Radiant recommendation

Discussion in 'HVAC Heating & Cooling' started by Tesla1957, Oct 17, 2020.

  1. Tesla1957

    Tesla1957 New Member

    Oct 17, 2020
    Dexter Maine
    I have a Rinnai V65 propane water heater, it's 150K BTU in Dexter Maine. The current heat source is oil forced hot air, which is horrible. I have radiant heat in a home in Rockwood Maine which I installed 15 years ago as a kit which I bought from Radiantec. I installed and then hired a professional to check and then start up system. I want to remove the FHA system and install the radiant heat using current water heater. Just 2 of us, 1150 square foot one floor home with a basement, will heat the basement in the future. From my calculations the WH is more than enough but I'm concerned about the cycling. My plan is just one zone for whole house with 250 foot circuits. 3 bds, 1 bath.
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Most DIY radiant systems (including Radiantec's cookie-cutter designs) based on tankless water heaters are insanely over pumped and over-fired resulting in excessive cycling. With low thermal mass radiation such as staple-ups or WarmBoard etc cycling is going to be worse than with a radiant slab or radiators that have some water volume to work with.

    The first order of business is to get a handle on the design heat load. It's possible to measure the heat load reasonably accurately using wintertime fill-ups and weather data between fill-ups on your existing hot air system using the methods outlined here. (Take a free trial subscription to read the details.) If you are willing to share your fill-up dates and amount, thermostat settings, and a ZIP code I can run that napkin math on the forum here. You can use Bangor's 99% temperature bin of -2F as the design temp, or maybe -5F, but not any colder than that (yes, I know it gets quite a bit colder than that, but less than 1% of the hours in a year.) Upsizing the output of the system by more than 20% from the load at the 99% design temp leads to more cycling and lower efficiency, for essentially a zero uptick in comfort.

    A typical reasonably tight 1150 foot 2x4/R13 house with clear glass double-panes over a 1150' full basement (not a walk-out), with NO foundation insulation will come in around 25-30,000 BTU/hr, or as low as 20,000 BTU/hr if the basement walls are tight and fully insulated. A tight 2x6/R19 with low-E windows and an insulated basement would come in under 20,000 BTU/hr. But run your fuel use numbers (wintertime use only) against heating degree-days as a sanity check. If you are on a regular fill-up service that stamps a "K-factor" on the billing slips, a few wintertime K-factors is enough information to put firm bounds on the potential heat load.

    I'll wait for the load numbers rather than chase down any number of potential dead-end paths.
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