INDIVIDUAL ROOM CONTROL APPLICATIONS

In some hea t pumps, a minim um of f timer pr events a compr essor r estar t for thr ee to five min utes. After shutdo wn, heat pump operation must not resume until pressures equalize between the suction and disc har ge sides of the compr essor. Shor t cycling the hea t pump ma y result in compr essor dama ge.

A two-sta ge, high-speed compr essor can pr ovide ca pacity contr ol for maxim um hea t pump ef ficiency. The r oom ther mosta t contr ols the indi vidual compr essor sta ges. The ther mosta t second stage contr ols the auxiliar y heat (for air -sour ce heat pumps) and compr essor sta ge two. When the outdoor ther mosta t contact c loses as outdoor temper atur e falls to its setpoint, the auxiliar y heat ener gizes with the compr essor (F ig. 38).

THERMOSTAT

STAGE 1

STAGE 2

HEAT PUMP

POWER

OUTDOOR AIR

THERMOSTAT

AUXILIARY

HEAT C3023

Fig. 38. Heat Pump Staging.

Defrost cycling is typicall y used w hen outdoor temper atur es are below 7

°

C, because the outdoor coil may operate below freezing and frost can form on the coil when the unit is in the heating mode. Frost inhibits airflow through the heat pump and degrades unit perf ormance . To remove the fr ost, the hea t pump momentar ily switches to the cooling mode . Hot g as fr om the compressor is directed to the outdoor coil, and the frost melts.

Typicall y the unit c ycles ever y 90 min utes for four to eight minutes to defrost. Instead of time-initiated defrost, some models use demand defr ost, which cycles to the cooling mode by measur ing chang es in the airf low acr oss the outdoor coil.

CONTROL LOOPS

Hea t pumps can use a v ar iety of methods to c hang e betw een cooling and hea ting, including a tw o-position r oom ther mosta t and man ual c hang eover.

Assuming automa tic chang eover betw een hea ting and cooling, on a r ise in space temper atur e, a tw o-position r oom ther mosta t senses the temper atur e rise and c ycles the hea t pump in the cooling mode . When space temper atur e falls below the deadband , the fir st sta ge of hea ting c ycles the compr essor in the hea ting mode . A fur ther dr op in space temper atur e br ings on additional thermostat heating stages to turn on supplemental electric resistance heaters.

In some hea t pumps, the c hang eover v alve cycles with the compressor in either the heating or cooling mode. In others, the chang eover valve remains in the hea ting or cooling position as long as space temperature is in the appropriate range.

Wher e a centr al water plant pr ovides hea ting/cooling sour ce water f or the hea t pumps, water contr ol is often pr ovided to conser ve water w hen the hea t pump c ycles off and w hen the water temper atur e is excessive for the load . Valves ma y be refrigerant pressure (may be furnished with the heat pump) or temper atur e contr olled. Temper atur e contr olled valves must close when the compr essor is of f, but m ust ha ve a minim um open position anytime the heat pump operates in order to keep water f low acr oss the water sensor .

Heat pump system controls must be carefully coordinated with the hea t pump man ufactur er and the w ater plant system

(on air -to-water hea t pumps).

422 ENGINEERING MANUAL OF AUTOMATIC CONTROL

INDIVIDUAL ROOM CONTROL APPLICATIONS

INDIVIDUAL ROOM CONTROL AUTOMATION

On automa ted jobs with a g r aphic BMS , ATUs ar e usuall y shown on a f loor plan similar to F igur e 39.

The Figur e 39 example sho ws a graphic of the souther n half of a floor with 30 VAV boxes and their associa ted space temper atur es. Selecting an y VAV box would pr oduce a g raphic of tha t bo x, similar to those pr eviously shown in this section, and all specif ied da ta suc h as space temper atur e setpoints, minimum and maximum airflow setpoints, etc.

ELEVATOR

LOBBY

V = VAV BOX

V

23.7

V

23.7

V

23.7

V

23.7

V

23.7

MEN

V

23.7

23.7

V

23.7

V

V

23.7

V

23.7

V

23.7

V

23.7

V

23.7

V

23.7

23.7

V

V

23.7

23.7

V

V

23.7

23.7

V

23.7

V

23.7

V

23.7

V 23.7

V

V

23.7

23.7

V

23.7

V

V

23.7

23.7

V

V

23.7

V

23.7

V

23.7

TENTH FLOOR SOUTH

Fig. 39. ATU System Floor Plan Graphic

M10532

ENGINEERING MANUAL OF AUTOMATIC CONTROL

423

INDIVIDUAL ROOM CONTROL APPLICATIONS

HOT WATER PLANT CONSIDERATIONS

See Chiller , Boiler, and Distr ibution System Contr ol

Applica tions section f or ad ditional inf orma tion on hot w ater system control.

Because unit v entila tor s have lar ge OA damper s in close pr oximity to the w ater coils, the hot w ater pump should r un anytime the O A temper atur e is less than fr eezing, independent of hot water temper atur e.

Unit ventila tor and f an coil unit hot w ater temper atur es should be r eset based upon the O A temper atur e to ma tch the hot w ater temper atur e to the load . This allows much smoother contr ol, and k eeps adequa te water v elocities in the hot w ater coils if modula ting valves ar e used. Dual schedules ar e recommended with signif icantl y war mer w ater pr ovided dur ing unoccupied per iods to aid in the con vection and w ar m-up ef for ts. A tr iple water temper atur e schedule ma y be consider ed wher e the convection mode has a higher w ater temper atur e than w ar mup or occupied modes. The setpoint shift betw een modes should be r amped o ver a 20 to 30 min ute dur ation.

The hot w ater pump should also r un dur ing unoccupied periods on a drop in room temperature to about 16

°

C. In this mode the coils will act as a r adia tor and ad d hea t to the b uilding per imeter without r unning the unit v entila tor f ans.

Unit Ventila tor s and f an coil units and their associa ted hea ting and cooling plants should be started by a global optimum start pr ogram with the O A damper c losed until occupanc y time .

424 ENGINEERING MANUAL OF AUTOMATIC CONTROL

SMOKE MANAGEMENT FUNDAMENTALS

ENGINEERING

INFORMATION

ENGINEERING MANUAL OF AUTOMATIC CONTROL

425

426 ENGINEERING MANUAL OF AUTOMATIC CONTROL