High-Efficiency Home Heating: Page 2 of 2

Intermediate

Inside this Article

High-Efficiency Home Heating
Which heating system is the most energy-efficient and cost-effective choice for your home?
Items to weatherize your home
Before you invest in a new space-heating system, be sure to weatherize your home.
High-Efficiency Home Heating
Items to weatherize your home

Because of the inefficiencies of generating electricity by burning coal or gas, along with some transmission losses, it takes about 3 units of source energy to produce 1 unit of site energy. In other words, utility grid power is about 33% efficient. When fossil fuels are burned on-site, conversion losses are much lower. The site-to-source conversion factor for natural gas is 1.092 (1.092 units of source energy for every 1 unit of site energy). The ratio is 1.158 for fuel oil and 1.151 for propane.

Let’s compare the real efficiencies of electric baseboard heat and a run-of-the-mill gas furnace with an annual fuel utilization efficiency (AFUE) of 78%. When the site–source ratio of 1.092 is applied, the 78% AFUE furnace is 70% efficient on a source basis, meaning that for every 100 Btu of energy potential in the raw fuel, 70 Btu are delivered in the form of heat.

On a site basis, electricity is roughly 100% efficient because virtually all of the site energy is converted into heat. But on a source basis, gas is more than twice as efficient, roughly 70% compared to 30%.

Comparing site and source energy gives a better picture of heating efficiency, and a more accurate measure of the environmental impact of using different types of fuel.

Manual J for Heat Loss

Oversized heating equipment costs more initially and will not operate as efficiently as a correctly sized appliance. Equipment that’s too small won’t provide adequate heating. So the goal should be to be to install equipment that’s chosen carefully on the basis of actual heating loads rather than guesswork.

The standard for many years has been “Manual J,” published by the Air Conditioning Contractors of America, which allows HVAC contractors to do the necessary heat load calculations and recommend the right heating system. A corresponding set of calculations in “Manual D” is for sizing ductwork in houses with forced-air heating and cooling systems.

Contractors once filled out the worksheets by hand, but there are now a number of computer programs that do the work. A variety of factors are plugged in, including type and amount of insulation, window type, lighting, what appliances will be in the house, and how much air leakage there is.

A Manual J calculation (or an approved equivalent) is now required in new construction by the International Residential Code. But there is ample anecdotal evidence that many HVAC contractors don’t use Manual J, sizing the system on the square footage of the house instead. Homeowners investing in new heating equipment should insist the calculations be performed.

Access

Scott Gibson writes about green building design and energy efficiency for a number of publications and blogs, including Fine Homebuilding magazine and GreenBuildingAdvisor.com. He and his wife live in southern Maine.

Comments (2)

Forrest Jones's picture

Compliments to the author for the research and for simplifying this down to useful information that we can use to compare apples to apples. May I add a some thoughts, some pros and some cons in addition to those in the article as to, "How do we define, and what do we consider to be True Efficiency." (And let me apologize in advance for the long comment here.)

The most important aspect of a heating system is to "get the heat to the person without heating the rest of the house or the outdoors." That being said, the optimal system is not necessarily the one that has the highest efficiency rating, but rather a combination of: unit efficiency, individual thermostatically controlled rooms, efficient distribution (ducting or individual room heat), additional heat recovery ventilation system, and ease of locating/installation. Before we even start, let me state that every new home should have a Heat Recovery Ventilation (HRV) to save 80% of the heat that is produced, but would otherwise be lost due to required house ventilation. The tighter a house is, the more important whole house ventilation is.

