Removing radiators replacing with radiant floor heat

mattyt1984

I have remodeled 2 rooms both 10 by 10 feet. They both had cast iron radiators. I want to put pex between the joist to heat these areas. I have 3/4 inch supply and return for the radiators so I plan to run pex directly out of these with 1 loop in each room. Will this work and how do I know how many feet of pex to run? Both floors are 3/4 sub floor one room has pergo on top and the other ceramic tile.

PC7060

You’ll need to determine the heating requirement for the room/area at the design temperature for your area. The design temperature is published by ASHRAE; take a look here. 

https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

The deign temperature feeds into the BTU heat loss calculations.  Once you know the heat loss you can determine how much piping and the required water temperature. 

bburd

To continue heating your house evenly,  you will need to zone the radiant floors separately from the cast-iron radiators that remain in your system.

Cast iron radiators on a hot water system hold heat for a very long time due to the high thermal mass of the cast-iron and the water inside. Radiant floors have much less thermal mass. 

If you put the whole thing on one zone, the rooms with radiant floors will be colder than the rooms with cast-iron radiators.

This happens when buildings are partially remodeled and in some rooms, cast-iron radiators are removed and replaced with fin tube or other modern radiators with much less thermal mass. The remodeled rooms end up much colder than the others unless they are zoned separately.

Ironman

In addition to what’s been stated, you’ll need to use a good quality extruded aluminum heat transfer plates (not the beer can type).

The heat loss calc should have been your first step since a radiant floor may or may not provide sufficient heat. With good quality transfer plates, you can expect about 20 btus per sq. ft. output, depending upon your floor coverings.

Another reason that you can’t just hook on to the existing risers is that water takes the path of least resistance. Adding a 150’ loop of tubing where there was once a radiator that virtually had no resistance to flow is gonna cause the flow to take an easier path.

PC7060

Personally, I went the other way and added radiators to an addition I am doing to 1920s house. Bought old ones from architecture salvage place and plan have the radiator cleaned powder coated! Love the ways these thing hold the heat!