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Pros and Cons of Radiant Floor Heating

Usafbair | Posted in Mechanicals on

I live in Western New York (Climate Zone 5) in a 1880 balloon framed, stacked stone foundation ~3000 sq ft farmhouse.  Currently the house has a hydronic system with a brand new high efficiency boiler, Amtrol Boilermate indirect water heater and cast rads.  I also recently installed a wood burning insert that is able to solely heat the house for approximately 3 months a year, and augments the boiler the rest of the year.  The house has clapboard siding covered by cement asbestos shingles, there’s no sheathing, true 2×4 walls with poorly blown in cellulose insulation, almost total lack of attic air sealing and insulation.  As you would imagine the house is quite leaky/drafty.  There are significant and obvious improvements that will be made with the envelope and insulation, but it will never qualify as a well insulated/tight home and that’s okay.

We recently went over to a neighbor’s 1840 stone farmhouse and were enjoying the warmth of his newly installed radiant floor.  I had initially dismissed this as a possibility, but the experience has me reconsidering.
My basic idea is to run pex radiant floor heating along the floor joists between the basement and the first floor.  The pex would be attached directly to the underside of the finished flooring on the first floor (the flooring is pine T&G w/o subfloor).  The joist bays are open and easily accessible in the basement, it’s a full basement and will not be finished.  I’ve been working in the basement and it’s relatively dry (40-55% typically) and cool with a constant temperature around 50-60 degrees.  I would run the system in a few zones.  I would likely not install radiant in the second floor as I’d have to rip out quite a bit of structure to install.
I think this plan has several advantages from what we currently have.  Chiefly, it’s hidden and would provide even heating.  As I mentioned, we currently have cast iron radiators.  I actually like the look of them and an alternate plan would be to refinish and keep them in place.  The drawbacks are they take up a lot of space and provide uneven heating.  Radiant floor solves both of these problems.  Additionally, given the specifics of my installation I think this would be fairly easy to install and maintain.
A few questions:
-I’ve seen some articles/conversation on GBA and similar forums dismissive of in-floor radiant as it doesn’t make the floor noticeably warm for a space like a bathroom or kitchen (which is typically why people install them), and there are easier alternatives to install (like baseboard).  What do you think of the heating system for an old leaky farmhouse?
-From an environmental/efficiency perspective is this a green way to heat the house?
-Is there need and would you recommend installation of a radiant barrier or insulation below the runs of pex in order to “direct” the heat to the first floor?  The intent is definitely not to heat the basement.
-Any good resources on the specifics of installation for a system like this?  Size/type of pex, number of runs per floor, other recommended equipment for the system, etc.
Thanks everyone.
Andrew

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Replies

  1. Paul Wiedefeld | | #1

    Hi Andrew,
    A lot here! Some top level questions before the installation stuff:
    1. What’s the heat loss of the first floor and what’s its sqft? There’s a maximum btu/sqft a radiant floor can do. If your cast iron radiators are generously sized, they could hit similar efficiency numbers (comes from low water temperatures).
    2. Would you consider insulating the full basement instead? You’ll insulate under the pex if you install it, but insulating the whole basement will make the basement warmer and likewise bring the floor temp up without a radiant floor. Likewise, would help the cast iron condense more.
    3. Would you consider adding floor registers for a heat pump on the first floor? This is in regards to the “ From an environmental /efficiency perspective is this a green way to heat the house?” question. The short answer is not particularly, the long answer is it depends, but not particularly. It offers slightly higher efficiency if the cast rads can’t use condensing return temps the whole winter. Radiant floors would help if you ever switch to an air to water heat pump, which are niche right now. An air to air heat pump is the green way to heat a house. A wood gasification boiler is too.

    The technical details are much easier than these! Caleffi Idronics is a great place to start.

    1. Usafbair | | #2

      Paul,

      Thanks for the detailed reply, I appreciate you taking the time to help me work this out.

      1. I am unsure of the specific heat loss of the first floor. We've got original single pane windows on the house with the standard triple track storms. We did get a blower door test soon after moving in, the initial test resulted in an error due to inability to de-pressurize the house. After closing a few doors it resulted, I can't quite recall the exact number but it was horrible. The first floor is approximately 1650 sq ft.

      2. I have gone back and forth with the idea of insulating the basement. A couple issues/misgivings - the house is balloon framed but the sill plates are timbers. I remember an article/posting (I believe by one of the bigger names on the site) discussing basement insulation or old house insulation that did bring up some concerns for timber rot as a result of cut and cobble insulation. There is a rat slab on the floor and it is not insulated, no current plans to rip that out and replace. The other concern would be insulating a field stone stacked foundation. A lot of the discussion I have seen for basement insulation are typically for a CMU, block, poured concrete wall. I'm unsure of the practicality of doing it on a field stone foundation. Plus I'm unsure if I want to go that route for aesthetic, historical preservation reasons.

      3. I'm definitely open to the idea and for a lot of the reasons I indicated in the original post it would be easy to install the registers and the rest of the structure for a heat pump.

