Zoning Doesn't Save Energy - Right? Wrong!
By Hal Alles
Last month I wrote about zoning a residential forced-air heating and cooling system by sensing the temperature in every room and controlling the airflow to every vent &mdash even for bathrooms. A central computer controls everything to keep every room the "right" temperature all of the time, no matter the conditions inside or out. Most people agree this would be comfortable, but will it save energy? Studies using typical two- or three-zone systems show only small savings at best, sometimes even small increases.
But a control zone for every room is different, especially when used with ducts sized to satisfy the peak loads for each room and properly sized two-stage equipment that adjusts airflow to match capacity. Up to 40% savings can be achieved in larger single-family homes (verified by an independent study of before/after utility bills).
How is this possible? Assume just the right volume of conditioned air is delivered to each room to exactly meet its needs and that the efficiency is constant. (Contact me if you want details about how this is done.) There are three primary ways energy is saved.
1. The thermostat in a home is usually set to keep the hardest-to-satisfy person comfortable in the hardest-to-condition room. In larger homes, the cooling temperature may need to be set to 73º to get 77º in rooms that get sunshine, while those rooms without sunshine will be only 69º. While they may be satisfied with 77º, they think they need 73º. If all the rooms are controlled to 77º, then the energy savings is equivalent to setting the central thermostat 4º higher. Likewise, the heating temperature may need to be set to 72º to get 68º in rooms with two or three outside walls. Maintaining 68º in every room is equivalent to setting the central thermostat 4º lower. Setting the thermostat to use less conditioning saves energy.
2. Circulation can move heat by mixing air from a warm room with air from a cool room, helping balance temperatures without using conditioning. Selectively controlling circulation to just the warmest and coolest rooms is as much as five times more effective than normal circulation. For example, if a fireplace heats a room while other rooms need a little heating, directing circulation to just the coolest rooms and the fireplace room will cool the fireplace room and warm the coolest rooms. When a heat cycle is needed, the fireplace room receives no airflow. Directed circulation saves energy.
3. When rooms are not occupied, they can be conditioned less. An interior wall provides a layer of R-4 insulation plus the dead air space in the room. When every room is controlled, bedrooms can be set back during the day, whether they are located upstairs or downstairs. The nursery or a bedroom used as a home office can be conditioned while the other bedrooms are not. Special purpose rooms for exercising, hobbies, home theaters, etc., can be conditioned a few hours per day, or even a few hours per year. Only the occupied bedrooms need to be conditioned at night. Reducing conditioning when rooms are unoccupied saves energy.
Motivating homeowners to "program" conditioning schedules requires ease of use. A well-designed interface using a graphics touch screen can make it easy. Reporting the actual cost of the energy used and predicting how setting changes will affect their utility bill can also provide the motivation.
Controlling the temperature in every room provides incredible comfort for everyone in the family &mdash and saves energy for the environment. It is possible to be "green" without compromising comfort.
September 2007 Builder Architect Edition Issue
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You Don't Notice the Perfect Temperature
By Hal Alles
Your temperature sensors are part of your warning system, closely related to your pain sensors. It's nature's way of telling you there is danger; if you don't pay attention, your body will try to protect you. You jerk away from something that's too hot or too cold, shiver or sweat when it is less dangerous. But more often, the wrong temperature is just plain uncomfortable and annoying. Be a little too warm all day long and it will probably make you grumpy.
Think of all the things you and your family use in your home because the temperature is not "right." Of course, there is the thermostat &mdash you can always adjust it up or down to change the temperature in the room you want to use. This is OK if you don't mind wasting energy or making everyone else in the home even more uncomfortable.
Sweaters and blankets are common ways to fight the chill. You can fire up that fireplace you bought for "ambiance." There are space heaters and electric blankets, maybe even a hair dryer. Exercise helps too.
After taking off all of the clothes you dare, there are still things you can use to keep you cool. There are fans, either manual or electric. Iced drinks, water spray and wet towels can help &mdash just don't try sitting in front of an open refrigerator.
When the temperature is perfect, you typically don't notice or think about it. Imagine walking from room to room in your home and not having the sunny side rooms warmer than the shady side rooms, or walking up the stairs without the temperature rising a degree every few steps, or working in your home office all day without it getting hot, or watching a movie in you theater room and not fanning yourself before the end &mdash doing absolutely nothing to make the temperature just right in every room of your home. Imagine everyone in your home experiencing (actually not experiencing) the same thing at the same time. How long would it take before you stopped noticing?
