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Mixing radiant into old cast iron system

fattyfatty Posts: 54Member
Hi Everyone,

   Long time reader and occasional poster.

 I'd like to put in an entirely new system in an old house. I'd like to keep all the radiators and the excellently fitted old iron pipe while dropping in a new cast-iron boiler and 2 new thin-slab radiant zones.

I have done the heatloss calcs, layed out the pex, pressure tested, fitted slab sensors (tekmar) and laid the dry bed.

Now I'm selecting products to do the controls. I'm set on a traditional Weil-Mclain gold series boiler, but the rest is flexible.

I'd like to come up with a simple combination of controls to handle these 3 zones.

1 Large Cast Iron Radiant Zone

2 Barrier Pex Radiant Slabs

I was considering using set point thermostats in each of the slab zones, a programable thermostat in the main cast-iron zone.

I've built one copper and one steel system in the past, but have never used pex. I'm familiar with Taco and Tekmar controls.

My main concerns are coming up with a sytem with minimal components that can safely reduce the temperature of the cast-iron boiler for the PEX, protect the boiler from cold returns and ensure even supply of water when needed to all 3 zones. 

In the past I have used primary/secondary piping but with the PEX i'm not really sure how it's going to look.

Thanks for reading and for your time,

A budding radiant nut,

Tom Blair


  • ChrisChris Posts: 2,869Member
    If Your Looking For Simple

    I would use a simple boiler reset control w/DHW capability like a honeywell aquatrol or tekmar equiv. For the radiant I would look at using a Taco Radiant Mixing Block. The RMB will give you boiler protection and you can zone both radiant zones off it. It will make a real simple system and give you both the reset for the boiler (cast iron zone) and lower water temp for the radiant. IF you really want to get some deep reset. Run primary/secondary and use 2 Taco RMB's.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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