*>I'm building a house and am looking for a computer model (Linux or*

*>Windows, free or inexpensive) that can *accurately* predict my HVAC*

*>heating loads given my exact residential floor plan, building envelope*

*>construction, glazing, occupancy and location (near Reno).*

You might write your own. Third-graders can learn BASIC :-)

Here's a start, which reads the Reno Typical Meterological

Year (TMY2) hourly weather data file (available from NREL's

web site) and makes another simplified file called "winter."

10 PI=4*ATN(1):SCREEN 9:KEY OFF:CLS

20 LINE (0,0)-(639,349),,B:DF=.434

30 FOR TR= 60 TO 80 STEP 10'temp ref lines

40 LINE (0,349-5*(TR-10))-(639,349-5*(TR-10)):NEXT

50 F$="23185.tm2"'NREL TMY2 file name (Reno)

60 OPEN F$ FOR INPUT AS #1

70 OPEN "winter" FOR OUTPUT AS #2

80 LINE INPUT#1,S$'read header

90 CITY$=MID$(S$,8,25)

100 LAT=VAL(MID$(S$,40,2))+VAL(MID$(S$,43,2))/60

110 LON=VAL(MID$(S$,48,3))+VAL(MID$(S$,52,2))/60

120 PRINT#2,CITY$,LAT,LON

130 GOSUB 190

140 PASS=1

150 CLOSE #1

160 OPEN F$ FOR INPUT AS #1

170 LINE INPUT#1,S$'ignore header

180 GOSUB 190

190 FOR H=1 TO 8760'hour of year

200 LINE INPUT#1,S$

210 MONTH=VAL(MID$(S$,4,2))'month of year (1-12)

220 IF PASS=0 AND MONTH<10 GOTO 610

230 IF PASS=1 AND MONTH=2 THEN END

240 WH=H+8760*PASS

250 DAY=VAL(MID$(S$,6,2))'day of month

260 HOUR=VAL(MID$(S$,8,2))-.5'hour of day

270 N=H/24'day of year (1 to 365)

280 TDB=VAL(MID$(S$,68,4))*.18+32'dry bulb temp (F)

290 PSET(DF*(WH-8030),349-5*(TDB-10))

300 IGLOH=VAL(MID$(S$,18,4))*.317'global horizontal radiation (Btu/ft^2)

310 PRINT#2,MONTH;DAY;HOUR;TDB;IGLOH;

320 IDIF=VAL(MID$(S$,30,4))*.317'diffuse horizontal radiation (Btu/ft^2)

330 IDIR=VAL(MID$(S$,24,4))*.317'direct normal radiation (Btu/ft^2)

340 L=PI*LAT/180'Phila latitude (radians)

350 T=HOUR'solar time (EST)

360 X=-SIN(PI*23.45/180)*COS(2*PI*(N+10)/365.25)

370 D=ATN(X/SQR(-X*X+1))'sin^-1(x) = declination (radians)

380 W=2*PI*(T-12)/24'hour angle (radians)

390 X=COS(L)*COS(D)*COS(W)+SIN(L)*SIN(D)

400 THETAS=-ATN(X/SQR(-X*X+1))+PI/2'cos^-1(x) = sun zenith angle (radians)

410 X=COS(D)*SIN(W)/SIN(THETAS)

420 IF X^2 >=1 THEN PHIS=-1.570796327#:GOTO 440

430 PHIS=ATN(X/SQR(-X*X+1))'sin^-1(x) = sun azimuth angle (radians)

440 FOR PHIPD=0 TO 180 STEP 90'azimuth angle of plane (degrees)

450 PHIP=PI*PHIPD/180

460 X=SIN(THETAS)*COS(PHIS-PHIP)

470 THETAI=-ATN(X/SQR(-X*X+1))+PI/2'incidence angle to surface (radians)

480 IF THETAI>=PI/2 THEN THETAI=PI/2

490 RHOG=.2'ground reflectance

500 IGLOP=IDIR*COS(THETAI)+IDIF/2+IGLOH*RHOG/2'radiation on surface (Btu/ft^2)

510 PRINT#2,IGLOP;

520 'IF PHIPD = 0 THEN PSET(H-8030,349-IGLOP)

