solar space heating
Overview
Solar space heating systems are larger than those that would be used for solar domestic hot water heating in the same building. Space heating is a larger heating load, so everything is larger- more collectors, more storage, etc. Additionally, the economics of solar space heating are different as well. Space heating is a seasonal load, so solar is only displacing fuel during the heating months. A solar domestic hot water (DHW) system displaces fuel year round. Correspondingly, the payback period for solar hot water space heating systems is longer- usually over 10 years with today’s moderate fuel prices. The exception would be if there is a pool to heat in the summer. In this case, the payback is much shorter since pool heating is well suited to solar thermal and the fuel savings are large. As with solar DHW systems, the collectors, tanks, and other components are very long lived with space heating systems and the only moving parts are pumps and the occasional zone or mix valve. Most solar space heating systems also have domestic hot water preheating functionality since the incremental hardware and costs to do this are not significant and the payback for that portion of the installation expense is excellent. Solar space heating and DHW systems are referred to variously as hybrid, combination, or combi systems. They are the fastest growing segment of the solar thermal market, and the largest portion of NESHW’s sales.
If you need a new boiler, heating system, or are considering a radiant floor, then having NESHW install these items in conjunction with a solar hot water heating system would make the entire project subject to the 30% 'solar' investment tax credit. This could effectively discount a new high effeciency boiler install, for example, by several thousand dollars.
Solar space heating collector array, Duxbury
Basic system design
Any solar thermal system (DHW or space heating) needs to work in conjunction with a ‘conventional’ space heating system and fuel source. For all practical purposes, a solar thermal system cannot meet 100% of the heating needs of any building. Usually, we will design a system to meet 30 to 50% of the load (‘solar fraction’), with the conventional system in place to provide the balance. There is a point of diminishing returns with respect to size. No matter how large a system is, there will always be extended periods of no solar gain when the back-up system will need to be engaged. The intent then, when designing a system is to make sure that the expensive solar hardware is optimally used and that we are not adding solar capacity to try to capture increasingly small increments of total solar fraction. As with most mechanical systems, there is also a point at which the system is ‘too small’, and the additional expense associated with a solar space heating system (relative to a DHW ‘only’ system) is not justifiable. We have found that combination systems begin to make sense at about 150 to 200 square feet of collector area which is about 6 flat panels or 100 evacuated tubes. 
NESHW solar space heating design
System types
The starting point for solar space heating is the existing (old construction) or the proposed (new construction) heating system. Any solar hot water collector has varying efficiency which is dependent on the ‘inlet’ temperature of the collector. And despite quality or even the collector type (flat plate or evacuated tubes), all collectors work best with the coolest possible inlet temperature. Primarily, this is due to the ambient losses of the system- the closest the solar loop’s operating temperature is to the surrounding air, the less thermal losses will occur and the higher the system efficiency. Parenthetically, this is why unglazed, black rubber type pool collectors are so effective. They are trying to heat water to 80 degrees or so, while the surrounding air is basically the same temperature. Thus, unglazed pool collectors have no insulation at all and collect massive amounts of energy on a square foot basis. The solar pool heating thermodynamics are also relevant to glazed (tube or flat plate) collectors. The lower the target temp, the more efficient the system and the more btu’s are collected for a given amount of collector area or given system cost unit. Low temperature space heating means radiant. Radiant can be flooring, or radiant wall panels. Medium temperature space heating is hydroair, and will also work with solar. High temperature heating is baseboard or steam convectors, which require alterations to work with solar. Solar thermal will work with any heating system, but the payback/ROI can vary considerably.
Solar radiant space heating
Radiant floors and radiant wall panels operate with water temperatures as low as 90 degrees. This means that the portion of time where solar heated water can meet the demand is the greatest relative to other heating systems. A typical project would entail installing radiant wall panels and plumbing the new panels to the existing boiler and a new solar storage tank and heat exchanger. A separate controller decides when the heating system runs on ‘solar’ or the boiler depending on the temperature of the solar storage tank, and in some cases the outdoor temperature. In solar mode, an injection pump starts or a diverter valve switches positions such that the solar heated water is circulated through the system. In either case, there is a mix valve that isolates the potentially very hot solar water or the boiler water from the cooler radiant system. This mix valve can be a simple ‘set-point’ type, or it could vary the radiant’s systems water temperature as a function of the outdoor temperature (outdoor reset).
In some cases, the solar heated storage water is not suitable for circulating in the buildings hydronic heating system. This can be because the solar storage water is stored in an unpressurized ‘atmospheric’ tank (used for large drain-back solar loop designs), or because the existing heating system uses antifreeze (usually if there is a hydro air air handler in an unheated space such as the attic). In these cases, the solar thermal energy is transferred to the heating system via a heat exchanger. Most designs use simple ‘coil in the tank’ type heat exchangers, but we use brazed flat plate heat exchangers which are more efficient and reduce the electrical load otherwise needed to pump fluid through 50 or even 100’ of heat exchanger coil.
Solar radiant floor
Hydroair heating
Hydroair is a ‘medium’ temperature heating design. The solar plumbing to an existing or new heating system is identical to radiant heat. Hydroair can also be tweaked to function with slightly lower temperature hot water by increasing the hydronic flow rate through the air handler or by simply moving larger volumes of air to deliver an equivalent quantity of thermal energy.
