We have written this article to help our clients better understand heat, heat transfer, radiant heating systems, and what we currently offer in order to provide the best product on the market at the best value.
Heat
Heat is energy transferred from one body or object to another. Heat migrates spontaneously based upon temperature differentials. The transfer occurs from an object of higher temperature to an object of lower temperature and is known as thermal transfer.
If we take a cast iron pan which has been heated to 100° and leave it in a closed room which is at 70°, the heat from the pan will be transferred into the room until the pan has cooled and the room has warmed. The two objects, the pan and the air in the room, experience an exchange of energy with the pan losing energy and the air gaining energy in the form of heat.
If we take that same cast iron pan, cool it outdoors in the winter to 20°, and bring it in to that same 70° room, the energy will flow from the warmer air into the colder pan until both the pan and the air have reached the same temperature, somewhere between the two. Once they have equilibrated, energy will no longer flow in the form of heat. The air in the room will be cooler than when it started out and the pan will be warmer, but they will both be the same temperature.
There are many variables at play in real situations, but for these highly simplified examples we can calculate the temperatures that will be settled upon if we know the mass of both the air in the room and the mass of the pan along with their starting point temperatures. The ratio of the two masses is what determines the eventual temperature outcome. In other words, in a large room with a small cast iron pan, the room has a higher mass and the ending temperature will lean towards the starting temperature of the room. But if we replace the cast iron pan with a 2 ton block of iron in that same sized room, then the heated iron block, with a much larger mass, will win in tipping the scales of where the final temperature ends up. Therefore, the ending temperatures of the experiment are based upon the temperature differentials and the mass of the two objects which are attempting to equilibrate.
Material Density
If you walk into a room that is 70° with a tile floor and a rug in the middle of the room and take your shoes off you will perceive that the tile floor is cold and upon walking onto the rug you will conclude that the rug is warm. Your body is 98.6° so there is a temperature differential between you and everything in the room (let’s assume that the room has been at 70° for many days so all of the room is the same temperature, including the air). The air is absorbing your heat energy, the tile floor is doing the same, and so is the rug, so why does the rug feel warm while the tile feels cold? The answer is material density.
The tile is far denser than the rug. Your foot comes into contact with the tile and your body’s heat is lost to the highly concentrated molecules of the tile. Then you step onto the rug. It is a lower density material with fewer molecules and therefore less actual material contact between your body and what you are standing on so there is less heat loss between those two bodies in contact. Believe it or not, the rug is exactly the same temperature as the tile floor. It is simply far less dense and so far less efficient at removing your heat.
(Until you are able to show me the secret, hidden heater that continues to keep all carpets and rugs warmer than the floors that they sit on, I think we should rest soundly upon this explanation).
But why do more dense materials transfer heat more readily? Dense materials are materials with molecules packed more closely together. This means that more molecules are touching each other. The higher the number of molecules to accept or deliver energy, the more rapidly heat will be transferred.
Imagine molecules as sponges and water as heat. The more sponges you have, the more water you pick up and the more water you can store for transfer later on to something drier. That is how materials accept, store, and pass heat.
Structures
In the middle of winter, your home normally exists at a higher temperature than the surrounding outside world and it is clear that the surrounding outside world is a much higher mass. From our examples, it should be apparent that your home is the hot pan and the outside world is the cooler room. Your home will lose heat to the outside world until it has reached equilibrium with that outside world. In other words, if you stop introducing heat into your home, it will cool while warming the outside world until they have reached the same temperature based upon the ratio of their two masses (the ratio is “your home mass / mass of the world”). In effect, the only way your home gets cold at night is by heating the entire world. There are only two ways to keep your home warm while it is cold outside:
1) stop the flow of heat from a higher temperature body (the inside of your home) to that of a lower temperature body (the outside world)
2) continually introduce new heat in a form that will transfer to the interior mass of your home to offset that loss to the outside world
It should not come as a shock to you that most structures are designed around the second scenario. This occurs primarily because it is easier to produce heat than it is to stop it from leaving. (It is also cheaper initially but becomes more expensive as time goes on). Since the transfer of heat is based upon high density materials in contact with temperature differentials, good home design must focus on the densities and locations of materials used for construction of the structure’s shell.
