Some further thoughts on Setbacks

Jamie Hall

A thread which could also be titled "Favourite topic #1, revisited" (well, maybe not favourite -- over on the steam side, pressure and pressuretrols are right up there these days!).



Having given the whole thing a good bit of thought...



In theory, a setback -- of any duration -- should save some energy.  Not, however, as much as might be immediately obvious.  While the temperature is declining and ramping back up, the heat loss (and hence the required heat input) will be proportional to the average temperature differential between the original set temperature differential and the lowest temperature differential reached (which may or may not be the setback temperature differential, depending on the heat capacity of the structure, the setback temperature, and the length of time required to ramp back up).  Clearly this is less heat than what would have been required, had the temperature not been set back.  After the structure has dropped to the setback temperature, the heat loss and heat input will be proportional to the setback temperature differential.  Again, a savings.  So it is reasonable to say that yes, in theory, a setback will save energy.  The amount of energy saved, however, is less than what would appear to be the case, considering only the setback itself -- it depends on the heat capacity of the house, the duration of the setback, and the capacity of the heat source to ramp back up.



Where things get complicated, though, is when the efficiency of the heat source is factored in.  Most steam boilers cycle on pressure when working on a recovery, but do not do so when maintaining a temperature; this entails a loss in combustion efficiency which depends on the cycle duration, the burner, and the boiler.  Some, but not all, hydronic systems also operate at a higher temperature when trying to recover from a setback.  This may drop their efficiency somewhat -- again, depending on the temperatures involved and the boiler and the way they adjust output temperature (if they do) in relation to apparent load.  In neither case, however, is efficiency increased by a recovery.



Will the net result be a savings of energy from the lower heat loss during the setback, or an increase in energy from the lower efficiency during recovery?  Ah... right.  As I said in an earlier post, "it depends" on all the above factors -- and the optimum setback (if any) from the energy standpoint will vary for each specific system.  Generalizations would appear to be dangerous.



I might note for the scorched air crowd that it is my understanding that the efficiencies of such monsters does not change much with output temperature of the unit, nor with cycle length -- so it is likely that the general reaction of "setbacks always save energy" may well apply to scorched air systems.



I would further note, as I mentioned in my earlier post, that some space applications might benefit from setbacks, but that setbacks might be a poor idea for other reasons, such as occupant health, damage to artifacts and objects, etc.  (one might add here that if the setback is enough to allow condensation to form on windows, walls, or artifacts, the damage can be catastrophic).



What fun!



Comments?

AlexR

setback and standby loss

When I asked about this in Strictly Steam, someone claimed that the primary source of inefficiency when a boiler short cycles is the standby loss- the heat the boiler loses to the cold basement when it's not running.  As I think about it more, that doesn't make any sense.



Imagine two systems, one with an ideal modulating boiler and another with a normal residential boiler.  Both have been running for a bit, the radiators are hot, and the pressure starts to rise.  The modulating boiler now cuts back and runs at, say, 50% output to keep the pressure constant at a few ounces.  The boiler's temperature is internally about 212F and externally somewhat less. 



Now think about the boiler you actually have.  It cycles on and off.  Over a long enough time, it produces the same amount of steam (and therefore same output energy) as the modulating boiler.  The internal temperature will also be about 212 and the external temperature a bit less.  However, both boilers are hot and producing steam for the same length of time (since they produced the same total amount of steam, that is, the amount the radiators needed to condense to satisfy the thermostat) and there's no appreciable difference in the boiler's external temperature and therefore no appreciable difference in standby heat loss to the basement.



 The non-modulating boiler may work a bit harder since it has to produce some steam at 8oz or 1psi and therefore needs a higher temperature than the other boiler at its constant few ounces.  Perhaps that's the source of the inefficiency? 



I did do some simulations about heat loss from the boiler and pipes into the basement.  Comparing two hour long cycles per day to four half hour cycles showed a whopping 2% savings, which is probably smaller than the error from my incomplete knowledge of thermodynamics.

Mike Kusiak_2

Standby loss

The standby loss does not only consist of the heat loss to the basement, but also the loss out the flue and up the chimney.



What you say makes sense as far as the boiler jacket loss of heat to the basement, since the boiler is at around 212F in both cases. The heat loss should be about the same. The additional loss comes into play with the heat that goes up the chimney during standby. If an automatic vent damper is used, this heat enters the basement instead of the chimney, but it still does not contribute to the steam produced to heat the building.



