Mixing copper-fin and cast iron?

[dick osa]

This is long-winded, but please bear with me; I need clarification! I recently added a kickspace heater to my kitchen because the previous owners removed all the radiators when they renovated the space. The rest of the house is heated with hot water in cast iron radiators. The kickspace heater has a fan unit that turns itself on at 130 degrees so as not to create any drafts. I installed the unit on the return side of the circuit (actually it’s last on the circuit) because it was just easier and it seems to just lose about 20 or so degrees on the return trip. Anyway, it does okay on the cold days and so-so on the moderate days (below 25) and never turns on days above 35. I called tech services at Slant-Fin (the manufacturer) and he said that in order to successfully mate copper-fin and cast iron radiators that I would need to somehow mix some of the return water with the supply side so as to force the boiler to run hotter (max. 200 on the aquastat). He said this needs to be done because cast iron runs about 110-120 and copper-fin runs about 180-190. He said that there are controls to do it electronically if I wanted to make it easier or just set it up manually with a couple of valves, but I’d have to constantly fiddle with it so as to not cook myself out with too hot cast iron radiators. Is he right or is he pulling my leg? The owner of the plumbing supply shop where I bought the heater has never heard of this and that he’s never had any problems mixing the two systems. Any advice would be greatly appreciated! Also any leads on a control system if needed please! Thanks in advance.

[AKPlumber]

I’m not real familiar with cast iron radiators, so hopefully someone else can answer your questions more satisfactorily.
But, it would make sense that the temp ranges between fin tube and the radiators would be different, since fin tube is nothing usually but type M copper with radiating fins and the louvres, whereas cast iron would take a higher temperature of flow to get to the same output.
One thing I would suggest is to zone off the fin tube and cast iron separately first off, so you can control between the two. You don’t mention how many (if any) zones you have going. At any rate, it would’nt be difficult to insert an additional zone control and wire it in.
Mixing return with supply could cause more problems than it’s worth. I would suggest getting a by-pass loop in if this is done for proper purging capability. But this is where I’m stuck, other than converting to all fin tube or getting the fin tube zoned. Hopefully someone else can offer the answer you need.

[Medichef]

That’s for the try AK! I have one zone looped in this 85 year old house. But I would like to work out some sort of separation so that I can use the same kickspace heater when I redo my bathroom. It’s a lot cheaper than radiant floor. The cast iron actually uses less temp (110-120) than the copper-fin (180-190) because of the large mass (it retains the heat longer.) Ironically, if I were to use radiant floor in my bathroom redo it would use the same temp as the cast iron.

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