regulating temperature settings at night

moores

Our service man told us that we should only vary the temperature when we are gone our at night by 2 degrees in order to get the best bang for your buck.

true or false?

BobC

Setback

With steam heat, deep setbacks are counterproductive. Most people on this board recommend no more than 4-5 degrees. I use a 4 degree setback myself but steam sytems are not all the same.



Bob

Rod

Setback

Hi- The thought is that with steam you use more fuel making up the temperature difference than just maintaining the original temperature.

- Rod

haaljo

Depends on your boiler and lifestyle

If your boiler is 50 years old without modern vent damper technology and uses oil, then radical setback may be best; especially with moderate outside air temperature. I mean no steam heat for days at a time and instead using electric radiant heaters in rooms in which you watch tv, etc.

I have two of these electric heaters that look like microwave dishes. 750 watts/hour = ~8 cents/hour in Boston. Bedrooms, no heat when it's over 30 degrees in my house.

Therm_lag

The "recovery" myth

We discuss whether one can set temperature back "too much" and somehow negate energy savings as the "recovery myth" in my Residential Energy Technology course. Of the 22 students In the most recent class, all believed.  After we discuss myths at beginning of semester we ask students to explain the myth at mid-term, and then let students pick a couple of myths and ask them how they would explan to others why the myth doesn't hold water. 

Energy loss from a building depends on loss rate (Uo) times temperature difference times time.  For a given Uo and time, any reduction in temperature reduces the quantity of heat that moves outdoors.  Whether heat moves from indoor contents to air and then outdoors is not the issue, as this can only happen as fast as temperature indoors drops.  (So, houses with lots of indoor mass, or with good insulation and low air exchange save less.)  As Indoor temperature drops, temperature difference to outdoors is less, and heat flow to outdoors is reduced regardless where the heat comes from.

Running equipment a while to restore temperature keeps equipment at steady state efficiency longer than if it cycled with no or smaller set-back.  Follow the heat.  It is a myth that more energy will be used for recovery than would have been used to maintain higher indoor temperature.  We find in our class that about 1.5 hrs is required to dispel the myth, so powerful is its "common sense" appeal.  See the Canadian study of two identical houses.at   > http://www.cmhc-schl.gc.ca/odpub/pdf/63816.pdf

Mike Kusiak_2

Recovery and pressure cycling

While I fully agree with your analysis of setback and energy savings due to the lower difference between indoor and outdoor temperatures, there may be a special case with certain steam systems.



In the situation where the boiler is significantly oversized compared to the radiation, deep setbacks may be less productive. Under normal operation, the boiler may be able to fire continuously for the whole call for heat without cycling on pressure. When coming out of setback however, the boiler may run for such a long period that all the radiation is filled with steam and the burner rapidly cycles on pressure. If the pressure cycling occurs for a long period of time, the efficiency gain of the setback period may be negated by the loss of efficiency during rapid cycling. This loss of efficiency may be significant, especially with a power burner which has a pre and post purge of the combustion chamber on each firing cycle.



If the boiler and radiation are precisely matched so that pressure cycling does not occur during setback recovery, then the energy savings of larger setbacks should be more fully realized.

Jamie Hall

I do love academics...

it would be so nice if the real world and their wonderful world coincided.  Unfortunately, at least by the records (meticulously kept) on the building I supervise indicate, they don't.  Pity...

jpf321

so .. let me see...

Therm_Lag is saying setbacks work because long firing is efficient firing?



Mike K, is saying that in steam (and oversized boilers) long firing leads to pressure cycling which leads to inefficient short firings. But well matched boilers might not cycle on pressure so long firing = efficient firing.



Jamie is saying that he doesn't use setbacks (or maybe very small ones?)



I'm going to stick with constant temp (0deg setback, but 0.5deg "swing"). I guess.

Mike Kusiak_2

Optimum setback

My feeling is that regarding efficiency, the optimum amount of setback for a steam system is the deepest one which will not cause cycling on pressure during recovery. If pressure cycling occurs even during normal operation maintaining constant temperature, then any setback at all would seem to be counterproductive.



Since every system is different, It follows that no sweeping generalizations can be made. Each system has to be evaluated for its specific operational characteristics, and only then can a determination of of optimum setback be made, if one is to be used at all.

Therm_lag

seems what I'm hearing . . .

In the case where boiler heats building without cycling on pressure I hear agreement that set-back savings will be achieved with steam as any hvac/building system:  reduce the driving force for energy loss to the outdoors.  Where cycling on pressure occurs during pick-up or recovery, I hear there will be a trade-off between cycling loss and energy saved by allowing temperature difference, building to outdoors, to drop.  A next step:  set firing rate to reduce in response to pressure, reducing firing rate before system fills (and venting radiators in relation to room loss).  Would this allow the full potential of setback?  Lowering the firing rate might produce savings greater than seen in fixed input furnaces and boilers.  Certainly, the time allowed for  recovery must account for the relationship between system capacity and loss rate. 

Peterson illustrates fill/pressure relationship, and there is much discussion of multi-stage firing through numerous threads.  MNCEE research > http://www.mncee.org/pdf/tech_pubs/85-8.pdf  fig. 7

Comments on Canadian research that illustrates increased energy savings with larger set-back and longer set-back?

