old cast iron radiators primary/secondary layout question

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ColinMcM

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New Utica combi boiler in basement. I am reinstalling three old cast iron radiators on third floor for 600 sq ft tenant apartment (bedroom, bathroom, LR/kitchen). The rads had previously been supplied by steel pipes exposed in rooms running vertical through first and second floors (brick house). Old pipes were removed as we redesigned and changed heating system in our unit, which is on floors 1 and 2, adding central air with a hydrocoil and forced air floor ducts as well as hyper heat minisplits for the bedrooms.
I plan to run 2 lengths of 3/4" orange pex through a chase in the walls, vertical up to 3rd floor. These will be tied into the primary loop (1" copper) off the boiler in basement with a monoflow tee, which had been used for each radiator previously on only the return side. Question: do I need a monoflow tee on both supply and return in this case, since it's a pretty far distance, about 25 feet vertical rise? Then I want to create a supply manifold with 3 supply branches off it in the floor joist space of the 3rd floor, and a return manifold with 3 return branches, for the 3 rads. The old rads had large steel pipe reduced down to 1/2" copper on both supply and return sides. Question is do I need monoflow tees on those branches off the manifolds? And what kind of spacing between branches is permissible? Also how far horizontally can I plan to run in the floor joist space, to get to the radiator locations, without running into trouble with heat distribution?
Does this plan sound reasonable? Anything else I should add to the system? Thanks for any advice.
 
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Breplum

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u might try the heating help/the wall site.
maybe use oxygen barrier pex too.
fyi, old water piping was steel, not cast iron that I've ever heard of. cast iron is drainage piping material
 

Fitter30

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Pex must have a oxygen barrier and might have their own pump. Why use mono flow tees just pipe them reverse return ( first in last out ) with its own pump check valve and thermostat. Combi boiler is only efficient when return water is under 140° above it drops to 87%. What kind of device are you using for air removal?
 

John Gayewski

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Don't use the monoflow tees. They have limits and 25ft sounds like it will exceed their capabilities (although I'd have to look to be sure). Monoflow systems need to be perfectly sized for a system and aren't flexible.

Your gonna want a secondary pump for this system. Just put two tees close together in the primary loop and put a pump with a spring check. It'll depend on how your primary loop is setup but this is a general common setup. You should actually buy a valve that has two taps in it so you can bleed the loop.
 

ColinMcM

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Thank you for the advice guys, and the valve link. It is oxygen barrier pex that I have, a big roll of it, hoping I have enough already... I like the idea of the pump. Which pump would you use for this size system? How do I make that pump talk to the boiler?
3rd floor (1BR apt) is going to be the only heat load on that combi boiler. Our owner occupied unit has its own boiler so the tenant will pay their own gas bill for heat and hot water. In that case I don't need any signal relay correct, just the thermostat? The copper primary loop previously went all the way around the basement of the house but I'm making it smaller now, will only be about 25 linear feet of 1" copper with the boiler in the middle of loop. Should the circulating pump be right by the start of the supply 3/4 pex branch that goes to 3rd floor? Are the manifolds I proposed unnecessary? If they are indeed necessary, can I make the manifold out of the same 3/4 pex? Is there a simpler design that will work instead of manifolds, like running the pex through the radiator in the LR/kitchen first (biggest space) then the BR rad, then WC rad, just a chain of rads arranged by reducing square footage of heated space? Or do I go with reverse return like you said (Last In First Out, with a supply and return manifold). Is it still a good idea to do the biggest room first with the manifolds? I just made a quick drawing of the reverse return as I understand it... One other complication is that the vertical chase I have available to pass through the 1st and 2nd floors is very close to the locations of the BR and WC rads on 3rd floor, but not that close to the LR rad, the largest space. So no matter what I will have a pretty long horizontal run of piping to that one rad's supply and return connections through the floor joists. Any concerns with that?

3rd floor heat drawing.jpg
 

John Gayewski

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Your pump is sized based on the heat loss for the space and the longest run of tubing.

You need to know how much output (btu's) you want each radiator to output. The problem with going with no manifold is the pump most likley need to be digger and even with a reverse return system or won't be perfectly balanced and WILL need balancing valves. So you can either balance the radiators in each room or at a manifold.

If you pipe the radiators in series you can use a smaller pump but will need to account for less output on down the line. First radiator gets 180 supply temp second gets 170 supply temp third gets 160 supply temp.
 
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