Replacement immersion heater on very tepid water why not heating fully?

your element has burnt out and needs replacing.

You could have a defective gas control valve or a bad thermocouple. Also check the venturi tubes again. sometimes cob webs collect in them and it may be difficult to see unless you remove them and check.

Good luck,

Marty

Water heater can be added to any incoming water line, either on hot or cold.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Under-counter water heater located near tub will supply hot water just like ordinary household water heater. But it will not reheat water that circulates inside the spa tub.
Otherwise, a spa heater is made with electric element that can work with 120V or 240V. Water from tub circulates through tank that holds element. Element must be fully immersed in water.
To retrofit spa tub with this heater would require cutting water line near pump, running both ends of water line into tank with element.
Then of course you would need to retrofit adjustable thermostat to control when element is on. And integrate element operation into air control switch at tub that can override thermostat so element is not heating when tub is not in use.
Unless there is a pre-packaged kit to retrofit heater, I think adding heater would be difficult.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

elements are bad

Hi,
Heating of water output depends upon the wattage of the unit and the condition of the water source (how cold). The lower the wattage the slower heating capacity at the same time the colder of the water source it is harder to keep warm. Also to consider the quality of the newly installed heater. Contact the manufacturer and demand a site visit as possible to check personally the performance of there unit.
Hope it may help you.
Have a nice day!
JDVillanueva

not insulated enough try insulating outlet line

I would not have both on at the same time . We never did .

SOUNDS LIKE THE CARTRIDGE ASSEMBLY IS BUGGERED UP. PART #46463.
MAYBE THIS WILL HELP:
All parts and finishes of the Delta faucet are warranted to the original consumer purchaser to be free from defects in material and workmanship for as long as the original consumer purchaser owns their home. Delta recommends using a professional plumber for all installation and repair.
Delta will replace, FREE OF CHARGE, during the warranty period, any part or finish that proves defective in material and/or workmanship under normal installation, use and service.
Replacement parts may be obtained by calling 1‑800‑345‑DELTA (3358) or by writing:
Delta Faucet Company Product Service 55 E. 111th StreetIndianapolis, IN 46280

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