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Radiant Panel Sizing

Brad White_63
Brad White_63 Member Posts: 24
The hours of maximum temperature operation are few enough that with outdoor reset properly tracking, you will still save fuel the vast majority of the time. Find your "ouch" point and maximize your radiation but don't sweat it if you do not get them all down to operate at 120 degrees. Just make sure that all radiators are proportional (IOW all work at the same water temperature to meet their room's heat losses).

What your question illustrates is not that panel radiators get large, -they do- but that radiant floors (the biggest radiators you can get) are really the best use of ModCon boilers. Panel radiators are, IMHO, the next best thing.

You have discovered not so much a disconnect but a point of diminishing returns.


  • Heatmeister_2
    Heatmeister_2 Member Posts: 88
    Radiant Panel Sizing

    As I research just how much heat I will need for output AND try to keep it low enough to increase efficiency, I have dicovered a disconnect.

    MOD/CON boilers really work best in low operating temp environments, so designing the panels around lower temps means MUCH bigger sizes than what are quoted by the manufacturer because they quote them at anywhere from 180-215 Degrees depending on the brand. But if you design around 140 degrees to get the big benefits of MOD/CON boilers the panel size goes up from 2-3X's the size. That's right 2-3X's. Cost increase in not comensurate with size, but at some point you gotta say......"Man that is a big panel"....... big and obstrusive. My bedroom has a load of over 9100BTU. I would have a huge panel in there at lower operating temps. If you design at higher temps like 180 degrees to keep the size down that then rules out Vitodens which is what I was looking at getting. Does that then diminish the advantages of high efficiency boilers.

    What are the Pros using as design temps for Radiant Panel applications. If 180 is being used......should I reconsider the boiler type and stick with cast iron. If so should I just then stick with inexpensive baseboard too???

    I am not to keen on have a 3foot by 5foot panel under my window in order to get an efficent design temp at 140degrees. Copper fin baseboard and castiron baseboard are much less abstrusive (and inexpensive) albeit less efficient. How much efficiency do you think I will give up going with higher temps in either application??

    FYI I did a repour in my basement so I will be running low temps there.

    Also what other MOD/CON Boilers can run at high temps say over 170 or more?? I hear Trinity can go to 200.
  • Ron Schroeder
    Ron Schroeder Member Posts: 998

    Why not split the difference, the bigger you make the panel the more often you will be able to get the advantage, just remember you only need max output 5% of the winter.
  • Heatmeister_2
    Heatmeister_2 Member Posts: 88
    Good point

    So, In my situation, given that the home is 1950's with Thick floors 1 1/2" (subfloor with oak flooring), how often you would then recommend to the home owner to combine retrofit staple up with panels vs. just panel or copper fin. B/C diminishing returns factor in on cost of installation as well.

    On a relative basis, panels are cheap vs. staple up(materials+labor). Staple up should achieve 1/3 needed BTU output, But will cost much thinks?
  • ScottMP
    ScottMP Member Posts: 5,884
    I may not have this right But

    If you size the panels for 180 at worse case, you'll only hit that a fews weeks a year. The rest of the time they'll be running at lower water temps with reset. That puts you in the condesning mod.

    Is that not right ?

    By the way, talking cost. I think you need to get a quote for Cast Iron baseboard :) It aint Cheap.


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  • stephen_4
    stephen_4 Member Posts: 22

    use the vitodens.. use 160 design temp at your coldest degree day...that does not make the panel to big and you will only have a couple of days a year it hits that cold condensing all of the rest. this winter in our area it never even hit coldest day it would have been condensing almost all season.
  • Brad White_63
    Brad White_63 Member Posts: 24
    180 Degree Water

    Yes, you are right, Scott, a ModCon will work reasonably with 180 degree design temperatures (176 or whatever the maximum is) most of the time. The design temperatures only occur about 1% of the time, certainly not over 5% and the range where proportional reset gets you below condensing (130 degree return water at the very least) is better than 50 percent. Not great, but there is a benefit.

    The key is not that you can run less than 180/160 supply return, but how often can you run below the 130/110 S/R line where condensing really occurs.

    My design philosophy is to extend that range to more of the year and not settle for 50% of the time. I like 100% of the time if I can, and who would not? Fact is, most reset schedules are not as aggressive as they could be (see Mike Thies work). But if I can design for 140 degree supply water or average water on the coldest day, I am fairly happy and the radiator sizes are rational. Bigger, but rational. This of course is absent radiant floors where I can use 120 degree water or less.

    The Brookhaven National Labs study (regarding 180 degree fin-tube) was to illustrate and prove that there is a ModCon retrofit market with payback benefits. It did prove it but with provisos that the reset be made. I saw that study as being a "feel good about having copper baseboard, there is hope", but would not take it as the way I would design if I had a blank slate to start from.
  • Brad White_63
    Brad White_63 Member Posts: 24
    Thick Floors!

    Yes, you could use a plate and staple up system but as you note, there is less benefit for cost than other means. Nice supplemental touch if you can do it, but you have over 2 inches of wood with an R value of over 2.5 even if no carpet. Not bad, just that balance point of warmer water versus protecting your floor.

    Panel radiators are your best bet for the cost, IMHO.
  • Dave_22
    Dave_22 Member Posts: 232

    You could always break it up to 2 rads in that room. I have done it- and you don't have 1 giant rad to stare at. I designed at 170/140. Never saw it run that high though- 155 on the coldest day with a Buderus GB-142 and room sensor.
  • Tony_23
    Tony_23 Member Posts: 1,033

    The Ultra will go to 190. I think 176 is from the GB series with the stock control.

    I have multiple bbd retrofits w/ Ultras that avg 40% reductions in consumption. One commercial install that replaced two CI "pin" style boilers w/ one Ultra230 and re-piping of the zone/circ distribution for proper flow that is now using 1/3 of their former usage :) Due to setbacks deeper than I'm comfortable with, coupled with short recovery requirements, I didn't use the ODR. I wonder how a different control strategy using ODR and lesser setbacks would affect the consumption, but they won't let me change a thing :)

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  • Brad White_63
    Brad White_63 Member Posts: 24

    is also from Viessmann Vitodens and Monitor MZ, probably arbitrary to coincide with a Celsius equivalent so could be off a degree or so with a soft conversion. It all gets down to flow rate. I think the manufacturers like W-M that acknowledge 180 or 190 degrees know their market -proper retrofits that will save where they can but serve where they must.

    The setback versus reset debate rages on. I am going to side with Mike T. next year and keep the house at a more constant but perhaps lower temperature, setback only for long absences and TRV's on all radiators. Save recovery for the gas log stove :)
  • Ted_9
    Ted_9 Member Posts: 1,718

    You could alway work backwards. Start with what size you are willing to have in your home and do the math to see if they work to heat the space.

    check out this excel program I found from someone on the wall.

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  • i like that sizer!

    to answer Heatmeisters original question though its been my experience that panel rads don't get too terribly large when sizing for low temp:

    multiply your rads reported output @180 deg by .605 to get what the output would be at 140 deg.

    and if they DO get too unweildly for acertain area most manufacturers offer double panel rads for greater output in a smaller package.

    and for the record, condensing boilers will still condense at higher operating temperatures as long as they see low return temps/high delta tee.
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