Radiant Heat vs Forced Air

Discussion in 'Designing, building, making and powering your life' started by TLP, Feb 24, 2015.

  1. TLP

    TLP Junior Member

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    I won't go into all the human biology or anatomy the book I am reading " Breathable Walls" goes into to proof this conclusion but how many agree that radiant heat is more comfortable than hot forced air that needs to cycle more often due to high insulation and low mass designs? I find the later very uncomfortable especially with hot and cold spots throughout the home. How many when they do their HVAC loads calculations take into account human factors that drop or increase the load? The second statement below I find absolutely true. The way the sun heats on a cold day with fresh air outside would be a good example of this.

    https://www.breathingwalls.com/index.html

    [​IMG]
     
  2. Topher

    Topher Junior Member

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    Greetings,

    While I agree that radiant heat feels more comfortable *to me* than forced air heat, that is only the start of the conversation about how we should maintain comfort in the face of our methods for doing so, changing the environment.

    First, air contains very little heat for its temperature. It is vastly easier to raise the temperature of your whole house's air supply to 75°F, than to raise the temperature of the walls of the room you are in, to 70°F.
    Second, warm air in contact with your walls will heat those wall, and cool the air. Conversely, warm walls will heat the air. So maintaining a difference between air temperature and wall temperature is going to require energy.
    Third, air needs to be refreshed by bringing in outside air, thus absent mechanical devices to exchange the heat for outgoing air to incoming air, (HRVs), the heat put into the air in your house is going out in a few hours.
    Fourth, heat is cheaper at different times of day, thermal mass is a cheap, easy way of maintaining heat levels, and using the cheapest form of heat. Thermal mass is closely correlated with radiant heat.

    Thank You Kindly,
    Topher
     
  3. TLP

    TLP Junior Member

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    Response to Topher

    Greeting’s Topher, thanks for the reply. I was wondering why people do not find this topic as interesting as I do.

    Topher: “While I agree that radiant heat feels more comfortable *to me* than forced air heat, that is only the start of the conversation about how we should maintain comfort in the face of our methods for doing so, changing the environment.”

    *to me* - not just to you but to the vast majority of the population, reason being the way the brain (the real thermostat) responds to skin sensors. A lot of HVAC pro’s don’t factor human comfort into the equation nor does manual J load calculator, that is one reason the majority of people are not comfortable in their homes.

    https://www.healthyheating.com/Thermal_Comfort_Working_Copy/HH_physiology_4_nerves.htm#.VQgXHI7F_UV

    “That's why radiant skin losses from the neck, head, and hands plus conductive losses through the feet in contact with cool floors contribute to discomfort more than any other area of the body. It is the #1 reason why radiant based HVAC systems are the ideal natural solution to improving ones indoor environmental quality. Blowing air may be cheap and low cost but not and will never be a human based solution to comfort conditioning.”

    https://www.healthyheating.com/Thermal_Comfort_Working_Copy/HH_physiology_4_nerves.htm#.VQgXHI7F_UV

    Indoor Environment Quality is defined below. Note what makes most satisfied and dissatisfied. Thermal comfort is not top of the list, it is a “state of mind” not building. Check out the link to how they did they survey, I’d recommend everyone explore this site beginning with Human Physiology 1-5 on left, that takes a human factors approach to designing HVAC vs building that does not satisfy the average mind. The mind is the thermometer and perfect, not wall mounted and limited. Air temp is a poor indication of comfort.

    https://www.healthyheating.com/Defintion_of_indoor_environmental_quality.htm#.VQgOhI7F_UU

    “We know it's possible to feel cool even cold when the thermostat says otherwise. That tells us the typical wall mount thermostat is a poor representative of our feelings. The phrase we use to describe what the body actually perceives is operative temperature which is a combination of the air temperature (what the thermostats says) and the average of all surface temperatures (what your skin feels) or what is called the mean radiant temperature. “

    This is true,

    “We're seeing it more often where IAQ is used as a surrogate or proxy for indoor environmental quality but this in our not so humble opinion is clearly misleading and effectively dilutes the term just as the incorrect use of green and sustainability have led to the dilution of their meanings.”
    IAQ is the misnomer and only part of the equation for human comfort,

    IEQ = IAQ +ITQ + ILQ + ISQ + IOQ + IVQ

    Topher: “First, air contains very little heat for its temperature. It is vastly easier to raise the temperature of your whole house's air supply to 75°F, than to raise the temperature of the walls of the room you are in, to 70°F.”

