Or when I’m told that open-system, cross-connected potable-hydronic systems are so great because they give you free air conditioning and increase the capacity of your water heater by as much as 50%. All of which can be done without any risk to your health because all the incoming cold water will be run through the radiant heating system on its way to the water heater and, therefore, prevent stagnation of potable water along with tempering the colder well or municipal water?

Those were the claims made during a heated e-mail exchange in which I was told, “You just don’t get it!” In yet another e-mail regarding open-system construction, I was told I’m “just a dumb (expletive deleted) plumber.” We’ll address the slam about being a plumber in another column. Where’s my sign?

Aside from the potential health risks associated with open-system construction (see Legionnaires' disease by Dave Yates), there’s the issue from last month’s column regarding pressure losses and balanced flow in plumbing systems using potable PEX water lines. It’s an issue ignored by many proponents of these systems, but one that exposes the customer to potential scalding or poor delivery pressures as downstream or lower floor fixtures are used simultaneously with the upstream or upper floor fixtures.

Now before anyone gets his boxers in a bunch, let’s take a minute to examine some hard realities. Line lengths for 1/2-in. PEX hydronic systems are often 250 ft. Systems connected to a combined-use water heater will have fairly small Btuh loads in order to permit bathing without losing too much capacity, right? If not, they need their own dedicated heat source.

At 250 ft. and using the chart from last month’s column, which shows a .208 pressure drop per foot with a 4-GPM suggested maximum flow rate, these systems are dealing with a potential 52-PSI drop during peak flow conditions! That’s before adding losses due to elevation changes and line lengths to fixtures. Here’s your sign.

But let’s change that to heating an area with 1,000 ft. of 1/2-in. PEX using four 250-ft. loops attached to a manifold.

The shower on the third floor (loss of 10.4 PSI for the change in elevation of 24 ft.) is being used by Mr. Jones who loves his high-flow, 8-GPM showerhead. He’s adjusted the faucet to compensate for that long four-way-split-run pressure loss and the differential from the lower resistance to flow in the cold water line. He’s using an 80%/20% mix of hot/cold because of the 55°F well water and the heating system, which was already running, has lowered his storage temperature.

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