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Notes on residential fuel use: Thermostat and window management
Energy and Buildings, 1 (1977) 99 - 101
99
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Technical Note Notes on Residential Fuel Use: Thermostat and Window Management
(1) For the same house modeled in various climates, the absolute value of the fuel savings should be almost independent of the climate. We note a saving o f 90 therms for both Washington, D.C., and Travis Air Force Base (located between San Francisco and Sacramento, Calif.). We can compare this saving of 90 therms each winter, which is equivalent to 0.5 gallon o f fuel oil each night, with the oil needed to produce electricity to p o w e r an electric blanket overnight, namely 0.05 gallon, or ten times less. Therefore, if an electric blanket will make larger temperature setback more acceptable, by all means the use of electric blankets should be encouraged. For commercial space, temperature setbacks to 35 - 40 °F should be considered for the dramatic energy saving potential. Table 1 shows a saving of 40 - 50% in fuel for a commercial building with the thermostat set to 50 °F at 6 PM on weekdays and starting at 1 PM on Saturdays. Full setback (protected only against freezing) to 35 °F saves a b o u t another 10% in Washington, D.C., b u t very little in northen California, since the building seIdom cools below 50 °F by morning. During the sample year for Wahsington, D.C., used in the program, the single-glazed building cooled to 38 °F only six times.
A R T H U R H. R O S E N F E L D
Department of Physics and Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720 (U.S.A.)
Our computer model shows that night temperature setback to 60 °F in homes saves up to 30% o f fuel in warm climates. In commercial buildings night~weekend setback to 50 or 40 °F saves 0.33 - 0.5 the fuel. Permanent rereflective film, e.g. Scotchtint, reflects valuable winter heat. If used as a shade, managed to admit winter sunlight, 25% fuel savings can be achieved.
It is c o m m o n practice to reduce the thermostat setting to 55 - 60 °F at night in residences and over nights and weekends in commercial buildings. In fact, this is an effective way to save fuel. Table 1 is a display o f comp u t e d fuel savings for several temperature setback levels. The t o p t w o entries of Table 1 show that even an 8 °F setback to 60 °F in a h o m e will save 10 - 20% in fuel. T w o remarks a b o u t moderate residential setback may be of interest. TABLE 1
Fuel savings for several temperature setback levels in a 1600 ft 2 residential or commercial space* Temperature setback schedule
Winter fuel requirement Washington, D.C. 4650 degree-day
Travis AFB, Calif. 2600 degree-day
(therms)
(%)
(therms)
(%)
Residential N o setback Night setback to 60 °F
812 720
100 89
510 420
100 82
Commercial N O setback Night/weekend setback to 50 °F Night/weekend setback to 37 °F
378 235 206
100 62 54
240 132 129
100 55 54
*The space is assumed to have 20% glass area in each wall, single-glazing, and a base daily daytime temperature of 68 °F.
100 TABLE 2 Winter fuel c o n s u m p t i o n for various w i n d o w - m a n a g e m e n t strategies. Calculations for a single-glazed Washington, D. C. house, w i t h 20% w i n d o w area in each wall. The reflective film is a s s u m e d to be P-18 S c o t c h t i n t with a shading c o e f f i c i e n t o f 24%. In the s u m m e r t h e S c o t c h t i n t saves a b o u t 1000 t o n h o u r s o f air c o n d i t i o n i n g ($50 electric savings).