    [​IMG]

    Right, air is a poor performer for buildings in part due to its low heat capacity. Walls are heated by radiant heat transfer, not air. This explains why air temperatures can be low outside but the sun makes us feel warmer than air temps. Put your back to a hot or cold window in a set air temp to get a feel for this. Radiant heat transfer to our body and surrounding walls is spontaneous, how fast the mass in walls heat to capacity depends on emissivity, resistivity, absorptivity, reflectivity, and conductivity. We also need to consider in cooling, color and absorptivity which is an thermo-optical property. Conductivity of the steel mass would be faster than wood for example. At the same temperature, wood and steel feel different on floors as sensed by the feet. A mirror, low e coating’s, polished metal have high reflectivity, low emissivity and absorptivity. Mass that is highly absorptive that reflects little will heat fast, but to capacity can take a while depending on heat transfer calculations. Once the mass is at or close to capacity the time it takes to emit is faster. Some homes can take a week to get to capacity, and at the same time keep the home warm or cool in power outages longer than a forced air system can.

    "The thermal absorptance represents the fraction of incident radiation that is absorbed by the material or is the proportion of radiation absorbed vs reflected at each wavelength. An absorptivity of 1.00 implies that the material is 100% efficient at absorbing radiant energy. An absorptivity of 0.20 implies that the material absorbs only 20% of that which it is capable of absorbing with the balance being reflected."

    "There is a corresponding relationship between emissivity and absorptivity in long wave radiation typical of room temperatures where materials having high emissivities also have high absorptivities; but it's not a perfect relationship because unlike long wave radiant emissions in heating where colour is irrelevant, color is very important in absorptivity of short wave (solar) radiation for radiant cooling systems, i.e. darker colours will have higher absorptivity than lighter colors. "

    You can read more about all this here: https://www.healthyheating.com/Radiant_Design_Guide/Floor-covering-R-values.htm#Radiant Flooring Guide

    Here is a discussion, convection wall transfer is negligible because it occurs from air (fluid) or flux flow tangent to the surface, not perpendicular. https://www.physicsforums.com/threads/radiation-heat-transfer-between-air-or-wall.357880/

    Topher: Second, warm air in contact with your walls will heat those wall, and cool the air. Conversely, warm walls will heat the air. So maintaining a difference between air temperature and wall temperature is going to require energy.

    Incorrect, air does not heat walls. The best place to get energy is from the sun in short wave form, design floors and walls according, in many climate zones may be all you need especially if you use solar water collectors with food grade clycol and a heat exchanger driven by PV. A mass design will cut loads over 75%.

    Topher: “Third, air needs to be refreshed by bringing in outside air, thus absent mechanical devices to exchange the heat for outgoing air to incoming air, (HRVs), the heat put into the air in your house is going out in a few hours.”

    There are many natural ways to get air exchanges without mechanical devices such as HRV/s. Breathable Walls should exchange air in small amounts and dry vapor without significant heat or cool losses. This is a topic for another thread. I have one here with a lot of quantitative analysis, hope the mods don’t mind the link to another permie site: https://www.permies.com/t/43637/natural-building/Breathable-Walls

    If more than natural is need I’d recommend an ERV. Hydronic radiation combined with the right indoor hygroscopic materials needs no humidification or ERV like forced air systems do.

    Topher: “Fourth, heat is cheaper at different times of day, thermal mass is a cheap, easy way of maintaining heat levels, and using the cheapest form of heat. Thermal mass is closely correlated with radiant heat. “

    Agree, other than mass being a "cheap" form of building, some have incurred more initial cost compared highly insulated with a short 3-5 year pay back (EG: concrete walls) . It also depends on local resources, passive solar and active designs. The “lag time” you are referring to is normally between 6-12 hours that can shift peak loads away from surcharges some grids charge.

