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Replacing hot water radiators with under floor radiant?

Hello, we are redoing our (very old) home room by room and one project I am considering was replacing the old hot water radiators with a plex system under the floors.

Can I simply swap out the radiators as I go? (yes, I know "simple" is a dangerous word here). I mean, can I leave the boiler and supply and drain pipes as is (large copper), and simply pop the radiator out and run under the floor instead? This would allow me to continue to use most of the existing system and the long delays between each room wouldn't be an issue (we're gutting each room to studs).

I presume at the least I'd need a valve to bleed the air out, same as each radiator on the top floor has.


  • Gordy
    Gordy Member Posts: 9,546
    edited February 2015
    It sounds simple but it's not.

    First you need to do a room by room heat loss to be sure the radiant can carry the load of each room with out supplemental heat.

    Radiant uses lower water temps than a radiator based system so the supply water needs a mixing valve to reduce those water temps to the radiant floor panels. Doing this room by room over a lengthy time frame would require repiping every time you switched a room over. Since you have to maintain higher water temps to the rads to get their full output.

    Next you have to decide on the type of radiant panel system. Over the top with plates gives you higher output with lower water temps. The water temps increase, and output decreases as you move that panel system below the floor, and remove plates from the equation.

    That's a start, why are you ditching the rads?

    What you could do once you know radiant can handle the load, and tube spacing is determined for the panel system selected. You could install the tubing leave the radiator system operational, and when all floors are done make the switch.

    Also ceiling radiant is an excellent option. Don't know your remodeling extent of ceilings are being redone.
  • RobertBenda
    RobertBenda Member Posts: 2
    Thank you.

    I have a faulty radiator on one end of the house (kitchen/bathroom) and that's an issue in Northern Minnesota :)

    Since we're already working on that kitchen, and will be doing a floor leveling compound, I had hoped I could lay in a radiant system. Though all the rest of the house would be under the sub floor, if I did it.

    The additional motivation is, I am laying in 2x4 walls, missing from my exterior walls (so my 2nd floor and attic all rest on the brick exterior), means I have to move all the radiators as I go since there isn't room behind them for the new walls.

    With what you tell me, perhaps I'll just fix it and move on.