      That being said, if I was going to install a new system I'd be really interested in the ROI and how much of an advantage from an efficiency/green perspective the additional system would provide. I know that's such a tough question to answer because of all the specifics that I haven't provided. But in general terms - if the new heat pump is 10-15k vs 1-2k (really not sure if that's correct, I'd do most of the install myself) for the radiant floor am I going to make up that difference over how many years in savings with the more efficient system? How much better is the heat pump for the environment?

      Thanks again.

      1. Paul Wiedefeld | | #3

        Thanks for the detailed reply!

        https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
        If you can isolate a winter month without much wood burning, you might get close using this.

        The radiant floor materials bill is going to be rather steep: ~2000 ft of oxygen barrier pex, about the same for transfer plates, insulation, a manifold, a mixing valve to supply the floor with a different temperature than the radiators upstairs and a circulator (if a single zone, otherwise many thermostats and either zone valves or more circulators). It can totally be done, the Idronics series can guide the way.

        The payoff emissions wise of a heat pump is huge in NY, which has a very clean grid: at about .4lbs CO2 /kWh, a heat pump is about 66% less polluting.

        1. Usafbair | | #4

          I calculated out the heating load, let me know if my math was off:

          From 1/20/2020 - 2/20/2020 we used 303.6 therms (w/o wood burning)
          We have a Navien NHB-150, AFUE is 95%
          99% outside design temperature is 5
          303.6 x 0.95=288.42
          288.42 x 100,000 BTU/Therm = 28.842 million BTU
          HDD for this period 1140.5 for 65 degrees
          HDD for this period 985.5 for 60 degrees
          288.42 MMBTU / 1140.5 = 25,289 BTU per degree day, 1054 per degree hour
          288.42 MMBTU / 985.5 = 29,266 BTU per degree day, 1219 per degree hour

          1. Usafbair | | #5

            I took a look at the data I've kept for Navien boiler pre wood burning insert and post. It's a little variable but burning daily decreased Therms by between 20 - 50% depending on the month.

          2. Paul Wiedefeld | | #7

            Just to clarify, that was 303.6 therms and you didn't burn wood for that month?

            If so, that puts your heat loss at ~63kBtu. At about 19 btu/sqft for the first floor, a radiant floor can do that. However, with the blown in insulation and storm windows, you could have sufficient cast iron radiation already to run low temperatures, in which case the radiant floor wouldn't add any efficiency gains. Idronics 25 shows how to take an inventory of the house's existing radiators.

  2. Expert Member
    Akos | | #6

    The reason you see people steered away from radiant in high performance homes is that heating loads are so low that the slab never gets warm enough to be noticed.

    I think your place falls into the opposite issues. Looks like you are close to 30 BTU/sqft which is well above the max of a radiant floor setup. At the temperature you need to heat the house, you won't be able to walk barefoot on the floor never mind warping the heck out of your flooring.

    If you can get the house down to around 15BTU/sqft radiant heat can definitely work. A well set up low temp heated floor would reduce your return water temperature and get you a couple of percent extra efficiency on your modcon. It would never save enough energy to have any ROI.

    If tackling the heat loss of your place is too much, an in-between option is to keep some of the larger rads in the house and eliminate some of the smaller ones that are really in the way. You can than replace the heat lost in those section with radiant floors.

    For adding on to an existing high temperature setup, I find UltraFin works great as you don't need a separate low temperature heating loop. I've even had luck with directly replacing a rad with a parallel run of pex. As long as you watch the pressure drop, you can get enough flow rate to work without needing a separate pump or zone manifold.

    1. Usafbair | | #9

      Thanks for the reply Akos. Can you explain how you and Paul are getting the heat loss number? Also there was a significant difference in both of your numbers what accounts for the differing amounts? The house is approximately 3300 square feet total, 2 floors with a unfinished 1600 square foot cellar/basement.

      The plan is definitely to improve the envelope. I'm going to air seal the attic and add an appropriate amount of insulation. Honestly, the current R-value of the attic insulation is probably around R-6 as it consists solely of an extremely dirty/compressed single layer of very old fiberglass batt insulation. Just bringing that up to code will probably make an enormous difference.

      1. Expert Member
        Akos | | #10

        Paul is correct, I used the wrong balance point.

        The problem still is that in an older leaky structure, most of your heat will be supplied by the main floor emitters, you won't get there with floor heat. I've tried this and had to air seal and add additional insulation for the floor heat to carry the place.

        You need to get the house in a better shape for it to work.

        If you go through and check your main floor rad output and the max water temperature you see in the winter time to see how far off you are. For wood floor you want to limit the max surface temperature to around 80F.

        If the existing system keeps the rooms somewhat even temperature, you can compare the BTU output of the main floor and 2nd floor rads to see how much of the overall load each supplies.

  3. Usafbair | | #8

    Paul to your reply in #7, you're correct the 303.6 was without burning.

    For January 2020 without wood burning therms 194
    For January 2021 with wood burning therms 101

    For February 2020 without wood burning therms 303.6
    For February 2021 with burning therms 235

    Thank you for the idronics recommendation I was looking through their hydronic basics document (#4 I think) that's quite the resource.

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