Dustbowldan

Installation 1996; heater supplies hot water to a tub, four sinks, a bidet and two stall showers.

Water-heater plumbing incorporates
a check valve in the cold water supply line.

Hot water recirculation-return line tees into the cold supply line between the check valve and the water-heater.

A Grundfos pump, in the recirculation-return line, sends hot water into the cold supply line, thence back into the water heater.

Simple, wot?

The trouble:

Heated water intermittently invades the cold water supply line [[I've traced it n/l/t twenty feet; then pipes disappear into floor insulation]] ; on the wrong side of the check valve; occasionally warm water can come out of a cold water faucet. Bad Juju!

Five hypothetical failure points:

1] the check valve gets stuck open, allowing the recirculation-return water to flow backward through the check valve into the cold water supply system?

2] Each shower has a single-handle Grohe rotary ceramic disc? valve that controls flow and mix of hot/cold water.

2] Two sinks have Price-Pfister single handle mixer "pull-out" spray spouts. The control has a part called a 'cartridge'; a mystery to me.

Possibly a faulty shower/sink mixer valve, while in the 'off', or closed, position, might still allow hot water to flow from the hot supply into the cold supply, due to pressure differential caused by Grundfos recirculation pump?

Seems to me: a brass check valve is an extremely simple gadget. Such a simple device should be much less likely to malfunction than more complex late 20th century mixer valve, right?

Question: Has anyone encountered this type of failure of these three devices?

How can I test these devices to figure out just what path the hot water uses to get into the cold supply, and what I must replace?

I'd prefer doing something simpler than replacing one brass check valve plus four sink/shower mixer-valves.

Please advise, friends.

DBD