Heating System Design
The house is in Vermont at about 44 degrees North Latitude. It has three full floors and about 3500ft2 (about 325m2) of floorspace.
The heating system design consists of several major components and subsystems. Almost all of these have their own page (or pages). This page is intended to serve as a high-level overview of the major components. Links to more detailed pages are included.
The oil boiler, baseboards, hot water heater, and hot tub have been in place for years, The wood boiler was added in November of 2005, and the balance was installed during the summer and fall of 2006. As with all projects, it will probably never be done - there are already plans for an additional low-temperature radiant heat zone, smart window shades, heat extraction from refrigeration systems......
Basic System Description
The oil fired boiler and the wood boiler are connected in parallel, each with their own circulator pump - either one can provide heat, and they are functionally interchangeable. Both are designed to operate with a water jacket temperature of 80°C (180°F). Check valves ensure that there is no reverse flow through either boiler when it is not operating.
A heavily insulated 880 gallon external heat storage tank is connected with its own circulator and zone valve. There is a bidirectional heat exchanger coil in this tank - it can operate either as a heat source (in parallel to the two boilers) or as another zone, accepting heat from either boiler.
Space heating is accomplished via hot water baseboards. The baseboards are divided into three zones. Each zone has a thermostat and a motor operated zone valve.
In addition to the heating zones, domestic hot water is provided from a 40 gallon tank which has an internal heat exchanger coil connected as a fourth zone. 'Makeup' water for the water heater is preheated by a heat exchanger coil in the external storage tank. A pair of mixing valves ensure that hot water supplied to the house is at the correct temperature.
There is also a hot tub which is heated via another heat exchanger, which is connected as a fifth zone.
A solar panel array can heat the external storage tank via a dedicated heat exchanger coil. These panels operate by thermosiphoning - if the water in the panel is warmer than the water in the tank, the warm panel water rises and the cooler water from the tank falls, establishing a self-regulating flow through the panels.
The wood boiler has it's own controller, which controls fan speed in order to maintain the selected water jacket temperature. If the temperature is above 65°C (about 150°F), the circulator pump is turned on. If the temperature drops below 60°C, the circulator pump is turned off.
The heat storage tank, 7260-based controller, and the solar panel were not part of the system during the winter of 2005/2006.
All three heating zones are equipped with Carrier Debonair 33cs071 commercial thermostats. These digital thermostats are powered by the 24Vac signal that runs the furnace / boiler control system, so they require no batteries. They are two-stage thermostats with programmable deadband and externally controllable setback.
Deadband is the temperature range that the thermostat will try to maintain. A two degree deadband and a setpoint of 70°F means that the thermostat will call for heat when the temperature drops to 68°F, and will stop calling for heat when the temperature reaches 70°F.
Two stage means that there are two outputs. The first output is activated when the temperature drops to the setpoint minus the deadband (68°F in our example). The second-stage output is activated when the temperature drops another two degrees - 66°F in our example. The second stage output will probably be used as part of the decision-making process for future automatic switchover from wood to oil.
Setback provides a way to tell the thermostat to operate at a different setpoint. This is typically used in a commercial setting to reduce heating demand when the building is unoccupied. On the thermostats, the setback temperature is programmable, and setback is enabled with an external switch closure - actually, by applying 24Vac to a terminal on the thermostat.
In this application, the wood/oil switch activates a relay that puts the thermostats into setback mode when oil heat is selected. This way, the house is automatically controlled to a warmer temperature when heating with wood, and a cooler temperature when heating with oil.
In designing the house, enough baseboards were planned to maintain 72°F with an outside temperature of -30°F. Because the house is well insulated, this worked out to 52' of baseboard for a 3500 ft2 house. While this calculation has proved to be pretty accurate, it's not enough baseboard to change the house temperature quickly. Worse, baseboard output is very sensitive to water temperature. Baseboards are designed for operation at 180°F. At 160°F, baseboards lose almost 25% of their output. This is a real problem if we want to heat the house with water that's not quite at full temperature.
The current plan is to add a radiant heat zone for the main floor. Radiant heat zones operate on much lower water temperatures, and it's not a problem if the top floor gets a little cooler as long as the main floor is comfortable.
A 550 gallon hot tub was modified to be heated with hot water rather than electricity. The electrical heating element was disconnected and a 220Vac relay was put in it's place. The relay contacts serve the same function as thermostat contacts. The hot tub has a low-volume circulator pump that circulates water through the heating element. This loop was re-plumbed to circulate through a water-to-water heat exchanger, which is connected as an additional zone in the heating system.