What is solar space heating and does it work?
It is often assumed that solar can be used to heat water for a hydronic space heating system such as radiant floors or a forced air furnace with a fan coil. While this is possible, it is important to remember that in Oregon we get the least amount of sun in the winter when we would need it for space heating.
It is often assumed that solar can be used to heat water for a hydronic space heating system such as radiant floors or a forced air furnace with a fan coil. While this is possible, it is important to remember that in Oregon we get the least amount of sun in the winter when we would need it for space heating. Therefore, the following considerations should be taken in the design of a solar-assisted hydronic space heating system:
- Avoid overheating in the summer. Typically residential solar water heaters are designed to provide 100% of the domestic hot water needs in the summer, and to preheat water any amount possible in the winter. Therefore, in order to provide extra heat in the wintertime for space heating, additional collectors will be added. These will produce much more hot water than can be used in the summer, which can lead to maintenance problems. The system must be designed for protection from summer overheating.
- Use a low-temperature heating system. A hydronic space heating system can benefit from solar preheating only if the temperature of the solar-heated water is higher than the temperature in the heating loop. Therefore, hydronic heating systems that operate at lower water temperatures (i.e. 80-90°F) will have more opportunity to benefit from solar than those operating at higher temperatures (13-150°F).
- Install control logic to use or bypass the solar as appropriate. If the temperature of the water coming out of your heating loop is higher than the temperature in the solar storage tank, the heating loop should bypass the solar tank and send the water directly to the backup water heater to be boosted back up to the pre-set temperature. This avoids filling your storage tank with electric-, gas- or propane-heated water, which would ruin the efficacy of your solar water heater. This temperature comparison can be made with a differential controller that operates a valve or pump. A hydronic system heated only by solar would not need such controls, but would need a supplemental heating system.