There is also the cycling efficiency loss of starting the burner and pre and post purge which blow cold air through the combustion chamber twice during each heating cycle. These losses will be significantly higher with the cycling burner when compared to the modulating burner.

icesailor

Setbacks:

What you are saying is probably more true for a steam system where you need 212 degrees just to start. But I think that Hydronic FHW systems are different. You can't manipulate the steam temperature and you can the hot water temperature. I'm not an engineer but I do my own heat loss calculations and radiation out of necessity. I design for 180 degrees knowing that I can increase the output of the radiation by increasing the temperature of the water. When I started doing it (design), I was using so little radiation compared to when I had a sales person from the supply house, that I worried I was doing something wrong. Like I said in a post somewhere here, you manipulate the outside temperature with set back thermostats. The issue was that with indoor/outdoor reset, you can't use setback thermostats because when the heat comes back on and the set back becomes set up, the water isn't hot enough and it will not raise the temperature in the structure unless you raise the water temperature. I guess you can only raise the steam temperature slightly by raising the system pressure.

What I personally find about set back thermostats is that the thermostat will be lowered in the home and people will be comfortable when it is done automatically. And if they are setting it themselves without setbacks, they will set it at a higher temperature.

I have been using them for over 35 years. I have installed them in customers houses. I checked the fuel use and compared it to degree day factors. In every case, there was a noticeable savings in fuel. I never had ANYONE complain that they didn't work.

NRT_Rob

I always love the

'no one ever complains so it works' argument. it's really bad design to maximum water temps at 180. Double the emitter, run at 140, and add significant efficiency. it's a better choice for your clients in the medium term.



There are two aspects to setback not really addressed in this post:



1. Setback reduces comfort. Air thermostats may recover quickly, but the mass of the room doesn't recover so fast. So a "setback recovery 70" is not the same as a consistent 70 in comfort. This effect varies with emitter though: it's a big deal with convective/forced air system that take *forever* to heat up the MRT of the room, but if you have radiant or radiators, it's possible the higher operating temp of the radiant panel will offset the cooler surfaces elsewhere to some degree.



2. Setback recovery, for many installed boilers/furnaces, can be GOOD for cycling efficiency. Reason being, most installed boilers and furnaces are far too big for the loads they service, and this can reduce cycling by concentrating the demands into a heavy demand period. this would depend on having enough emitter to handle the output of the unit.



But for properly sized heat plants, in modern, tight construction, setback is a marginal benefit.

Jamie Hall

I totally agree

with NRT_Bob here,  On both points.  The first one -- comfort -- is one which I don't think is given as much play as it needs to have.  It is sort of part of the hair shirt argument -- and I'm often inclined to wonder just how much discomfort folks are willing to put up with for the sake of a percentage point or two savings.  But then, I'm an older chap, and with creaking joints comfort starts to become more important!  It sort of goes along, though, with the caution I was making about certain types of objects (my own experience in this is musical instruments; specifically pianos and harpsichords) not liking temperature changes.  Pianos aren't too bad that way; so long as the humidity is reasonable (around 50% RH) a daily swing of say 5 degrees won't throw a good piano out of tune that fast (say two to three months)(which, for the heating inclined, translates these days into about a gallon a day in terms of oil cost...).  On the other hand, such a swing will throw a harpsichord out of tune in two or three days -- or less.  Oil paintings are less affected, although temperature swings aren't good for them, and books even less so.



On the second point, quite true also.  Which is partly why I note that the optimum setback -- if any -- is very much a function of the heating plant installed.

Gordy

Rob nails it

 Totally agree with Robs philosophy  on this subject that is constantly debated.



Put it this way if your total heating season fuel bill is 1500.00 which is a high figure for a 2000 sf home. Say you save 5% setting back that translates to 75.00. for a 5 month season or, 15.00 a month, or .50 cents a day. Is that not a valid expense for comfort? It is for me.



So in the end technically you can save money when setting back but there is a sacrifice in comfort.

Charlie from wmass

Since set backs work best with force air

and forced air is the least comfortable heat to start with then who would notice a bit less comfort? If set back was so good why is continuous circulation so popular in Europe where fuel costs are so high? People seldom complain when they have nothing to measure what they have against. I size for 170 degree water but for -20 outside air and 70 degree inside air. this gives room for a good amount of reset in the shoulder months and a warm home in that week in January or February that gets to -15 or -20.

Mike Kusiak_2

Setbacks and comfort

There is another side of the comfort issue also. Personally, I use a small setback of about  2 or 3 degrees, not for economy, but because of comfort. We like to wake up to a warm house, but like it cool for sleeping, so a nighttime setback works perfectly for my family. The small overshoot in morning temp is a bonus as its slightly warmer and more comfortable when getting out of bed, than when maintaining temp during the day.



Rob's point about the long burn coming out of setback is another bonus with my system. With oversize cast iron boiler and rads, the system runs continuously for an hour or two in morning recovery, heating up all the water and iron, and then coasts until the late morning or afternoon depending on outside temp. Just about optimum as I see it.



System dependent, and a matter of preference, but the is also a place for small setbacks as far as comfort is concerned.