Practical products are now available that control a standard forced air HVAC system so the temperature in every room of a home is independently controlled, including even small rooms like bathrooms. If your standard for temperature control is set by living in a typical 2,500-square-foot, two-story home, the first few days might seem miraculous. But within a few weeks, you would become more and more accustomed to not noticing and soon would not notice the perfect temperature at all. It would be like living with a nagging pain for years, and then, all of a sudden, it was gone &mdash at least until you experienced a friend or neighbor's home without perfect temperature control. Then you would think, "How odd!"
Now, building homes with the perfect temperature in every room just makes perfect sense, and especially because it can also reduce heating and cooling cost by up to 40%. I will write about that next month.
August 2007 Builder Architect Edition Issue
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A Home with Only One Light Switch?!
By Hal Alles
Imagine a home with only one centrally located light switch so every light is either on or off. Anyone needing light in any room needs to turn on every light; anyone wanting the light off in any room needs to turn off every light. My guess is the lights would usually be on and family members would use blindfolds when they wanted personal darkness. I think most people would agree this is not sensible.
However, this is exactly how a thermostat controls a typical forced air heating and cooling system. One central thermostat turns on the heating or cooling when the temperature at the thermostat reaches the set point; then the furnace or air conditioner runs until the temperature at the thermostat changes a degree or two. The control is all on or all off in every room. When on, the airflow to each room is fixed, no matter what the temperature is or what temperature is wanted.
When conditioned air is delivered to a room, an equal amount of air must leave the room. The temperature of the air that leaves a room represents the temperature of that room. All of the air that leaves the rooms mixes on its way to the return, reaching an "average" temperature. A thermostat can only control the temperature where it measures the temperature. When the thermostat is located near the return, the thermostat controls the average temperature.
In a room, if the supply of conditioned air is different than the need for conditioned air, the temperature moves away from the average. Each room has a limited effect on the average temperature, so it has a limited effect on how much conditioned air it receives. In the summer, as rooms on the sunny side warm up and cause more cooling, rooms on the shady side must cool down to keep the average the same. Hot rooms don't get enough, cold rooms get too much.
Control using a single thermostat is fatally flawed. It is based on the false assumption that the need for conditioning varies the same way for all rooms. But that is obviously not true. For example, 1 square foot of west-facing window needs about 5 CFM of continuous cooling (or 2 CFM less heating) to compensate for solar radiation. So, a 4 x 5 window requires about 100 CFM (6" duct) more continuous cooling when the sun shines than when it does not. That becomes 200 CFM (8" duct) if the A/C needs to run only 50% of the time to maintain the average temperature. Everything makes heat. Each person needs about 12 CFM, and each 100 watts of light or equipment needs about 17 CFM.
Host a Sunday dinner for your family in a sunny dining room, and you may be serving a hot dessert &mdash even if it's ice cream. You will probably be sweating after a few hours working in your home office. The kids will be grumpy after playing in the bonus room over the garage. To be safe, keep your baby's crib near the thermostat.
There are many compelling reasons to have temperature control in every room. There is only one reason to have a single thermostat for an entire house &mdash it is cheaper to build.
July 2007 Builder Architect Edition Issue
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Where Are the Returns?
By Hal Alles
This is not about elections or unwanted gifts. It is about one (of many) reason residential HVAC systems don't work as well or as efficiently as they could. Forced air heating and cooling systems operate like a conveyor belt for heat. Airflow is the belt that carries heat between the HVAC equipment and the rooms. The blower (fan) in the air handler (usually called the furnace) drives this conveyor, pushing airflow to many supply vents around the house and pulling air back through one (or a few) centrally located returns. To load heat onto the conveyor, the temperature of the airflow is increased. To unload heat from the conveyor, the temperature of the airflow is decreased. The furnace loads heat onto the conveyor and it is removed near the supply vents (to heat); the A/C unloads heat that is loaded near the supply vents (to cool). The change in temperature between the return air and the supply air is called "delta TO," and is proportional to the heat transferred. HVAC equipment consumes about twice the energy to produce twice the delta TO for a fixed airflow.
