Belgiumboy - can you give some more info on the pump model - the UPS 200 covers a number of different pumps. If you can give the actual one I will look up the curves.
This does sound like cavitation as the others have pointed out. This can really damage a pump in fairly short time. To get around it there are a few things you can try:
1.) try reducing temperature although there are limits - if your boilers aren't designed to run in condensing mode they won't last long at low temperature operation.
2.) Reduce pump speed - all UPS have a three speed selector, make sure yours is running at the slowest.
3.) reduce resistance at the pump negative pressure side - make sure the inlet valves are fully open etc,
4.) increase resistance on the pump delivery side - daft as it may seem, if you can partially close a valve downstream of the pump the inlet absolute pressure will rise and may stop the cavitation.
5.) The expensive option - fit an inverter to slow the pump down.
6.) Change the pump - to cope with the full available output of the two 53 kW boilers the pump will need to move about 2.3l/s against your total system resistance.
Hope this is some help.
This does sound like cavitation as the others have pointed out. This can really damage a pump in fairly short time. To get around it there are a few things you can try:
1.) try reducing temperature although there are limits - if your boilers aren't designed to run in condensing mode they won't last long at low temperature operation.
2.) Reduce pump speed - all UPS have a three speed selector, make sure yours is running at the slowest.
3.) reduce resistance at the pump negative pressure side - make sure the inlet valves are fully open etc,
4.) increase resistance on the pump delivery side - daft as it may seem, if you can partially close a valve downstream of the pump the inlet absolute pressure will rise and may stop the cavitation.
5.) The expensive option - fit an inverter to slow the pump down.
6.) Change the pump - to cope with the full available output of the two 53 kW boilers the pump will need to move about 2.3l/s against your total system resistance.
Hope this is some help.