or replacing only the heat the house is losing to the environment. This allows modern equipment to get the equivalent of 60-mpg city and 80 mpg highway. With multi-stage and modulating equipment, highway use can be achieved most of the time, without setback trickery. In fact, setback trickery REDUCES efficiency by asking this new equipment to switch from cruise control to full throttle.
Technology Changes, Strategy Needs To Change With It
Modern equipment design works more efficiently when running on LOWER settings. In Germany, for years now code design has required 150 degree radiant design instead of 180 degree design. They found there are tremendous energy savings and comfort improvements from delivering cooler temperatures. This in part led to the development of modern modulating or “Mod-Con (iv)” equipment. It is widely understood that lower return temperatures dramatically improve equipment efficiency. Colder supply means colder return which means more heat pulled from combustion, which means higher efficiency. More heat can be removed from combustion due to a larger heat exchanger to delivered BTU ratio, and lower return temperatures.
It is now generally recognized that perfect sizing and load matching is the most efficient approach. Providing no more than the heat lost, and running continuously, is the goal the industry is moving closer to. Increased heat exchanger size (condensing) and smart controls modulate output, which reduces and sometimes completely avoids inefficient shut down periods.
Your Friends With Radiators Knew Setback Didn’t Work
Even 40 years ago setback was not as effective for cast iron radiator heat. Hi Mass hot water situations are less affected by leaky shell and mechanical ventilation magnifying leakage is not a factor. Also dramatic adjustments in temperature lead to control issues.
BTU loss through a tight, well insulated shell at differentials of a few degrees is and has always been fairly insignificant. Households today are dramatically tighter, so Delta-t (v) stack loss (vi) savings due to setback are also insignificant.
Remember, savings from setback was not from reduced losses; it was from tricking the equipment into longer more efficient run times and thus more efficient delivery. With the development of High Efficiency Modulating Equipment combined with Hi Mass delivery setback does the opposite of what it did before. Instead of allowing the equipment to operate at optimum efficiency, it has the effect of putting the pedal to the floor. Recovering from setback no longer cheats short cycling and shutdown losses; technology has fixed those problems.
Mass And Control
In High Mass situations use of setback can seriously impact control. Typically the thermostat calls until air temperature is satisfied. If returning from a large setback the boiler puts a tremendous amount of heat into the radiators before the air warms up enough to tell it to stop (again, high throttle city driving). This heat doesn’t stop just because the boiler shuts off.
Once the thermostat is finally satisfied these BTU’s continue to transfer into the living space quicker than they are lost. This flywheel effect causes wasteful overshooting. Also, this approach somewhat nullifies the benefit of a modulating boiler by having it fire on high, then shut down. Outdoor reset attempts to temper this, but at the expense of slowing recovery from setback.
Comfort, The True Driver Of Energy Cost
This is the whole point of heating. We are throwing money at spaces to make them comfortable to occupy. Is there a large chair or couch people occupy? Allowing a space to drop to uncomfortable temperatures means furniture gets cold. While it may not take long to warm air from setback, large objects (walls, furniture, etc.) are still cold for a long time. These objects will pull heat from occupants making them uncomfortable. This discomfort creates the need for higher temperatures to compensate negating any savings that might have occurred.
There are a tremendous number of variables affecting heat loss in any space, so the strategy to maximize comfort and efficiency for any given space will take time and thought to achieve. That said, the technology of heat generation has made huge gains. Understanding how a heat generating plant and a heat distribution system work best together can have dramatic impact on comfort, control, and consumption.
Setback strategy forces modern equipment to drive in the city for 2 major heat generation requirement periods a day. Now recovering from setback causes throttle up, which reduces efficiency by increasing return temperatures and reducing the heat exchanger surface area to BTU ratio. Setback is very hard to control, which often means putting more heat to a space than is necessary. Set back cools everything off, which forces the equipment to start from zero and means more recovery to provide comfort. Setback means the equipment has to ramp up to recover. The greater the setback strategy, the more the equipment needs to run full out.
Always beware of conclusions based upon partial information, avoid rules of thumb or at least understand to which circumstances they apply.
If you have a Mod Con, match your boiler’s water temperatures to your house’s heat loss and leave the temps set. Let it run on low. If you use setback, use it delicately. I use mine to control when the radiant peaks occur to enhance comfort, particularly for my bathroom floors. If you want to use aggressive setback your Mod Con will have to provide higher temperatures to the radiators than would be necessary without setbacks. It is unlikely setback will accomplish anything other than making the achievement of comfort complicated.
If you have a ground or air source heat pump, using setback will often mean that recovery uses very expensive backup resistance electric heat. People with heat pumps usually have a clear understanding that setback does not save them money.
The best way to understand how operator behavior (and error) might apply to your situation is to have a home energy audit.
Read the Energy Vanguard response:
If You Think Thermostat Setbacks Don't Save Energy, You're Wrong!
Thanks to Bob Siegel, Margie Campaigne, and Liz Cameron for their help editing this paper.
(i) High Mass - This refers to heating systems that have a lot of mass. Typically hot water with cast iron radiators, systems that heat concrete floors.
(ii) Building Shell or Envelope – This refers to the thermal and pressure boundary of a building. Outside walls, ceilings, floors, generally where indoors and outdoors meet.
(iii) Short Cycling – This occurs when equipment turns on and runs for a very short period, then shuts off. This can lead to very high energy bills and frequent repair costs.
(iv) Mod Con is short for Modulating Condensing Boiler. These boilers not only pull a tremendous amount of heat out of exhaust gases due to their ability to lower those gas temperatures to the point where they condense (state change), they also have the ability to throttle up and down instead of simply running at full throttle.
(v) Delta-T – This refers to difference in temperature. Heat goes to cold (envision a magnet pulling steel), the bigger the difference or delta, the faster that movement or pull. Delta T refers to temperature difference. As the temperature difference from outside to inside increases the upward pressure, thus air leakage or change, increases.
(vi) Stack Loss – As the temperature difference from outside to inside (delta t) increases, the upward pressure on the air increases. Think Hot Air Balloon. As this pressure increases, air leakage out of any holes increases as well.
Photo of window by Wetsun from flickr.com, used under a Creative Commons license.