For the conveyor to work, the air needs to get from the supply vents to the return. The halls and open areas of the house function as big ducts to get the air from the supply vents to the return. But what if the return path is blocked by a closed door? The airflow through the supply vent still forces air into the room, but without an easy path back, the airflow is reduced as the pressure in the room increases, so the room receives less conditioned air. The pressure increase forces air out of the room through windows, outlets, light fixtures, bathroom vents, etc. Room temperature air is forced out to condition the great outdoors.
Even worse, the escaped air is replaced by the same amount of air from outside. A slight vacuum is created, pulling outside air in through the easiest path, perhaps another bathroom vent, windows, exterior doors, etc. This replacement air requires extra conditioning to get it to room temperature, so energy is wasted. For example, to remove the added heat when it is 100° outside and 75° inside, the A/C needs to condition a volume of air about equal to the air from outside, wasting that portion of its capacity. With many doors closed and poor returns, this waste can be over 20%.
So how much return is enough? The cross section area of the return path should be at least 20% larger than the supply. The largest available return path is usually the gap under the door. A 6-inch duct can supply about 100 cfm and has a cross section of about 28 square inches, so the gap needs to be more than 1 inch - not likely to find that in a new home. Some new homes have up to five vents serving the master suite, so the gap needs to be more than 5 inches - clearly impractical. The best solutions are "pass through vents" or "jumper ducts." These allow the supply air to easily pass from a closed room into the open area with the return. It is a small addition to get a big improvement in comfort and efficiency. Faulty returns will still occur in elections, but they should not occur in homes.
June 2007 Builder Architect Edition Issue
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The Boiling Frog
By Hal Alles
I think everyone has heard how frogs behave differently when thrown into boiling water (they jump out) versus when swimming in water that is slow heated to boiling (they die). This may be stretching the analogy, but I believe the force of the free market on builders and home buyers has unwittingly been conducting an experiment on the behavior of consumers as they are brought to a slow boil. It is a classic contest between "bling" versus "bones" when houses are designed to minimize the cost of the things that are hidden (the bones) while spending on the things that are seen (the bling). Beauty is only skin deep, but ugly goes to the bone.
The HVAC system is one of the "bones" in a home; there is little to be seen, yet most designers object to what is visible - the vents. But unlike most other "bones" in a home, the HVAC system is entirely responsible for satisfying one of the five basic senses. It has impact on every person in the house in whatever room they are in. When it's dark and people can't see the bling, they can still feel the temperature. So how poorly does the HVAC system need to perform before home buyers will not buy the home? That is the experiment the free market is conducting. The HVAC system design has been compromised over time to enable an ever-lower bid. Ducts are made smaller, returns are eliminated, supply vents are eliminated or located to save time and materials, and larger areas and more rooms are put under the control of a single thermostat. The low-cost bidder usually wins, which usually means the most compromised HVAC system is installed.
After decades of intense price competition between builders and their HVAC contractors, the water may be getting close to boiling. Some builders conduct a customer satisfaction survey a year after purchase that includes the question "How is the temperature control?" In too many of these surveys, temperature control is the largest single source of dissatisfaction. Temperature-related problems are a major source of warranty claims - up to 90% for some builders. Not much is done, or can be done, other than provide temporary relief by rebalancing. Homeowners are told, "It is working the way it is supposed to work." The message is clear: "No need to ever complain or call again."
Many homeowners accept avoiding certain rooms many months of the year because they are too uncomfortable. They should get a refund from the builder for getting only partial use of the space they purchased. But will the home buyer ever exercise a basic survival instinct - to not buy a home without a written warranty covering temperature control? Will they ever demand and get guaranteed performance that is enforceable? Or will they accept ever-lower HVAC performance? I'm afraid I know the answer - the frogs die. But then again, people are not frogs, and frogs don't have attorneys.
May 2007 Builder Architect Edition Issue
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How a Thermostat Is Like a Teeter-Totter
By Hal Alles
I went to a three-room country grade school with playground equipment that today would be mistaken for weapons of mass destruction. Our parents never complained because it was still safer than the farm equipment we operated when we weren't in school. One of our favorites was a big teeter-totter made from a 14-foot piece of 2x12 pivoted about 3 feet off the ground. It was great fun to bounce a playmate off their seat for the 6-foot fall to the ground. Sometimes a third playmate would join in by standing at the pivot point, shifting their weight to make the launch off the seat faster and higher. Not much happened in the middle, but the ends bounced up and down like crazy.
