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# Flow rate - Stumped!

Member Posts: 10
I have a granfos pump Ups15-35fc connected on the hot water side of my boiler. After the pump the water splits into the heat exchanger and to domestic hot water. Out of the heat exchanger it connects back to the Cold supply of the boiler. This creates a recirculation loop when the pump is turned on by a thermostat.

I am trying to resize the heat exchanger I need to support my radiant system. To do this I need flow rates of hot and cold into the HEE.I have my pump flow verse head loss chart, but I cannot figure out the head loss in that circulation loop. Is it the approximate 6 ft of 3/4 copper pipe + the HE + the boiler + fittings? I keep doing the math and coming up with a head loss figure that exceeds the pumps rating which I know can't be true.

• Member Posts: 18,973
I don't suppose you have some handy valves that you could use to measure the head gained across the pump directly?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 10
I might be able to place one directly before the pump (between the boiler and pump) but after...not easy...how far away is safe? I can do on the cold into the boiler..
• Member Posts: 18,973
The big variables are the boiler itself and the heat exchanger. So if you could find a place somewhere between the pump and the heat exchanger you could at least get the head loss through that and through the boiler. Piping losses are probably pretty minor in comparison -- so that should get you into the ballpark, anyway.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 605
another approach is based on BTUs and delta T. If you know the output of the boiler you can approximate the flow based on the temperature rise.
• Member Posts: 10
> @SuperJ said:
> another approach is based on BTUs and delta T. If you know the output of the boiler you can approximate the flow based on the temperature rise.

199k BTU boiler. Is the delta between the water temp out and then temp when it's recirculated back into the boiler?
• Member Posts: 605
BTU = GPM x DeltaT(degF) x500
GPM=BTU/(DeltaTx500)

You have to adjust the 199,000 for efficiency. Multiple it by 0.8for 80% efficiency for example.