Pipe and header sizing

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Design: Solar Heating of Buildings and Domestic Hot Water

MIL-HDBK-1003/13A

Differential thermostats are available with high temperature protection and

low temperature (freeze) protection. High temperature protection is

important, especially in evacuated tube collectors, in that it prevents a

very "hot" collector from suddenly receiving a supply of cold water thus

producing a thermal shock that could damage the collector components.

Another type of control called proportional control is available. It is

similar to the ON/OFF differential controller in operation. The difference

is that the proportional controller changes the threshold ON and OFF points

and controls the flow such that less than full flow can be achieved if the

sun is at less than full intensity. The advantage is that the proportional

control can "turn on" the system when the other controller (the ON/OFF type)

is waiting for more sun to become available. This is an advantage on cloudy

days and early morning start ups. Overall system efficiency is increased

slightly with the proportional control. These controls are more expensive

and one such experiment at NCEL has shown that proportional controls result

in considerably more cycling of the pump motor which could shorten pump life.

It is recommended that the control manufacturer be consulted on this point

before a proportional control is used.

As the building requires heat, other controls must direct pumps or blowers to

provide heat from the storage tank to the load. This control is the

conventional thermostat. The same room thermostat may control the auxiliary

heater; however, a delay timer or a two-step room thermostat must be

incorporated into the auxiliary heater control circuit so that the auxiliary

heat will not come on if heat is available from storage. Ten minutes has

been suggested as a typical time delay before auxiliary heat comes on. Some

manufacturers supply combination thermostat and solar system controls.

2.9 Piping, pumps, valves.

2.9.1 Pipe and header sizing. Piping should be designed for low pressure

drop. All exposed piping should be well insulated with approved weather-

resistant insulation. Dielectric unions should be used at connections

between dissimilar metals. Rubber or silicone hose used for connections must

be of a high temperature type. The pipe ends should have ferrules to provide

a good seal with the hose. In low pressure systems, spring type clamps are

preferred because they compensate for thermal expansion. Copper pipe is

preferred to galvanized steel due to its longer life expectancy and relative

ease of installation. Thermal expansion should be provided for all piping or

hard tubing. Pipe sizing should be in accordance with recognized methods,

but for most installations the following estimates are reasonable:

For a single row of parallel collectors (see Figure 2-6) with "X"

number of branches, 0.5 gpm flow per collector, water or 50% glycol

as heat transfer fluid.

Up to 3 collectors

1/2-inch headers

4 to 7 collectors

3/4-inch headers

8 to 12 collectors

1-inch headers

13 to 18 collectors

1-1/4-inch headers

More than 19 collectors

1-1/2-inch or larger (size for each

design)