What is the min. distance between receptacles and Hydronic (hot water) heat register?

It is not specified. Receptacles can’t be above wall/baseboard mounted heating units but they can be installed as a listed part of the unit. Regular “radiatiors” are not addressed

What Greg says is true, but only for “electric” baseboard heat.
I cannot say for sure about straight hydronic heat. I do know that they put hot water baseboards under receptacles all the time and there has never been any cause to avoid this.

See NEC 210.52, including the FPN.

The heated water in a hydronic baseboard(non-electric) unit is in the 105 -140 deg. F range. The NEC does not appear to address these units and as Speedy says it’s common to see receptacles mouted aove them. A quick call to the local AHJ wouldn’t hurt and may avoid problems.

In many homes with baseboard convectors, often every lineal foot of exterior wall space is covered by a hydronic convector. It is unavoidable to have receptacles over them, and it is not an issue with any model code text that I’m aware of, be it electrical or mechanical.

In Canada, Marc, you’re right. Don’t know about US codes.

Thanks Petey. I was talking about “electric” wall and baseboard heaters but I didn’t make it clear.
Heat is not a big thing here so I am not real good with terminology. Usually the only heat we will have is resistive, either a wall/baseboard unit or strip heaters in the A/C air handler. It gets turned on a few times a year. Don’t be shocked if a puff of smoke or a burning smell comes out of the ducts when you test them. It may be a year’s worth of dust burning off the toaster wire.