P e r m a n e n t film, S.C. = 24% Clear single-glazing Roller shades: Closed 11 PM - 7 AM Closed 5 PM - 7 AM Double-glazing
U-value ( B t u / h r - f t 2 . °F)
Winter fuel (therms)
(%)
0.935 1.1
800 700
114 100
0.51 0.51 0.65
650 620 590
93 89 84
We recommend this conservation technique as cheap, effective and painless. Several manufacturers are currently marketing aluminized self-adhesive plastic film with a transmission coefficient of about 20%, which will convert a clear glass window into a "solar c o n t r o l " window. In an air-conditioned home it reduces solar heat gain in summer enough to pay for itself by reduced electric bills. (In Washington, D.C., applied to a window which would otherwise receive direct sunlight, it saves 25 - 35~/ft 2 each summer*). In any home, the film reduces glare and summer heat, and prevents fabric from fading. The catch is that in the winter it also reduces solar heat gain, and according to our calculations, causes our single-glazed Washington house to use 100 therms more gas ($30 at 30d/therm), or 75 gallons more fuel oil. The summer savings are $50 - 75. We now ask how much fuel can be saved if the reflective film is not applied permanently, but installed as a roller shade or Venetian blind, closed all day during the summer, but open on sunny winter days. If the shade (or blind) is confined in a track so as to produce a dead air space between the window and the closed shade, a single-glazed window is converted to an unusually good storm window, with a U-value improved by a factor o f two. Consequently, it pays handsomely to close the shades after sunset in winter. Table 2 shows the fuel consumption for various management strategies for a Washing*It also saves t h e electric utility a peak d e m a n d o f a b o u t 10 W/ft 2 o f film.
ton, D.C., house. If the base case is taken to be permanently adhesive film, total savings of nearly 180 therms (25%) can be achieved, in addition to the summer savings. Of the 180 therms, two-thirds come from solar heat gain during the day and one-third from the betterinsulated windows at night. Figure 1 shows the hour-by-hour thermal advantages of the roller shade over the permanent film. The printouts cover four days in January and April 1962. Nights are easily picked out because the thermostat setting, labeled TLO, dips to 60 °F from 11 PM to 7 AM. 'It is assumed that the roller shades are not closed until 11 PM and are opened again in the morning. (A conservationist household could, of course, close t h e m at dusk, and keep them closed on overcast days, thus achieving a performance even better than that o f double glazing.) The full curve, with the dots below it, represents the hourly furnace load with the permanent film rejecting three-fourths of the sunlight. The broken curve is the furnace load when the shades are opened during the day, and closed at night so as to improve the Uvalue of the windows. Note the dramatic contribution o f sunlight, which almost replaces the furnace on January 2 and 4, and for most of April. Sears sells the self-adhesive Scotchtint for 70¢/ft 2, but does not offer shades. Wholesale prices for film are 50¢/ft 2 for the self-adhesive form, 25¢/ft 2 for the shade material. Dealers charge $1 - 3 to install the self-adhesive film. Shades can be made more cheaply. Sears, for instance, sells cloth shades for as little as $1.50 per window.
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Fig. 1. C o m p u t e r printout comparing heating loads for permanent v s . manageable reflective film w i t h a shading c o e f f i c i e n t o f 24%. The full curve (joining the t o p s o f the c o l u m n s o f dots) is the h o u r l y furnace load w i t h perm a n e n t film. The b r o k e n curve is the load if the film 'is o n a shade w h i c h can be o p e n e d during the day, but is closed from 11 PM to 7 AM to convert the single-glazed w i n d o w s to storm w i n d o w s • The T's s h o w the outside temperature• The same p r i n t o u t for a Washington, D.C., house w a s used for Fig. 3 o f the article b y Dean and R o s e n f e l d , see page 22.
We have n o t y e t e s t i m a t e d t h e c o s t o f t h e tracks t o p r o v i d e t h e dead air space, b u t w e c o n c l u d e that solar c o n t r o l shades c o s t n o m o r e t h a n p e r m a n e n t film, and o f f e r c o n s i d e rable t h e r m a l advantages. O t h e r m a n a g e a b l e , t h e r m a l l y a s y m m e t r i c , shades and blinds are
discussed e l s e w h e r e in this report b y Silverstein and R o s e n f e l d . We t h a n k L o u F. M a s o n i c k , Solar C o n t r o l P r o d u c t s , 3M C o m p a n y , for his help and suggestions.