Baseboard heating
Baseboard is a high temperature system which cannot directly be tied to solar hot water systems. By adding additional baseboard though, a space can be heated with lower temperature hydronic water. Baseboard radiates heat (convection, really) primarily as a function of two variables; inlet water temp and baseboard length. To deliver an equivalent amount of BTUs, one can decrease one variable but the other must also be increased.
In existing construction, it is seldom possible to increase the amount of baseboard radiator to work with lower temperature solar heated water. So if the existing heating system is baseboard, we will usually install a radiant floor if there’s access to the first floor joist bays from the basement, or we will install one or more small fan coil units that will operate from lower temperature water. These units can be installed in cabinet ‘toe kick’ spaces, in the floor, or in the wall. In most cases, we will also hook up the new heating system (floor or fan coil units) to the existing boiler so that continuous heat is available, even if there is no solar heat available.
Outdoor reset
Outdoor reset is a heating system control that varies the heat delivered to a space as a function of the outdoor temperature. In practical terms, this means that the water temperature from a boiler is hotter when the outdoor temperature is low. The building is losing heat more rapidly in colder temperatures so that heat must be replaced more quickly. Conversely, a radiant system may adequately replace the heat loss by circulating relatively cool water during the transition months when the outdoor temp is in the 40s and 50s. So even though the year round solar fraction of a system may only be 40%, 100% of the heat load can be provided by solar during these periods. Outdoor rest is a critical component of solar hot water heating systems since it provides a greater opportunity for the medium temperature solar heated water to be used to heat the space. Many solar hot water space heating systems, however, rely on simple ‘set-point’ control. This means that whenever the solar storage tanks exceeds some set point (130f, for example), then a diverter valve switches position and the heating system is then ‘solar’ until the set point is undershot by some increment and the diverter valve switches again. These systems do not account for the fact that a building’s heat loss and heat gain are very much a function of the outdoor temperature, and the temperature the system operates at should be continually adjusted.
Tekmar outdoor reset control
Non condensing and condensing boilers
If a low temperature heating system is used with a non-condensing type boiler, boiler protection plumbing must be added to keep cool water (less than 135) from returning to the boiler. Most boilers are non-condensing and as such cool return water will cause flue gas to condense to a corrosive liquid. Techniques for isolating the boiler from low temperature return water include primary/secondary loop plumbing and 4 way mix valves.
Buderus non-condensing boiler
Condensing boilers can operate over a broad range of temperatures and there is no need for boiler protection techniques. Low mass, modulating condensing boilers (Mod-Con) offer an excellent opportunity for integration with solar, since one need only to set up a sensor and injection pump scheme to deliver solar heated water to the hydronic system based on the temperature at the return main to the boiler. Whenever the solar storage tank water temperature exceeds the return temperature, the solar heated water is simply injected into the system (directly or with a heat exchanger intermediary). The boiler’s aquastat senses hot water and does not fire. This removes all of the control, mix and bypass plumbing that would otherwise be necessary. The existing boiler’s outdoor reset control will already have made a decision about what temperature water to deliver to the system, and the solar system need only to ‘tag along’ by simply monitoring a single sensor value and comparing it to the solar storage. In these cases, there is no interaction at all between the solar controls and the existing heating system or boiler controls. The only plumbing interface is a pair of ‘closely spaced tees’ which are the supply and return to the secondary heat injection loop from the solar storage tank.
Buderus modulating/condensing boiler (wall hung)
Economics
With existing oil based heating systems, a hybrid DHW/space heating system will have about an 11 year payback with oil at $3 and 8% fuel inflation. Gas heat will produce a 13 year payback. If there is a swimming pool then the payback can be as little as 7 years. Electric heat, of course, has the quickest payback. There are several software packages for evaluating the performance of solar hot water DHW or combination systems and the economics. These are T*SOL, RETScreen, and F-chart. NESHW has all of them and will provide excellent documentation with any proposal. Below is a sample T*SOL summary page;
NESHW analysis software (T*SOL)
Sample NESHW system photo
The photo below shows the mechanical side of an NESHW combination solar hot water system. This system is relatively complex as it supplies hot water to a domestic hot water preheat tank, and two heating systems (radiant and hydro air) that operate at two different tempertures. It also uses outdoor reset controls to modulate the water temperature to the heating systems and to determine if the heating system should operate in 'solar' or 'boiler' mode. The existing boiler was non condensing, so primary/secondary loop plumbing was installed along with boiler protection (via a 4 way mix valve). The system uses 125 evacuated tubes, an 80 gallon domestic hot water preheat tank, and a 200 gallon hydronic buffer tank (pictured). This type of design with two levels of outdoor reset logic is the most advanced and efficient system available. Since the primary heating system is oil, the payback is about 11 years.
Solar heating buffer tank and mechanicals, Hopkinton
Marblehead project
This unique project will be the largest residential solar thermal installation in New England. The home is an extensive renovation of a historic ocean side mansion. The mechanical systems are state of the art and the solar thermal component will service space heating (hydro air and radiant), DHW, and pool heating. NESHW will install an advanced data logging system that will allow remote, wirelsss, monitoring and control of the system.
Marblehead project