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Endurance 110 (old style blue) hot water recirculation

We use a UP-15-18B Grundfos pump 1/2" line to reticulate the hot water throughout the house--boiler in basement with one story above.Worked fine when the Endurance was hooked up to a storage tank, but due to solar conversion the boiler now makes the hot water directly from the heat exchanger. Problem is that when the circ pump is on, for recirc only w/out demand, the boiler high limits constantly. I have cleaned both sides of the heat exchanger with INTEC and the problem persists. I think this pump cannot handle the flow rate needed by the exchanger. What size bronze or stainless pump would keep the flow rate better matched to the exchanger? Or should I just install the larger heat exchanger sitting in the shop for 20 years?


  • Snowmelt
    Snowmelt Member Posts: 1,416

    The 1/2 pipe to one inch add a 009 or 0011.

    What's your head pressure?
  • rayplumber
    rayplumber Member Posts: 8
    Endurance EBP recirculation woes

    This is an existing house on slab --supplies are 3/4" PEX and the hot water return is 1/2" PEX. I am more concerned with the head on the boiler heat exchanger,which isn't listed anywhere, than the piping since the boiler works great as long as there is a load ie: water demand. The problem comes when the aquastat on the recirc pump "calls" and there is no demand then the boiler high limits. Not enough flow through the exchanger . I don't think these boilers were ever intended to work with a recirculation set-up. Their diagrams in the manuals are vague That is one reason I always used a tank set up. But here we are using the boiler as designed. A 009 will probably take care of any flow issues but seems like such an expensive fix. I was hoping for a smaller higher head pump. But my experience with the Endurance is that most solutions will be expensive!