99
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Technical Note Notes on Residential Fuel Use: Thermostat and Window Management
(1) For the same house modeled in various climates, the absolute value of the fuel savings should be almost independent of the climate. We note a saving o f 90 therms for both Washington, D.C., and Travis Air Force Base (located between San Francisco and Sacramento, Calif.). We can compare this saving of 90 therms each winter, which is equivalent to 0.5 gallon o f fuel oil each night, with the oil needed to produce electricity to p o w e r an electric blanket overnight, namely 0.05 gallon, or ten times less. Therefore, if an electric blanket will make larger temperature setback more acceptable, by all means the use of electric blankets should be encouraged. For commercial space, temperature setbacks to 35 - 40 °F should be considered for the dramatic energy saving potential. Table 1 shows a saving of 40 - 50% in fuel for a commercial building with the thermostat set to 50 °F at 6 PM on weekdays and starting at 1 PM on Saturdays. Full setback (protected only against freezing) to 35 °F saves a b o u t another 10% in Washington, D.C., b u t very little in northen California, since the building seIdom cools below 50 °F by morning. During the sample year for Wahsington, D.C., used in the program, the single-glazed building cooled to 38 °F only six times.
A R T H U R H. R O S E N F E L D
Department of Physics and Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720 (U.S.A.)
Our computer model shows that night temperature setback to 60 °F in homes saves up to 30% o f fuel in warm climates. In commercial buildings night~weekend setback to 50 or 40 °F saves 0.33 - 0.5 the fuel. Permanent rereflective film, e.g. Scotchtint, reflects valuable winter heat. If used as a shade, managed to admit winter sunlight, 25% fuel savings can be achieved.
It is c o m m o n practice to reduce the thermostat setting to 55 - 60 °F at night in residences and over nights and weekends in commercial buildings. In fact, this is an effective way to save fuel. Table 1 is a display o f comp u t e d fuel savings for several temperature setback levels. The t o p t w o entries of Table 1 show that even an 8 °F setback to 60 °F in a h o m e will save 10 - 20% in fuel. T w o remarks a b o u t moderate residential setback may be of interest. TABLE 1
Fuel savings for several temperature setback levels in a 1600 ft 2 residential or commercial space* Temperature setback schedule
Winter fuel requirement Washington, D.C. 4650 degree-day
Travis AFB, Calif. 2600 degree-day
(therms)
(%)
(therms)
(%)
Residential N o setback Night setback to 60 °F
812 720
100 89
510 420
100 82
Commercial N O setback Night/weekend setback to 50 °F Night/weekend setback to 37 °F
378 235 206
100 62 54
240 132 129
100 55 54
*The space is assumed to have 20% glass area in each wall, single-glazing, and a base daily daytime temperature of 68 °F.
100 TABLE 2 Winter fuel c o n s u m p t i o n for various w i n d o w - m a n a g e m e n t strategies. Calculations for a single-glazed Washington, D. C. house, w i t h 20% w i n d o w area in each wall. The reflective film is a s s u m e d to be P-18 S c o t c h t i n t with a shading c o e f f i c i e n t o f 24%. In the s u m m e r t h e S c o t c h t i n t saves a b o u t 1000 t o n h o u r s o f air c o n d i t i o n i n g ($50 electric savings).