Resistance to Heat Transfer
This causes us to ask about the ways in which we can stop thermal transfer. A single pane of glass is very dense. It transfers heat from one side to the other efficiently, meaning that is has a very low resistance to heat transfer. That is where we get the term “R-value”. When something has a high R-value, it has a high resistance to thermal transfer and when it has a low R-value it has a low resistance to thermal transfer.
Cast iron is very dense. Tile floors and concrete filled block walls are very dense. They pass heat readily. They have a very low “R-value.”
Fiberglass insulation, Styrofoam board, and cellulose insulation have a very low density. It possesses excellent resistance to passing heat and therefore has a very high “R-value.”
Focusing on material density is the key to stopping heat loss from one area to another. Ensuring that the perimeter of the building has a structurally sound envelope yet a low density core will decrease thermal transfer and allow a home to retain any gained heat for a longer period of time. Simply put, the higher the R-value of the overall structure, the more improved its ability to maintain the energy within that structure.
Radiant Heating Systems
Radiant heating systems use high temperature water in tubing which is placed within a concrete slab to transfer the heat from the water to the mass of the concrete. If the water has a higher temperature than the concrete, then, as we have seen, the cooler concrete will absorb the heat from the passing water until the water and the concrete are the same temperature (or until the room temperature has reached the set thermostat goal).
There are four elements to any radiant heating system:
1) Heat Production (gas boiler, solar collectors, or a combination)
2) Delivery System (thermostats, valves, tank, pumps, and tubing)
3) Thermal Mass (concrete slab)
4) Resistance to Thermal Transfer (insulation)
Thermostats determine the air temperature in a room and if the room’s air temperature is lower than the thermostat’s set temperature, the thermostat closes a circuit and sends an electrical signal to request hot water from the storage tank (or from the on-demand boiler) to be sent to the tubing. The pump will activate, valves will open for that thermostat location, and heated water will flow through the tubing. As the hot water flows through the tubing, it passes through the high mass concrete which is colder than the water in the tubing and the heat energy migrates from the hot water to the concrete, raising the temperature of the concrete and decreasing the temperature of the water in the tubing. The dense, high mass concrete increases in temperature and since it is in contact with the cooler air of the room, it will transfer that heat energy to the air. The coldest air in the room happens to be sitting right upon the floor. It is heated by the slab, becomes warmer and therefore less dense than it was a moment earlier. The higher density air (heavier, colder air) in the vicinity is drawn downward by the gravitational attraction of the mass of the entire planet and the warmer air merely gets out of the way, heading up in the room until it reaches air of the same temperature, losing heat to the cooler air along the journey. (We have been taught all our lives that hot air rises. Technically, it is more correct to say that cold air falls due to gravity. Hot air just gets out of the way.) This process continues until the air in the room at the elevation of the thermostat rises to the temperature set on the thermostat. At that moment, the thermostat opens a circuit. The pump stops, the valves close, and the boiler stops producing heated water.
Now, it would remain at this stage forever if the room did not lose heat to the outside world, but it does. The envelope, or outer shell of the home, is in direct contact with both the outside world and the air of the room. It touches both.That contact means that heat will be transferred from the higher temperature air in the room to the lower temperature world outside. Once the room temperature has decreased past the thermostat’s set temperature, the whole cycle will start again.
Thermal Mass
The thermal mass, the concrete slab, should be thought of as a battery; the larger a battery is the greater the storage capability of that battery. The same is true for the concrete slab of the heating system. The greater the mass of the slab, the greater its storage capability will be. If we increase the mass of the slab, then the energy required to bring it up to a given temperature increases as well, but that energy will be available later on towards heating the home as room temperatures decrease.
You can see that we have a triangle of effects:
Higher Mass – Higher Energy Requirements – Longer Periods of Heat Return.
If you increase the mass, it leads to increased energy requirements to heat that mass but results in longer periods of heating available from that mass.
The converse is true:
Lower Mass – Lower Energy Requirements – Shorter Periods of Heat Return.
If you decrease mass, it leads to decreased energy requirements to heat that mass but results in shorter periods of heating available from that mass.
There is one conclusion that we can derive from this section: if the energy to heat the thermal mass is free (solar gain) then we want to have a large thermal mass to continue to help heat the room after the source of the heat has gone away. However, if we are using fossil fuels to heat the thermal mass alone, then the added efficiencies of a larger thermal mass does not apply because we must pay to heat it in the first place.
