Sidearm WH with controls?

This tank (blue tank on the right) was attached to the boiler, so I’m assuming it’s the sidearm water heater. Is the control knob to regulate temperature? How does it do that?

Turn it up to call for more heat…just another thermostat, I believe…but I don’t know what a “sidearm” water heater is.

The boiler heats the water and this tank is basically a storage tank for hot water, so it’s not actually a water heater. “Sidearm” is the term used around here…
water comes out of the boiler at way too high a temperature for use at plumbing fixtures. Because there is no actual heating mechanism I’m wondering how the tank supplies water at proper temperatures. All I can think of is that it might mix cold water into the hot water supply pipes somehow.

It appears that the tank contains a heat exchanger, and boiler water is circulated through the heat exchanger coil in the tank by a pump. The thermostat on the tank controls the pump. This provides for domestic water temperature control and isolates the domestic (drinking) water from the boiler water, which may be contaminated from circulation through the heating system, or perhaps even contain water treatment chemicals.

This illustrated site will help you gain additional knowledge on indirect water heaters.

I see them every day.

Thanks, James and David.
Nice to see you posting again David. I hope things are back in order.

Thanks…I’m finally caught up after returning home from EGYPT.