Document Lp5a3nomxjE8DLzpbBXqK06ag

884 CHAPTER 39 1954 Guide be actuated by outdoor temperature (load) to determine the basic tem perature of the heating medium supplied to the radiant surface. To allow for variations in internal load, an inside thermostat should be used as a high limit to reduce further the heat input, if necessary. If a thermally light radiant surface is used,; controls may be applied in the same manner as for typical convection heating. The terms thermally heavy and thermally light, referring to capacity for heat storage, are comparative and descriptive rather than exact. For example, a concrete .floor panel in a frame structure, without insulation would represent a heavy panel in a light structure. A frame type (metal lath and plaster) panel in a concrete structure would represent -a light panel in a heavy structure.- As indicated previously, a heavy panel in a heavy structure provides comfortable conditions if outside controls are used in addition to the inside thermostat. But if a heavy panel is used in a light structure, rapid changes in outdoor conditions may cause discomfort in spite of outdoor controls, because the structure reacts so much more rapidly than the radiant heating surface. Since thermally heavy radiant surfaces introduce considerable lag in the heating system, it is desirable that a control system be capable of maintaining the lag at a minimum. One method of reducing the lag in heat output is to utilize design water temperature in the panel, thus causing maximum rate of heat from the water in-the pipe coils through the concrete slab to the surface of the panel; and into the space when heat is needed. Satisfactory control can be obtained only if the control system is sensitive enough to regulate the heat output of the panels with sufficient precision to prevent over-heating, and to anticipate heat demand as affected by out side weather conditions. Circuit Balance In addition to a thermostatically controlled device for modulating the temperature of the circulating water, it is advantageous to insert in each circuit a locked flow'control or adjustable resistance to give uniform condi tions throughout all rooms. Owing to unforeseen difficulties with varying frictional losses in pipes, emission factor, and exposures, it is an advantage to be able to regulate permanently the flow through each circuit by means of a key operated valve as indicated in Fig. 3. Compensation for Increase of MRT Due to the increase in-MRT (mean radiant temperature) within a panel , heated space, which necessarily takes place as the heating load increases; the air temperature should theoretically be lowered to maintain comfort. In ordinary structures, with normal infiltration loads, the required reduc tion in air temperature is not great arid a conventional fixed control point room thermostat may be used. If a large infiltration load exists, or if un tempered mechanical ventilation is employed, a thermostat with variable control point should be considered. Because of the relationship between' MRT and air temperature in the space (and the variable MRT from point to point in the space) a conventional type of room thermostat (either, fixed or variable control point as previously determined) measuring prin cipally air temperature, should provide simple and satisfactory control. ` Lowered Night Temperature In general, lowered night temperatures will produce very unsatisfactory" conditions with any panel healing system. Heavy panels such as concrete" floors cannot possibly respond to either increase or decrease in temperature Automatic Control 885 within any-reasonable time.. Light panels such as'plastef ceilings or walls may respond;'with sufficient rapidity to changes in temperature, so>that moderately`satisfactory results- can be expected- front lowered 'night'temA peratures: However, very little fuel saving-can be expected-- even with these fight panels. . Typical Control Systems /H There are many methods of control which are used^in connection with panel heating systems. Some systems provide continuous circulation of the water and vary the water temperature to counteract variations in the outdoor temperature, space temperature, or a combination of these tem peratures. SeveraTsystems of Control depend upon intermittent circula tion of the water,' either by- intermittent pump operation1 or' the closing and opening of valves, in combination with either- constant water temperature or with variations in the: water temperature.: ROOM CONTROL TO MIXING VALVE OR Fig. 3. Panel Heating Control System Fig. 3 shows a typical arrangement of the essential parts of a simple' panel heating system, which has several possible variations, depending upon the equipment used. The heating unit -controls and-safety-devices are not shown; These devices are described under-Residential- Control Systems. One system of control-uses a self-contained three-way mixing valve operated by an outdoor thermostat to maintain the water temperature in inverse proportion to changes in the outdoor temperature. - The indoor thermostat stops and starts the water circulator as required to maintain the space temperature. If more than one zone is to-be: ^controlled, a separate indoor thermostat is used to control operation of a separate circulator for each zone. - - Another arrangement has a' self-contained three-way water-mixing valve controlled by an outdoor thermostat in conjunction with an indoor heatsensitive element. This instrument may be a-blackened-copper sphere Perhaps 8 in. in diameter that wili 'overcome thevoutdoor thermostat in fluence on the mixing valve if the indoor temperature varies too much. Another scheme employs a three-way water mixing valve operated by an electric motor. The impulses from the outdoor thermostat, indoor thermostat and an aquastat in 'the mixed water all are electronically co ordinated to control the mixing valve motor. Fig. 4 shows a typical electronic control system arranged for circulator