Heat - when to set back

[---MIKE---]

I have a small log home. The logs are efficient insulators plus they act as a heat sink. My heat is baseboard hot water and I have two thermostats controlled by a homemade switcher. The "day" thermostat is set to 65=B0 and the night one is 57=B0. This does save some oil. My question is about daytime if I leave the house. Does it make sense to go on the lower setting for just four hours? It would use less oil during that time but then it would use more to "catch up" when I get home. What would be the minimum number of hours that would give the best savings on oil?

---MIKE---

>In the White Mountains of New Hampshire >> (44=B0 15' N - Elevation 1580')

[RBM]

good article on setback stats:

---MIKE---

[Edwin Pawlowski]

common misconception associated with programmable thermostats is that a furnace works harder than normal to warm the home back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. Years of research and numerous studies have shown that the fuel required to reheat a home is roughly equal to the fuel saved as the home drops to the lower temperature. This will result in fuel savings between the times the temperature stabilizes at the lower level and the next time heat is needed. The longer the house remains at the lower temperature, the more energy saved.

[Joseph Meehan]

Just not worth it. There is very very little "catch up" effect and sometimes it is even positive. Of course if you are going to be gone for

10 hours .... :-)

[Broderick Crawford ililililil]

When you are home you should burn wood preventing the furnace from running at all and for some time after you leave. If your wood is free it's 100% savings. I buy my wood and like my money staying local instead of going into the hands of the terrorists. Wood is cheaper but I would still burn it if it cost more. Stacking and carrying wood is good exercise too.

[Stormin Mormon]

The heat you pay for is the heat you lose.

The hotter the indoors, the faster you lose heat.

The colder the indoors, the slower you lose heat. Even a couple hours reduces your heat bill

[Tony Hwang]

Hi, First using programmable 'stat, decide what you want to do. Most comfort, most saving or compromise. Then program the 'stat and tweak it to your liking.

[newman]

common misconception associated with programmable thermostats is that a furnace works harder than normal to warm the home back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. Years of research and numerous studies have shown that the fuel required to reheat a home is roughly equal to the fuel saved as the home drops to the lower temperature. This will result in fuel savings between the times the temperature stabilizes at the lower level and the next time heat is needed. The longer the house remains at the lower temperature, the more energy saved.

[nicksanspam]

Got physics? :-)

There's no minimum time. Turning the heat down always saves energy.

Nick

[Joe]

I'm not getting this, can you explain further?

[John Barry]

He probably can't, because at best it's a working hypothesis.

Based on scientology or something, not on math/physics.

John

Joe wrote:

[Cshenk]

"---MIKE---" wrote I have a small log home. The logs are efficient insulators plus they act as a heat sink. My heat is baseboard hot water and I have two thermostats controlled by a homemade switcher. The "day" thermostat is set to 65° and the night one is 57°. This does save some oil. My question is about daytime if I leave the house. Does it make sense to go on the lower setting for just four hours? It would use less oil during that time but then it would use more to "catch up" when I get home. What would be the minimum number of hours that would give the best savings on oil?

It would depend on how much drop in temp you get over 1 hour, and 4 hours.

2 scenarios here:

1- when you shut down at night to 57, it takes 3 hours or more before the temp hits 57 but in the morning when you bring it up, it rewarms pretty much in 15 mins or less.

- may not save you that much

2- when you shut down at night to 57, it takes 1 hour or less to hit that temp and takes at least 15 mins, maybe more to rewarm.

- will save you money

[Dave Martindale]

Of course, you need to take into account that this talks about *energy* used, not cost. If energy costs the same any time of the day, no matter what the temperature differential (typical for gas or oil heat), then energy and cost are the same. But there are some systems where this isn't true. Pure resistive electric heat can cost different amounts at different times of the day in some locations.

And heat obtained from a heat pump gets a lot more expensive if the house gets cold enough to bring on the resistance heaters instead of just running the compressor (that's a case where it actually uses more energy for the same heat output, since the resistance heater is much less efficient than the heat pump heat). You may be better off letting the heat pump run some during the day to keep the house warm enough so that when the setpoint comes up at 5 PM (or whenever), the resistance heaters are not used.

So the minimum-cost option can depend on the system type.

Dave

[Sanity]

What difference does it make how long it takes to drop to 57? The furnace is not working at all at that time. Only when the temp is reset to 65 the next morning does the furnace come on continuously till the temperature in the house climbs to 65.

[nicksanspam]

Nonsense. Any setback saves energy, no matter how short.

This is 300-year-old high-school physics :-)

Nick

[Edwin Pawlowski]

Is there any limit to what the setback temperature should be?

[Cshenk]

"Joe" wrote

It takes a certain amount of time to reheat back to your 65 in the morning. It takes a certain time to drop back to 57 when you cut the heat back.

Unless we know how long those 2 are, no one can answer your question as it depends on the insulation and heating efficiency of your home.

The best any can do with what was posted is ask how long it takes to drop to

57, and how long it takes to heat back to 65.

[trader4]

Assuming, as someone earlier pointed out, that fuel costs are constant, which is usually the case. For example, if you had a situation with a heat pump where by turning it down too much it resulted in secondary electric kicking in during recovery, it could negate the cost savings. Or if you had differing rates for electricity for a heat pump during different periods, etc.

Only what you can tolerate from a comfort level when you're there and what the house can tolerate when you're not. If I'm away for an extended period, I set it down to 45. One factor is as the house cycles, you may get more drywall cracking do to expansion/contraction.

[AZ Nomad]

Freezing. If you can't feel the difference, let it set back to 35-40 degrees. You don't want it to get cold enough to burst the plumbing.

I personally prefer something above 60 at night.

[salty]

If you manage to lower the "average temp over time", then you are saving oil. The lower the average temp, the lower the oil usage. Yes, it's really that simple.