Turn down the thermostat?

Scott Kuchta

First let me say that I am reading Dan's book, "We Got Steam Heat!" and it is terrific. The money spent on the book will be repaid many times over - after I insulate my mains! Little did I know.
Anyway my question is this: I have an oil boiler one-pipe steam system with radiators. When leaving the house for work, say for 10-12 hours, or going away for the weekend, should we or should we not turn the thermostat down to "save oil"? I've heard many different views on the subject. The one I've been going with is to turn it down no more than 4 degrees from where it mormally is when we're home. what is your view on the subject and does it depend on the type of heating system you have? This will settle an argument at work.
Thanks.

Firedragon_4

This one of the best explanations

out there:

http://www.mge.com/home/appliances/thermostat.htm

This ain't bad either:
http://content.honeywell.com/yourhome/ptc-thermostats/savings/savings.htm

And last, but not least:
http://www.eere.energy.gov/consumerinfo/factsheets/thermo.html

This should nail down the bet, hope it was for big bucks!!!

Mike T., Swampeast MO

From That First Link

"Myths:

It costs as much or more to heat a home back up after a setback. (Or to cool the house after a summer setup.)
Fact: The longer your house remains at the lower temperature, the more heat you save."

Note the ALL-IMPORTANT qualification in that "Fact". The house must remain at the lower temperature to save energy.

If the boiler does not come on during your setback, you are using too much setback as the house is NEVER being maintained at the lower temperature!

Recommended setting of 85° during the summer when sleeping??? Obviously that's not written with Swampeast MO in mind!

Firedragon_4

The great thing about

burning fossil fuels is that all of the burners are 100% efficient. They burn nothing when they're off and all of the fuel put through when when they're on.

Efficiency is what you make of it, but you couldn't get me to part with my Chronotherms.

Jeff_17

I wouldn't part with

mine either! The heat works best when it is lowered SOMEWHAT and then raised SOMEWHAT. How do we define that? Individual homes are different. If you lower too much on a very cold day, that boiler will run a long time to warm that house back up to your temp....and then...no matter how good your therm...overshooting of the room temp. Those cast iron rads heat well...and LONG!

.

This question gets asked once every week or so at this time of year. A search will locate other threads.

Short answer: setback saves money, no ifs or buts. There is a "however", though: it doesn't save much money. Thermostat vendors overstate how much money is saved.

There is no harm in a larger setback. Set it back 20F if you like, as long as pipes don't freeze. The business of "it costs more to heat it back up than to keep it hot" is total and absolute nonsense.

Dave_16

Thermostat programs

I think you're going to have to just experiment a bit on your own to see how long it takes your house to recover from various setback temps. Obviously, a well-insulated house will cool-down and warm-up faster. I have poorly-insulated house, with a combination of baseboard and under-floor hot water heat. I allow the baseboard zones to cool 4 degrees during the day and overnight, but I only drop the radiant floor heat 2-3 degrees since it holds its heat better and takes longer to heat back up. I tried various values until the house warmed back up within one hour. Also, try not to fight Mother Nature by heating up too early or late in the day - I start my weekday warm-up gradually after about 1-2 pm, after the sun reaches its apex and has a chance to warm up the house a bit. I also start my lower level zone first, since some heat will always migrate upwards, thus giving the upper level zone a little extra help. Finally, we keep the upper zone set 1 or 2 degrees lower based on our personal usage of the space.

Fred Harwood

Setback

Yes, this thread has legs. And energy cost is important to us all. As is comfort. Large setbacks on a daily basis in winter, for example, chill the house and potty seat. Don't set back the temperature to the point of shocking tender parts.

Dale

Steam setback

Well maintained steam really heats quickly and if in good shape will bring back condensate quickly and quietly after a setback. No one in the house all day? Set back to 60 or whatever. What temp do you want it when you get home? My old WA oversize time was one hour, slightly oversized (1/2 input of old) is 2 hours. I want the heat on when I get up to shower so I set back so it will run steady then and then off till I get home. I don't have a 7 day setback so if I'm home on the weekends I manual override. Push the setback till the fuel savings/comfort compromise is what you want.

Dale

Summer temps

We really push getting AC that's 10% less than the heat gain. Allows the unit to run and dehumidify so the summer indoor air in our humid ( although not as humid as yours) area is low enough to make you feel comfortable at 82 or so. Problem is to convince people that running all the time at 90 is what it's supposed to do and that opening the windows with the AC on isn't the best idea. With the new 13SEER mandate the longer cycle time will save even more money. And, now we can add another benefit of under or at least correctly sizing, good for keeping the mold down too.

Mike T., Swampeast MO

That's What I Really Like About High-Pressure Units

Particularly in this climate.

They're generally sized well as nobody wants to run more of those expensive ducts than necessary--especially in a retrofit.

While it's true that they take a bit of an SEER efficiency "hit", it's more than made up for by the high humidity removal. Every such system I've felt around here is significantly more comfortable at a higher thermostat setting.

Cooling systems around here (particularly older ones) are generally massively oversized and are able to lower the air temp quite rapidly even after being set up during the day. Problem is that you have to run it VERY cool at night to keep humidity down.

