Hi Skipper, thanks for your reply. There is/are no temperature mixer valve/s. All faucets are two handle; no single handle mixers. The only on off valves are at the tank and the hot/cold manifold under the sink. This is very basic, tank to filter to pump to branch one to water heater and one to cold water manifold. The hot return from the heater goes to the hot water manifold.
Can anyone tell me if they have the on/off valve at the water tank partially closed to restrict the flow?
I meant a tempering valve (pic).
It would be near or attached to the water heater.
I'll assume there isn't one.
Ok, heat up the water in the heater. Then, with the pump off, open all hot fixtures (only).
Turn pump on, let water flow 10 seconds. Turn pump off.
Close all fixtures.
Turn pump on and pressurize the system. Open only the galley sink hot side. See if hot is steady.
If that works, I'll explain why.
If it doesn't, the only 2 things left would be (1) a cracked or corroded manifold, IF the same manifold is attached to both hot cold.
Or (2) you're sucking air at a connection between the tank and pump, without it leaking water.
Edit: whoops, air would cause your issue only if there's a tempering valve.
BTW don't restrict the flow from the tank. It won't help and will only burden the pump.