Armstrong House Project: Solar Combination Space Heating and Domestic Hot Water
Updated: January 2011
A solar thermal collection and storage system is the common basis for the combination space heating and hot water system. Solar thermal energy will be augmented by a propane fired, tank-less, instantaneous hot water heater. Additional space heating will be supplemented by a high efficiency wood burning stove.
The combination system is comprised of 3 main systems:
- Solar thermal collection and storage (currently two panels on the roof – to be expanded in the future).
- Domestic hot water system (DHW) (storage, distribution and propane augmentation of solar heated water).
- Radiant space heating system (low temperature (70-95 degree) water from solar and propane tankless water heater.
This system is comprised of solar thermal panels, solar fluid pump and control station, and solar thermal storage tank. This system collects energy from the sun and stores it in the form of hot water. System is filled with freeze protected solar fluid (food grade glycol) and maintains complete separation from both heating fluid (water) and potable domestic water. Heat is transferred from the solar fluid to the solar storage tank using a double walled heat exchanger coil.
System functions on a differential controller that utilizes one sensor on the solar thermal panels and one sensor on the solar storage tank. When the solar thermal panels are hotter than the solar storage tank, the pump activates and draws solar fluid through the solar thermal panels and into the exchanger coil where the heat is added to the solar storage tank. When the solar storage tank becomes the same temperature as the solar thermal panels, the pump is deactivated.
Domestic Water System
Prior to contact with any of our systems, well water proceeds through a system dirt separator, particulate filter and scale inhibitor. Once filtered, domestic water is piped into the solar storage tank and to the cold water manifold. Water in the solar tank will be heated by the solar thermal panels and solar heat exchanger. Water piped to the cold water manifold will proceed directly to each point of use fixture via a homerun PEX plumbing topology. Only two fittings are used between the fixture and manifold; one connecting the piping to the manifold, the other connecting the piping to the fixture. Potential for failure is greatly minimized as are pressure issues relating to the use of more than one fixture at any given time.
Domestic hot water (DHW) temperature is regulated by a motorized diverting valve. By way of a setpoint controller and tank sensor, this valve measures the temperature of the water in the tank and either automatically directs adequately stored solar hot water straight to a tempering anti-scald valve or diverts it through the propane tankless water heater to be heated and then sent to the tempering anti-scald valve. The tempering mixing valve mixes the hot water from either the tankless propane heater or directly from the solar storage tank with the cold water to achieve the desired temperature of ~120 degrees. Upon achieving this set temperature hot water is piped to an efficient distribution manifold where it can proceed to the all the fixtures that use hot water such as showers and sinks.
Radiant heat system
This system contains a mixing station, controls and a distribution system for the solar thermal energy once it has reached the heating system. Low temperature water ranging between 70 and 95 degrees will circulate through the 3600 linear feet of 3/8 inch PEX tubing at a constant rate of ~2.0 gallons per minute. Heating fluid temperature is mixed specific to building heat loss directly related to outdoor temperature by the outdoor reset controller. At 12 degrees outside, the heating fluid will be maintained at 95 degrees, the system design temperature. When it is 50 degrees outside, the system might only need 70 degree water to maintain the interior temperature.
The outdoor reset controller is an important part of the heat delivery system. It enables the temperature of the hot water to be regulated precisely in relation to the outdoor temperature (i.e. the actual heating demand) versus a design temperature that assumes a fixed outdoor temperature in winter. This allows the heating system to deliver heat smoothly and continuously at a rate equal to the heat loss from the building. In conventional space heating systems, the amount of heat delivered to the building is regulated by simply starting and stopping flow of hot water through the heat emitters. It’s like driving a car by repeatedly alternating between full throttle and coasting. It’ll get you there, but the ride is not as smooth as it could be and it wastes a lot of fuel.
There are 24 heating loop circuits which are connected to two 12 loop stainless steel distribution manifolds. The PEX heating loops are outfitted with 2000 square feet of aluminum heat transfer plates and are buried in floors walls, ceilings, a staircase and other surfaces. An outdoor reset controller, outdoor temperature sensor, primary heat loop pump and motorized mixing valve will determine what temperature water the radiant system requires and from where to get that energy. If the heating system needs only moderate temperature water to heat the house, the motorized mixing valve will directly route the water collected from the solar thermal system to the heating system. If all the usable solar thermal energy has already been depleted by either the domestic hot water demand or heating system, the controller will automatically initiate a call to the propane fired, tank-less water heater. This call will indirectly heat the radiant heating fluid by heating the DHW in the solar storage tank through which it passes. The heating system controller will exhaust all usable solar thermal energy collected before resorting to propane powered heater. Depending on the ultimate number of solar thermal panels installed in the future, the potential exists to entirely eliminate using propane to assist with space heating and hot water.
Zoning is to be achieved by on/off control of each heating loop circuit. Each room has between one and four heating circuits and is wired for its own thermostat. Additionally each room is thermally isolated and provisioned with usage and occupancy sensors as well as internet connectivity. As such, the Armstrong house will be able to control and adjust its consumption of heat energy on an extremely micro level.