I am using a hot water heater to heat the in floor heat in my shop, I am having trouble getting the water hot enough, I have been told to wire the heater so both elements come on at the same time,, I have 10-3 wire and a 40 amp. Breaker,, Two 3000 watt elements and 40 gallons of water. How do I wire the heater so they both come on at the same time?

Thx.

First off, #10 wire is too small to be on a 40 amp breaker. This is not a safe condition, and should be a maximum of a 30 amp CB.

Wiring a unit with 6000 watts at 240 volts will total 25 amps.

A 30 amp circuit for a water heater is to be derated by 80%, which is 24 amps, so that is not allowed.

You need to change the wiring to #8 copper for a 40 amp circuit to continue on with your plan.

Next you need to check the rating of the thermostat in the heater to see if it can handle 6000 watts.

I'm curious. How large is the shop? What is the estimated btu's needed for this shop? How large is the hot water heater circuit? In most circumstances if you are sized properly, the water heater will keep up under two conditions: It's sized properly and you don't turn it off when unoccupied. Are you using baseboard or in floor radiant?