Argument against high setbacks
(I don’t recommend setting back your thermostat more than 2-3 degrees (and not below 65) unless you have a leaky home with over sized, inefficient equipment)
Setback benefits developed in the 70’s as a result of the energy crisis and technological advances in controls. These benefits are assumed to be a prescription for energy savings in a broad range of heating applications. It is important to understand why this energy saving “trick” was effective.
Energy savings results from setback were due to a combination of factors. Tremendously leaky shell conditions, leaky duct work magnifying air losses, and a complete lack of insulation was the norm and all contributed to savings from setback. But the primary savings came from the ability of oversized, inefficient equipment to dramatically improve delivery efficiency during 2 daily recovery periods.
This was due to the two significantly increased run-times providing a huge portion of the household’s daily heat generation requirement, which in turn dramatically reduced daily short cycling. Before setback, equipment would run full speed for a short period, then shut down, run, then shut down. After setback it would run and run and run twice a day, be off for a significant time as temps fell during set back, and run intermittently for a short time in between.
This new approach was analogous to driving an automobile 100 miles a day. Imagine this old automobile got 5 mpg city and 20 mpg highway. Setback was akin to planning your route so 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), not 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 running 80 mph or off, our new automobile analogy has a throttle. Modern equipment gets 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.
Modern equipment design works MORE efficiently when running on LOWER settings. It is widely understood that lower return temperatures dramatically improve equipment efficiency. More heat can be removed from combustion due to larger heat exchanger to delivered BTU ratio, and lower return temperatures. In Germany code design is requiring 150 degree radiant design instead of 180 degree design because of the energy savings cooler temperatures affords when combined with ModCon equipment. Colder supply means colder return which means more efficiency.
It is generally accepted that perfect sizing is the most efficient approach. Generating no more than the heat lost and running continuously is the goal the industry is moving closer to. New High Efficiency equipment is designed to run on the highway all the time, only ramping up to city consumption when excess load occurs (returning from setback, indirect hot water recovery, etc…) Modern equipment design and proper sizing gets us ever closer to this target. Colder, more even supply makes recovery difficult. You want fast recovery; you better heat that water UP.
Modern equipment design has almost completely eliminated start up and shut down losses through mechanical exhaust, but they still exist. Increased heat exchanger size (condensing) and smart controls modulate output which reduces and sometimes completely avoids shut down periods. Hi Mass hot water situations are less affected by leaky shell because mechanical ventilation magnifying leakage is not a factor, so there is little measurable advantage to setback there. Also, BTU loss through an insulated shell at differentials of a few degrees is and has always been fairly insignificant, again, little advantage. Households are dramatically tighter, so delta-t stack loss savings due to setback are 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.
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
This is the whole point of heating. We are throwing money at a space to make it comfortable to occupy. Allowing a space to drop to uncomfortable temperatures means that even after air temperatures recover, large objects (walls, furniture, etc..) are still cold, and will pull heat from occupants. This means discomfort, or the need for higher temperatures to compensate. Is there a large chair or couch people occupy?
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 ModCon, match your boiler’s temperatures to your house, leave the temps set and let it run on low. If you use setback, use it delicately, to control when the radiant peaks occur to enhance comfort. If you want to use aggressive setback your ModCon 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.
