I've done a lot of work with both cast iron rads and the steel panels rads (literally 1000s of them) and I will say that the jury is still out as to which one is more efficient, but I know which one is heavier, LOL. I just did a A2W HP with oversized stelrads and have been able to keep this relatively under-insulated house to a max water temp of 40C so perfect for HPs. The oversizing of the rads wasn't that bad either.
The HPs are all 410A because that is mostly what is commercially available but if I could use propane and sell the unit, I would.
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