Expansion tank leak in radiant heat system

[RunningBum]

Hoping for some help from someone handy here.

I've got a radiant heat system with a leak in or around the expansion tank. I shut off the water main and the steady trickle from the bottom of the tank stops after a few minutes. I know the tank needs replacing, and there's a maintenance company that has been responsive for me and should be able to handle it.

My immediate issue is, how can I keep the main (cold) water on but bypass the water heating system and with it, the expansion tank?

I've shut off all of the valves I can see between the big main tank and the boiler (expansion tank is between the two), and the pipes leading to the floor lines it serves. I'm pretty sure I've got the water input into the main tank off, but it's a little confusing because I've got a recirculating pump too, but I've closed the valve there too. I've also cut power to the boiler and recirculating pump

So it seems like I should be able to turn the main water on, but when I do I get the leak again. Even in the 30 seconds I have the water on it leaks right away, and keeps a pretty steady trickle for about 5 minutes. It's only a few inches in a bucket, but I don't know why the expansion tank would be getting that much water right away.

Is anyone familiar with this kind of system who can tell me what I've missed? I don't mind missing a shower until Monday but would love to have cold water. I connected a hose to a spigot that I guess is used to bleed the system so I can at least direct water to a drain if the only solution is to turn water on for a minute when I need it.

Thanks for any help.

 

[RunningBum]

When I read about replacing the expansion tank, it's suggested that someone pretty handy with plumbing can do it themselves, otherwise leave it to a pro. I don't consider myself too handy, and there's a further complication that I'm leaving for the week Monday afternoon so I don't want to leave my amateur work and come home to a mess later.