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Old 03-20-2011, 11:34 AM   #1
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Cold water tees into hot water circuit?


I am trying to determine why I have issues with hot water ever since I turned on the infloor heating last fall. I started tracing all the water lines, and have attached a diagram, but perhaps someone can explain to me why the mixer valve, and why there is a direct tie between the cold and hot lines??

The main issue is if the forced air furnace throttles down at night (reduce temp to 62*) in the morning I have a nice hot shower. If I leave the temp up on the furnace (70*), the shower will be cool in the morning.

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Old 03-20-2011, 11:54 AM   #2
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Cold water tees into hot water circuit?


In floor heating uses lower temperature water than baseboards/radiators. Some return water is mixed in with the hot water to reduce it's temperature.

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Old 03-20-2011, 11:59 AM   #3
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Cold water tees into hot water circuit?


That makes sense but there's no way for the cold water to make its way back past the mix valve, and into the hot water circuit?
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Old 03-20-2011, 07:12 PM   #4
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Cold water tees into hot water circuit?


Are you sure your diagram represents your system? You show one pair of inlet/outlet pipes coming out of the boiler and serving both the floor radiant heating pipes and the hot water faucets. This is inherently incorrect.

Potable water is not supposed to go 'round and 'round in the floor radiant heating pipes. Rather the boiler (not a water heater tank) is supposed to have a separate pair of inlet/outlet pipes (usually about 1-1/2" to 2-1/2" diameter) where boiler water feeds the floor radiant pipes and returns to the boiler.

For a tankless furnace water heating system, potable water is supposed to enter a coil submerged in the boiler water and come out its own outlet and up to the hot water faucets. It does not mix with boiler water. These inlet/outlet pipes are usually about 3/4".


There is one pipe, usually about 1/2" from the potable cold water supply into the boiler with a valve in it, to replenish the boiler water periodically, seldom more often than a small amount once a week.

If your system is as your diagram shows than it is a jury rig and the results are unpredictable.
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Last edited by AllanJ; 03-21-2011 at 02:19 PM.
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Old 03-22-2011, 06:11 AM   #5
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Cold water tees into hot water circuit?


Yes the diagram is correct. I did do a bunch of research before buying this house regarding this, and it is not the ideal way, but it was also not that uncommon. Throughout the spring, summer, & fall that loop is closed off completley, and does require a good flushing when opened in the fall.
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Old 03-22-2011, 09:22 AM   #6
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Cold water tees into hot water circuit?


Add a check valve just under the 3/4 Mix so water can go (as shown in the diagram) up only. Then in the event of pressure imbalances, cold water can't come from the green line into the potable (or domestic) hot water lines. The 3/4 mix unit may have a built in check valve but given the symptoms you describe that check valve may have failed. In your diagram the green line is the return to the boiler.

Alternate location for check valve: In the green line so water can go only to the left.

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Last edited by AllanJ; 03-22-2011 at 09:27 AM.
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