High Efficiency Space Heating
ReVision Energy specializes in designing renewable energy heating systems for high-efficiency homes where the heating load is low (relative to the needs of most homes in New England).
Typically the relative efficiency of houses falls into one of three broad categories:
- Conventional homes (code built)
- Well-insulated homes (better than code built, Energy Star® homes)
- High performance homes (near net-zero or passive homes)
Why Heating Requires a Look at the Entire House
The amount of energy (and thus the cost) it takes to heat your house is determined primarily by the house itself. In other words, homes are built to require greater or lesser amounts of heat, based on decisions made in the design process.
For this reason the design of a clean, efficient and cost effective heating system actually begins with the design and construction of the home itself. That means paying attention to all the details building a home - project siting, building envelope and insulation, air sealing, etc.
The Integrated Design Process
We believe strongly in the benefits of the Integrated Design Process - an approach where homeowner, architect, builder, energy modeler and the mechanical system designer/installation contractor (that's us!) work collaboratively on a building from early on in the design phase.
By collecting input from all the stakeholders early in the design process, this approach tends to lead to the best possible decisions and also keeps total costs down for the homeowner by eliminating costly rework or design changes.
Matching Heating Options with Heating Load
Based on the discussion above, it follows that differently built homes will require differently sized mechanical systems. There is no "best" heating system out there, but instead a range of options based on the characteristics and heating requirements of a house.
The chart below estimates the heating load required from a 3000 sq. foot home in Southern Maine, given some different choices in the building process:
After determining the yearly heat load of a building, we can evaluate different heating options to see which make sense:
- * - For supplemental, not main heat
- ** - May be expensive on propane
- *** - Wood only with storage
- **** - Solar hot water can only be used as a supplement, not as only form of heat - more on this below
The key takeaway is that as the heating needs of a house decrease, larger and more elaborate mechanical systems make less economic sense. In a truly high-performance house, there are a number of routes to achieve net-zero emissions.
Heating Options for Conventional Homes
"Conventional homes" refers to a broad category of homes that range from drafty, turn-of-the-century farmhouses to new construction that is code-compliant but not especially energy efficient.
These homes typically have R values of 11-19 in the walls, and 20-25 in the ceiling. They may not be total energy hogs, but neither do they have especially low energy demands. Some higher-efficiency buildings may fall into this class simply because of their square footage (space takes energy to heat, even if that space is well-insulated).
Solar Heating for Conventional Homes
In most conventional homes, solar heating options are uneconomical. However, grid-tied solar and solar domestic hot water systems are excellent ways to reduce fossil fuel usage in these homes and correspondingly reduce energy bills.
Nearly any household can generate 80-90% of their hot water from the sun, and/or install grid-tied PV to offset fossil-fuel based grid electricity.
Wood and Pellet Boilers
If you're committed to reducing fossil fuels used to heat your home, there are a number of routes, ranging from weatherization to optimizing the existing mechanical services.
However, there's often a limit to how much the BTU requirements of the house can be reduced by weatherization efforts. In these situations, some fuel will be consumed in fairly large quantities, so we encourage these homeowners to seriously consider burning local fuel - cord wood and pellets!
While there is an up-front premium to wood and pellet heating equipment, the fuel itself is inexpensive and plentiful. Wood pellets are also the ultimate solar collector - trees store solar energy all summer long that is later accessible as heat energy by burning the bio matter.
Heating Options for Well Insulated Homes
This class of homes exceeds the expectations of building codes and may be built to Energy Star ® standards. Their heating needs are considerably lower than a conventionally constructed home of similar size, but are not designed to meet net-zero or passive house standards.
Condensing Natural Gas/Propane Boilers
Condensing gas boilers achieve a remarkable 95% efficiency rating by condensing exhaust gases as water vapor and recovering additional heat from them. Their incredible performance makes them a great choice for homes that have reasonable heating needs.
Compared to an oil boiler, there are a number of benefits to using a condensing gas boiler:
- Condensing boilers are incredibly efficient at modulating their temperature, firing up only to meet the demand required, rather than most oil boilers which essentially operate at full throttle or nothing.
- A gas boiler produces both heat and on-demand hot water, with no wasted tank losses (oil boilers often sit in standby mode in the basement during summer months, wasting hundreds of gallons of oil annually).
- Small oil boilers are very difficult to find - running the risk of oversizing the boiler for the space and thereby contributing to a life of unnecessary and excessive fuel bills.
- High efficiency boilers exceed 95% annual fuel utilization efficiency (AFUE) and qualify for a Federal Tax Credit up to $1,500.
- Gas emits 20-30% fewer carbon emissions per gallon into the atmosphere than oil.
- Gas is a domestically produced product and does not have to be transported from overseas.
While propane is slightly more expensive per gallon than oil, the incredible boost in efficiency boasted by a propane boiler more than makes up the difference in fuel costs.
Propane boilers are a hydronic heating system, meaning the boiler heats water, and goes through a distribution system to release heat energy into the home. Typically this is done through radiant distribution through pex tubing so the cost of pex adds to the overall cost of the heating system.
Contact us for more information, or download the brochure for the Prestige Condensing Gas Boiler (PDF)
Heating Options for High Performance Homes
These homes are truly outstanding products of smart engineering, design, and process. They aim to achieve top industry ratings, such as net-zero, LEED Platinum, or passive house.
