Modern mechanical building systems rely on a constantly available and reliable supply of low-cost electricity. Readily available and relatively cheap power is taken for granted in the design of hvac systems.

We are able to do this because the energy used as electricity by heat-moving equipment like pumps, blowers, compressors, controls, and equipment safety circuits is a fraction of the gas, oil, solar, geothermal, etc., energy that is moved by the system.

However, even in conventional mechanical systems, the electricity used to run the process is far more valuable than the heat energy. Electricity produced by a remote power system is dramatically more expensive and far more difficult to come by than conventional power.

The economic success of the investment in the remote power system is determined almost entirely by how the power that is produced is used.

Importance of conservation

Investments in energy conservation virtually always provide better returns on investment than energy production strategies. Opportunities for energy conservation are found in the building structure, plumbing and water supply systems, and in the power production and mechanical systems.

Investments made in increasing the efficiency of the building and its systems enable power production systems and mechanical energy conversion systems to achieve their highest performance potential.

In this manner, efficient plumbing systems permit water supply systems to operate at their highest efficiency; intelligently designed and well-insulated buildings provide mechanical systems with lighter work; and so on.

More importantly, investments in water- and energy-saving appliances and lighting yield savings in terms of the purchase and operation of the remote power system and everything else that relies on it.

The reliability and performance of all building systems are linked through their reliance and cumulative effect on the power supply system. The reliable operation of some systems, like heating or water supply, can be critical to the building and its occupants.

Contractors need to know some basics about the supply system, so that their customers’ comfort systems will operate reliably.

Load calcs

It is important that the loads on all building components and systems that draw on the remote power system be accurately and realistically estimated.

Overestimating heating, cooling, or ventilation needs can result in power demands that are difficult or impossible to support with some power supply systems like solar, wind, or hydro systems.

It quickly becomes prohibitively expensive to provide fuel to remote installations, and to operate diesel- or propane-fired generators. This is especially true when the remote power system must provide power to buildings and building systems that have been designed without respect to the unique requirements imposed by isolation from the power grid.

Renewable Sources

Remote energy sources include photovoltaic (PV) solar systems, wind-electric systems, and micro-hydroelectric generators.

Solar, wind, and hydro produce intermittent dc power, usually at 12, 24, or 48 V, which can be used directly or stored in batteries. Usually, 6- or 12-V lead acid or gel cell batteries are configured in series strings to match the input voltage. The series strings are wired in parallel into battery arrays to store the power required.

Immediate use of dc power directly from the source represents the simplest, most efficient use of power, but requires that the supply of power be coincident with the load. Examples include solar-electric water pumping or solar energy supplied to air cooling or ventilation systems.

Storing power in batteries increases the cost of that power dramatically.

Batteries are expensive to buy, maintain, and operate. They work at very low efficiencies. Good battery systems can be expensive, but they’re worth it.

There are several disadvantages to using dc power. There is a limited selection of efficient dc devices such as pumps, fans, and controls. Low-voltage dc will require wire gauges that result in more expensive wire runs or unacceptable power losses.


Most remote power systems require ac power. Inverters are devices that convert dc into ac power.

This conversion increases the compatibility of the power system with conventional components and devices, but increases cost while decreasing reliability and efficiency.

Inverter technology has improved rapidly in recent years, however, and efficient inverters are available in a variety of sizes. These units can produce power that is cleaner and purer than line power, and can be used to power even the most demanding appliances.

Modern inverters and power system controls can sense low battery voltage and automatically start a generator either in response to a load or to charge the batteries.

Inverters are available with outputs from 75 to 5,500 W ac and input voltages of 12, 24, and 48 vdc. Some inverters may be staged or stacked for higher output wattage. For large projects, the electrical loads are divided into several circuits that are served by individual inverters.

Fuel-fired generators

It is difficult for a remote home of any size to do entirely without fuel-fired generators.

These generators can provide relatively large amounts of ac or dc power on demand. Substantial amounts of energy can be contained in the propane or diesel storage tank.

The addition of a generator to a remote power system increases the reliability of the power system. Gas generators produce a lot of power when operating, and are inexpensive to buy relative to renewable power sources of comparable peak output.

However, generator systems have many disadvantages. They convert fuel to electricity at very low efficiencies. This is reflected in the waste heat and air pollution they produce, as well as the noise they create.

Fuel is expensive to buy, and liquid fossil fuel prices are variable and even volatile. The availability of fuel to remote locations is often problematic, especially in certain seasons. Generators imply substantial maintenance when compared with other renewable energy sources.

Generator systems can be made more efficient. Cogeneration is a means of integrating water-cooled generators into the heating system. Fuel use efficiency is increased when the normally wasted heat is used to produce space heat, space cooling, or for domestic hot water.

