Re-filling hot water system

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    • #275196
      Jeff Duncan

      What is the correct procedure to re-fill and bleed a Hot Water heating system after a complete drain of the system.?The system has a Weil-Mclean boiler, installed in 1996,circulating pump and a basketball size expansion tank mounted near the boiler.This is in an older house, with cast iron radiators and we are heating all of 2 floors and part of a 3rd floor.Any help would be greatly appreciated.Thank you.

    • #291375
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      Each radiator is an air trap. You do not want to trap pressurized air in all the radiators. To be careful, open the vent on the highest radiator and place a diverter to let water that might come out fall into a bucket.

      In the basement, lift the relief valve on the boiler and let water in; when water comes out of the relief valve, close the relief. Permit the system to fill slowly until water comes out the highest vent (Did I have to say someone would have to be watching upstairs? – maybe yourself if you are doing this alone?.

      Go to the basement, close the water fill valve. Note the pressure on the boiler gauge (you can even mark it with a piece of tape or magic marker.) Go upstairs to the first floor and let air out of each radiator on that floor until you see water. Go back to the basement and open the fill valve. Watch the highest point vent again. Repeat the air purge on the second floor.

      In the basement open the valve to slowly add water til it shows at the highest vent again. Close the vent, go to the basement and permit the boiler gauge to go to to whatever psig the pressure reducing valve permits, the pressure should go to 4 or more pounds higher than the gauge mark you noted (if there is no pressure reducing valve fill to 4 psig higher than the mark) then shut off the water. The pressure should not go higher than 25 psig or the relief valve may dump water. Let air out of the highest radiators.

      This eliminates most of the air from the system without turning radiators into air-filled pressure vessels.

      Heat the system and run the circulator so air bubbles will then be collected in the radiators once more. Let the system cool, the pressure should remain at least 4 pounds higher than the mark on the gauge (if not, you might have to adjust the pressure reducing valve upward to reach the added 4 psig that lets you remove air in the future.) In turn, from the bottom up, vent remaining air from each individual radiator.

      Just pressurizing the system without doing each floor in stages this way makes each radiator a potential explosive device from the trapped air – no need to worry about pressurized water below 212F, water will not expand explosively if released.

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