Let the boat sit a year. I usually prime the sea strainer with water and get the system running. Three of the four exhaust manifolds wont move water. Last time it did this I backflushed backwards and calcium came out. This time it wont work.

What can I pour down in the exhaust manifold water jackets to eat/dissolve the calcium?

They are Dana Marine, the manifold is aluminum and the riser is stainless.

i have used 3 different brands. here is one

RYDLYME Marine: The Ultimate Biodegradable Marine Descaler! | RydLyme Marine

Here is another

Marine Products ? Bright Bay Products

Bup, just pour it in?

On the bottom inlet hoses its easy, drain them (since they are on the bottom) just fill with Rydlyme and lift the hose. But the manifold is different. I put water in it to start it... Get the water out or pour in the Rydlyme in with the residual water?

Only thing I can think of is run it till its warm and get it hot (ish) and hopefully the water evaporates?

See if any one in area does engine block boiling. That should work. Or a big ultrasonic tank

Okay, got it out.

Drained the bottom lines with gravity, pour in Zep calcium, lime & rust remover up top in the manifold and in the bottom feed lines and held them up.. Let it sit 24 hours then put a fitting on the feed bottom lines with a fitting and 15PSI - each one blew through in about a minute. Then I drained it and filled the system with Zep calcium, lime & rust and circulated it with a cheap transfer pump. The insides look almost new.