Hydronic radiant floor help please

Discussion in 'HVAC Heating & Cooling' started by Mark Cralen, Jul 7, 2014.

  1. Mark Cralen

    Mark Cralen New Member

    Jul 7, 2014
    I'm trying to install an open hydronic floor heating system to heat a 300 sq ft room in my basement using my existing hot water heater. I followed a plumbing schematic I found online, but it does not work. The pressure gauges in the system are maxed out at 75psi, so the pressure is higher than that. If the pressure reducing valve is on, I get no flow at all and the hot water pressure in my house is reduced. If I turn the pressure reducing valve to bypass, I can get flow of about 0.5 gpm, which is too low and under conditions that are not right for the system. The schematic is posted below.

    Here are my questions:
    1. Is there anything I can do to make this open system work better?
    2. Should I give up on the open system and install a heat exchanger? I think this will allow me to drop the pressure in the heating system and maintain the pressure in my house.
    3. Is there something else obvious to an expert that I am not seeing or forgetting to ask?

    Thanks for any help you can give.

    Attached Files:

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    I was hoping one of the pros would jump in...

    But, I'm pretty sure that the cold supply is not shown properly on your diagram, it does probably go to the WH directly.

    Your diagram shows that both the heating loops and the normal house hot water are fed from the same tank. This can be a major issue, as in the off season when you're not heating the area, you'll have stagnant water in those loops. Also, to work on an open system like that, you cannot use the normal heating circulators and fixtures...they'll fail because of the dissolved gases in the water supply (it's mostly the oxygen, but CO2 isn't great, either). A WH can be used as a cheap boiler for heating purposes, but unless it was specifically designed for it, isn't a great choice.

    What size piping are the heating loops? WHat size pipe is feeding it? NOte that if there is any air trapped in the loops, it is very hard to impossible to get it to circulate properly, but street supply pressure should be plenty to force the water all through the system. You may need to add a drain/purge valve to get that flow while locking the zone valves open initially, but since there is always some dissolved gases in the potable water, it may accumulate there again. You may need an air extracter, something like a Spirovent.

    I'm not certain, but I do not think what you have would pass code, and those loops can harbor bacteria and start to grow things that you will not want in your supply going to the house fixtures. So, I think you have more problems than you realize.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    I'm guessing he has a crimp in the line (or a gazillion feet of 3/8" PEX) to only get 0.5 gpm of flow even with the PRV bypassed.

    The symbol used in the schematic for the PRV looks like a temperature & pressure relief valve, which is something SHOULD be at the hot side of the hot water heater, but the outlet of it should be to a drain, not in series with the loop. Putting the PRV in series with the pumped loop is guaranteed to keep the pump from providing any significant flow.

    If the static pressure of the cold feed is 75psi+ should be reduced to something "reasonable" like 25-50psi on the house plumbing, but located between the incoming cold supply and the system, not in series with the loop.

    But if you have slow/no potable hot water flow when the valve is bypassed as-configured, you have a hard restriction somewhere in heating loop. Putting a heat exchanger in the system and keeping the heating loop out of the potable path will help your slow potable hot water condition, but it won't overcome the heating side restriction.
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