In most cases I think the conclusion of the article that setting the thermostat back at night reduces energy consumption is correct. I think that there are multi-mode heating systems where the second mode (used to boost heat output when the main mode is not keeping up) is so inefficient that a setback can result in higher overall energy usage.
The most likely example is common in my area. The heating system consists of a primary mode of a heat pump, with a secondary mode of electric heating coils to boost heat output. The electric heating coils (often called emergency mode in this case) are very inefficient relative the efficiency of the heat pump. A common thermostat checks the difference between house temperature and set point temperature, and if they differ by more than a certain number of degrees it turns on the second mode (emergency mode) of heating.
The setback approach in a house with this type of heating system can cause the emergency mode to be invoked each morning when the setback is turned off. If this causes the very inefficient electric heat to be used instead of the heat pump, then there is very good chance that any savings benefit of the reduced usage overnight is outweighed by the inefficient mode of reheating in the morning.
dennishiding, when you start a thread here, it’s helpful to other posters if you provide a link to the column you’re discussing. Yeah, it’s on the front page now, but in a week or two it will be hidden in the Archives. So, providing a link saves searching time and helps keep us all on the same page.
There are a lot of systems that Cecil’s article didn’t address, if you want to get picky. Our town hall has a new dual-system that is highly computer controlled. Supposedly it analyses your request and figures out which system to use and for how long. If you are in a hurry, it will use the system that is fastest; if you want economy, it may be slower to heat or cool, but if there is enough time, it will start the cycle so the desired temp is reached at the time you want.
Still, underneath all of that computer malarky, it is just a system that is on or off, like Cecil said.
Exactly what I’m saying is that is not just on or off. A multi-mode system has different methods of generating heat, and those different methods are not equally efficient.
If the system is on in electric heating mode it takes 1000 Watts to dissipate 1000 Watts of heat into your house, but if instead you were running the heat pump mode it only takes 250-333 Watts to move 1000 Watts worth of heat from outside to inside your home. If the large set point differential when the thermostat set back is removed in the morning causes your thermostat to think it has to turn on the electric heat to catch up, you could be paying 4 times more for that burst of heat in the morning than if you’d let the heat pump just keep that heat in the house all night.
Exactly what I said, using our town hall’s system as an illustration.
Exactly what I said. If you have two systems, it is possible to use one or the other or both depending on whether you want fast and expensive heat, or slow and cheap. Either you let the computer calculate or do it by hand.
Anyway you look at it, each system is either on or off. I don’t doubt that other systems are possible, but the mentions in this thread of continuously-variable heat amounts seem to be limited to heat pumps.
So I just had a new furnace installed a few months ago (forced-air fuel oil). It’s either on or off, no in between (thermostat controlled, of couse). The only thing I can control otherwise is the fan speed.
Modern boilers (for hot water, not steam, heating) are often “modulating boilers”, which means a large number of stages—essentially continuous firing rate. Examples of brands that sell modulating boilers include Viessmann (premium German brand) and Munchkin (cheap US brand).
My Munchkin rarely goes off in cold weather. (In warmer weather it can’t modulate low enough so it does go on and off, but modulation allows it to avoid short-cycling. Instead it runs for long periods at low fire.)
If the heat loss from your house was constant, then it wouldn’t matter either way, because it would take exactly the same amount of energy to reheat the house in (a) as it took to maintain the temperature in (b).
But the heat loss in your house isn’t constant. You transfer more heat when the temperature difference is greater than you do when the temperature difference is smaller. That is what makes (a) more efficient.
The one exception to this is exactly what dennishiding is talking about in the OP, which is when you have some sort of auxiliary heating system that is not as efficient from a cost point of view. From a purely thermodynamics heat transfer point of view, setting the thermostat back is still more efficient, but if the later reheating causes the less efficient heating system to kick on, you may be adding most of that heat energy from a much less cost efficient source. The difference in cost for running the auxiliary heating system may be greater than the heating cost that you saved by setting the thermostat back, so you could end up losing money on the deal.
Some multimode heating systems are smart enough to account for thermostat setback and won’t immediately kick the auxiliary heat on just because the thermostat’s setpoint changed.
In addition to multimode systems there are other exceptions like radiant floor systems. Setback is rational on systems where air is the medium (except in auxiliary mode) because air has a low heat capacity. If you’re heating through a medium that has high heat capacity like concrete you’re going to waste a lot energy trying to recover heat loss in a short period of time.