This is like what happens with the temperature control in most homes. The one central thermostat controls the temperature at the thermostat. With luck, it is located near the central air return where the temperature is the average of the whole house. The thermostat is at the pivot point of a temperature teeter-totter. When running, the HVAC equipment produces a fixed amount of conditioned air that is distributed in fixed proportions to each of the rooms by the duct system. If the HVAC contractor did a good job of design and installation, this airflow to each room will be about right for the average loads in each room. However, the actual loads are seldom equal to the average since the loads vary dramatically due to sunshine, weather, people, lights, electrical equipment and appliances, etc. These can change the need for conditioning by more than a factor of 3. The mismatch between loads and the amount of conditioned air delivered causes the room-to-room temperatures to vary about 10 degrees in single-story homes and 15 to 20 degrees in multiple-story homes.
This is most apparent during sunny days when air conditioning is needed. The heat load on the sunny side of the house is typically three times the load on the shady side. The rooms on the sunny side warm up, so the return air warms up. But this air is mixed with the return air from the shady side. The thermostat senses some increase in temperature and runs the A/C longer and more often. However, cool air goes to both the sunny side and the shady side, so both the sunny and shady side rooms are cooled until the average return temperature at the thermostat is the set temperature. But the sunny side will never reach this set temperature because the return air from the needlessly cooled shady side lowers the average. The rooms are at the end of a temperature teeter-totter. Hot rooms are balanced by cold rooms. The central thermostat is like the playmate at the pivot point - it is a smooth ride while the rooms are out of control. Too bad we don't spend more time living by the thermostat. Fixed airflow and a central thermostat is a weapon of mass destruction to the comfort in the rooms. A good countermeasure is a well-designed and installed zoning system.
April 2007 Builder Architect Edition Issue
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Our Poor Relative HVAC
By Hal Alles
Most of us have a poor relative. They are good people and try hard, but some how they never get respect, a break or get ahead. Of all the trades in the building industry, HVAC is our poor relative. Next to keeping dry, keeping warm is the most important thing in a shelter. For our ancestors, using fire came shortly after using caves. Why is it that today HVAC has such a lowly status that a $1 million house has an HVAC system that costs less than $10,000, essentially the same system that the most basic house has? This is especially odd considering how on a customer satisfaction survey, a question like "How is the temperature control in your new home?" always gets the lowest score. Why is it that when a home buyer adds $100,000-plus in options to a home, $0 goes to a better HVAC system?
We all accept that people will buy at least some clothing for looks rather than comfort. Women's shoes are a good example. A little discomfort to make a fashion statement at a party is worth it to most people. But when they get home, they can get comfortable. When a home is primarily designed to make a "design" statement while neglecting comfort, where does the homeowner go after the party to get comfortable? Worse yet, the home may not be comfortable during the party. Even in modest homes, the living and dining rooms may only be used when entertaining. However, the HVAC system is never designed to cope with many people in these rooms. It is ironic (sad, frustrating, maddening, etc.) that the only times these rooms are used, they get uncomfortable.
Why does the average America house-hold use about 3.5 times as much energy for conditioning as the average Japanese household? The average Japanese home is about half the size of the average American home, but this can only account for less than half of the difference. The rest is because the Japanese only condition the rooms they are using. Americans condition every room all of the time, even though most of those rooms are unoccupied most of the time. We control conditioning with a single central thermostat that ensures some rooms are over-conditioned while others are under-conditioned.
Most energy codes and rebate programs are obsessed with controlling lights. I suspect that is because as children, we were all yelled at about turning off the lights. But what about HVAC? It typically takes more energy to condition a room than to light it. There are dozens of light controls in a home for every HVAC control. If each room needs a light controller (switch), why doesn't it need an HVAC controller?
This is a typical poor relative story. Despite the logic, despite the advice, despite the head shaking and well wishing, they somehow never get respect, a break or get ahead. The intent of this department is to focus on our poor relative HVAC, to understand the problems they attempt to solve, to understand their predicament, to offer support and to actually offer some solutions. I hope you find this department interesting and useful.
March 2007 Builder Architect Edition Issue