replacing a low water cutoff on gas furnace

Old 11-01-10, 02:56 PM
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Join Date: Oct 2007
Location: Pennsylvania
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replacing a low water cutoff on gas furnace

I need to replace a low water cut off on my gas furnace. I need a McDonnell 6667. Does a 6700 also work, I think its a 6700,might just be a 67 series. I found 1 online, but it says its the head???? only. Is this all that is needed.

I'm kinda lost here when it comes to furnaces, a local plumber said it would cost $300 for the part alone . I found 1 at pex for half the price but does'nt even look like the 1 I need. Then there's the head only on ebay for half of pex price.

Any help would be greatly appreciated. Thanks

Old 11-01-10, 03:17 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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For future reference a "furnace" usually means a fuel burning device that heats air and with a blower distributes it through the house through ducts. What you have is a boiler.

Some pictures of your present installation would help us to help you. Is your system steam or forced hot water? What is the exact reason for replacing the existing low water cut out? If it is only the switch or the float, those parts are easily replaceable but if the casting is broken you need a new unit. Be prepared as McDonnell-Miller parts are not cheap.

To post pictures you need to first upload the pictures to a photo hosting site such as or and then post the public URLs for the pictures (or album) here. More pictures are always better than fewer. Please have CLEAR pictures and have both close up pictures and ones from a far enough distance that we can see how the various parts are interconnected.

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