Rod

Real World vs Theoretical

I think we're all very aware that having a controlled variable output burner would be a great benefit. However this is the real world and we have to work with what we have. That means working with piping systems that are old enough to collect social security, boilers that are just big tea kettles, burners which have about as much control as a military flame thrower and what controls we have, are sixty year old technology with springs and electrical contacts and the high percentage of them don't work right out of the box.

Until they come up at the residential level with a fully modulating burner, with microprocessor control, available in gas and oil,  what might be theoretically possible has no real meaning to us at all. 

Just my two cents.

- Rod 

jpf321

appendix 3???

fantastic paper .. really interesting .. makes me rethink alot of things, and helps other things fall into place .. but where oh where is Appendix 3? anyone? anyone?



thanks for sharing this interesting resource. now i have to go rip out all my pipes and weigh them. what other treasures like this do you have .. i crave more.

ansky

Setback

We can review studies all day that try to prove a setback savings one way or the other, but I did my own "study" in the real world two winters ago.  For a month, I lowered my thermostat about 6 degrees when I left for work in the morning, and then raised it back up when I returned home about 10 hours later.  I did this every day for a month.  All I can say is I almost had a heart attack when I opened up my gas bill at the end of the month.  My gas usage was more than double what it was when I kept the temperature nearly constant. On top of that I came home to a cold house every day and had to wait for it to heat back up.  So maybe in some instances people will have a savings by using a setback.  But it certainly doesn't work for everyone, like myself.

FJL

Will change and compare

For the next billing period (mid Nov thru mid Dec) I will experiment and leave the temp at the same setting and see what results I get.  If nothing else, I'll be totally warm all the time!

Therm_lag

flip flop

Try changing from no set-back to set-back week-by-week, say Sunday night.  Record utility use.  Retrieve the hi/lo temps for each day (Sunday newspaper?) and average for the week.  Graph use against temperature for both cases, as seen at Graphical Load Estimating Method on the Wall.    Flip-flopping for a few months should account for weather variation as well as changes in habits.  Avoid holidays, use of supplemental heat, and the myriad of other actions that can raise or lower utility use to isolate only the difference from reducing delta T between indoors and outdoors.   It's not a large change.  Canadians observed 6.5% in highly insulated house.  Note that difference in weather-related energy use is "diluted" by non-weather (base) use.

I have spent hours debating set-back savings.  I am open to any insights on this and have no interest in proving anyone wrong or reinforcing a particular opinion.  It would be great to put set-back savings to bed!

Mike Kusiak_2

Canadian study

Something to note about the Canadian study is that it is based on houses with mid efficiency (non-condensing) forced hot air systems. I have a feeling that such hot air heating systems will benefit more from setback than will hydronic systems. Since most homes in the USA are equipped with hot air heating, perhaps this is why it is claimed in general that deep setbacks will significantly increase energy efficiency.



The situation with steam systems regarding the tradeoff between setback and recovery has been discussed above, but I believe a similar tradeoff exists with hot water systems also. Consider the case of a hot water heating system with a condensing boiler, maintaining constant steady-state room temperature. The boiler will be running at a low firing rate with low return water temperatures. This is the ideal situation for a condensing boiler and will most likely be operating at an efficiency in the  95% -97% range. Now lets look at what happens when the same system recovers from a deep setback. In order to achieve recovery in a reasonable amount of time, the firing rate and water temperature will have to increase, most likely with return water temperatures above the condensing range. Once the boiler stops condensing, there is an immediate decrease in efficiency of almost 10%, more than offsetting any possible savings from the setback. So, at least in this instance, setback makes no sense at all. This is admittedly a worst case situation, but the general analysis is valid.



So, while the Canadian study may show energy savings in the specific case described, it does not follow that similar savings will result with other types of systems different from the one actually studied.

jpf321

6.5% what?

savings? greater consumption? difference?

haaljo

Condensing boiler not in the cards for steam.

Too bad; And also the modulating burner thing for home steamers. I'd like one of each.

The Canadian study is a great reference.

The laws of thermodynamics should be rewritten.

Coming home to a cool house is good for you.

Steam heat owners should have a radiant electrc heater or two.

Charlie from wmass

If Jamie's record do not support savings

From set back then you can bet that it will not save you money with a steam system. 4 degrees or less and you can be moderately comfortable. Many homes with steam also have less than ideal r factors in the walls. If your how is like a thermos then set backs can really save you money. If it is like a colander not so much on the savings.

Polycarp

underfiring/undersizing

We've heard a couple of times on the Wall about the benefits of underfiring/undersizing boilers.  I wonder if this approach would help limit/eliminate the inefficiencies that plague some steam systems in recovery.  

haaljo

Building envelope and weatherization is the key

It has to be commissioned as that industry has to overcome previous reputation, at least in the Boston area. It's about $20K to button up an "average" house big time. Lot sweat equity can be done. To get homeonwers to shell out even $10K for things they can't see and a relatively small change in lifestyle is very difficult.

Prefer granite countertops.

haaljo

Define undersizing a new steam boiler

If you have an installer put a steam boiler sized "perfectly" for the EDR and fail then are you willing to shut off or TRV some radiators? The Contractor certainly dosn't want to be callled back and whinned at.

But oversizing new steam boilers happens all the time.

Underfiring is a good question. When I see boilers on this board that are using $4+ worth of oil/hour, I wonder if getting the firing rate reduced as much as possible has an attractive payback. You get what you pay for and has to be done by a tech with fancy instruments and other means of magic.