Issue - have almost no pressure when more than one faucet is open on hot. Also base board heaters are not worming up. Cold water has plenty of pressure.

House is 4 years old running on well. Heater is an line water heater (T-KD20).

Issue started up 4 days ago after draining parts of it and replacing the expansion tank (old one had leaked a bit a while back but stopped. We had an extra expansion tank on had so figured it was time to replace it any ways.). We had heat for a day or two and then it stopped again. We then replaced the motor in our circulator pump (TACO 007). Still no heat. Next we flushed the heater with vinegar. Still no heat to the base boards.

Through all of this if you turn on any of the faucets, hot water will kick on or if you drain from either the intake to the heat loops or the return the hot water will kick on.

I was able to get the house up to heat and the valves for the heat loops closed. Then when heat in house drooped the opened back up. So I believe all the electronics are working correctly.

I think the issue is there is not enough pressure in the system to kick on the heater for the hot water loops.

Any ides how to test this theory and or pin point the part that is causing the issue.

Thanks in advance for the help!