Does it really use less energy/cost less to leave radiators on?

[Gray Matter]

I live in a 126-year-old drafty house (not sure if that's relevant).  I've been told by several people that it uses less energy/costs less to leave my heat up rather than turn it up and down, something about it taking more energy to heat the water and iron radiators back up than to leave them at a constant temperature.

I've been ignoring that advice for 10 years, because it doesn't make sense to me that it should use less energy to keep my house at 65 degrees all the time than to keep it at 58 degrees 20 hours a day (while sleeping and at work) and at 65 only four hours a day.

But then I read it in a reputable magazine, but without any explanation as to why.  I'm still skeptical, but admit that I know very little about physics (is this even a physics question?) or energy, so thought I'd check in here.

[jr1029]

Wow I am so looking forward to an answer to this question from someone knowledgeable, as it has been bugging me for years!

Last year I tried experimenting both ways and couldn't see a clear difference month to month. But that could have to do with outside temps - not a controlled experiment. Will be checking back eagerly!

[Cecil]

When it comes down to it, you can think of your house as a black box. Heat escapes, and you have to replace it. You pay only to replace the heat that escapes. (If that doesn't make sense, consider that if your house was perfectly sealed and insulated your heating bill would be $0).

So then, it turns out that the rate at which heat leaves is proportional to the difference between your house and the outside. If your house is warmer, heat leaves faster, and you have to make more heat to replace it than if your house was cooler.

All the other effects (length of time taken to heat the radiator back up, etc) are red herrings.

Short answer, the higher your thermostat, the higher the bill, on an ongoing basis.

[jr1029]

what about the fact that my heat tends to "overshoot" the temp on the thermostat? if i keep it at 63, it stays at 63.
but, in getting from 58 (nightime setting) to 63 (daytime setting) I often see a 66 before the radiators cool down, even after the furnace is off. does that change the answer?

[Posthumane]

The overshoot might affect it since the energy loss at 66 is greater than at 63, but you can fix that with a better control algorithm. If you are just setting it manually you may be able to set your thermostat to a couple degrees lower than your planned temperature and then raise it up after the overshoot starts to head back down, if that makes sense.

What Cecil said is spot on. Energy loss is proportional to temperature difference between two bodies (i.e. your house and the rest of the outside world) so the closer you get your house temp to the outside temp, the better. It is possible that the heat/cool cycle could be less efficient if you had a furnace that used a less efficient heating method to raise the temperature quickly and a more efficient cycle to maintain it, but most home furnaces only have two states (heating or not heating) so that is generally not an issue.

[Greg]

A radiant heat system is often more efficient if set to a temp and left alone.  This is because it's not as reactive, meaning it takes longer for it to reach your set temp, and it takes longer for it to react when that temp is reached.  Sometimes it depends on your thermostat, some thermostats can be set to accommodate different types of heat systems and can "anticipate" when the set temp will be reached.  It takes a lot of heat to warm up the water and metal, but it also gives it off for a long time as well. 

As an alternative you could try choosing a set point of say 62 and leaving it there.  Might make for warm nights though.

But if it's really drafty and not well-insulated this might be sort of moot, and any real savings will be obtained by tightening up the house.

I have radiant floor heat and so experience some of this problem, but also my house is only about 7 years old and so it's well air sealed and insulated.

An example of what you have heard about radiator systems might be supported by what I saw when I was a renter in buildings with radiator heat.  It seemed like they were always on, and we didn't have individual thermostats beyond a knob on any given radiator that we could turn if it turned at all, and opening windows.  My thinking is, knowing how cheap my landlords were, that if it saved money to turn it on and off they would have.

[jr1029]

very interesting, thanks. i am a renter so not a whole lot I can do about insulation. window film, i suppose - but actually the windows are quite good and new, it's the doors and walls themselves that have no insulation!

I think part of my problem is we have old radiators and a new programmable thermostat intended to work with forced air - the radiators still give off heat for a solid hour after the furnace is off. (AND, take 30 minutes to heat up, once i turn it on). I might just go with a steady temp all winter.

[dragoncar]

No

[kimmarg]

I live in a 126-year-old drafty house (not sure if that's relevant). 

YES very relevant. Any minutia of keeping it at 58 or 64 is far overblown by the crazy leakage. Get a home energy audit. Or light a stick on incense and walk around, everywhere you see the smoke blow you have a draft. Buy spray foam, door gaskets, shrink wrap kits for windows. All the options I just listed can offer a substantial improvement for around $20

[Greg]

I think part of my problem is we have old radiators and a new programmable thermostat intended to work with forced air

Check out the directions that came with the thermostat.  Look online if you didn't install it.  Most programmable thermostats have a way of setting the anticipation and other settings for various types of heat.  The setting for hydronic radiators will turn it on sooner and off sooner due to the lag time involved.  This lag time is referred to as "anticipation."