With properly sized conventional systems, people often complain that they are too small and can't deal with the heat. Why? Because they've let the place get WAY too hot in the heat of the day and want that "instant" temperature drop. Their comfort level goes down and the electric bills go up! With significantly less "setback", their comfort goes up and bills go down! (I have the electric bills and complaints/compliments from two different renters to prove this!)

billygoat22

I've run my tstat in both manual and program and haven't noticed much difference in power consumption. I downsized the unit from 2 1/2 tom to 2 tons when I switched the unit to a heatpump, and at 90 outside I run 78 and get 45% rh in the house.
Like any a/c, it doesn't dehumidify if it doesn't run. At 78 setting that happens much of the time so now I'm runnug a whole house dehumidifier to take up the slack on mild days.
The manual D calls for over 28000 btu even with the insulation upgrades I've made, but I feel even a 1 1/2 ton could do well if you used manual only tstat and didn't reset the temp down in the hot afternoon hours.

Scott Kuchta

Turn down the thermostat?

Thanks for everyone's input here. Looks like I got the thermostat turned down but opened Pandora's Box! The argument against me at work was the old "it takes a lot more to heat up the cold house again - you're not saving anything" view. Apparently that's been shot down by actual research. So I guess I won the bet, not money, donuts for the office! Even better. Thanks again.

Uni R

Thermal expansion

Another thing against larger setbacks is the daily expansion and contraction that the house goes through. Your hardwood flooring is expanding and contracting, pianos and fine furniture get stressed, and the biggest issue is your drywall. Expand and contract the walls and ceilings on a daily basis and pretty soon you'll be seeing the heads of drywall screws. I would never go more than 5 degrees unless you are setting it back for several days.

Firedragon_4

You're kidding, right???

jackchips_2

Interesting Uni.

I have been setting back my Warm Air system to 60 degrees at night and while we work for over 25 years.

Should I start worrying about the screws? :-)

Uni R

Nah...

Nah, but I would have rather said "eventually" than "pretty soon". I noticed it happening in my own home when I used to use a 10 to 12 degree setback. That also meant that the relative humidity inside the house changed about 5% each cycle as well. Anecdotal for sure, but I never expected to set off anyone's BS-meter.

Mike T., Swampeast MO

Sketches show where you get fuel savings and where you don't. If you haven't maintained that lowered setpoint for some period of time there is no energy savings--in fact there's probably a touch of waste as the space will tend to overshoot quite a few times before it again goes into balance at the higher setting.

How rapidly the house looses heat (and thus is able to be maintained at the lower setting) depends on outside temperature, insulation, infiltration, etc.

In general the warmer the weather, the less setback you should use--either that or make certain that the setback period is long enough that the house is actually maintained at that temperature.

The better the thermal performance of the house, the slower it looses heat. So again, in general, the better the insulation/infiltration the less setback or the longer the period.

Also, many of the programmable thermostats now use a sort of "learning" logic whereby the thermostat strives to achieve the temperature you select at the time you select. If your setback periods are close and the house is loosing little heat, there can be some really goofy conflicts wherein the heat may be shut down before the thermostat ever reaches your higher setpoint. e.g. you've set back to 65° at night, want 72° at 6:00 a.m. and then want your daytime setting of 60° at 8:00 a.m. If the house doesn't loose 12° in two hours, there's a problem and I'm not certain what a "smart" stat will do...

Ron Schroeder_2

Mike,

I am going to agree with your sketch except that the second graph should be labeled "Less Savings". Even thou the temperature doesn't reach a steady state, the average temperature durring the setback period is still less than before the setback period. Durring that time, the house will loose less BTUs to the outside than it would have lost with no setback. The total energy needed to be put back into the system will still be somewhat less than the energy that would have been needed to keep it at a steady state. It will be even more true if the warm up reduces short cycling.

Ron

Mike T., Swampeast MO

Mean Radiant Temperature

comes into play here in a big way.

As the temperature in the space drops, so does the mean radiant temperature.

If you have your mean radiant temperature in reasonable balance before the setback period, you must raise the MRT back to that balance level for the same level of comfort at the same AIR temperature. Remember--the air in the space holds very little heat--it's the objects in the space that have the ability to store heat. It's not the air in the space that really heats colder infiltrating air--it's the objects. Were this not the case, even an extremely (almost unhealthily) tight home with .333 ACH would be at outdoor ambient temp in only 3 hours!

If you haven't raised the MRT back to a reasonable balance after setback you'll have that extremely well-known problem of "cold 70°". Your heating appliance will cycle much more rapidly until it gets the MRT into balance. The occupants will want more heat in the air than is truly needed for comfort because the MRT is low.

Granted, the MRT won't drop quite as fast as air temperature, but neither will it rise as fast as air temperature. MRT is a "lagging indicator".

I'll still say that unless you actually maintain your lower temperature for some period of time you have not saved energy for the same comfort level.

Set back unused spaces as deeply as possible without possibility of freezing. In occupied spaces on a daily cycle, keep your setback fairly low--how much has EVERYTHING to do with the weather and loss characteristics.

jerry scharf_2

wood is changed by moisture more that heat

Uni,

Go and talk with a musician or a flooring person. It's the moisture in the wood that drives the expansion and contraction not the heat. As for sheetrock, it's mostly ground rock, so I don't think it has much change for moisture or temperature in the ranges we are talking about. Plastics are a whole different thing, with lots of potential for differnetial temperature expansion...

jerry