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Reno Zone 5A: repair/replace hydronic baseboards or switch to underfloor radiant?

greengalt | Posted in General Questions on

First, thank you for all of the knowledge shared on this forum; it’s been very useful as we’ve improved our home’s efficiency and comfort!

After struggling to find a definitive answer through research, my questions are:
1. Does underfloor radiant provide efficiency gains over hydronic baseboards?
2. Is there a noticeable improvement in comfort?
3. What else I should consider?

HOME CONSTRUCTION
Climate Zone 5A: Southern NH
1989 contemporary with large first floor and primarily cathedral ceilings (see photo) 
– 2×6 walls with R19 fiberglass batts and exterior R5 XPS
– R38 fiberglass batts in cathedral ceilings –inadequate given ice dam issues.
– Small accessible attic upgraded to R50 cellulose fill and air sealed
– Finished basement (drop ceiling), below grade only exterior R5 XPS. (Planning to upgrade with interior XPS where possible)

MECHANICALS
1989 oil boiler (109k BTU/hr) with two zones + indirect hot water HTP tank
– Fin baseboards with polybutylene tubing that is beginning to leak in spots
2020 Mitsubishi Hyper Heat Mini Splits:
– 36k BTU/hr multi head for 1st floor Master BR and 2 2nd floor units
– 18k BTU/hr head in the basement
2021 Mendota propane fireplace (To be installed) rated to 41k BTU/hr output 

OTHER CONSIDERATIONS
– The next project on the list is replacing flooring throughout the first floor giving us the best opportunity to do this project.
– Mini-splits have been great (My room by room Manual J seems to have been adequate) but as the monthly average temp drops to 30F they are approaching the cost of oil. I hypothesize that this is due to their placement high on the wall (in rooms with high ceilings); we set them above 75F to get the room to be ~69F. My hope was the heat from the basement unit would rise to heat the first floor, but even with the basement door open, not enough heat transfer is occurring. A contributing factor is the heat loss through the under insulated below grade walls.
– I had considered removing the baseboards and boiler, but given the recent cold snap I think that is unwise. Given the infrastructure in place I like the redundancy of two heat sources. I’m also unconvinced there is a better solution for our home’s hot water supply than the indirect hot water. I don’t think a HPHW will keep up with our demand–I would prioritize keeping our hot water supply capability over more efficient options.
– Given the drafts in the house, when it’s really cold I prefer the baseboard heat from a comfort standpoint, although we have waited a long time to turn them on.
I plan to upgrade the boiler in either case and may downsize to better match the load.

TLDR: Should I replace my leaking hydronic baseboards with underfloor radiant heat or repair the baseboards and use the money saved somewhere else? If you were me, what else would you consider? Thank you in advance!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Do you have a room by room load calculation to work with? Without the basics there is no way to know if a radiant floor can adequately heat the place on it's own.

    How good is the room to room temperature balance when heating with the existing baseboards? If it's pretty good you can use the baseboard sizing in each room as a reasonable guide for what it would take to replace it.

    >"My hope was the heat from the basement unit would rise to heat the first floor, but even with the basement door open, not enough heat transfer is occurring."

    That's actually a good thing- it means you don't have a lot of hot air escaping out of the top of the house, pulling cold air into the basement.

    Are the basement walls insulated? (They should be- if not, that's far more important than radiant vs. fin-tube from an efficiency point of view.)

    1. greengalt | | #2

      Thanks Dana!
      1. Yes, room by room is attached. If I go radiant I plan to pay someone to design the zones and conduit layout.
      2. Fairly good temperature balance across the house. The three rooms that were the coldest (basement, master bedroom, and bedroom over the garage) got three of the four head units.
      3. Great point about basement heat rising. I didn't think about the chimney effect.
      4. Above grade basement walls are R19+R5, below grade which is 75% of the space is only exterior R5. Adding insulation to the inside has been spec'd, just need to get it scheduled.

  2. Expert Member
    NICK KEENAN | | #3

    I would think the most economical solution would be to fix the existing baseboards. Are the baseboards leaking, the fittings, or the pipe?

    It's a complicated question whether underfloor heat would be more efficient, but I would say probably not. Underfloor systems typically run at a lower water temperature, and with certain boilers that means they can run more efficiently. But if you're keeping the existing boiler that probably isn't a factor.

  3. greengalt | | #4

    Thanks DC! The baseboards themselves have developed pinhole leaks and since the PB tubing is prone to failure (and obsolete) I plan to replace that as well. Agreed it's the most economical option. I just want to confirm before I shut the book on radiant.

  4. charlie_sullivan | | #5

    With your existing boiler, the ability to run floors at lower temperatures doesn't affect efficiency much. Eventually, we'll need to shift from using fossil-fuel fired boilers to heat pumps to avoid climate catastrophe. At that point, you will get an efficiency benefit by lowering the delivery temperature. You might wait and see what incentives become available to make that transition before you spend money on a new oil boiler.

    Another advantage of floors for heating is that your furniture positioning become unconstrained. You can block some of the baseboards without a problem but you start to lose efficiency what you start blocking too many.

    If you haven't yet bought your XPS, please consider a more climate-friendly option--ordinary XPS is by far the worst building material in that regard. Options include:

    * EPS instead of XPS--a little lower R-value per inch, but you can get thicker and still come out ahead cost-wise. Or get graphite-infused EPS (e.g. "neopor") to about get the same initial R/inch as XPS, and better long term.

    * Reclaimed XPS, which you are diverting from a landfill.

    * Polyiso

    * Newly available (by special order in NH) "NGX" from Owens Corning, a version of XPS made with a more climate-friendly blowing agent. It's still worse than the options above on climate impact, but if you are set on using new XPS for some reason, it reduces the impact by a factor of 10.

    1. greengalt | | #6

      Charlie, great call on the XPS! I double checked the quote for the basement insulation and polyiso is spec'd.
      It sounds like there's no efficiency advantage to switching to hydronic now, but there could be in the future. Thanks!

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