Laars EBP110 external bypass question and DHW problem

Back in December, I replaced the old water boiler/heater in my apt with a Laars EBP110 Combination boiler. In the beginning, we had a lockout problem due to the ignitor/flame sensor assembly failing prematurely. Laars assigned a contractor to diagnose and he was able to resolve the problem. He also advised us to have an external bypass installed. He didnt really say why other than the unit would eventually suffer an F-4 error, supply sensor error, if we didnt. He also mentioned that the piping connecting to the unit was somewhat undersized.



I did some research about the external bypass. The manual says external bypass should be installed if there is a system pump. There is a system pump on the system return piping. I believe the external bypass is supposed to relieve pressure on the system caused by the system pump and undersized piping. The pressure gauge on the Laars unit reads 26 psi when it should be around 15 psi. A gurgling noise can be heard coming from the baseboards whenever there is a heat call.



Another problem we have is with Domestic Hot Water calls. I have a feeling this is also related to the lack of external bypass. Whenever someone takes a shower while the heat is on, a loud rattling noise can be heard coming from the Laars unit and the pressure needle fluctuates wildly. I believe the rattling is from the anti-condensation valve trying to close for the DHW call but unable to due to the high pressure (26 psi) in the system. The unit short cycles as well. There is alternating periods of hot and cold water in the shower and then just cold water.



I have tried turning off the thermostat that controls the system pump prior to anyone taking a shower to avoid this problem. The rattling dies down signicantly but is still there. The unit also short cycles a couple of times before firing normally. There is still alternating periods of hot and cold water in the shower.



A permanent solution would be to remove the system pump negating the need for external bypass as the Laars unit has an internal pump capable of driving the whole system. Equivalently, i thought maybe i could just disconnect the system pump from thermostat and power. Not sure if the anti-condensation valve is damaged and needs to be replaced. Any help, suggestions, insights would be greatly appreciated.