Re-piping and re-zoning radiant heat system

farm_shenanigans

Good evening everyone. I live in WNY in a 140 year old farmhouse. We heat with a boiler and cast iron radiators. Previously the boiler was fuel oil but recently switched over to natural gas. We have a Navien NHB-150 boiler on a single zone system. The system is piped with old black iron or cast iron pipes with a trunk and branch system. We have American Radiator Company Rococco Cast Iron Radiators. They are three column radiators approximately 9" wide and 38" tall. The number of sections per radiator varies from 11 to 19. The calculated heat emission rate for the radiators varies from approximately 5000 BTU to 9000 BTU. The longest distance from one of the radiators to the boiler is 60'. We currently have 8 radiators but plan on adding up to 5 more (7 on the first floor, 6 on the second).

The system performs okay but I'm looking to re-pipe to add headroom to the basement, and to address poor heating with some of the radiators farthest from the boiler, as well as move some of the radiators around.

My plan is to switch over to a two zone system (one for the first floor rads and one for the second floor rads). Plan is to use a manifold/circulator pump per zone and to use hydronic pex for the home runs.

To that end I have a couple of questions I was looking for help with:

1. From doing some research on this website as well as some others, it seems that the appropriate PEX size for the radiator home runs is 1/2 inch. 1/2 inch would accommodate up to 10000 BTUs and I don't have more than 9000 BTUs. Would 1/2" pex be correct or should I increase size? What would the cost/benefit be of going to 3/4" other than cost of material?
2. I currently have 3 circulator pumps (Taco 007) on the system. Per the diagram, #1 on the return side from the boiler mate indirect hot water heater, #2 on the return side by the boiler and #3 on the supply side prior to the junction of the heat emitter piping and the boiler mate piping. I'm having a hard time understanding how the three circulators interact. Is it correct that circulator 1 activates when DHW is called for and pulls the hot water from the boiler through the boiler mate. Circulator 2 activates when the boiler turns on for any reason to circulate water through the system. Circulator 3 activates when heat is called for the rads (but also pushes it through the boilermate). Aren't 2 and 3 redundant? Given that I'm planning on adding two manifolds each with a circulator pump could I use any of the circulator pumps on the current system for the new set-up?
3. My plan with the new manifolds is to continue the 1.5" copper piping on the supply side after the flo control valve to a circulator pump (Taco 007?), pipe goes down to 1" and then to the manifold. I plan on purchasing the manifolds pre made from one of the big companies (uponor, rehau, caleffi, etc) and then running the 1/2" pex to the boilers.

Thank you for your help, time and expertise.

Andrew

Jamie Hall

Basic idea is fine.

That said, I (at least) would not use PEX on the supply side, or if I did I would support it in troughs, not pipe hangers. At anything much over 110 PEX tends to get floppy and sag over time…

hot_rod

Maybe that is just a simplified drawing, but you need an air sep and I highly recommend a magnetic dirt separator with old iron or steel pipe systems.

You can easily push 1.5 gpm through 1/2" pex for short runs. Here is an example of an 80' loop of 1/2" pex at 1.5 gpm. So 15,000 BTU/hr if you need to.

The PPI website has a flow calculator to allow you to play with flow rates, gpm, velocity, head, etc.

https://plasticpipecalculator.com/PressureDropHeadLoss.aspx

You can run hydronic, oxygen barrier pex to 180F without any issues, allow for expansion as you run it. Support every 24" and you will be fine.

With a cast iron boiler you could get away with a 1 pump system, zone valves for the zones and indirect. Some of the newer smaller size cast boilers have minimum flow rates, older ones work with minimum or no flow.

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