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Chemical treatment for radiator system

Hello all.

I have a water rad heated home, and have removed ~6 gallons of water from the heating system. This was done in order to drain an upper rad which had no shut-offs, so I could remove it to do some work in the room.

When I bought the home, the previous owner did tell me that there was kerosene in the rad system, to prevent corrosion I believe. When I bleed the rads, and more recently when I partially drained the system, I do smell something which could be kerosene.

Now that I have the rad off and have drained ~6 gallons from the system, I was thinking to add more chemical back in before I hook up the rad again. I was planning on doing this by pouring some amount of some chemical into one of the pipes to which this rad was connected, before reconnecting it.

I have a few questions:

1. Someone on The Wall suggested kerosene was bad, and that propylene glycol was more widely used. I suspect that propylene glycol is used as an antifreeze, whereas I thought the kerosene was in there to help prevent corrosion. The home has two heating zones, one of which is forced air. The home is occupied year-round, and freezing is generally not an issue. Comments?

2. How much chemical (what concentration) should be in the system? If I've removed 6 gallons, how much chemical should I add back in?

3. If I add the chemical into one of the pipes for the (currently removed) radiator, is one (supply/return) preferred?

4. Which chemical to add?

5. We have hard water in the area, and I understand that this can actually cause problems with glycols, since it can reduce the effectiveness of the rust-inhibitor. I also have galvanized pipe for much of the heating system, which apparently reacts with glycol to form sludge.

Thanks for any advice you can share.

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