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radiant heat vs forced air


    BILL GARNETT Member Posts: 12
    radiant heat vs forced air

    would anyone have documentation in regards why a radiantly heated structure is more efficient than the same structure heated with forced air-thanks big bill
  • Rich L.
    Rich L. Member Posts: 414
    THE debate!

    There is quite the ongoing debate on this one Bill, try searching the archives. You can support which ever side you're on with that info.

    However going from the personal comfort approach, I've lived in both, live in my second radiant heated home now and I'll never go back! The comfort is second to none, hands down. IMHO

    Rich L
  • Brad White
    Brad White Member Posts: 2,398
    Start with cost of delivery

    An air system (moving an insulator, no less) requires much more horsepower per BTU than a hydronic (water-based) system.

    A 50,000 BTUH furnace moving 1,200 CFM at 1.00 inches of total static pressure and at 60 percent efficiency will require about a 1/3 HP motor drawing about 230 Watts.

    The same 50,000 BTUH load using hot water at a 20 degree drop can be delivered by 5.0 GPM, say at 8 feet of head and at 35% pump efficiency (small circulators are not terribly efficient you see). This small circulator would require 0.03 brake HP or 21 Watts. Even if doubled, you are still ahead, every hour you are heating.

    And without much effort, that 5.0 GPM can be used in many ways to serve different rooms. Every room can have it's own control.

    You can do that with air too (with dampers) but seldom is it done beyond splitting one large furnace into two, maybe three zones.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Paul Pollets
    Paul Pollets Member Posts: 3,656

    ...the difference between Convective heating (heating air molecules) vs. Radiant heating (heating objects, rather than air molecules. The radiant wave is more efficient than the convective method of trying to heat air molecules, which have no mass, hence more energy is required to heat the space.

    Once the radiant wave warms all the objects in the space, they too become "radiators" of a sort. Large concrete or lightweight slabs become giant radiators that require relatively low temps to stay warm. A modulating boiler heating a space with 100 deg. water on the coldest day of the year, is not uncommon. I've seen designs that heat slabs with 80deg water when the outside temp is zero.

    Robert Bean has waxed eloquently about the efficiencies at healthyheating.com

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  • ER
    ER Member Posts: 27

    Where? That's as important a question as anything.
    Is AC part of the equation?

    What kind of floors? If they really want carpet radiant is going to take a hit.

    retrofit an older poorly insulated place or in a new insulated, sealed, controlled ventilation.

    I think you need some important details before anything can really be said.

  • Brad White
    Brad White Member Posts: 2,398
    I think this has been

    framed as a "heating system comparison" with "all other things being equal". By this I mean that regardless of the condition of the envelope (which we all want to be the best), how do the systems stack up.

    Not to dismiss your excellent points of course. Still, I would trade deep pile shag carpet for bare radiant floors any day. :)
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • mitchb
    mitchb Member Posts: 19
    ITS NOT!!!

    A building with a 50,000 btu loss @ design temperature will lose 50,000btus @ design temperature whether you are using radiaant, forced air, or sheep dung space heating...

    I think your question should be

    "Why do buildings with radiant heat cost less to OPERATE."

    There is no change to the building envellope based on heating system.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Not sure thats true

    Just a theory but - consider a forced air system,- depending on the return location, its like blowing into a paper bag. You are pressurizing the discharge room unless there is a return there. Wherever the return actually is, you are creating a negative pressure zone. Either way, I believe we are increasing infiltration losses in both locations. This doesn't happen with radiant heat.

    We put returns into bedrooms on the better FA installations. Consider there is no return, the door is closed, the forced air system is on, trying to dump 600 cfm + into the room, it isn't going to work properly. Operative word here....forced - not passive.

    Don't we use a higher infiltration (air change value) for heatloss where forced air is involved? If so, the heatloss for that structure is higher.

    When we calculate heat losses, the intent is to determine the necessary heat energy that needs to be delivered to that area to keep the occupants comfortable at design conditions. Certain factors will change, based on the delivery medium we choose - radiant, forced air, or baseboard.

    Over the years, attending seminars, factory classes, etc. I've been taught to use a lower indoor design temp when radiant heat is used - depending on who you talk to, its between 65 -68F. For a forced air scenario, I usually use 70F for indoor design and 1.0 -1.5 ach. If we use a higher indoor design temp, and higher air change factor, when compared to radiant, by default, the same structure will have a higher btu heat load.

    Another reason forced air has a higher heat loss for the same structure is delta T. A forced air system stacks the air - hot air rises - the temperature at the ceilings can be 85 -95 or higher. This creates a higher delta T between the inside and outside surfaces - hence greater heat loss for the force air system.

    To address the question - efficiency, the radiant system can be designed around much lower boiler temps - lower temps usually translate into higher combustion efficiencies. As Brad points out, if you look at the cost of heat delivery, the hydronic boiler systems is significantly less and easily zoned. Use a modcon boiler with ODR, ...I bet we are talking about 25 -35% energy savings or more.

    In my opinion, for a like structures, I think forced air will have a higher btu load - 5-10% and be substantially lower in overall system efficiency. I have admit though, I don't recall reading a definitive article comparing the two.
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