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Sizing pipes for panel radiator system

I'm an American living in Slovakia. After ten years in the countryside here, we've gotten tired of maintaining fires in wood stoves to keep warm, so I'm designing and installing a radiator system to be driven by a condensing gas boiler. I don't really have a good way to find a pro to do it (my wife's aunt hired someone to do their house, and they have a kitchen that is always too hot and another room always too cold). For better or for worse, I've been a DIY guy all of my life, and studied electric engineering, so I've been studying everything I can find now (in English, Czech and Slovak), and I think I've got most everything figured out, as best as possible.
My main nagging issue, is that while I've designed flow rates with a standardized local "average lowest winter temperature" (-13°C), and dimensioning radiators according to water temperature 55°C/45°C, which is what is used in new installations in most European countries now, but leaves me the option of upping the boiler output temperature in the event that we get a severe cold (which has happened twice over the last decade), but the flipside is, when its fall or during mild days in the winter, is it really an issue that the velocity of water in the pipes will be low? Because I've read warnings that it needs to stay above a certain amount to keep air bubbles mixed in the water, but is this really a big issue?


  • mattmia2
    mattmia2 Member Posts: 9,637
    You can use this style of radiator valve to connect the radiators, it will allow you to keep a higher flow in the loop and will let you adjust to balance the radiators:


    There is a minimum flow you need to maintain through the boiler, if it isn't primary-secondary, your loop flow needs to maintain that.

    If you set up the near boiler piping so that you are pumping away from the boiler and you put a microbubble resorber type air eliminator between the pump and the boiler, any trapped air will eventually dissolve and come out of solution at the microbubble scrubber as long as you purge or bleed enough air to get the system circulating.
  • mvickers
    mvickers Member Posts: 30
    Cast iron radiators?
  • rus1107
    rus1107 Member Posts: 1
    Hi! I am living in Europe, and familiar with hydronic systems, because I am designing it.
    Usually we calculate heat load for the coldest period of the year, further based on this data we select heating emitters (radiators, underfloor heating systems etc.). In your case heating load calculated for 33°C difference (20°C inside the room and -13°C outside). Logically, if outside warmer than in design day we need proportionally less heat. We have 3 methods to control heat output from the emitters:
      1) Variable flow rate
      2) Variable temperature rate
      3) Combined
    For your comfort, I recommend using a combined method. This will work next way - on the radiator we install a thermostatic head (restricts the maximal temperature in the room), for the boiler we install the outdoor reset control. Outdoor reset control will automatically change the flow temperature which goes from the boiler to the heat emitters. Therefore over a year we will have near the same flow rates (and enough high velocities too) and comfortable temperature inside the rooms (thanks to thermostatic head).

    P.S. If you have cold and hot rooms it means that the installer does not preform hydraulics balancing for correct heat dissipation.