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Variable speed pumps

Simply Rad
Simply Rad Member Posts: 176
I installed a Grundfos Magma 32-100 pump for a full house radiant floor system and I am amazed how little energy is used to distribute so much heat! The system is currently pump 16 GPM@ 13' and is ONLY using 63 watts!!!!!!! This is incredible. These pumps are huge energy savers considering I am using continuous circulation distribution. The home is 10,000 ft2 with a heat loss of 160,000 btus. Amazing to think an old school 100 watt light bulb uses more energy than the system pump. Also, something to consider is the fact I have been using the Viessmann Vitocrossal CU3A medium mass boiler that I use direct pumping. Meaning, the pressure drop is so low, non restrictive and has 20 gallons of storage, that I am using one pump for the radiant floors and one for the DHW. Super simple and highest efficiencies since there is no mixing the return water....direct return. I love this boiler and system

Cheers Jeffrey
Jeffrey Campbell


  • kjb1057
    kjb1057 Member Posts: 1
    Right on! Your a very smart individual
  • hot_rod
    hot_rod Member Posts: 20,828
    Nice work!
    It's nice to have access to all that data also.

    I sent you a couple PM about that radiation sensor, not sure you received them?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Shahrdad
    Shahrdad Member Posts: 119
    Jeffrey, how many BTU's does your Vitocrossal have? I have a 123 year old house with an old gravity system and a 1970s cast iron boiler (196,000 BTU output), and I'm considering changing to the Vitocrossal. Could you tell me more about your system? I have cast iron radiators that are lukewarm most of the winter and the house stays very comfortable. I know the biggest Vitocrossal 300 CU3A has an intake of 199 BTUs, which at 95% efficiency should be an out put of around 190K BTUs. I think that should be enough for my old house, but not totally sure. I see contractors installing huge boilers for houses way smaller than mine in my neighborhood, and I don't want to go that route.
  • Jamie Hall
    Jamie Hall Member Posts: 22,196
    It might be better, @Shahrdad , to start your own thread. Your question isn't really about variable speed pumps, after all, but replacing a boiler -- a very very different project.

    That said, you are clearly guessing at what size boiler you might need. There is only one way to determine that: do -- or have done -- a Manual J heat loss calculation for the house. Slant/Fin has a very nice calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/ which is remarkably accurate -- and easy to use.

    You are quite right in thinking that too big a boiler isn't a good idea. In fact, it is a financial catastrophe. Do it right.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Robert_25
    Robert_25 Member Posts: 522
    I installed Grundfos Alpha circulators in my system this fall.  I have been very surprised at how little power they use.  I think lots of systems are over pumped.