530 NEXT PHIPD

540 PHIP=PI*270/180

550 X=SIN(THETAS)*COS(PHIS-PHIP)

560 THETAI=-ATN(X/SQR(-X*X+1))+PI/2'incidence angle to surface (radians)

570 IF THETAI>=PI/2 THEN THETAI=PI/2

580 IGLOP=IDIR*COS(THETAI)+IDIF/2+IGLOH*RHOG/2'radiation on surface (Btu/ft^2)

590 PRINT#2,IGLOP

600 IF HOUR=.5 THEN LINE (DF*(WH-8030),349)-(DF*(WH-8030),345)

610 NEXT H

620 RETURN

Here's the start of the winter file, with the month, day, hour, dry bulb

temperature (F), and Btu/ft^2 of sun falling on the ground and south, west,

north, and east walls for the months of October through January. The worst-

case months are December and January. I start simulating in October to make

sure house temperatures have stabilized by December.

RENO NV 39.5 119.7833

10 1 .5 44.06 0 0 0 0 0

10 1 1.5 43.52 0 0 0 0 0

10 1 2.5 42.08 0 0 0 0 0

10 1 3.5 39.74 0 0 0 0 0

10 1 4.5 39.74 0 0 0 0 0

10 1 5.5 40.28 .634 .2433778 .2218999 .2218999 .8492084

10 1 6.5 41 20.288 20.40661 6.466783 6.466783 110.9118

10 1 7.5 48.92 79.884 78.2263 16.23037 16.23037 211.7582

10 1 8.5 57.02 142.333 137.1752 25.48676 25.48676 228.2724

10 1 9.5 64.94 194.638 186.9535 33.09476 33.09476 201.9825

10 1 10.5 69.98001 232.361 223.7174 38.29355 38.29355 149.5433

10 1 11.5 75.02001 247.577 240.1501 40.44915 40.44915 78.724

10 1 12.5 80.06001 245.041 240.2293 78.39368 40.03705 40.03705

10 1 13.5 80.06001 221.583 221.1326 147.2574 36.74025 36.74025

10 1 14.5 80.06001 178.154 183.0438 197.6003 30.65386 30.65386

10 1 15.5 80.06001 121.094 130.7123 218.4883 22.25336 22.25336

10 1 16.5 74.12 56.743 67.68118 185.0582 12.64827 12.64827

10 1 17.5 68 11.412 10.40487 49.15384 4.311193 4.311193

10 1 18.5 62.06 0 0 0 0 0

10 1 19.5 60.08 0 0 0 0 0

10 1 20.5 57.92 0 0 0 0 0

10 1 21.5 55.94 0 0 0 0 0

10 1 22.5 51.26 0 0 0 0 0

10 1 23.5 46.76 0 0 0 0 0

10 2 .5 42.08 0 0 0 0 0

...

Here's a program that simulates an 8' R32 cube with an R2 south wall

with 80% solar transmission and a huge thermal mass using the winter

file as an input. There are much more efficient solar heating schemes

(eg those using simple air heaters or low-thermal-mass sunspaces), but

this direct gain scheme is simple to simulate with a 21 line program.

10 CLS:SCREEN 9:LINE (0,0)-(639,349),,B:DF=.43

20 FOR TR TO 70 STEP 10'plot temp ref lines

30 LINE (0,349-5*(TR-10))-(639,349-5*(TR-10)):NEXT

40 RV2'wall R-value

50 C 000!'house capacitance (Btu/F)

Increasing the R-value or capacitance raises the worst-case temperature...

60 TIp:TIMIN00'Initial house air temps (F)

70 OPEN "winter" FOR INPUT AS #1:LINE INPUT#1,H$

80 INPUT#1,MONTH,DAY,HOUR,TA,SH,SS,SW,SN,SE

100 SOLGAIN=.8*64*SS-(TI-TA)*32'net south window solar gain (Btu)

The solar gain is 80% of the south sun minus the heat loss to the outdoors.

