December of 07 we had a new boiler installed - Burnham MPO with 6 zones run by individual Taco 00R multi-speed pumps. One zone is for indirect domestic hot water and the rest for space heating. All worked more or less well - at least there were no mechanical problems. We did have problems with heat delivery so we worked on insulation and control of air infiltration. We also added a hydrocoil when a new AC system was installed in October. The hydrocoil is a separate pumped zone and supports a central humidifier as well as supplying aux heat. When we did that, we also corrected a problem involving two baseboard heaters not being on the circuit controlled by the appropriate thermostat. All the initial plumbing was ¾" copper. The hydrocoil was plumbed with a combination of 1" copper and 1" Pex. The original install was by one contractor and modifications last fall by another.
Now we do have mechanical problems - repeated air blocks that prevent one or more zones from circulating requiring repeated zone flushing. (At least every zone can be separately closed on each end and the boiler can also be closed off making it easy to purge, but it's still a pain to have to keep doing it --- plus we introduce oxygen that will cause deterioration of the system eventually.)
The most recent contractor suggested the problem was an incorrectly installed air scoop. The scoop was mounted just after the boiler with the expansion tank below - that's OK. But it was at the end of a straight run of 12" and not the required 18". He suggested replacing it with a SpiroVent because he said it doesn't require a straight run and is a better system for removing air. It did help some - now only single zones quit and we have gone up to 10 days without having to purge. Better, but still bad.
The contractor initially suggested that we should close the supply valve for make-up water a few days after the initial setup and purge. That way we would know if a leak developed in the system. Then when we had repeated problems, he suggested we keep it open. Keeping it open may have helped, but not enough to be sure it did anything.
One more point, while we were gone for 3 days over Thanksgiving the system completely jammed so no zones were circulating despite the pumps running constantly. This seems to have happened just as we left so the pumps operated against a complete block for 3 days - one pump was dead and had to be replaced.
The contractor says our system is "over-pumped" so the pumps suck in air. But the system worked for year - up until the recent mods.