I have a weil-mclain boiler with four zones and an indirect hot water tank. How do I purge/flush each of the zones? I would guess the following:

  1. Turn off the gas and electricity to boiler and connect garden hose from [UNKNOWN-VALVE?] to sewer access.
  2. Close all manual valves on the right side except for the desired zone.
  3. Open [UNKNOWN-VALVE?] above and allow to run until no more dirty water or air bubbles.
  4. Close [UNKNOWN-VALVE?] and then go back to step 2 for the other zones.

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2 Answers 2


You don't.

All this is going to do is flush out the old rust and deoxgenated water to replace it with new oxygenated water to make new rust. It will be just as "dirty" in a week or so. If your system has a combination of cast-iron and copper parts, you want to keep the old deoxygentated water right where it is so you don't make any new rust, or only the smallest possible amount if you need to top up.

A properly sealed loop non-leaking system does not need to be "flushed" - until you are doing something major like replacing the boiler.


You might want to purge air out of a zone if it isn't delivering heat.

In that case, you open the PRV to allow more water into the system while having a hose hooked up to the return side of the zone you are interested in purging and having the return to boiler after the drain turned off.

This will force water through the zone and back to the return, once you have hot water coming out this drain you know the water has made it from the boiler to the return from the zone and would be getting back to the boiler. You can then turn off the drain, open the return and turn off the prv. My pressure reduce valve has a little lever to add more water/pressure to the system. Too much water / pressure will cause the water to pressure relief valve to dump some water.

  • Which screw valve do I hook the hose up to? There's one all the way at the bottom after the hot water tank joins and one right above the ball valve after the heating zones combine?
    – Nate
    Commented Dec 9, 2022 at 15:29
  • Your setup isn't ideal. The best practice is to have a drain valve for each zone so you could individually flush them. You have 4 zones. If I had to do this I'd put a drain valve on each zone before the return shutoff so you can verify flow through each zone. Commented Dec 9, 2022 at 17:59
  • I suppose you can make it work by closing all return zones except 1 and close the return to the boiler. Then you open the prv to allow more water into the system which can't return to the boiler and forces water out the drain... once the water runs hot you've flushed that zone. Commented Dec 9, 2022 at 18:04
  • Which valve is the return to the boiler? The one after the heating zones but before the hot water return?
    – Nate
    Commented Dec 11, 2022 at 16:38

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