    Below is a study showing a survey of where hydronic radiation is the most comfortable. Warm walls, cool ceiling's are by far more comfortable since unlike floors they radiate the entire body and the feet are not instrumental causing varicose veins, not lower third. The cool ceiling takes down "asymmetry" difference as shown in graph, top=bottom, left-right, or hot-cold drafts forced air is notorious for. Europeans are heading this way. You can find micro-tube mats on the internet you embedded in earth plaster that keeps the walls from evaporation. Again, very effective since water transport heat 1000 times faster than air, this type of temperature control follows the model of human capillary receptacles.

    There is a lot more to all this it would take a book.

    Thanks again for the input.

    [​IMG]
     
  4. TLP

    TLP Junior Member

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    Response to Topher

    Greeting’s Topher, thanks for the reply. I was wondering why people do not find this topic as interesting as I do.

    “While I agree that radiant heat feels more comfortable *to me* than forced air heat, that is only the start of the conversation about how we should maintain comfort in the face of our methods for doing so, changing the environment.”

    *to me* - not just to you but to the vast majority of the population, reason being the way the brain (the real thermostat) responds to skin sensors. A lot of HVAC pro’s don’t factor human comfort into the equation nor does manual J load calculator, that is one reason the majority of people are not comfortable in their homes.

    https://www.healthyheating.com/Thermal_Comfort_Working_Copy/HH_physiology_4_nerves.htm#.VQgXHI7F_UV

    “That's why radiant skin losses from the neck, head, and hands plus conductive losses through the feet in contact with cool floors contribute to discomfort more than any other area of the body. It is the #1 reason why radiant based HVAC systems are the ideal natural solution to improving ones indoor environmental quality. Blowing air may be cheap and low cost but not and will never be a human based solution to comfort conditioning.”

    https://www.healthyheating.com/Thermal_Comfort_Working_Copy/HH_physiology_4_nerves.htm#.VQgXHI7F_UV

    Indoor Environment Quality is defined below. Note what makes most satisfied and dissatisfied. Thermal comfort is not top of the list, it is a “state of mind” not building. Check out the link to how they did they survey, I’d recommend everyone explore this site beginning with Human Physiology 1-5 on left, that takes a human factors approach to designing HVAC vs building that does not satisfy the average mind. The mind is the thermometer and perfect, not wall mounted and limited. Air temp is a poor indication of comfort.

    https://www.healthyheating.com/Defintion_of_indoor_environmental_quality.htm#.VQgOhI7F_UU

    “We know it's possible to feel cool even cold when the thermostat says otherwise. That tells us the typical wall mount thermostat is a poor representative of our feelings. The phrase we use to describe what the body actually perceives is operative temperature which is a combination of the air temperature (what the thermostats says) and the average of all surface temperatures (what your skin feels) or what is called the mean radiant temperature. “

    This is true,

    “We're seeing it more often where IAQ is used as a surrogate or proxy for indoor environmental quality but this in our not so humble opinion is clearly misleading and effectively dilutes the term just as the incorrect use of green and sustainability have led to the dilution of their meanings.”
    IAQ is the misnomer and only part of the equation for human comfort,

    IEQ = IAQ +ITQ + ILQ + ISQ + IOQ + IVQ

    Topher: “First, air contains very little heat for its temperature. It is vastly easier to raise the temperature of your whole house's air supply to 75°F, than to raise the temperature of the walls of the room you are in, to 70°F.”

    Right, air is a poor performer for buildings in part due to its low heat capacity. See attached. Walls are heated by radiant heat transfer, not air. This explains why air temperatures can be low outside but the sun makes us feel warmer than air temps. Put your back to a hot or cold window in a set air temp to get a feel for this. Radiant heat transfer to our body and surrounding walls is spontaneous, how fast the mass in walls heat to capacity depends on emissivity, resistivity, absorptivity, reflectivity, and conductivity. We also need to consider in cooling, color and absorptivity which is an thermo-optical property of the floor. Conductivity of the steel mass would be faster than wood for example. At the same temperature, wood and steel feel different on floors as sensed by the feet. A mirror, low e coating’s, polished metal have high reflectivity, low emissivity and absorptivity. Mass that is highly absorptive that reflects little will heat fast, but to capacity can take a while depending on heat transfer calculations. Once the mass is at or close to capacity the time it takes to emit is faster. Some homes can take a week to get to capacity, and at the same time keep the home warm or cool in power outages longer than a forced air system can.

