Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Newbie Hydronic questions

Icarus Member Posts: 143
Newbie here,

I have been reading through a number of threads and have found them to be most helpful. I am a semi retired general contractor. My current project is a new (to me) 1895 house. It currently has an old boiler in an out building, running two zones through fin tube. I want to install a E50C Rinnai combi boiler. I want to run the water temp cool enough to condense as much as possible. I have figured to run the loops at ~150f. I will have about 4 zones including 2 infloor radiant . So here are the first of what will be (I suspect) many questions.

I have done a heat loss/heat load calc. and need between 20-30k BTUs. (temperate climate, Pacific NW) the house has high ceilings, but is pretty well insulated given its age. There is also a 30K btu free standing fireplace.

I have figured a derate for most emitters of about .69 to account for the lower water temp. Is this a good ball park number? Given a choice, would you all agree to go with some sort of radiators instead of fintube? Am I better oversizing any emitter than undersizing?

My first choice would be traditional Cast iron radiators, but there are few used ones in our neck of the woods, as they are not common. (Forced air is common!) What is the best brand/design style at a reasonable cost. I like the look of the Hudson Reed traditional school house radiators, but Im open to anything.

I guess that is all for now,

thanks once again for any help,



  • Icarus
    Icarus Member Posts: 143
    more newbie questions

    Just an add on to the thoughts above. My other alternative is to do a air source heat pump, running at low temps, with the Rinnai as a back up. thoughts
  • SWEI
    SWEI Member Posts: 7,356
    Radiant floor heat

    should not need anywhere near 150ºF if done properly.  I would suggest designing the RFH and then sizing plate radiators to match that if possible.  Otherwise, you will need two reset curves.
  • Icarus
    Icarus Member Posts: 143
    Sorry for the reply delay

    Sorry for the delay in responding, I had trouble with signing in.

    I'll confess my ignorance, and have to ask, what is RFH?

  • Rich_49
    Rich_49 Member Posts: 2,721

    is Radiant Floor Heating
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
    PLUMMER Member Posts: 42
    Verify boiler efficiency first

    Be sure to verify at what temps and firing rates the boiler is efficient. I seen a chart for a WM mod con, and it was most efficient or meets its efficiency rating at full fire rate and output of 85-95 degrees. Lower turn down firing rates dropped off more than I expected, below the 91%. And as soon as the output temp went over 118-122 degrees it was back down to 90 and dropping.

    If someone can post one of these charts for reference, tilli find one that's eye can post or link. My numbers are not exact, going by memory, but not far off.

    Fin tube derate of .38 at 140 degrees is on their site charts. Good job pre planning your project
  • Icarus
    Icarus Member Posts: 143
    Rinnai ef curve

    the boiler that I am considering is this:


    I can't find any ef numbers relative to incoming water temp, but since it is a mod con, I would assume that as long as the temp was low enough (sub 140F) it should be just as efficient at any firing rate.

    Since the total high output capacity of the boiler is well in excess of the BTU load of the house, it would seem logical to design the emitter output for each room based on the worst case situation. That said, since worst case (~10f) is rare, and there is the fireplace perhaps undersizing a bit might be a bit more efficient?

    Would you suggest zone valves or TRVs for individual zone control?

    Once again, thanks for the help, Im sure I will need more advice as the time comes closer?


    PS Thanks for the definition
  • Icarus
    Icarus Member Posts: 143
    Outlet-inlet temps

    So, to carry the conversation on a bit. If i were to use a 13-140 outlet temp, and try for a 120 return temp, would I be happy with fin tube, flat panel or conventional radiators, (sized properly) or is the loop temp just too low?

    Of course reading as much as I can, it seems very much of the advantage of a mod con goes away if you can't have it condense as much as possible. Given my climate, (average winter temps in the 40s ) with an out door reset it should be able to do. The question is will I be happy with it?

  • SWEI
    SWEI Member Posts: 7,356
    Water temp and efficiency

    Your system will be simpler if you can design for a single water temperature instead of two.  Size the RFH first and then pick radiators using that supply temp.

    Radiators (either panel type or cast iron) work far better with low water temps than fin-tube baseboard will.

    With a mod/con boiler, lower return water temps produce higher combustion efficiency.  Lower firing rates (burner modulating down) also produce higher combustion efficiency.  Condensation happens as a side effect of this higher efficiency.  There is no "quantum leap" when condensation starts -- but instead, a continuum of roughly 86% to 98% depending on the conditions above.
  • Icarus
    Icarus Member Posts: 143

    I am beginning to lean towards doing a simple electric radiant in the bathroom (the only in floor heat. I will probably then design the boiler temp for in the existing fin tube, and then change out the fin tube to radiators as time and money allow. There are two existing loops of fin tube, and an ancient Vailent boiler(50k) keeps up fine, with a temp of 170. I should be able to dial down the water temp from the Rinnai to suit. I assume that the outdoor reset would help with that.

    If i did go with in floor hydronic in the bath(s) would a 3 way mixing valve be a pretty good alternative? What is the ideal temp for in floor radiant under tile, for a small (40 sq ft) area?

    Once again thanks for all the help and input,

  • Snowmelt
    Snowmelt Member Posts: 1,387

    Yes 3 way valve is what you need to damper down the temp. What material are you putting down on the floor as far as temp it's a big formula of what you want there is no specific answer.

    You want your floor surface area 75 degrees

    Ultimately you want your subfloor 115 but can be as high as 145 degrees.

    ps temp = (btu/square foot / 2) +SP temp.
  • SWEI
    SWEI Member Posts: 7,356
    Sizing and such

    Start with a room-by-room heat loss.  Then measure your existing baseboard and see what water temp it needs to be happy on a design day.  If that number is high, you might consider augmenting or replacing it at some point.  If you're going to use panel rads for the rest of the house, I'd suggest sizing them at 140ºF if practical.

    Does the bathroom have any exterior walls?  If so, electric radiant could cost a bit to run.  Back to the heat loss calc -- be sure to bump up the indoor design temp on the bathroom a bit, say 75ºF.  Radiant towel heaters are one of the finer things in life, even in summer.  The electric ones cost about 1/3rd of what the hydronic ones do, but they can run all year without firing up the boiler.  If that bath has exterior walls, you might want both heat sources in it.
  • Icarus
    Icarus Member Posts: 143
    Still thinking

    I would only use the floor heat to warm the floor, using a Rad or towel bar hydronic to essentially heat the room, so the electric shouldn't be too spends to run. The room has no outside walls. It will have ceramic tile on the floor. I was looking at warm board, or Upnor system.