Heat takes a long time to reach some radiators in a hot water system
I have a 3 family house (3 stories) with an oil fired, hot water system with a single zone. I have a circulator pump, but one particular stack of radiators (1 cast iron radiator on each floor all fed off the same risers) takes forever to get hot. When i turned the system on last month, i kicked the heat up to 80 while bleeding the radiators in the entire house. Most radiators required very little bleeding to get hot water. However, this one stack of radiators all needed to be bled for about 20 minutes each before getting hot water. These 3 radiators used to work great but as of last year started to have issues. I'm wondering if perhaps my circulator pump is on its way out or if i need to add an additional circulator pump on this one particular stack. There are no leaks in the system at all and all risers are exposed as they pass between floors. Any advice would be greatly appreciated.
The entire house is heated by cast iron radiators. I am having issues with this one particular set of radiators that come off the same set of risers between the floors. Every other radiator in the house is fine. The pressure on the system is usually around 22-25. I have a TACO 007-F5 circulator pump on the system today. I was told by another heating contractor that it is possible that the circulator pump is on its way out and that i can replace it with a new high-velocity pump (around $100 for the pump). I know for a fact that this particular set of radiators used to get extremely hot about a year or so ago.
One more thing to note is that this is a single zone system in a 3 family house. I have the honeywell thermostat mounted in the basement and i am using the Honeywell remote sensor that was installed on the first floor.