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Best Hot Water Tank Temperature

06honda Member Posts: 15
edited December 2017 in Radiant Heating
My home is a one level slab and is heated with a Natural Gas Direct Power Vent hot water tank for the infloor radiant heating. All is working but I am wondering if my HWT temp is high enough, it is currently set at 120 F. My understanding is the water goes into the floor at or around 115 F - 118 F, based on the thermostat that measures the water in the piping. See image as this has been the reading during the winter when system runs for the past few winters. Just wondering if 120 F is cutting a little close for its operation. Temp in pic is after it has not run for a bit. So would increasing the HWT to 125 F have any benefit or cause the system to work better. Or just leave it at 120 F and increase room thermostats a bit more than they are. Some times it feels a little cool but wall thermostats are usually on the money and steady. Floors do get warm but never stay warm 24/7. It's a little different after having used a furnace for years to get use to it. Other thing is we have lots of hot water so turning it up to 125 F will bring it hot much faster as the house is only 1080 sq feet so we would need to be careful with that. Right now it takes about 25 seconds for the hot water to be felt in the kitchen tap.

** Our wall thermostats in the house are usually set from 66 - 70 F.


  • BobC
    BobC Member Posts: 5,473
    You need 135 degrees to reliably kill legionella, 120 can allow it to grow. I would set the domestic hot water to 140 and add a mixer to brig it down to safer temperatures. This will also allow you to draw more hot water than the 120 setting.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
    bob eck06honda
  • Rich_49
    Rich_49 Member Posts: 2,766
    You are using a water heater for domestic hot water ( plumbing) and space heating ( radiant floor ) ?

    If this is the case we'll all have more questions . There is a right and wrong way to do this successfully .
    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
  • kcopp
    kcopp Member Posts: 4,397
    edited December 2017
    Sounds to me he is using the WH only as heat. Not taking water for domestic use off the tank.
    Leave it at 120. If its not broken why mess w/ it....
    JUGHNE Member Posts: 11,021
    "....30 seconds for hot water to be felt at the kitchen tap"
    Do you mean the kitchen sink faucet? What heats your water for the sinks and shower?
  • HomerJSmith
    HomerJSmith Member Posts: 2,407
    You can't reliably use a w/h for domestic HW and radiant space heating without some form of mixing valve. There is a lot of information that you are leaving out. Radiant in-floor heat must be planned correctly and more importantly done correctly. Size of the tubes, spacing (pattern layout), placement in the slab, floor covering, etc., etc., etc. Very rarely do I see slab temperatures higher than 120 deg in a well planned system.

    You don't want to be adjusting the Thermostat up or down, just set it and forget it. In-floor is a slow acting system, unlike panel radiators and fin-tube BB.

    There should be a heat exchanger separating the radiant from the Domestic Water Heater. In the summer the radiant is not being used and is a dead leg in the DHW system and at those temperatures harbor legionella. Just saying.