Hydronic heating systems can readily store heat in (water) storage tanks for use at a later time, and are sufficiently versatile as to be able to supply heat normally wasted to all of these other loads. The ability to store heat is limited by several factors, and cogeneration schemes will only recover some fraction of the heat wasted.

Again, the extent to which the heating load is coincident with the operation of the generator strongly influences the effectiveness of cogeneration.


Practical remote power systems incorporate a renewable energy source (usually a PV system) and a diesel or propane fuel-fired generator into a system called a photogenset.

The fuel-fired generator provides the system with reliability and high power outputs on demand, while the renewable energy system is sized to produce the bulk of the power used.

The energy produced by the renewable system is “free,” and offsets over time the cost of the more-expensive electricity produced by the fuel-fired system.

The voltage of the dc power produced by the renewable energy system is regulated and used to charge battery arrays. Power is routed to the building from the batteries by means of dc and ac load centers, which provide means for a junction of the incoming power to various load circuits. These connections are generally made through fuses or circuit breakers as appropriate to protect the load wiring and appliances.

Power can be drawn from the batteries and sent to a dc load center directly, or it can be sent through an inverter that provides ac power at 120 V to an ac load center. This breaker box distributes the power to ac loads by conventional means. Special transformers are required to step voltage to 220 vac.

More relevant to hvac applications may be the ability to dedicate small inverters to particular tasks. Early inverters ran at very poor efficiencies — less than 60%. Modern, solid-state inverters can achieve peak efficiencies over 90%, and typically operate at efficiencies of 80% to 90% when delivering their rated output.

However, their efficiency drops off when less than their rated power output is drawn. The inverter itself draws power and very small loads “wake up” large inverters and operate them at poor efficiencies, which rapidly depletes battery banks. These are called “phantom loads.”

Inverter efficiency is also dependent on the quality of the ac power delivered. True sine wave inverters are 5% to 10% less efficient than their modified square wave counterparts.

However, there are distinct advantages to pure sine wave power, especially when used to power ac motors, which will run cooler and last longer when driven by true sine wave inverters.

Also, the surge current rating of many inverters may easily be exceeded when starting certain ac motors, especially those that serve prime movers like pump and fan motors. The starting surge of many ac motors may be 10 times the running current.

Overloading the inverter may result in everything from voltage dips in the rest of the system to shut down. Wherever possible, prime movers should be adapted to dc motors.

Dedicating inverters to particular pieces of hvac equipment allows the inverter to be sized to operate at maximum efficiency and provides the power supply system with a measure of redundancy and increased reliability.

Small inverters may be used to couple batteries to efficient ac pumps, blowers, or compressor motors to perform specific tasks. Some motors have particular starting requirements, especially in terms of their starting surge currents, which may tax a central inverter serving other loads.

Remote heating

Space heating systems can be much less reliant on electricity than cooling systems, because heating systems can be independent of the need to move air.

Cooling, ventilation, and humidification processes require air movement so that the air itself can be treated or conditioned.

However, it is not necessary to move the air in a building to provide heating comfort. Recirculating the air mass of the building uses much more electrical power than is necessary to simply move the heat from a fuel to the space.

A wood-burning stove, for example, provides for the combustion of fuel and delivery of heat into the space with no power draw. Freestanding wood stoves are likely the prevalent means of heating small remote homes today.

All early central heating systems circulated air, water, or steam by means of gravity. A gravity heating system uses gravity and the difference in density between a hot fluid and a cool fluid to circulate the fluid through the system. The energy that drives the air movement in these systems comes from the heating fuel.

The energy density of water with respect to air makes hydronic heating systems the only real choice for central heating systems in remote homes.

Using forced hot water circulation, heat can be moved from the source to a variety of heating loads, using a small fraction of the power needed to recirculate air through the building. Most of the benefits of central hot water heating become available to the remote system, and can all be implemented with minimal power draw.

Heat delivery can be controlled and regulated by zoning or mixing. A central hot water production system can serve a variety of heating needs, with very low delivery costs. Heat can be moved by a variety of terminal devices such as radiant floors, hot water baseboard, panel radiators, towel racks, etc., that are entirely passive with respect to their use of power.

Hot water may be supplied for heating distribution by a variety of fuel sources including propane, oil, solid fuel, and solar thermal systems.

Other than the circulator, many fuel sources do not require power to supply heat passively to the system. For example, propane and some solid-fuel boilers can be used as natural draft appliances, whereas fuel oil-fired equipment will require a powered oil burner. Solar thermal systems will require the use of a small circulator to drive the solar thermal array.

Hot water systems easily provide for storage of heat in insulated heat storage tanks. Solar thermal and solid-fuel-fired systems can take advantage of heat storage to manage their input to the system.

