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Laars Mascott - Baxi 380 HT - 4 heating zones + DHW - shared infos and questions

brasjorge Member Posts: 1
The attached picture shows our boiler (in Simsbury, near Hartford, CT).
The installation was done in 2006 as a DIY project with a Laars Mascot.
I chose then to not install a auto-feed valve so I monitored the water pressure on the boiler and filled it a bit every 6 months if necessary (there's a faucet on the boiler itself to do that).
Each zone valve is controlled by a room thermostat.

It ran smoothly till 2011 when the unit was changed to a Baxi 380 HT.
Back in 2011 the water pressure started dropping daily (so my renters had to turn the faucet on the boiler to get DHW back) then an error code E133 appeared...
I was living in France then and couldn't do much from there. The house was rented and the only plumber we found willing to do something changed the whole unit - still not sure yet what was changed, the Baxi HT 380 is a copycat of the Laars Mascot, look for Baxi 380 HT parts and they are the same for the Laars Mascot!).

So now it's a Baxi 380 HT and I moved back to my house this past summer 2014.
Running OK since 2011 so.
But this past couple of months (Mar-Apr 2015) the water pressure started to drop randomly and daily; again!
The pressure relief valve was leaking water which never occurred before. So I first changed it. Didn't solve the problem.
Then I changed the external expansion tank. It seems it was shot (no leaks but can't fill it with air). But the water pressure lasted only a few days with the new expansion tank. So it didn't solve the problem.
Now I got a new internal expansion tank and will change it soon.

Meanwhile I saw that when a heating valve opens the boiler's water pressure goes back to normal.
For instance, when the boiler is off due to the lack of water pressure (no pressure, no DHW, error E119), if I turn one heating zone valve on the pressure goes back up and the boiler starts!
This gave me a hint and I simply disconnected one zone valve and manually opened it (Taco zone valve).
Bingo! Now the water pressure never drops and the problem is solved!
So, it seems that this boiler needs an open heating circuit to be happy.

Looking further at the boiler's installation I see the supplemental external tank is located at the return end of the heating. Mine is at the heating flow.
Could this be the culprit? Look at the boilers diagram and the three way valve located on the heating flow.

This type of heaters are all over France (I lived in houses with wall hung condensing boilers for 6 years in France) and I never saw any system with zone valves. It's all setup with open circuits and thermo-valves at the radiators that regulate the heat flow.
It makes for a much much simpler setup. Look at my pic: remove the zone valves and everything is clearer!
Could the zones also be overloading the expansion tanks and making them a short life?

Any other ideas?