Because of the incredible commitment to efficiency, the heating requirements are so low that very modest mechanical systems can meet the heating needs.
Resistive Electric Heating
Resistive electric heat consists of either an electric baseboard or an electric boiler supplying hydronic distribution (i.e. radiant floors).
While electric heat is cost-prohibitive for conventional homes, an ultra high-performance home needs so little heat that electric can actually be a sensible approach.
With fewer moving parts, and no need for refueling, electricity is extremely convenient and already part of a home's design. The carbon emissions created by coal power plants can be offset by installing grid-tied solar, allowing the home to become truly "net-zero."
Air Source Heat Pumps
Another electric heating option are air source heat pumps. By extracting heat from the outside air and using it in a reverse refrigeration cycle, the heat pump can generate 2-3 units of heat for every unit of electricity it consumes. This is like having an resistive electric heater operating at 200-300% efficiency and that ratio is commonly referred to as the coefficient of performance (COP) of 2 -3.
Like resistive electric, the best way to make an air source heat pump "green" is to use grid-tied solar electricity to offset the energy use. We size the system based on yearly load, so your PV system will bank excess energy generated in the summertime, so you can use it in the winter.
Historically air source heat pumps only worked in moderate heating climates like Florida or the Mid Atlantic, but thanks to new refrigeration technology, the latest generation of air source heat pumps work down to temperatures as low as -6 degrees F at exceptionally high COP's.
There are two types of air source heat pumps that we install:
- Air/air heat pumps - Heat the indoor air directly.
- Air/Water heat pumps - Heat water which is subsequently used in a hydronic distribution system.
Air to Water Heat Pump - The Daikin Altherma
The air to water heat pump is like a boiler in that it heats water, but it does so without burning anything. Instead it uses a refrigeration cycle to pull heat out of the outdoor air and uses that to heat the water which is then circulated through the floor and radiators.
The Daikin Altherma is the best of these units and comes in a variety of sizes to meet a home's load (more info via PDF).
Because the Daikin doesn't burn fuel for heat, the only operating cost of the heat pump is the electricity to run the pumps and the refrigeration cycle.
Our good friends at Northeast Radiant Technology (NRT) recently installed an Altherma heat pump for their office building; case study here: Heat Pump and Rooftop Solar Puts Northeast Radiant on Path to Net Zero.
Air to Air (Ductless Mini Split) Heat PumpsThis heating system heats air, not water, so a home doesn't need a hydronic distribution system at all. Instead, a number of inside units blow warm (or cool) air to condition the space as necessary.
Ductless mini splits, like air to water heat pumps, use heat in the outdoor air to heat a home's living space, but instead of doing it through the circulation of warm water, they heat the air via wall mounted units: http://www.mitsubishicomfort.com/
For a 2,500 square foot building with a fairly open concept we generally install 2 of these heat pumps (one per floor) and then count on natural convection and the air exchange/heat recovery system to equalize the heat throughout the space. If needed, we will install simple supplemental resistive electric heat in the spaces that end up not heating as well as needed with the heat pumps alone (often this is suggested for at least the bathrooms).
Once nice advantage of this type of system is that you automatically get an efficient air conditioner as part of your heating system.
What About Ground-Source Heat Pumps? (Geothermal)
Ground-source heat pumps (AKA geothermal) have generated a lot of buzz in recent years, but we find that for homes with a low enough electric load (<50,000btu/hr), we can design a simpler and more reliable system with a free fuel source to run the system for the next 30-50yrs - the sun!
In these designs, we will install resistive electric base board, electric boiler or an air source heatpump combined with a large grid-tied solar electric (PV) array.
Why use grid-tied solar for heating?
- Affordability - Solar electric systems are more affordable than ever before, with great incentives and historic low pricing on photovoltaic panels. By choosing solar electric, you pre buy your fuel for the next 30-50yrs and insure yourself against future energy price increases.
- Reliability - Unlike geothermal, solar electric panels are maintenance-free, warrantied for 25 years, and are expected to last 30+ years. We back all of our systems with 24/7 emergency service.
- Convenience - Your PV system will be generating electric credits all summer long which can be used to heat your home in the winter. One ReVision employee put it this way - you can split and stack wood, OR you can go to the beach and let the sun do the work for you!
Solar Space Heating
We're often asked whether we can integrate our solar hot water systems with conventional boilers to produce energy for space heating. Solar space heating is doable, but has a few special requirements:
- Solar space heating only makes sense in higher performing homes
- Solar space heating requires low temperature distributed (e.g. radiant floor) heating
- Solar space heating requires excellent southern exposure
For retrofits, solar hot water is rarely a viable option. Solar is not great at heating water to high temperatures for baseboard heat, and even homes with existing radiant heat are usually not insulated enough to make the investment in a solar space heating system worthwhile (however, domestic solar hot water remains an excellent choice).
See our Solar Space Heating (PDF) document for more details.
In homes where solar space heating is viable, it is still a supplementary form of heat - with the added benefit that it will also produce over 90% of the home's domestic hot water, year round!
Since solar availability is at its lowest when heating needs are at their highest, solar space heating requires a form of backup heat. Solar space heating can work well when paired with a high efficiency gas, propane, or wood boiler.
Interested in talking to a solar professional about your high performance home? Contact us today or see some of these other resources: