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Low temp radiators and mod-con efficiency

FHVs [usually] come in two varieties.

One modulates via slab sensor. Such is generally only suggested for fairly small spaces--up to about 100 sq.ft. of heated floor area--think a reasonble bathroom.

Another version modulates via room air temperature. The practical limit for a single FHV is about 10,000 btu/hr. While multiple FHVs could technically be used in the same room, you'd likely wind up with only a single FHV providing the vast majority of the heat in most weather conditions. Perhaps not too much of a problem (other than potential "striping") if you've interleaved the tubing, but otherwise you'd likely wind up with only certain sections of the floor being heated.

Because FHVs (like TRVs) can ONLY regulate flow, they allow <I>unlimited</I> panel temperature if you "crank the operator". In other words if you don't have a decent reset curve you could do the <A href="http://www.amazon.com/Demon-Seed-Julie-Christie/dp/B000A0GOFU/ref=pd_bbs_sr_1/104-6384430-4388761?ie=UTF8&s=dvd&qid=1175117955&sr=1-1">Demon Seed</A> thing and cook eggs on the kitchen floor.

For this reason, the [better] FHV valve bodies has a built-in flow-setter adjustment. This allows you to keep head loss (and thus flow) similar regardless of the tube length in individual circuits. The purpose of such is to limit the ability of a relatively small floor area to overheat if the user cranks the operator.

My offer of free design assistance still stands for any reasonable contractor or <I>highly informed</I> homeowner who wants to use FHVs, Warmboard and a Vitodens.


  • Gene Davis_2
    Gene Davis_2 Member Posts: 71
    More opinions, please!

    We're the GC that will build a couple small spec houses, to be sold to vacation home buyers, in ski country. We want to go with a heating package that uses a small mod-con as the plant, for supplying heat to the in-slab PEX of the walkout level slab, and to panel radiators for the main floor above.

    Why panel rads for the mainfloor and not underfloor tubing? The choice is cost-driven, as we'll simply spend more on materials and probably labor to get enough tubing and plates under the floor, plus the rads or baseboard required as supplemental heat in the high loss areas where there is lots of glass.

    So, with the choice being panel rads for the main floor, our pro is recommending we size and place rads that will all use 120F water at design temp (-30F), in order to be able to realize the mod-con's condensing mode efficiency.

    Our in-slab loops will all use water at 120F and less.

    We'll likely use a SuperStor 45 for DHW, for which the mod-con will provide heat.

    My call for opinions is on the matter of the panel rads use of 120F max temp water. It'll require a whole lot more rad surface to deliver the required heat, than if we design the system as a two-temp one and have the ability of doing, say, 145F water to the rads at design temp.

    Our pro says the fuel use efficiency will offset the higher cost of more radiation surface.

    How true might this be, and how common is it to do radiation from panels (or even baseboard) at a temp of 120F and lower?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Crunch the Numbers Gene

    Firstly, despite previous ribbing, I applaud you for going the extra mile for your buyers unknown.

    My personal standard basis is using a 120 degree average water temperature (140 in, 100 out) for ModCon-based design. Remember, average, not supply temperature is key. I can adjust this and sometimes do, to the realities of budget and wall space for example.

    The ideal would be to have one temperature do the entire building if you can. You would have to calculate how much surface output from the floor and let that set your maximum water temperature, then size the radiators accordingly. If you do not do that, then prepare for a mixing station of some sort, even rudimentary.

    Now, the economics of it: First cost at a higher temperature versus first cost at the lower temperature gets you a number.

    The payback engine is in hours of operation at the lower outdoor temperatures. You may find that the coldest design bin (in which you are trying to capture the difference between 140 degee average water and 120 degree average water) comprises only, say, 1 percent of the heating season. I suspect payback will be a long time.

    Attached is the weather data for the old Plattsburgh AFB, probably pretty close to you or close enough. See the hours of occurance at each temperature bin as a place to start. See pages 5, 6, 7, 8 and 9 of the attached. Page 9 is a summary.

    The absolute coldest, between -26 and -30 is only one hour on average per year. The 99.6% design temperature of -11 occurs only about 19 hours per average year. The median of extreme lows of -20 occurs but 9 hours. Hoohah.

    Let me know if there is a more nearby location to you and I may have the data. How far are you from Plattsburgh as a point of reference?

    In the end, if cost and wall space matter, you can size the radiators for a higher temperature and put some of that savings into better radiant floor control, which you may have already.

  • jp_2
    jp_2 Member Posts: 1,935
    lets see the math

    the offset in efficiency will cover the extra radiation costs by lowering design temp 25F ?

    I'd like to see his/your math on this one. seems as long as your return water temps are in the range of condensation, 25F doesn't seem like much, how often do you see -30F and how good is the skiing then?
  • Gene Davis_2
    Gene Davis_2 Member Posts: 71
    Colder than Plattsburgh

    Brad, we are doing the work with the nearest village being Ausable Forks, NY, perhaps 25 miles from Plattsburgh AFB, which is in the Lake Champlain valley.

    As I advised my pro (who is a few states away), when he found the only data available was Plattsburgh, our building location is a little higher in elevation and not in the Champlain valley, which typically has milder weather. Saranac Lake NY may be a location where data is available, and my guess is that our location has weather temps right between SL and Plattsburgh AFB.

    My pro seems to be driven by the desire to design a one-low-temp package, and I am all for it. I'm just trying to understand the design parameters, the efficiencies or lack thereof, and the tradeoffs.

  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Pretty Darn Close, Gene

    Understood that the microclimate will differ, but close enough to get you an idea. I figure if the floor temperature water governs then go for it. Here is what I have for NY locations as far as weather data:


    ALBANY COUNTY 725180


    BUFFALO 725280




    JAMESTOWN 725235




    NIAGARA FALLS 725287





    UTICA/ONEIDA 725197

    WATERTOWN 726227

    WHITE PLAINS 725037

  • John Ketterman
    John Ketterman Member Posts: 187

    I would advise designing for 150F supply. That will require nearly double the radiation compared to 180F, and you'll still condense in most circumstances. Going from 150F to 120F is overkill, and will require nearly doubling the radiation compared to 150F. That second doubling will not lead to any significant payback.

    I am assuming you will use outdoor reset, so the water won't be 150F except on a design day.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    I agree

    The radiation is a one-time expense and the hours at the hotter water are limited. Condensing will occur 95% of the time if not more. Still, there are bragging rights to heating with tepid water. :)
  • Uni R_2
    Uni R_2 Member Posts: 589
    Since it's a retreat...

    Since it's a retreat that will probably have an oversized boiler btween 50 MBH and 100 MBH, I'd try and take advantage of that horsepower for allowing the quick recovery of internal temps. Obviously this will help the upper levels more than the slab - low mass can have its advantages as well!

    I'd design closer to 120° than 150°, use a FHV or some other simple device to protect the slab from high temps, and then on occasion you take advantage of those big panels full of much hotter water. Whenever the occupants show up and want to get the place up to temp quickly, the system can then take advantage of the excess power of the boiler.

    Likewise, if they freeze their Speedo©-clad butts off while snorkelling in deep snow they may want to boost the inside temperature for a while when they get back to the chalet. The lower design temps will allow the heating system to be more responsive without hampering the efficiency excessively.
  • ALH_4
    ALH_4 Member Posts: 1,790
    Radiator Sizing

    140°F is the lowest I have ever been able to justify sizing radiators. Even at that, they are nearly twice as big as they would be for 180°F. 120°F makes for some very large radiators.
  • Gene Davis_2
    Gene Davis_2 Member Posts: 71

    Forgive my lack of knowledge. What's a FHV? And what other simple device might be used?
  • Noel
    Noel Member Posts: 177

    floor heat valve.

    It's basicly a TRV with a remote (slab) sensor that will modulate the flow to that zone.

  • Plumb Bob
    Plumb Bob Member Posts: 97

    > nearly twice as big as they would be for 180°F.

    They would be more than twice as big.
  • ALH_4
    ALH_4 Member Posts: 1,790

    For 140°F I plan for 60% of rated output from the radiators. For 120°F I would plan for 40% of rated output.
  • J. Cricket_4
    J. Cricket_4 Member Posts: 1

    > For 140°F I plan for 60% of rated output from the

    > radiators. For 120°F I would plan for 40% of

    > rated output.

    You are right; I was confused.

  • the thing is, making a radiator larger is cheap. the major cost with radiators is in adding more units. but making an existing unit bigger isn't a big jump in cost.

    so if the total difference in cost between a 140+ sizing and a 120 sizing is something like $500, that's not a huge amount of money in the scheme of things, right?

    Even if it's a small efficiency benefit, it doesn't have to be huge to save $500 in a reasonable timeframe, and it's not a huge cost now. so why not do it?

    If $500 is a dealbreaker on the cost of a house, so be it. But in a normal situation, that's not a horrendous price increase to go from good to best (and maximum simplicity in controls).
  • ALH_4
    ALH_4 Member Posts: 1,790

    At 120°F the radiators do get to be relatively large and take over a wall.

    I'm not sure the price difference is only $500. Though it does depend on how many radiators there are.

  • It's slightly more, but the addition of mixing hardware eats up $200 for minimal mixing (pump and tempering valve, roughly, with shut offs/flanges etc) assuming you keep the radiators not TOO far from the slab temps so you won't be operating the slab bang-bang at 120 for half the heating season, instead just running it a bit warmer than needed, using the radiator reset curve chopped off at 120 for the slab.

    If you then need to add reset mixing for the slab because the radiator curve is much higher, well that's $350 and up. Plus more items to wire and more items to die or malfuction.

    Try it both ways sometime... just making them bigger for no reason may not be worthwhile, but if you can match another curve on the system to eliminate water temperatures.. Sometimes it makes sense and sometimes it doesn't.. for smaller areas though, it can. for larger room counts (more units), maybe not as much if multiple rooms have to add extra units. But if you're just enlarging the radiators... not *generally* such a big deal...
This discussion has been closed.