110 IH=SOLGAIN-(TI-TA)*5*64/RV

120 TI=TI+IH/C'find new house temperature (F)

Divide the net heatflow by the capacitance to find its temperature change.

130 IF TI>70 THEN TIp'limit house temp

An exhaust fan might enforce this 70 F upper comfort limit.

200 IF TI<TIMIN THEN TIMIN=TI:MMIN=MONTH:DMINY:HMIN=HOUR

The line above finds the worst-case minimum temperature over the year,

in this case 65.01 F, which occurs at 6:30 AM on 12/16...

210 IF MONTH OR MONTH GOTO 270'skip Oct and Nov display

220 SST=SST+SS'accumulate south wall sun

230 PSET(DF*H,349-5*(TA-10))'plot ambient temp

240 PSET(DF*H,349-5*(TI-10))'plot house temp

250 IF HOUR=.5 THEN LINE (DF*H,349)-(DF*H,345)

260 H=H+1'increment hour

270 IF MONTH=1 AND DAY1 AND HOUR#.5 THEN GOTO 280 ELSE GOTO 80

280 PRINT MMIN;DMIN;HMIN;TIMIN;C;SST/62

With just a few more lines you can add some internal heat gain from

electrical use, air leaks, thermal mass airfilm resistance, a furnace

(if needed :-), a whole-house fan for cooling at night in July (with

an average daily high and low of 91.9 and 51.3 F), and so on.

*>One that I've tried (eQuest) is wonderful in its detail and*

*>ease-of-use. Unfortunately, there must be a flaw in the underlying*

*>model calculation because increasing the south facing (double pane,*

*>high SHGC) glass from 12% to 70% *increases* the HVAC heating*

*>requirements.*

Windows lose heat at night and on cloudy days, as DJ mentioned.

You might try making this south window a simple air heater that loses

no heat at night, or move it to a low-thermal-mass sunspace that's

isolated from the living space and let warm air circulate between

the sunspace during the day and stop the circulation at night and

let the sunspace get cold, so the window loses little heat at night,

IF your simulation program permits. Lots of them don't, even DOE's

latest and greatest ENERGY 10.

Changing the window to an air heater in the cube above reduces the

required thermal mass from 20K to 3.5K Btu/F, for a 65 F min temp.

Putting in the ceiling with a larger temp swing reduces it further,

especially with a nighttime heat setback.

*>Other software (e.g. the Canadian Hot2000, RESFEN 3.1 )*

*>does not seem to allow one to input the exact floorplan, unique*

*>building materials (e.g. ICF's) and other data necessary for an*

*>accurate estimate of heating requirements. Some also have artificial*

*>limitations (e.g. unable to specify 80% of the south facing wall as*

*>windows)*

Those are the kinds of limitations and bugs and unclear assumptions

that led me to write my own hourly simulation programs. Also, being

able to change the fundamentals can give much deeper insight as to

what's going on, compared to canned programs.

Nick

> I'm building a house and am looking for a computer model (Linux or> Windows, free or inexpensive) that can *accurately* predict my HVAC> heating loads given my exact residential floor plan, building envelope> construction, glazing, occupancy and location (near Reno).> One that I've tried (eQuest) is wonderful in its detail and> ease-of-use. Unfortunately, there must be a flaw in the underlying> model calculation because increasing the south facing (double pane,> high SHGC) glass from 12% to 70% *increases* the HVAC heating> requirements. Other software (e.g. the Canadian Hot2000, RESFEN 3.1 )> does not seem to allow one to input the exact floorplan, unique> building materials (e.g. ICF's) and other data necessary for an> accurate estimate of heating requirements. Some also have artificial> limitations (e.g. unable to specify 80% of the south facing wall as> windows)> Can anyone suggest a good *accurate* program? What do energy> consultants use?