Short Cycling Boiler and circulation pumps


Old 11-23-17, 10:21 AM
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Question Short Cycling Boiler and circulation pumps

Just getting settled in my "new" ( to me ) house. A week or so ago the boiler stopped with an error about low flow. The main circulator is a three speed pump that was set to low so I moved it up to high to try and buy some time.

With the pump on high and a call for only hot water the boiler does not error but it cycles rapidly, as in on and off twice in 2min. I can hear the circulator pump running and it makes a little rattling noise, not good. I tried bleeding the system thinking it could be air lock but it did not make a difference.

When the boiler fires the water temperature gauge moves up until it reaches temp the boiler shuts off then the temperature quickly moves back down again. Here's a video I made of it cycling:

So this has me feeling like maybe the main circulator pump is bad and not flowing enough water. I started poking around to understand the pump that's in there and I'm not sure if I am doing this all correctly:

Here is a diagram of the entire system:
Boiler's head loss curve:
Hot water tank stats:
Circulator pump curve:

Seems like I need at least 8GPM for the 1.25" copper and the water tank recommends 8GPM as well. Water tank literature says at 8GPM there's 6ft of head, if I read the boiler curve right it says at 8GPM is ~7.5ft and there is 10ft of 1.25 copper pipe so that's ~.5ft. of head.
Looking at the current pump's curve I don't see how this could ever have moved 8GPM with a 14ft head.

The piping for this system is different than what I have seen before am I doing something wrong or is this pump just to small?


Last edited by geoff77; 11-23-17 at 10:28 AM. Reason: Formatting!
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