Heat storage also provides a means for cogeneration systems to supply waste heat from the diesel-propane generator to the space and water heating loads. PV systems and wind-electric systems may shed excess power to the hot water storage systems by means of heating elements.

Hot water design

It is important that the hydronic system be designed to be as efficient as possible.

“High-head” circulators should be avoided. “Porting out” the system piping, compared to a conventional forced hot water design, will allow heat to be moved at the lowest possible delivery cost and allow the use of the smallest circulators.

This is especially important if glycol antifreeze mixtures are used instead of water. Glycol decreases the ability of the system to move heat and the ability of the pump to move the water-glycol mixture. Lighter mixes of antifreeze than may be normally used can provide freeze protection while decreasing power draw during normal operation.

Select high-mass boilers or volume water heaters as a heat source that is not dependent on (relatively) high flow rates through low-mass heat exchangers. The boiler mass allows the (single) system circulator to be sized around the strict needs of the space heating system.

High-mass heat sources also decrease ignition system cycling, which promotes reliability and provides system controls with a stable heat supply.

For smaller heating loads and more economical packages, inexpensive, natural-draft water heaters work well in remote homes. For larger heating loads and higher quality systems, we have used Viessmann wet-leg, high-mass, propane-fired, natural-draft boilers equipped with millivolt (pilot light) ignition systems, with great success.

Use a standing pilot rather than electronic ignition system in the boiler or heat source. The fuel used in a gas pilot light is far less expensive than the remote power required to operate a modern, 24-vac boiler control system.

The continuous power draw of control systems of these kinds must be avoided. They operate inverter systems at poor efficiencies and draw down battery banks.

Vent by natural draft. Forced- and induced-draft fan motors are to be avoided unless absolutely necessary, as in oil firing.

Some heat sources require conventional ignition or combustion controls or motors. The use of heat storage allows for flexibility in managing the power draw of these devices vs. delivery of the heat energy to load.

Use non-electric mixing valves to modulate water temperatures for radiant floors and other low-temperature zones. Three-way mixing and diverting valves are available from several manufacturers, including ESBE and Sparco.

Select efficient electrical components. For example, some of the new generation 24-V, thermal zone valve motors offered by Roth and Braukmann draw as little as 70 mA (200 mA and 4 min to open) and can operate on dc power. This is more than 41/2 times less than the power draw of a conventional 24-vac, geared, motorized zone valve.

Use low-voltage dc where possible, especially for pumps and fans that are responsible for moving heat through the system. These motors will have the longest duty cycles and draw the most power from the batteries.

Several 12- and 24-vdc hot water circulators are available that are ideal for use in remote power systems. Some circulators, like the Hartell, use brushless dc motors magnetically coupled to pump heads. Brushless dc motors promise higher efficiency and reliability.

One new company, Universal Flow Control, is making available 24-vdc, brushless hot water circulators with built-in controls. The ability to vary motor speed is a feature of these electronic dc motors. The pumps take advantage of this variable-speed feature to use the pump as a control element to move heat so as to satisfy either a static water temperature setpoint or a dynamic setpoint that may be changed according to outside air temperature (reset control).

These pumps are available in two sizes, 40 and 100 VA. The dc injection pump and control allow designers to include outdoor reset features in their remote home systems.

Whenever possible, avoid 24-vac control systems. Transformers, relays, heat anticipators, electronic thermostats, and the like all present phantom loads to inverters and needlessly draw down battery banks.

Circulators can be directly switched by snap-action thermostats. Snap thermostats with contacts rated for 120 vac can reliably switch low-voltage dc current. Small dc circulators can also be switched directly through zone valve end switches which are generally rated for 2 to 10 amps ac.

Care must be taken not to exceed the end switch current capacity and to fuse-protect dc circulators. Switches rated for ac should be de-rated and tested for use with dc.

For larger heating systems, where the use of low-voltage ac controls may be either desirable or unavoidable, it may be especially wise to dedicate a small inverter to operate the control system.

New microprocessor-based controls, such as those manufactured by tekmar or Viessmann, take their timing signal from the ac wave form and may require true sine wave inverters. The ac controls can switch ac as well as dc motors and loads.

Doing more with less

Successful energy conservation doesn’t mean doing without; it means doing more with less.

It is possible to provide all the comforts of a modern lifestyle at a fraction of the typical, grid-based electricity use. Achieving this goal requires careful integration of the building, power supply, and hvac and other dependent systems.

The converse of this is also true. Designing buildings, power systems, and remote hvac systems according to standard practice for grid-connected homes, and relying entirely on gasoline-powered generators and ac inverters to produce the necessary power, courts some very real problems.