Replacing iron pipe / Non-filling radiator

motomars · May 11

Hi all. Hoping to get feedback on a couple issues regarding my two pipe hydronic heating system. Note that it has a pump/circulator and all of the radiators are old school cast iron, except two sections of newer cast iron baseboard radiators, as indicated in the attached diagram

First issue is that I'd like to move/replace two sections of the the original 2.5" iron pipe to get more headroom in my basement. In my diagram, I've circled where these pipes are. It's a 15' supply section and a 13' return section. Both connect to the boiler via 1.5" copper. My question is whether I can replace these sections with 1.5" (or smaller?) copper.

At the same time, I'd remove two sections of unused 1.5" iron (also circled) connected to the 2.5" lines (radiator was removed at some point).

My other issue is that the 3rd floor radiator highlighted in yellow in the diagram is almost impossible to get filled. The system is at the recommended pressure, but when I bleed this one radiator it eventually gets to the point where it's pushing out little to no air, with no water release. When the system is on, I can hear some sloshing in the radiator, confirming that it's not filled. Any thoughts on what might be causing this?

Thanks in advance.

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delcrossv · May 11

What pressure are you filling the system to?

Sure your gauge is accurate?

hot_rod · May 11

What size is your boiler? Did you run a heat load calc? If itn is a pumped system, you may not need 1-1/2" pipe.

1" copper pipe for example can move 8 gpm or more, so 80,000 btu/hr minimum.

The higherst point in the system? The top of the uppermost radiator, what is that distance form the expansion tank?

The dimension X .433. Then add 5 psi to that.

Example top radiator 18' above the boiler 18 X .433 =7.75 + 5 = 12.7

So 12- 15 psi should be adequate.

With large diameter pipe you need a lot of flow to purge air out, you may need to bypass the fill valve, or connect a hose to you water supply to get a good flush and purge

high point air.png

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mattmia2 · May 11

put in valves to purge the problem radiator rather than trying to bleed it. There is probably some section that loops in such a way that it is difficult to bleed by gravity.

motomars · May 11

Thanks all. Some more info: it's a Weil-McLain CGI-5. The highest point in the system is about 30' above the expansion tank. I've been running the system at about 18psi (I used a separate gauge to confirm the built-in gauge is accurate).

The W-M website indicates that for a size 5 boiler 1" supply/return piping is sufficient. So I guess that's that answer to the pipe size question - I'll be okay replacing the 2.5" iron sections with 1" copper?

Regarding the problem radiator, it sounds like I should try purging by pushing water through the system or using purge valves at the radiator. I can try those solutions when I get ready to restart the system.

motomars · May 11

One more question. If I'm going to replace the 2.5" iron with 1" pipe, is there any reason I shouldn't use 1" oxygen barrier pex rather than copper?

mattmia2 · May 11

you can use oxygen barrier pex. copper can be easier to work with in larger sizes and sweated copper can cost less if you need to use a lot of fittings. no reason not to mix and match. can use black iron too.

the sizing comes down to how much heat you have to move and the max flow that will be quiet and won't erode the pipe moving that amount of heat. you figure out the flow a section of pipe needs from the the amount of heat it needs t move then you find what size pipe can handle that flow. you may need a way to balance it if you are mixing large gravity pipes and minimum sized pipes. that usually can be done with the radiator valves.

EBEBRATT-Ed · May 11

Changing the pipe to a smaller size may upset the existing balancing. I am not saying you shouldn't go to a smaller pipe just be ready for it. I would not use Pex above 3/4" you will find Pex in the larger sizes difficult. I would use copper.

You need to look at the radiation size to determine which size pipe to use.

As far as the problem radiator goes put a 0-5 or 0-10 psi gauge on the radiator vent location and fill the system until that gauge reads 4-5 psi.

hot_rod · May 11

The size pipe really has to do with the size of the radiator and` the heat load of the room it is in. In some, many cases 1/2" pex is adequate. It can deliver 15,000 btu/hr

Here is a conservative pipe flow chart. Note the 1" pex is quite a bit less flow than 1" copper due to ID.

A home run with 1/2 pex to each radiator might be an option. Locate a manifold in a 2nd floor closet, feed it with 1" pex. Each radiator could be. zone with manifold actuators or TRVs at the radiators.

An example with a ∆P circulator.

mattmia2 · May 11

https://forum.heatinghelp.com/discussion/comment/1855285#Comment_1855285

or home runs from the risers where they drop in to the basement.

hot_rod · May 11

Useful info would be how hot do you need to supply the radiators on the coldest day. Backing down the SWT or using outdoor reset will help comfort and maximize boiler efficiency.

motomars · May 16

Thank you all for the feedback - it's much appreciated. I'm still assessing the situation, but one thing I realized is that my expansion tank is located on the output side of the circulator pump (which is on the supply side). As I understand it, this is a bad idea. Could this be contributing to the problem radiator? Either way, is this placement enough of a concern that I should re-plumb and do it right?

Big Ed_4 · May 16

What I would recommend with that third floor radiator is bumping up the pressure a few pounds , I think you have a broken gauge ..

Hope this helps

EBEBRATT-Ed · May 16

@motomars

its always better to pump away from the EX tank, But in a house with minimal pressure drop it doesn't matter that much

hot_rod · May 17

An example of what happens to dynamic pressure ( pump spinning) in a system with either low fill, or a circ develop in high pressure differential.

So in some cases the upper portion of the system could be running at negative pressure. The second slide shows a system with low fill pressure 5 static, notice - pressure starting just past 1/2 way around the loop.

These are just examples, you would plot your exact piping path to determine where you could go sub-atmospheric. Or gauge a few locations.

Any auto air vents at that location would allow air in, instead of out!

A gauge at that highest radiator should read at less 5 psi static. And ideally not drop, maybe increase when the circ runs. Pumping Away allows that to happen.

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Replacing iron pipe / Non-filling radiator

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