flow rate in converted gravity system with modcon

1bourbon

Looking at a converted gravity system. One zone of CI rads, totalling about 220k BTU/hr with 170F AWT. One zone of 3/4" fin-tube, about 30k BTU/hr with 170 AWT.

The CI rad zone is mostly 1.5-2" piping, with some 3-4" in the basement. Current circ is a Taco 120.

Want to replace the current boiler with a Knight. Heat loss is about 140k.

The questions are:

1) determining desired flow rate in CI rad zone. Standard calc shows a need for about 12-14 gpm to move 120-140k BTU/hr at 20F dT. That's less than the typical 2-4 ft/sec, but assuming trapped air will be bled at the rads. Is that ok?

2) If the CI rad zone is flowing at 12-14 gpm, and the fin-tube zone is flowing 3-4 gpm, the combined flow in the common piping is really close to what the Knight 150 would normally do for a 20F dT (a Taco 0011 would provide 14 gpm). Is that ok?

3) if the flow in the common piping is greater than the flow through the boiler loop, does that create a situation where closely-spaced tees are a liability? Should this be piped differently?

Thanks.

Brad White_203

In general

your flow rates are in range and flow is a forgiving variable.

In order of your questions:

1) No problem. Air will find a way out and if not all of it, what is left will be lonely in there, not causing any problem I can think of.

2) Not sure what the concern is here to you, Rob. The flow rates are pretty close to spec. It is not like you are doubling the flow or anything. Can you elaborate on your concerns? (I cannot at the moment speak to the pump selections for I do not have your head losses.)

3) Having greater flow in the common piping than the boiler loop is excellent for a condensing boiler. This assures that the coldest system water goes to the boiler. If this were reversed and higher boiler loop flow were present, the excess would pass right back to the boiler return and raise the return water temperature, slightly or not.

So long as your closely spaced tees have at least 6 to 8d upstream and downstream from each pair, I do not see any issues there.

I would ask that you consider coming off with the FTR zone first and have the CI zone on it's own mixing valve, assuming that the CI radiation is more abundant versus actual heat loss. They may well take the same water temperature, but one can never be sure.

1bourbon

cool (as it were)

Thanks Brad. My concerns have been allayed, and thus elaboration is unnecessary. Having not thought through a gravity conversion system before, your confirmation that flow is forgiving and we're not getting way out of spec has covered it.

Thanks again!