Hot water backflowing into domestic cold water

[rodopa]

My daughter just purchased a home which has an oil boiler w/ summer-winter hookup for domestic HW. There is also an electric HW heater.
A single Cold water line to the house feeds the domestic water feed on the oil burner and continues about 20 ft. to the cold water feed connection on the electric HW heater, as it should, then on to the kitchen.

Here's the situation. When she turns on cold water in her kitchen she gets a fair amout of warm to hot water before she gets somwhat cold water again.

My question: Shouldn't the cold water feed to the electric tank be the hot water output from the oil boiler?
Or, should there back some sort of backflow valve where the cold line feeds the electric boiler to prevent the hot water from flowing back?

Thanks!

Rich

 

[taydo]

It could be as simple as a hot and cold pipe to close together, the hot pipe then convects heat across to the static water in the cold feed.

 

[drgl]

I'd say it was that too-quite a few houses i've been in did it.........

 

[rodopa]

There's actually about a foot between the pipes ....
The cold feed is 3/4 ..... it T's to the electric h/w ... then ends up stepping down to 1/2 inch leading up to the kitchen about 4 ft. past the T....
I agree that it could be the hot water equalizing the cold water as it passes by the feed to the h/w ... as there's only about 4 ft. from the cold feed to the h/w heater.
Think a backflow fitting makes sense?

 

[oilman]

Shouldn't the cold water feed to the electric tank be the hot water output from the oil boiler?

No, the HW output from the boiler should go to a heating coil, not a water supply that feeds a tap. Though you haven't said what type of boiler you have. Might be ok if it's a combi.

The "backflow " is possibly expansion of water from the heater into the cold water mains. This is not a good idea, though common. Fit a non-return valve and then you will need an expansion vessel somewhere between that valve and the tap. You had better check all your regulations as well. There's plenty over here.