I have a hydronic heating system with a relatively powerful B & G circulator pump. This pump has a relay that is controlled by the furnace electronics; if there is no power to the furnace, there's no power to the pump.

This makes it tricky to purge air from the system via the technique of using the city water filler, a circulator, and the drain, thus bypassing the boiler.

I would like to be able to use the circulator when the furnace is off. I thought that the best way to do this would be to install a junction box with a UL listed DPDT switch (e.g. a Leviton 1286) that switched the circulator's feed from the normal furnace wiring to an electrically isolated hard-wired appliance cord of sufficient gauge that I would plug into a different circuit.

Can anyone think of problems with this?
Is this against the electrical code, or will it not apply so long as the appliance cord is not plugged into the different circuit.


2 Answers 2


For those who are interested in doing this:, I simplified the wiring diagram as illustrated below.

I cut the 120v lead to the circulator relay and wired it to a terminal block. From that terminal block, leads go to to one pole of a 3-way switch and to the relay. Likewise, the other side of the relay was cut and now goes to the other pole of the switch. The center pole of the switch goes to the circulator motor. This allows me to run the circulator without running the furnace. Total cost was less than $10.

wiring diagram


It's a bad idea actually. If it's a very old system with big radiators you don't purge the air unless the heat is running & the radiators are at least warm & only add water if a purge doesn't produce water, so you don't kill your expansion tank or its bladder.

If it's a new system, all of the above is done automatically to keep your expansion tank safe. If you're experiencing creaking, clicking & knocking sounds then something's broken. It could be as simple as re-pressuring your expansion tank bladder (they're very low pressure) or it could be a leak anywhere in the system.

Get an HVAC guy out instead, for a full testing, tuning & assessment.

  • I know of one leak already, and didn't get any air coming out on the top floor at the hyvents, and the radiator (baseboard) on the top floor seems not to be working. So either both hyvents are bad, or the system is down a bit of water; thought that circulating the water would help.
    – gbronner
    Jan 6, 2016 at 15:44
  • Oh very good. Yeah, it's low on water & hopefully that was the only leak & problem. Use the top floor as a gauge. Add just 5-seconds of water (half turn if a knob & not a lever) to see if you then start to get heat up there. If you can feel the baseboard's top & bottom pipe, assuming it's not just a single straight pipe, then you should feel that the top is still cooler. Then you can add another 5-seconds of water to check again & again & again until it's half or even mostly full. Let the system run for a few hours so all of that new cold water is heated to find the hyvents doing their job.
    – Iggy
    Jan 6, 2016 at 16:06
  • I over-added water, and some came out of the hy-vents at the top; it appeared to have a number of air bubbles I disconnected one of the main loops and managed to get the circulator to purge the air bubbles on the top floor (heard the whooshing sound from the hyvents), but it would be nice to be able to run the circulator without the furnace being on.
    – gbronner
    Jan 7, 2016 at 2:57
  • Very good! You know the hyvents are working properly & they should continue to bleed air & excess water until everything's evenly heated & hopefully back to stay at full equilibrium. Forget about the circulator purging idea it's a closed system & won't rust-out or anything like that from the air, which is just cooled steam. Everything but my boiler is 100-years old & it's never rusted in the slightest & it's ALL straight iron, not even steel & I only need to manually bleed it every few years.
    – Iggy
    Jan 7, 2016 at 3:14

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