    "The thermal absorptance represents the fraction of incident radiation that is absorbed by the material or is the proportion of radiation absorbed vs reflected at each wavelength. An absorptivity of 1.00 implies that the material is 100% efficient at absorbing radiant energy. An absorptivity of 0.20 implies that the material absorbs only 20% of that which it is capable of absorbing with the balance being reflected.
    There is a corresponding relationship between emissivity and absorptivity in long wave radiation typical of room temperatures where materials having high emissivities also have high absorptivities; but it's not a perfect relationship because unlike long wave radiant emissions in heating where colour is irrelevant, color is very important in absorptivity of short wave (solar) radiation for radiant cooling systems, i.e. darker colours will have higher absorptivity than lighter colors. "

    You can read more about all this here: https://www.healthyheating.com/Radiant_Design_Guide/Floor-covering-R-values.htm#Radiant Flooring Guide

    Here is a discussion, convection wall transfer is negligible because it occurs from air (fluid) or flux flow tangent to the surface, not perpendicular. https://www.physicsforums.com/threads/radiation-heat-transfer-between-air-or-wall.357880/

    Topher: Second, warm air in contact with your walls will heat those wall, and cool the air. Conversely, warm walls will heat the air. So maintaining a difference between air temperature and wall temperature is going to require energy.

    Incorrect, air does not heat walls. The best place to get energy is from the sun in short wave form, design floors and walls according, in many climate zones may be all you need especially if you use solar water collectors with food grade clycol and a heat exchanger drive by PV. A mass design will cut loads over 75%.

    Topher: “Third, air needs to be refreshed by bringing in outside air, thus absent mechanical devices to exchange the heat for outgoing air to incoming air, (HRVs), the heat put into the air in your house is going out in a few hours.”

    There are many natural ways to get air exchanges without mechanical devices such as HRV/s. Breathable Walls should exchange air in small amounts and dry vapor without significant heat or cool losses. This is a topic for another thread. I have one here with a lot of quantitative analysis, hope the mods don’t mind the link to another permie site: https://www.permies.com/t/43637/natural-building/Breathable-Walls

    If more than natural is need I’d recommend an ERV. Hydronic radiation combined with the right indoor hygroscopic materials needs no humidification or ERV like forced air systems do.

    Topher: “Fourth, heat is cheaper at different times of day, thermal mass is a cheap, easy way of maintaining heat levels, and using the cheapest form of heat. Thermal mass is closely correlated with radiant heat. “

    Agree, other than mass being a "cheap" form of building, some have incurred more initial cost compared highly insulated with a short 3-5 year pay back (EG: concrete walls) . It also depends on local resources, passive solar and active designs. The “lag time” you are referring to is normally between 6-12 hours that can shift peak loads away from surcharges some grids charge.

    Below is a study showing a survey of where hydronic radiation is the most comfortable. Warm walls, cool ceiling's, are by far more comfortable since unlike floors they radiate the entire body and the feet are not instrumental causing varicose veins, not lower third. The cool ceiling takes down "asymmetry" difference top=bottom as shown in graph, top-bottom, left-right, or hot-cold drafts forced air is notorious for. Europeans are heading this way. You can find micro-tube mats on the internet you embedd in earth plaster that keeps the walls from evaporating. Again, very effective since water transports heat 1000 times faster than air, this type of temperature control follows the model of human capillary receptacles.

    There is a lot more to all this it would take a book.

    Thanks again for the input.

    [​IMG]

    [​IMG]
     
  5. Topher

    Topher Junior Member

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    [Blatant Advertisement redacted.]

    "Incorrect, air does not heat walls."

    Of course it does. All anyone need do to check this for themselves is go into a house heated by hot air, and touch the walls. If air does not heat walls, those walls will be the OUTSIDE temperature. I predict that they won't be.

