The Solar Combi 101-Mini: for the cabin or guest house
By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.
Anyone who works with home-heating construction projects has probably heard this before. The most common response from the person in charge of the budget has got to be either; “Can’t you make it cheaper?” or “Can’t you make it less complicated?” or both.
A few years ago, I took this challenge to heart when a number of construction jobs crossed my desk that seemed to be ideal applications for a solar combisystem but seemed too small to justify the full-blown primary loop (flow center), multi-zone treatment that I use as my standard design for typical home heating projects. (I refer to this as Combi 101, mentioned in previous columns.) These smaller buildings typically resembled guest houses or cabins, and some of them were off-grid with photovoltaic (PV) electric systems.
The standard Combi 101 makes sense when connecting (1) solar heat with (2) a boiler or other backup heat source, (3) a DHW heat storage tank and (4) hydronic radiant floors (HRF), because it simplifies the interaction and control of these four most common components. To save on cost and complexity in a small building that is well-insulated with only a single heating zone, the solar heating system can be further condensed to only the essential functions.
Figure 1 shows a piping and wiring diagram with details combined from several “as-built” installation drawings from small solar heating systems installed between 2007 and 2009. Since the number and size of all components normally found in a Combi 101 system have been minimized here, the system is referred to as the “101-Mini”. Although this system was intended to be lower cost and simpler, it was designed to preserve the essential function and “feel” of the larger Combi 101 solar heating systems.
The essence of the Combi 101 controls
The most important features of a Solar Combi 101 control system allow it to function automatically and reliably, to provide heating comfort for the occupants, for both DHW and HRF, without any special knowledge required by the users for normal operation from day to day. This includes automatic backup heat, solar preheat, solar only and solar overheat control. These features are provided by the 101-Mini as follows: DHW heat is delivered directly to the in-tank heat exchanger by a low voltage circulator powered directly from a PV panel. The heat storage capacity of the potable water is also used to heat the floors through the in-tank heat exchanger.
Solar domestic hot water tank control (DHW) features include:
• Low-limit aqua stat (AQ), comes with the electric element in the hot water tank
• Solar heat storage ON, PV-powered circulator runs when sun is up
• Solar heat storage OFF, PV-powered circulator stops when sun is down
• High-limit, provided by thermal switch 4E117
• Safety high-limit, P & T relief valve on the DHW tank.
The heat storage capacity of masonry warm floors is used for solar heat storage, controlled within the human comfort range by a two-stage room thermostat. The room temperature is allowed to drift upwards when “free” solar heat (Stage 1) is available. A second low voltage circulator is switched ON and OFF to provide HRF heat.
Solar heated warm masonry floor zone control (HRF) features include:
• Auxiliary low-limit Stage 2 heat ON (backup heat is allowed)
• Minimum comfort temperature Stage 2 heat OFF (auxiliary OFF)
• Solar heat-banking allowed — Stage 1 solar room heat ON
• Maximum comfort temperature — Stage 1 solar room heat OFF
The solar glycol circulator is used to deliver heat directly to the DHW tank and the HRF space heating system with adjustable temperature priority, using low-limit controls. It is also used to cool the DHW tank by circulating after sunset when a high-limit temperature is reached in the tank.
Solar collector heat control features include:
• Solar collector low-limit, provided by thermal switch 4MY93
• Solar HRF heat banking low-limit, provided by thermal switch 4E116
• Evening heat dissipation high-limit, provided by thermal switch 4E117
Unique design features in the Solar Combi 101-Mini
In this design, piping and wiring are simplified and costs are cut by eliminating components and using low voltage circulators. This results in unexpected opportunities for both thermal and electrical efficiency. Here are some of the highlights:
There is no separate backup boiler. The solar heat storage tank, DHW and backup (electric) heat are all integrated into one unit.
The solar heat collection and distribution can be powered from AC or DC electrical sources. The pumps and controls can be powered by an automotive battery during an emergency.
Low voltage DC circulators (only two) allow low voltage switching and wiring.
The main heat exchanger between the solar collectors and the house is eliminated, which maximizes the solar heat transfer to the DHW and HRF loads.
The primary loop or “flow center” is not used here (to simplify the piping). This is not recommended in systems with more heating loads, more collectors or other added complexity of any kind.
Solar heat is joined to the floor heating system using two tees, shown as T1 and T2 on the diagram.
All “boiler fluid” is propylene glycol antifreeze mixture, even in the floor tubing.
The glycol expansion tank doubles as a small refill reservoir.
Snap disk or capillary tube thermal switches are used for low-limit and overheat protection. (Grainger part numbers are shown for example.)
There are no motorized zone valves or transformers to power them.
Room thermostats are two-stage, with programmable “solar” temperature swing.
Overheat protection is provided by absorbing heat into the DHW tank and releasing it back through the solar panels at night.
Some optional choices
The diagram presents the bare minimum (yet fully functional) Combi 101 system I would ever recommend. There are a number of options that can be added to this design to improve its response and adapt to other needs of a project. Here are some of the most common:
Upsize the tubing diameters to lower the pumping power needed (floors and solar heat supplies).
Upsize the collector and the DHW tank to increase the heat storage for night use.
The use of integral flow check (IFC) pump bodies can be substituted with a spring check valve at the pump outlet.
Multiple heat exchangers are available in top and bottom along with the electric heating element (e.g., Solar Contender tanks).
Integral gas burners are available with in-tank heat exchangers. These have been used (at greater expense) in locations where electric heat is not practical.
A two-tank DHW system (in series) can be used to separate the solar heat storage from the conventional heat source.
Upgrade the thermo-mechanical switches with electronic differential and set point controls.
An anti-stratification circulator is sometimes added to the water tank to stir the tank from top to bottom when a single heat exchanger in the tank is located near the bottom.
There are a number of compromises made in the Combi 101-Mini system that allow lower cost and greater simplicity at the expense of precise control and ideal thermal performance. This system is presented here mainly to illustrate the basic piping and control issues involved in any solar combisystem and are based on our experience with a number of successful field installations.
Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.