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I've just replaced my water heater and for the first time my house has a water heater drain pan. The washing machine waste line is nearby and I'd like to connect the water heater drain pan to this line before the trap. The proposed addition is shown in black:

enter image description here

The only pitfall I can think of is that if there's a blockage downstream of the tee, the washing machine water can back up into the black pipe instead of having to build all the way to the top of the green pipe.

Keeping in mind that the picture is not to scale, how can I determine what distances are safe for this configuration? If the trap were ten feet below floor level, would this be considered safe? Are there other issues I'm not considering?

The alternative involves a 12' run to a sump pit. This would crowd my crawlspace with a long, low drain line and might interfere with a future radon mitigation system connected to the sump pit. Please suggest another alternative if you have one.

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Oh, there's also a vent just downstream of the trap that I forgot to draw. –  ArgentoSapiens Mar 28 '13 at 0:54
    
It seams perfectly reasonable in your situation to have a drain pan pump. In an emergency, the pump could deliver water to a sink or up to the existing opening of the laundry drain if applicable. –  Edwin Mar 28 '13 at 6:50
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The configuration you pictured is not really a good idea. Waste water from the washer is going to flood the heater drain pipe, leaving nasty residues that could conceivably siphon back into the heater if connected to the heater drain valve. Not likely, but possible any time two drains are connected at different levels other than directly into the vented out flow pipe. The other problem will be that if the drain is a open pan drain only, washer waste water will back up into the pan. Washers tend to eject water quickly and can partially fill the pipe above the trap.

I would rather see you pump it up to the inlet of the washer drain.

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On top of it not being a good idea, it don't believe it meets minimum code requirements for plumbing in many regions. The code where I live requires washing machines and water heaters to have a seperate drain for both the appliance and the overflow. –  maple_shaft Mar 28 '13 at 11:15
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here, we are allowed to pump up to a washer drain. The sump pump from my condensing furnace pumps up to the washer drain and is to code. Just installed it last year. Some areas may be more strict however. But for practical purposes it would work fine. –  shirlock homes Mar 28 '13 at 12:36
    
Aha, now I see that the IPC requires the laundry standpipe to be individually trapped and requires the water heater drain pan to be connected to an indirect drain. It does not appear to allow a pump for this, but I'd be happy to be proven wrong. Otherwise I'll go to the sump pit. –  ArgentoSapiens Mar 28 '13 at 16:58
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