I was recently asked about heating a home with a radiant floor heating system that utilized solar hot water. This seems a counterintuitive at first since the sun only shines during the day and it shines for less than 12 hours during some of the coldest times of the year. However, we have to remember one of the key concepts of passive design: thermal mass. So it is possible to do when the home is designed for it.
Key factors to keep in mind:
- Thermal mass. This system works best with a 4-6 inch concrete slab with some insulation beneath to help regulate the concrete temperature. The thermal mass can help cool the home in summer.
- Maximize solar collectors for the coldest months in your climate.
- Maintain water circulation. It is possible to rely on thermosiphon, the natural tendency of warm water to rise and cool water to sink, to keep the system working. However, it is always a good idea to have a backup pump just in case.
- Back-up boiler. You don’t want to get stuck in a cold house. A boiler can be configured to only kick-in if the water in the system falls below a specified temperature.
- Protect the system from freezing. It is possible in cold climates for the water in the solar collector to freeze. Non-toxic Propylene glycol can be used on the system to reduce the already lower freezing temperature of moving water.
- Orientation. Good solar access for the panels and buildings should be part of the overall building design. Most windows should be placed in the south facing walls for direct solar gain in winter. The east side should have the second most windows to aid early morning heat gain. West and north facing windows should be minimized or eliminated in order to minimize afternoon overheating and general heat loss, respectively.
These are the primary concepts for a successful solar hot water radiant heating system. As always, for any alternative system to be successful it is best to incorporate it into the project as early as possible so that it is wholly integrated.