Now, we have to look at how to control the individual rooms so that we are heating the rooms where people are, instead of heating the whole house with a single thermostat. The best way of doing this is to use an individual heat source in each room, with separate thermostat controls in each of those rooms. And for layout, installation and user comfort, let's say that we don't want any electric baseboard heaters taking up half of the wall. Lets also make the rule of "no ducts through unconditioned space" (attic or crawlspaces). And a rule that the system has to heat up relatively quickly and shut down quickly. And lets agree that any type of chimney will vent valuable inside air regardless of dampers (especially fireplaces). Once these basic concepts are accepted. Then we have jumped above all of the High Efficiency systems that were previously mentioned in the article, but are now looking at Ultra-Efficiency and even approaching Net Zero buildings. For this type of system, I have only found one type of heating system that will meet these requirements, an Electric Radiant Ceiling Panel system (example: see Enerjoy Radiant Ceiling Panels at: http://sshcinc.com/). The particular benefits with radiant heat, is that you heating objects instead of the air. The occupant would instantly feel the warmth, just like stepping into the sunshine. It is also instant on/instant off. It can be controlled with a thermostat in each room as well as an on/off switch that can be turned off when leaving the room, and with a whole house switch at the front door. What makes the efficiency greater than the100% rating of electric heat, is not the source of the power, but rather the individual room controls and the instant on/off feature. In this way, you are only heating the room that your are in, and not the 20 other rooms in the house. It is absolutely silent, takes up no wall space, can be painted, needs no ducts, etc. Radiant heat from above, is the best way to go in my opinion, and I have yet to see a more "efficient" system. True, it does use electric power, but I would recommend solar photovoltaic panels on the roof, super insulate the walls, and then you would have a true, Net Zero House.

PS: Radiant concrete floor slabs are not really radiant (in my opinion) if they have anything on top of the concrete slab (carpet, wood flooring, beds, couches, etc.) and they take 12 hours to warm up. You only feel the radiant heat from them if you are barefoot and walking on a bare concrete floor. People have to leave them turned on at 72 degrees for the whole winter.

Fred Golden's picture

How can one compare the "efficiency" of any heating system to another? How about energy costs per year?

For the electric radiant system, I believe it will be hundreds of KW per year, and that can be really expensive in places like Southern California with it's 16 per KW rates when you exceed the minimal baseline amount.

How about 120 tube evacuated tube system hooked up to radiant floor heating? You would be using less than 1 KW per week to run the pumps, and less if they are 12 VDC powered by a battery and solar panel. You can not only store heat in a 300 to 600 gallon tank, but also within the concrete floor, it can store 30 Btu's per cubic foot of concrete heated by 10F.

What is wrong with two stage thermostats, one set point for minimal heating when the tank temp is below a certain threshold, and another - say 72 or 75F when the storage tank is over 130F.

I think you will find that electric heat in the ceiling will put nearly as much heat into the attic as the space below the heater.

Have you heard that electric resistance heating is 100% efficient? In that it will produce 3,400 Btu's per KW of energy burned in the system. Some heat loss can happen in a ducted system, as some power is consumed by the fan and not turned directly into heat. Also ductwork loss into a unheated attic or crawl space is also lost forever. Funny how some builders brag about the R-22 insulation in the walls, while insulating the very warmest air in the entire house with R-6 ductwork, thus leaking plenty of 110F heated air into the attics.

Heat pumps can transfer much more than 3,400 Btu's per KW of energy used. Some transfer well over 11,000 Btu's earning them a HPSF of 3.0 or more. This means it produced 3,400 X 3 Btu's of heat in the test lab per KW of energy used. I have seen advertizements for 25 SEER ductless heat pumps. This means it can transfer 25,000 Btu's of heat for 1 KW of electricity. That would be about 700% of the efficiency comparing the heat output from a 25 SEER heat pump to the radiant electric system.

So it seems that passive solar heating (open your window shades on the sunny side of the house) is the least expensive heating system. Active solar thermal will provide 100,000 Btu's of heat each day if the system is sized that large and for free if you are using 12 VDC pumps powered by a solar panel. Another active solar is a hot air box mounted on a roof or sunny south facing wall, that blows in warm air to the heated space. Both can have free fuel.

Earth Tube is another way to collect heat from the earth. Taking in 5F outside air in Colorado, the air can be pre-heated to 45F before reaching the air to air ERV in one Colorado home recently featured in Home Power Magazine.

Beyond that, electric consuming heating devices range from the super efficient ductless heat pumps to ducted heat pumps, and possibly geothermal heat pumps.

Wood fired exterior boilers will not consume any air from within a house, they are actually installed 10 to 30 feet away from the home to isolate it from CO intrusion, and any fumes. No heat loss through the chimney either.

What does it cost to heat a super insulated home? With some homes reaching beyond the new code minimums of R-22 walls and R-40 ceilings here in Oregon, they are barely using more than a candle to keep them warm in the winter. A dinner party with 10 guests can overheat a modern home on a 40F day. Heat loss in a modern code compliant home is very low. Air is sealed in tightly, and heat loss through windows is now minimized with tight seals and wide double panes, high insulation values.

Show or Hide All Comments

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading