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Combined radiant/forced air cheap system idea

I’m starting to hatch an idea for a residential heat/cooling distribution system. A drawing is the next step, but right now I’m trying to kill the idea before doing too much work. It stems from the following givens:

1. A growing trend is the use of so-called floor trusses. Think of a very stout joist but very open like a rectangular open web truss. The top and bottom chords are 2x4’s parallel to the floor surface.
2. Copper tubing with aluminum fins is very cheap.
3. Radiant floor heat is all the rage, and is the most comfortable and efficient. (Efficient because the actual air temp can be lower than in other systems)
4. It’s hard to cool a home with radiant floor heat unless you install a separate (redundant) forced air system.

Are you thinking what I’m thinking?

Run a bunch of fin tube in the joist cavity between floors. As the cavity is heated, the floor radiates heat above. Some heat would also radiate below. In cooling mode, the ceiling radiates “cool” to the rooms below, and also absorbs heat by convection.

Since air isn’t the best heat transfer medium, maybe this scenario won’t keep up at very cold or hot temperatures. However, just add a small blower and intake air, and you can get all the hot or cold air you might need—the joist cavity is now a big plenum. My limited knowledge of fire codes makes me think that you could only have the airflow connected to the rooms above the joist cavity—no penetrations in the ceiling below. If the house has an HRV system, it would only have to be connected to the joist cavity, not requiring its own duct system

I can think of many other advantages to this setup.

What are the downsides? Do any fire codes disallow this?

Comments

  • Ken_8
    Ken_8 Member Posts: 1,640
    Right church - wrong pew.

    A few of your assumptions are flawed e.g., "2. Copper tubing with aluminum fins is very cheap."

    Not so. Both are not only expensive, but alarmingly so!

    You then suggest, "4. It’s hard to cool a home with radiant floor heat unless you install a separate (redundant) forced air system."

    Define "hard" The principals of physics will always dictate that cool air drops and warm air rises. Radiant heat goes equally in all directions. Having said that, your dependence on convection to move cool air off the floor to the actual living space (typically 3 feet off the floor)puts us back into a forced air circulation mode. Ceiling fans? Perhaps. OK, I'll buy that.

    Condensation on floors? Most likely.

    Also remember, the principal function of A/C is remove moisture from humid air. Cooling is secondary. Where's the condensate going to go in your plan?

    If you ran cool water through the tubing, the condensation issues would rot the z-trusses in short order with mold and/or water stain issues above and below.

    I may be off on a few of the flaws I see in your design.

    But I've been wrong more than occassionally.

    Just ask my wife.

    But I don't see any way this could fly.

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  • Kevin_in_Denver
    Kevin_in_Denver Member Posts: 64
    Condensation is a good point

    I hadn't thought the condensation issue through completely, but consider this:
    A conventional central A/C system has a smallish coil that gets cold, usually dropping below the dewpoint. My concept would have a heat exchange area many times larger than that conventional coil. In the southwest at least, the chilled water temps. could be kept above the dewpoint at all times, and still get the required result if the blower I mentioned was running. The controls could be a headache.

    But this idea still may be worth pursuing even if you throw out the cooling mode.

    Cost-wise, this system would be cheaper than a baseboard hydronic system because you don't have to buy the fintube housings. You're saving floorspace as well. But you're getting a radiant floor system which is better than a baseboard hydronic system.
  • Robert O'Connor_7
    Robert O'Connor_7 Member Posts: 688
    ??

    NOT!
  • Weezbo
    Weezbo Member Posts: 6,232
    Kevin,....

    i am reluctant to say this however "Expensive Is expensive buh Cheap is not cheap, There are some hidden difugalties that would drive the idea "Home "...
  • Uni R
    Uni R Member Posts: 663
    Combined radiant/forced air cheap system idea

    For a good functioning frugal system consider baseboards (sized for a 140° design temp) using constant circulation with a condensing boiler. Use the HRV strategically. I.E. duct out of bathrooms or where there might be a kitty litter box. Duct in where you want fresh air (living room, bedrooms). Finally, if you have a nice open design consider a split ductless A/C system (or two or three) and size it as conservatively as you can and make sure its a good one like a Mitsubishi that has great airflow and dehumidification.


    The minimal ducting required is reserved strictly for the HRV and bath/kitchen vents. Absolutely no duct loss worries for your A/C. Pretty maintenance free actually and everything can be repaired very easily any time down the road should it need to be. The only downside or design consideration is that the A/C won't work through closed doors. This design assumes cooling for 3 months a year. For a long hot season, a central A/C system would be a better route because you would have a condensor farm on one of your walls. L
  • Nron_9
    Nron_9 Member Posts: 237
    cheep system

    they make a pex system that does this its called ultra fin ,we will see our first job soon , we have a client that installed this in there home and want us to install the boiler and control system , i hope the btu out puts are right well see ?? but for every 10 F you raise air temp in joist space above 140 F in tji joists you remove 30 % structural strength of the floor sysem ?? these were the numbers given by rep for product Hope this helps Nron
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Now THAT'S an interesting number...

    I'd think the structural engineer would have some say so in that arena...

    ME

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  • Constantin
    Constantin Member Posts: 3,782
    Hmmm

    As a manufacturing engineer, and someone who has seen baseboard up close, I take issue with the idea that the housing is the expensive part. Considering the materials, processes, etc. required to make the finned Al-Cu heat exchangers, my bet on largest system cost would be in that department.

    YMMV, of course.

    Instead of the sub-floor tack, I'd take a closer look at the Karo system. Radiant ceiling panels that are relatively simple to integrate, hide, etc. These panels could do a great job of removing sensible heat, while leaving a much smaller load for the AC system to handle (i.e. the remaining latent heat). In the southwest, my recollection is that humidity is not a major component of the heat load due to the very low humidity. Thus, a ceiling-based radiant system would allow you to heat and cool the place with relative ease.

    Furthermore, a large ceiling area allows you to run the system at a sufficiently high temperature to prevent dew while still removing a lot of heat from the room. I was pretty excited by the Karo system until I realized that there are few qualified installers, control strategies are still being developed, and that latent heat is a relatively big component in the NE. With the guidance of KISS in mind, I don't want to create a system sufficiently hard to service that only one guy in the surrounding 500 miles can do it.
  • Kevin_in_Denver
    Kevin_in_Denver Member Posts: 64
    whoa

    Do they have a website that explains more about this?
  • Troy_3
    Troy_3 Member Posts: 479
    TJI

    NRON you must elaborate. I would love to have that in writing. Do you know if they publish this figure?
  • Nron_2
    Nron_2 Member Posts: 4
    tji

    No i dont have the information in writing and as with everthing else im told I like to check other sourses to see if the information is correct , like to see where this goes some of the guys are engineeriers Nron
  • Nron_2
    Nron_2 Member Posts: 4
    tji

    No i dont have the information in writing and as with everthing else im told I like to check other sourses to see if the information is correct , like to see where this goes some of the guys are engineeriers Nron
  • combination radiant heating and cooling

    WORKS! I cool my primary heating loop in the summer using well water. I first took off a set of tees for a large fan coil unit, like a kick space heater, with a drip pan underneath to remove condensation and to provide additional cooling. next in line are my 2 radiant staple up zones which cool my floors to 68 degrees. ther is no condensate on the floors and no sign of mold around my joist after 3 summers of use. when i pull the insulation down during operation there is no condensate on the pex. during 100 degree weather 2 years ago my house was 70 degrees. radiant cooling is FAR more comfortable than air conditioning, your house is cool to the core. walking on a cold floors chills you quickly, just like walking on a cold floor in the winter. bob

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  • bert waller
    bert waller Member Posts: 4


    Radiant cooling is not a new idea. Many systems were installed in office type buildings in the '50's and some are still operating. Problems are (1) the radiant panels don't dehumidify, and can't be allowed to. The result is a humid clammy atmosphere, unless a seperate air system is installed to pick up the dehumidification load.

    The writer who says that his radiant cooling works fine without dehumidification is not to be believed. I've seen too many cases with discomfort caused by non-dehumidifying systems, even in arid places like New Mexico.

    Radiant cooling can be made to work, with careful design. The chilled water temperature going to the panels must not be allowed to go below interior air dew point, even during cool-down, to avoid condensation. Those controls plus the needed supplementary air system drove the cost of the old systems up, till they died a natural death due to economics.

    Now, a generation later we have a new batch of people who know nothing about past experience, and are eager to repeat old mistakes all over again. Plow up some new ground, guys!
  • Weezbo
    Weezbo Member Posts: 6,232
    See :)

    some stuff works however you gotta do your homework....work for free with some people who already determined the minor technicalities then you will have gained countless hours of your future in the way of not making mistakes and by doing things right the first go around.
  • don_30
    don_30 Member Posts: 1
    Bert

    I agree with you 100%.If I recall oh willis invented ac
    to remove the moisture,it just so happen you have to reduce senisble to get to that point.

    If it was only about removing the senible load,then no one
    would be having this conversation.

  • Matt Clina_2
    Matt Clina_2 Member Posts: 8
    Condensation etc.

    Keep in mind, that in the southwest, where your "cooling with chilled water that is just above the dewpoint" might work, they can get away with evaporative "swamp coolers" and get their cooling almost for free. As one of the other posters said, the dehumidification is the most important aspect of an a/c system.

