maple1
Minister of Fire
Could be too little flow on boiler side, or too much on the other side. If your temps are good on the load side when there's a load on, ya it's likely the boiler side. This is the tricky part of HX sizing & pumping determinations - and also when variable speed or multi-speed pumps come in handy. I like my variable load pump, and multi boiler pump. I don't have an HX, but would come in even handier if I did.