Keeping tstat at 57 vs 65 vs 72

I think those who say that are relying on some false analogy. Maybe that if you make the bath water gradually hotter to temp H it won't burn you but if you put your dry foot in water at temp H, it will feel like you're burning it. This is true (but the story about frogs not jumping out is false).

If your furnace was such that it got less efficient the longer it ran, so it was more efficent to run in 10 5-minute bursts than in 2 25-minute sessions, they woudl be right, but I don't know of any furnace or method of heating a home that is like that. Meanwhile, your 57^ house will be radiating and maybe even conducting less heat to the outside. So your method is more efficient.

I may go away for 25 days this January. Should I leave the heat at 68 so I won't have to make a big jump when I get back? What's the difference between 25 days and 12 hours?

What's the argument on the other side?

I've heard that too, but I don't see that it matters. Heat always seeks to level itself, thus, if the house is warmer than outside, it will try to move out. The greater the differential, the faster the exchange.

Will the furnace or boiler work harder? No, it is either off or on. When on, it burns the same amount of fuel per minute of run time. Again, depending on differential it may cycle more as temps get closer, especially on a water system.

On reason to minimize the differential is comfort. I want to be comfortable any time I'm home and I change it at night to a lower temp for sleeping.

About 24.5 days.

Oh, I thought this was a math quiz. It reminded me of the (old!) story of the boy who bought a baseball and a bat for a total of $1.10. The bat cost $1.00 more than the ball. How much did each item cost?

Most people will blurt out $1.00 and $0.10, which is of course wrong.

Jim Joyce snipped-for-privacy@none.invalid wrote in news: snipped-for-privacy@4ax.com:

My daughter says $1.05 and $0.05.

Your daughter digs algebra.

x + y = 1.10 x - y = 1

adding the two equations together, you get:

x = 1.05

Yeah, but your daughter is 38 years old.

She's a keeper.

Agreed. Comfort is king. I hate the sound of my wife when she's cold. What the world needs is a cheap closed-cell fire-proof spray foam.

Exactly, that's why setback works. If it's 30F outside, less heat is lost to the outside during periods when the inside temperature is 60 than when it's 70. You've cut the temperature delta from 40 to 30, a 25% reduction.

It's actually going to be a bit more efficient running it for longer periods. Each start up takes some heat to heat the furnace up, heat the ducts up, etc. Most of that is lost once it shuts off.

If the furnace in a closet next to the kitchen, its heat works its way into the rest of the house. If it's in the basement, part warms the walls which warm the dirt outside but part floats upstairs.

The heat from the ducts might not be detectable but it isn't lost. Warming the furnace and ducts slows how fast the house is heated when the furnace first turns on, but the warm ducts also slow how fast the house cools when the furnace is off. The heat may not be discernable but it can't get to the outside without going through the inside.

Not necessarily, my ducts (insulated) are in a ventilated crawl space.

Nighttime temps are in the low 30s (daytime mid 60s).

Furnace efficiency is better steady state than when starting up - which is why an over-spec furnace is not efficient. Maximum efficiency is acheived with a system sized to run as close as possible to 100% duty cycle at worst case (coldest anticipated) conditions. This is why

I suspect that the efficiency gain from heating all the time is not more than the reduced heat loss from having the house cool off significantly overnight and then get reheated hours later. Heat loss varies inversely with the temperature difference. My furnace hardly runs at all when set to 45 or 55 at night. Maybe not at all at 45, unless it is an unusually COLD Seattle night. In the morning, it does not take long to get the air in the house back up to 58 or 60, which is my usual setting. The walls probably take significantly longer.

Speaking of walls, a lot of how your heating system responds is related to where the thermostat is located and if (as usual) wall mounted, how well insulated that particular wall is, and what material is behind the void, i.e. masonry, wood framing, dryer exhaust vent, etc.

The thermostat should always be on a centrally located interior wall

- where insulation, material, etc have very little effect. Also it should be out of the "draft" of a return duct and out of the range if a heating or AC duct.