    Topher: “Third, air needs to be refreshed by bringing in outside air, thus absent mechanical devices to exchange the heat for outgoing air to incoming air, (HRVs), the heat put into the air in your house is going out in a few hours.”

    "There are many natural ways to get air exchanges without mechanical devices such as HRV/s. Breathable Walls should exchange air in small amounts and dry vapor without significant heat or cool losses."

    Sorry, the laws of thermodynamics disagree. The heat loss in a breathable wall is just not being measured. Breathable walls engender serious concerns about air quality and what to do should there be a failure in the wall and water infiltrates. Not recommended for the faint of engineering

    "If more than natural is need I’d recommend an ERV. Hydronic radiation combined with the right indoor hygroscopic materials needs no humidification or ERV like forced air systems do. "

    Whether you need an HRV or an ERV is totally dependent on your climate, and to a lesser extent your house. There is no universal answer in this case. Forced air systems are no more prone to needing humidification than radiant systems. This is a prevalent myth, that you can banish by thinking about how moisture in the air works. When we say we 'dry' something what we are doing is moving water from the item into the air (requiring heat), what do we mean when we 'dry' the air? The exact opposite, we move the water from the air to something else, thus it requires COLD. Thus no form of heating can possibly remove water from the air. Air borne is leaving the house ONLY through movement of that air to the outside, and replacement with dry cold outside air. What then happen is that air with little water already in it, gets heated, and the RELATIVE humidity drops. This happens no matter what form of heating is used.

    It should be noted that in a properly designed energy efficient house in my heating climate, DEhumidification is more likely to be needed than humidification. Humans produce enough humidity in their daily activities to compensate for the amount that leaves a well sealed house.


    "The “lag time” you are referring to is normally between 6-12 hours that can shift peak loads away from surcharges some grids charge. "

    The lag time is whatever it is (preferably DESIGNED to be).

    Thank You Kindly,
    Topher

    Energy Auditor
     
  6. Brian Knight

    Brian Knight Junior Member

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    I tend to agree with Topher on most of these points. Comparing radiant to forced air is a bit like comparing wood to stone. They each have pros and cons for any given situation.

    While I agree that radiant floors can be a more comfortable form of heat, I think that they and other radiant systems are potentially less comfortable than forced air. I like for my air temperature (the most descriptive variable for human comfort) to quickly respond to my mood, situation or state of rest. The lag time for thermostat response is much longer for radiant than forced air. This point tends to make radiant less comfortable to me.

    if you can feel the heat by touch of a radiant floor, it probably indicates a very poorly built thermal envelope. Like an envelope where the designer or builder placed the importance of thermal mass or "breathability" in exterior walls above air-sealing or continuous insulation.

    The other big issue with "radiant vs forced air" is cooling or perhaps more importantly, cooling with dehumidification. You cant cool with radiant systems. Most "forced air" systems in my region are heat pump based and I think heat pump technology is the fastest growing or most common space conditioning strategy in developed or developing countries throughout the world. I think most heat pump systems can be considered "forced-air": central ducted, PTAC, or mini-split, there are many ways of space conditioning with this technology and the energy efficiency and cost effectiveness of the newest VRF technology is often better than site-used low cost fossil fuels. Of course, you can always use the efficiency of heat pumps to power radiant systems but this is still pretty rare and it doesnt solve the biggest problem with radiant: you cant use them to cool and dehumidify.

    Forced-air heat pumps heat, cool and dehumidify. So unless you are in a climate that requires no cooling or dehumidification to stay comfortable, then radiant is a very expensive upgrade to add to your space conditioning needs. As Topher points out, a professionally designed system with a well built building envelope (airtight, continuously insulated) will not suffer the drawbacks that are so often described for typical forced air systems. With the right design and installations, most occupants have no idea when the "forced-air" system is even running.
     
  7. clr.ltr

    clr.ltr Junior Member

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    Great Thread, enjoyed reading. You both have great point's and wonderful info. Gave me a major insight on how to deal with my current issue of AC death and how / what to do about it. `Thanks.
     

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