    That being said, do not be discouraged. It is always a great idea to try to think outside the box.
  • Kevin_in_Denver
    Kevin_in_Denver Member Posts: 64
    Thanks for all the help so far

    I think I would kill the idea of cooling the joist cavity, I've already considered the swamp cooler. They work great in Colorado, but strangely, all new homes have central AC. That's another thread.
  • I didn't say

    Radiant cooling works without dehumidification, I said I have a large fan coil unit that works like the small coil in conventional air conditioning. Kevin is on the money when thinking the heat exchanger should be many times larger than a conventional coil. I made mine out of 100' of baseboard element, and let me tell you it removes a lot of humidity. I assume it lowers the dew point in my house so my floors don't condense. The result is not a humid clammy atmosphere; it is an extremely comfortable system. The cooling comfort of radiant cooling is comparable to radiant heating. It cools your house to the core. And you can believe EVERYTHING I ever say, bert. Bob Gagnon

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  • bert waller
    bert waller Member Posts: 4
    radiant cooling

    Ken, your picture shows a large hand-fabricated coil with a prop fan. I presume that this is the apparatus placed in the under-floor joist to cool the floor. I misunderstood your earlier explanation, so a different response is appropriate. Your apparatus is a large hand made fan coil unit. Frankly it looks like a crude and expensive way to get the results you want. Not shown in picture is the large drain pan needed. In my opinion you could get better results for much less money and fuss with a manufactured fan coil unit. I'm guessing that your fabricated coil will be in the trash after a couple of years when the fin-to-pipe bond had deteriorated. And how do you know if your coil has capacity to match the cooling load in the first place? That it has the optimal latent/sensible capacity? You get all that with a factory designed & built unit at lower cost. A no-brainer in my opinion. You do earn some points for good intentions, however.

    Bert Waller
  • Bert

    My fan coil unit dosen't cool the space beneath my joist; my suspended tube radiant system cools that space. the fan coil unit cools and moves the air around inside my house. placed in a closet it simply draws the air from one side of my house, passes it through the coils and cool air comes out into the other side of my house. I built that crude unit in my spare time using an $11 fan and left over baseboard element- you can't get less expensive than that. you are right about the sizing as I have on background in cooling, I have no idea what latent/sensible capicity even is. I simply made one unit out of 48' of element- that was not enough so I doubled it to 96'. the system has worked great for 3 seasons. I'm curious about the fin to pipe deteriorating, would this be because of the cooling application? I have not seen this with any heating system. Thanks for your help, Bob Gagnon

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  • Kevin_in_Denver
    Kevin_in_Denver Member Posts: 64
    fintube deterioration

    If you get condensate at the Al-Cu interface, galvanic corrosion will eat up the fins. It doesn't take long because the Al is so thin. In heating only systems, this never happens.
  • the fins

    do look a little chalky, but they seem flexable. do they use stainless fins for cooling? thanks again bob

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  • tombig
    tombig Member Posts: 291
    Bob

    > do look a little chalky, but they seem flexable.

    > do they use stainless fins for cooling? thanks

    > again bob

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    > HREF="http://www.heatinghelp.com/getListed.cfm?id=

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  • tombig
    tombig Member Posts: 291
    Bob

    Way to think outside the box. If 48' don't do it then 96' might. Edison tried hundreds of filament materials inventing the light bulb. Nothing wrong with good old trial and error in your own home. I know a guy who ran his hot gas line from his home AC compressor thru a heat exchanger/pool heater.

    Just curious, do you run the water down the drain or somewhere else? What is the well water temp? How much condensate on a humid day.
  • bob_25
    bob_25 Member Posts: 97
    Kevin how's this

    grab ya? A swamp cooler thats 100% efficient will discharge air that's at the ambient wet bulb, also the temperature of the water in the pan will be the wet bulb temperature. Out west this could be 50* maybe. The relative humidity of the air will be 100%, thus the name swamp cooler. What if you run the discharge air in one end of the attic and out the other. This would significantly lower the load. Now pump the water in the pan through a heat-x and then through the radiant system. The water will be above the dew point because the system is not 100% efficient. You could also run the water through a fan coil etc. The heat-x is to protect the system from the yucky water. This system would do nothing for the latent load. It would be very energy efficient. Just a thought. Way back in the late 60's I piped an intensive care wing of a hospital that had radiant heating and cooling panels on the ceiling. It was a four pipe set-up plus each room had a forced air system, I think Johnson did the controls big bucks! bob
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    That's an elegant way to eliminate condensation

    Very slick, sort of like using a cheap swamp cooler to replace a cooling tower.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • tombig

    The well water here is 56 degrees. After it cool my house I send the water out to my multiple zone sprinkler system, so if you have a big yard with a lot of watering needs the cooling is pratically free, except for running the circ. pumps and fans. This works out because when it's real hot, thats when you need the most watering. The fan coil unit gets a LOT of condensate on it when it is humid. the comfort level of this system is unbeleivable, and i'm guessing that if it was designed properly, it could save a lot of energy. bob

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  • Kevin_in_Denver
    Kevin_in_Denver Member Posts: 64
    Ultra-fin looks pretty good

    Thanks for pointing me toward Ultra-fin. I really don't think the hot joist space will be a structural issue. The website is

    www.ultra-fin.com/index.html

    This really looks like a nice compromise between comfort and cost, even ignoring cooling for now.

    Now I'm going to search around for the feasibility of using a water heater for the heat source.
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