Controls

Solar collectors should face the winter sunlight

Solar Heating Controls (PDF)
Proper controls and proper sensor placements are really quite important. Errors here or poor installation will seriously affect performance.

THE SOLAR HEATING CONTROL- You will need something to turn the pumps of the solar heating system on whenever solar energy is available and to turn them off when energy is no longer available.

We use a "differential temperature control" for this task. It is a solid state electrical device with two temperature sensors  Whenever the control senses that the temperature of one sensor is higher than the temperature of the other by a certain amount, it will close an electrical circuit and supply power to a device of your choice (usually a pump). The amount of this differential is adjustable.

The controller is also the device that will turn off the pump when a certain temperature differential is no longer present. The turn off differential can be adjusted separately from the turn on differential.

To set these differentials, we should first determine the conditions under which it would be desirable to run the system.

I propose that the if the amount of energy coming out of the collectors is equal to or greater than the electrical energy required to harvest it, the system should be "ON". Otherwise, the system should shut off to save wear and tear and electrical consumption. Then, select turn on points and turn off points that will accomplish this.

Generally speaking, the system should run if the fluid coming out of the collector is 5°F warmer than storage and should shut off if it is not.

We cannot however just set the differential at 5°F and let that bethat.

When the controller is deciding whether or not to turn on, the collectors will be usually be stagnating in low sun conditions, and the temperature will be higher than it would if it were running. If we just set the differential at five degrees, the pump will come on, and pump the warm HTF out of the solar collectors, and then shut off because there is not enough solar energy to maintain the 5°F differential under flow conditions.

This is called short cycling. It is not good for the components and it annoys the occupants of the building if they can hear the pump. The system would also short cycle in late afternoon when it is time it will turn off, and would run badly on partly cloudy days as well.

The solution is to set the "ON" differential at 10°F or so, and set the "OFF" differential at 5 degrees. Every system is a little different, and the actual settings should be determined on the site.

THE SLAB HEAT CONTROL - The "Solar Heating System with Domestic Hot Water and Radiant Slab Heating" will have a control to turn the slab heating "ON" or "OFF" according to the temperature within the building. We will use a thermostat, a zone control valve, and perhaps a safety control to prevent excess temperature HTF from entering the slab.

THE THERMOSTAT - Any good 24v thermostat will be fine.

THE ZONE CONTROL VALVE - Here we have to be more particular. We want a paddle type valve with a clock type motor; and not the more common electromagnetic solenoid type. Bear in mind that the zone valve will be on almost continuously for most of the heating season. The electromagnetic valves draw current whenever they are holding the valve open. This wastes electricity and the valve is not meant to work that hard. The clock type motor only draws electricity when it is operating.

SAFETY CONTROL (HIGH TEMPERATURE OVERRIDE) - The temperature of the solar HTF will be coupled fairly closely to the temperature of the heat storage medium (assuming that the heat exchanger is effective). When the slab storage is being heated, we would expect temperatures not to go above 100°F. But when domestic hot water only is being made temperatures of the solar HTF can get well above 200°F. Such temperatures are well above the rating of the slab heat exchanger tubing. The result of opening the slab heat control valve under these conditions is that the slab heat exchanger gets a shot of very hot HTF that is well above its rating, and is then "rescued" by cooler HTF.

Abusive testing conducted by Radiantec was not able to damage the slab heat exchanger in this manner. However, it seems wiser to prevent this occurrence with a device to override the thermostat when system temperatures are very high. We use a "set point control" to do this. This device is similar to the old aquastats except that it is solid state electronic. (A sensor is placed somewhere, and the controller turns something "ON" or "OFF" whenever a certain temperature is reached).

For this purpose, we will locate the sensor on the pipe coming down from the solar collectors and set the control so that it opened the thermostat circuit (and thus override the thermostat) whenever the HTF is higher than the rating of the Slab Heat Exchanger.

THE HEAT DUMP - Solar collectors are capable of making too much heat under certain conditions. We will first try to deal with this potential problem with good design. Solar collectors should face the winter sunlight, and the summer sunlight will strike the collectors at a considerable angle, and thus reduce the summer efficiency.

The "Solar Heating System with Domestic Hot Water and Radiant Panel Slab Heating" will shut off the heat flow to the slab by way of the thermostat whenever the building starts to get uncomfortably warm.  Then you are left with a great big solar domestic hot water heater. By design, solar collectors supplied by Radiantec are not greatly efficient at temperatures above 200°F, and in the summer, the sun strikes the panels at a considerable angle. But once in a while, we will have to do something so that too much hot water is not produced.

Some designers would simply shut off the solar pump whenever too much domestic hot water is being made. This is not a real good solution because stagnation of the solar panels is not good for them. It would be better to give the solar panels a little more work to do.

In order to do this, we will configure a small solenoid valve to automatically dump a little hot water down the drain whenever DHW temperatures are getting too high. We place a sensor at the outlet of the DHW heat exchanger and set the control so that the solenoid valve activates whenever the temperature at the sensor goes above 180oF and dumps hot water until the temperature falls to 178°F.

THERMOSTAT OVER-RIDE FOR SOLAR OPTION II SYSTEMS - The performance of Solar Option II systems can be compromised by limited heat storage capacity. The storage for Solar Option II systems is in large domestic hot water heaters. It is seldom cost effective to buy a lot of these heaters because they can be expensive and it also costs money to build a place to put them. But the building itself can store considerable heat energy between the minimum acceptable thermostat setting and the maximum tolerable temperature (perhaps between 60°F and 85°F).

When the domestic hot water heaters become quite warm, they are not able to store much more heat energy. This control automatically turns on the heat even if the thermostat is satisfied and allows additional heat to be stored in the building.