What is the science of setbacks and saving energy? Either way, you're firing boiler to maintain a t

CoachBoilermaker

Say it's 30F outside.

If you set the temp to be 70, your temps will range between say 69 to 73.
Overshoots to 73, then cools to 69, and boiler fires to maintain this range.

If you set the temp to be 60, your temps will range between say 59 to 63.
Overshoots to 63, then cools to 59, and boiler fires to maintain this range.

In both cases, you are having regular boiler cycles to maintain the temps in that 4 degree band. Either way, you're firing boiler to maintain a temp range.

So, where does the energy savings come from?

Is it due to the idea that higher temperature delta from outside will cool heat loss faster, resulting in more boiler cycles?

pecmsg

the wider the differential the faster the heat loss.
what are your design conditions?

delcrossv

IMHO, there's no advantage to setbacks with steam heat. E.g. 70 during the day, 65 at night.

What you're describing is deadband on your thermostat. That's necessary to keep your boiler from kicking on and off trying to maintain a very narrow temperature range.

Usually it's less than you're describing. Adjust your anticipator.

Jamie Hall

Under no wind conditions the heat loss rate of a structure is directly proportional to the temperature difference between inside and outside. Nice of it. Makes for easy math. (wind messes things up — oddly wind conditions will cause less of a difference in heat loss for large differentials than it does for small one).

As a very rough first approximation, then, a setback will make a difference, and the difference is related to the time weighted average temperature differential achieved — not the high or the low.

hot_rod

With hydronic systems you really want to use as low as possible SWT.

ODR can play a part in energy savings. I'd look into that that before setback.

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Wcurtis

I currently use 3 setbacks on steam heat and t-set at 2 CPH, each setback recovery jumps 1 degree (64 7:30 PM to 4:00 AM, 65 6:00 to noon, 66 to 7:30 PM), the CPH helps me weigh comfort with economy with a more stable temperature as the radiators do not cool all the way down and need to heat back up.

I would ensure your main venting is adequate as well, took me awhile to figure all this out

I must add, I have a Honeywell Vision Pro 8000 with the ability to learn the heating habits and will adjust (in theory) how long it takes to reach setpoint, this is why I mentioned the main venting which will affect this

jesmed1

Here's where I think setback helps most.

I maintain a 4-unit condo building in the Boston area heated by two over-sized oil boilers and a 100-year-old converted gravity system with cast iron radiators.

Both boilers are far too large for the heat loss of the building. In the deepest winter, they run from a cold start every 4 hours or so, run for 30-40 minutes, then shut down and cool off. Then 3-1/2 hours later the cycle repeats.

This is inefficient because the run times are relatively short, and can become even shorter (down to 20 minutes) in shoulder seasons. And every time the boiler heats up from cold start, it loses some residual heat up the flue after shutdown.

So my efforts to improve overall system efficiency have focused on having fewer boiler runs of longer average run time. @hot_rod is fond of sharing the Brookhaven boiler efficiency graph that shows the system efficiency of a cast iron boiler plummeting as its run times decrease. So I'm trying to move us to the right of that curve, with longer average run times.

I have two ways to do this. One way is to increase the "swing" or differential/deadband setting on the thermostats.

Another way is to create a large overnight "deadband" by programming in a setback of 3 degrees (my current setting.) With the large overnight deadband, this often eliminates one shorter overnight burn, and causes one longer burn in the morning of 1+ hours to recover.

So the increased swing and the overnight setback accomplish essentially the same thing: increasing the average boiler run time, while also reducing the total number of burns during a heating season. In theory, this shifts us to the right on the Brookhaven "system efficiency" graph.

This is the first season I'm experimenting with the overnight setback and a larger swing, so I can't yet say what the results are in fuel savings. But I do have a good 10 years of fuel consumption records from previous years with no setback and with lower swing settings, so I hope and expect to see measurable results by the end of the season.

CoachBoilermaker

Ok, so each degree loss at a higher range will occur faster and result in more fuel used.

70 to 69 will be faster than 62 to 61

Wcurtis

I received my December gas bill and used less therms for same heating period as last year with average temp this year 2 degrees colder and cheaper bill, also I had lower overnight temp setpoints 62 vs 64 and 65 5:00 PM to 7:30 PM.

These values do not seem like much but in my situation the gradual increase in temp with 2 CPH has made a noticeable difference

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1829852#Comment_1829852

Correct. The building loses less energy when the DT is 60 (assuming 0°F ambient) than when it is 70.

It also loses less energy as the temperature is falling from 70° to 60° (while the burner is off) AND it loses less energy when it is climbing from 60° to 70° while the burner is operating.

The claim that it is a wash to setback because it takes all the energy saved to recover is a common fallacy and will be repeated endlessly. This argument becomes somewhat more valid with a mod-con where it will operate less efficiently, at higher SWT, to recover.

CoachBoilermaker

GPT has this to say....

The energy savings from setting a lower thermostat temperature primarily comes from reducing the rate of heat loss from your home to the colder outside air. Here’s the reasoning:

1. Heat Loss is Proportional to the Temperature Difference

Heat naturally flows from warmer areas (inside your house) to cooler ones (outside). The greater the temperature difference, the faster this heat transfer occurs. If you set your thermostat to 60°F instead of 70°F, the temperature difference between inside (60°F) and outside (30°F) is smaller than it would be at 70°F, resulting in slower heat loss.

2. Boiler Runtime

Because your home loses heat more slowly at a lower indoor temperature, your boiler needs to fire less frequently to maintain the set temperature range. Even though the boiler cycles similarly to maintain the range (e.g., 59-63°F vs. 69-73°F), the overall heat demand is reduced due to slower heat loss at the lower set point.

3. Setbacks and Recovery

If you lower the thermostat further during unoccupied periods (e.g., down to 55°F at night), the heat loss is reduced even more. While there is some energy required to reheat the home, the overall energy savings from extended periods of reduced heat loss often outweigh the cost of reheating.

4. Thermal Mass

The energy needed to maintain a steady state (e.g., 60°F vs. 70°F) is lower because less heat is being lost to the surroundings. The boiler cycles to replenish less heat compared to maintaining a higher temperature.

Practical Example:

If the heat loss at 70°F is, say, 10,000 BTUs per hour, it might only be 7,000 BTUs per hour at 60°F. Over a day, that difference adds up to significant energy savings because the boiler doesn’t need to replace as much lost heat.

Key Insight:

The energy savings are not about the frequency of boiler cycles but rather about the amount of heat energy required to maintain a lower temperature difference with the outside environment.

ethicalpaul

not everyone is overshooting like you are with that crusty faulty old chronotherm

hot_rod

In simplest terms the 1st and 2nd law of thermodynamics:

Hot travels to cold, always.

The rate of heat transfer is driven by the delta T.

Hot_water_fan

i was worried GPT was smart for a few seconds! Then I saw point 4 🤣