In order to get cold or merely warm water to flow from ALL single handle faucets in my home I must turn the handles 'slightly' open. If I opt for turning to where the faucet says 'cold' it is already warm. I cannot turn it further for fear of burning.
I believe there must be something restricting the cold water that is allowing the hot water to take over.
I have a whole house filter at the water source but the plumber who installed it never placed a cut-off valve nearby.
Do I have to drain the entire house to flush out a possible cold water blockage?
And in doing so would that, or could that fix my problem?

Hi Reschzoo

Since cold water feeds the hot water heater and you have plenty of hot water a restriction in the cold water in unlikely. Also note that with most single lever shower valves if you have a drop in cold water pressure you will see a corresponding and equal drop in pressure in hot so my gut is telling me that you have more of a CROSS CONNECTION somewhere than a blockage or restriction, OK?

Here, a cross connection could occur as a result of any single lever faucet cartridge failing and allowing hot water to flow into the cold water system. To determine which faucet is at fault would require you to shut off a hot water shutoff under each sink and then open one at a time until you find which faucet causes the issue to return. The fix, of course, would then be to replace the defective mixing cartridge. If no faucets show up as the issue then the issue could be at the single lever shower faucet mixing cartridges, but showers are harder to isolate so we usually just end up replacing the cartridges and hope that resolves all.

Other places to look for cross connections include outside faucets that have a hot and cold connection, the washing machine solenoid valve or single lever washing machine valve (choked down to one hose), and any two handle faucet (such as a laundry faucet) with a single hose attached and left open (;)).

I'd check these sources for cross connections and see if you can resolve this step by step.

Good luck and let us know how you make out, OK?

Mark