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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?