Concrete Slabs
Generally speaking, concrete slabs in homes are 4” thick. They are 4” thick because everyone has always made residential slabs 4” thick not based upon any specific design requirement for that specific home but because it is a generally accepted standard. Not generally questioned, just generally accepted.
(Even though we say that the slab is 4” thick, the reality is that the 4” is a nominal dimension, meaning that the grade-work was most likely performed with a 2x4 as a depth determining screed. It is called a 2x4 but is, in fact, 1 ½” x 3 ½” so what you actually have under your feet is a 3 ½” concrete slab).
As mentioned, the slab thickness of 4” is a generally accepted standard for structural reasons. However, with the advent of radiant heating systems the slab is now serving double duty. It is still a structural component of the home but now it is also a component of the heating system but, for the most part, seems to be sadly forgotten as such in the design process. Companies rarely, if ever, discuss the appropriate thermal mass of a structure when designing the heating system and heating requirements. It is a critical component in developing a truly efficient system and must be taken into consideration if ultimate energy efficiency is the goal.
The NSC Heating System
When designing a heating system you must consider the square footage of the home, the cubic footage in regards to ceiling heights, the R-Values of walls and ceilings, and footing and slab insulation. All of these calculations will lead to a BTU rating required to heat the home. But the BTU rating of a heater is only the component that makes the heat, it is not the component that holds the heat and then re-radiates it into the structure. That is the thermal mass of the slab. If the thermal mass, the concrete slab, is the battery of the system, then the boiler is the generator. There are basically three types of heat generation:
1) Gas boiler
2) Passive solar heating
3) Active solar hot water collectors
The gas boiler is the easiest choice simply because it is always available for work. If it is cold at 3am, there will be heat. But that heat comes at a cost and that cost shows up as a fuel bill. The old physics adage that “there is no free energy” continues to be true in this age of upwardly mobile fossil fuel prices. The gas boiler heats the water required to heat the slab and that slab will then be the re-radiator for the home. But for all of its efficiencies, the gas boiler will always required fuel and will therefore always be an expense and a source of pollution.
The second, passive solar heating, is simple and can be attained through design and correct orientation of the structure to allow maximum solar access to the interior of the home and its thermal mass during the winter months and minimum solar access during the summer months. This method of heating is available as long as the sun is shining but its effects into the night are limited by the thermal mass of the home and the efficiency with which that mass can be heated passively.
Lastly, active solar hot water collectors can provide significant amounts of high temperature water to be used to transfer the daytime solar heat to the thermal mass of the structure, bringing the entire home up in temperature but it can do this only during daylight hours. However, like the passive solar option, the efficiency is directly linked to the design of the thermal mass of the structure and the structure’s ability to resist heat loss during the night.
Based upon all of the information provided above along with data which we have collected over years of operation from our current structures, we have determined that the best scenario for heating as efficiently as possible is as follows:
a) Design and construct a home with passive solar gain
b) Design and construct as air tight and high R-value a structure as budget will allow
c) Design and construct a high thermal mass slab with resistance to thermal transfer except into the living areas
d) Heat that high thermal mass with active solar hot water whenever there is solar heat to be acquired and use that energy to maintain the high thermal mass at a relatively high temperature
e) Use an on-demand gas boiler to supplement the passive and active solar heating for times when the radiant period is not long enough to get through the night or cloudy periods.
It is imperative that a large thermal mass be heated with passive and active solar heating and not fossil fuels. The thermal mass can only be increased for efficiency if the primary heating of that increased mass is to be free of charge: solar.
All of our structures at North Star Construction are passive solar designs, but we feel so strongly about these conclusions that we have pledged to incorporate both passive and active solar heating in combinations with a high efficiency on-demand gas boiler along with our newly engineered NSC Advanced Slab Design® into every structure we build as of January 1st 2008. We are committed to providing to our clients what we feel is the best heating system available.
Please read our next segment entitled NSC Advanced Slab Design® for more information regarding what we are currently providing towards heating and energy efficiency. Please contact us tour of one of our designs and to review our current available designs. We look forward to sharing our work with you. Go to www.nscnm.com for more links to information articles provided by North Star Construction or call us with questions at 1-505-660-2720, email us at info@nscnm.com for general inquiries, or for design and engineering specifics, please contact our designer at by emailing paul@nscnm.com. We appreciate your time, your attention, and your interest in North Star Construction’s Square House.
Monday, May 26, 2008
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