P e r m a n e n t film, S.C. = 24% Clear single-glazing Roller shades: Closed 11 PM - 7 AM Closed 5 PM - 7 AM Double-glazing
U-value ( B t u / h r - f t 2 . °F)
Winter fuel (therms)
(%)
0.935 1.1
800 700
114 100
0.51 0.51 0.65
650 620 590
93 89 84
We recommend this conservation technique as cheap, effective and painless. Several manufacturers are currently marketing aluminized self-adhesive plastic film with a transmission coefficient of about 20%, which will convert a clear glass window into a "solar c o n t r o l " window. In an air-conditioned home it reduces solar heat gain in summer enough to pay for itself by reduced electric bills. (In Washington, D.C., applied to a window which would otherwise receive direct sunlight, it saves 25 - 35~/ft 2 each summer*). In any home, the film reduces glare and summer heat, and prevents fabric from fading. The catch is that in the winter it also reduces solar heat gain, and according to our calculations, causes our single-glazed Washington house to use 100 therms more gas ($30 at 30d/therm), or 75 gallons more fuel oil. The summer savings are $50 - 75. We now ask how much fuel can be saved if the reflective film is not applied permanently, but installed as a roller shade or Venetian blind, closed all day during the summer, but open on sunny winter days. If the shade (or blind) is confined in a track so as to produce a dead air space between the window and the closed shade, a single-glazed window is converted to an unusually good storm window, with a U-value improved by a factor o f two. Consequently, it pays handsomely to close the shades after sunset in winter. Table 2 shows the fuel consumption for various management strategies for a Washing*It also saves t h e electric utility a peak d e m a n d o f a b o u t 10 W/ft 2 o f film.
ton, D.C., house. If the base case is taken to be permanently adhesive film, total savings of nearly 180 therms (25%) can be achieved, in addition to the summer savings. Of the 180 therms, two-thirds come from solar heat gain during the day and one-third from the betterinsulated windows at night. Figure 1 shows the hour-by-hour thermal advantages of the roller shade over the permanent film. The printouts cover four days in January and April 1962. Nights are easily picked out because the thermostat setting, labeled TLO, dips to 60 °F from 11 PM to 7 AM. 'It is assumed that the roller shades are not closed until 11 PM and are opened again in the morning. (A conservationist household could, of course, close t h e m at dusk, and keep them closed on overcast days, thus achieving a performance even better than that o f double glazing.) The full curve, with the dots below it, represents the hourly furnace load with the permanent film rejecting three-fourths of the sunlight. The broken curve is the furnace load when the shades are opened during the day, and closed at night so as to improve the Uvalue of the windows. Note the dramatic contribution o f sunlight, which almost replaces the furnace on January 2 and 4, and for most of April. Sears sells the self-adhesive Scotchtint for 70¢/ft 2, but does not offer shades. Wholesale prices for film are 50¢/ft 2 for the self-adhesive form, 25¢/ft 2 for the shade material. Dealers charge $1 - 3 to install the self-adhesive film. Shades can be made more cheaply. Sears, for instance, sells cloth shades for as little as $1.50 per window.
101 ~*~,* POIb'fS PeOTT~SO
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Fig. 1. C o m p u t e r printout comparing heating loads for permanent v s . manageable reflective film w i t h a shading c o e f f i c i e n t o f 24%. The full curve (joining the t o p s o f the c o l u m n s o f dots) is the h o u r l y furnace load w i t h perm a n e n t film. The b r o k e n curve is the load if the film 'is o n a shade w h i c h can be o p e n e d during the day, but is closed from 11 PM to 7 AM to convert the single-glazed w i n d o w s to storm w i n d o w s • The T's s h o w the outside temperature• The same p r i n t o u t for a Washington, D.C., house w a s used for Fig. 3 o f the article b y Dean and R o s e n f e l d , see page 22.
We have n o t y e t e s t i m a t e d t h e c o s t o f t h e tracks t o p r o v i d e t h e dead air space, b u t w e c o n c l u d e that solar c o n t r o l shades c o s t n o m o r e t h a n p e r m a n e n t film, and o f f e r c o n s i d e rable t h e r m a l advantages. O t h e r m a n a g e a b l e , t h e r m a l l y a s y m m e t r i c , shades and blinds are
discussed e l s e w h e r e in this report b y Silverstein and R o s e n f e l d . We t h a n k L o u F. M a s o n i c k , Solar C o n t r o l P r o d u c t s , 3M C o m p a n y , for his help and suggestions.