Cold radiator

[dbanks2552]

I own a 3 story house with a gas boiler and both cast iron and slant/fin type radiators. most of the radiators in the house run warm, but there are a couple of the slant/fin radiators that are cold. I bleed the radiators and all I get is cold water. I have tried bleeding a couple gallons out in the past and still I get no heat. We recently replaced our boiler with a high efficiency Weil-McClain unit. since then, one of the slant/fin radiators that wasn't working is now producing some heat, but there are two that used to work that are no longer working and one that never did and still does not work.
the house is 100 years old. is it possible if the radiators are off-level that the bleed valve is now not the highest point? would that matter? (I know that is not the case for at least one of the problem units as I checked level today.)
There is also one unit under a cabinet that does not appear to have any bleed valve at all. Does anyone have an opinion on saddle valve to try to bleed this. From what I can tell, they are prone to leak.
I spoke with my installer about the non-working radiators. He bled one of them of a few cc's of water and said see if that helps. It did not. He doesn't seem to have any good ideas about how to fix this.
The first floor is all cast iron and all works fine. the second floor has cast iron (all working) and some slant/fin. one is working and two are not. the third floor has two slant/fin. one is working, one is not (and has never worked).
The pressure in the system is about 20psi, I believe.
Is there some reason that I would not get hot water when bleeding? I guess I am not bleeding enough as they seem to be hooked up to the system since I am getting water. Is it possible the return line has been closed off somewhere so that when I bleed I get water but then without a loop, I get no circulation through the radiator.

Any thoughts? what other information would be helpful?

Thanks

[Shawn_from_DE]

First if the fin tube radiation is controlled by the same thermostat as the castiron radiation you will never have even temps. Castiron heats up slow and cools off slow, while fin tube heats up fast and cools off fast.

Without being there to see the system it's hard to figure out what's happening. Especially on older mixed systems like you have. But I will give you a small primer on hot water systems and how they work.

If the system is set up in series loops you may be air bound somewhere and need to purge the air out. A series loop is one continuous loop that travels into and out of each piece of radiation on the loop. Bleeding won't always move a large air pocket from a series loop. Do you have purge set ups down near the boiler or on the returns for each loop? In case you dont know, a purge set up is a valve to shut off flow on the return with a tee and boiler drain just up stream. You shut off the valve open the feeder bypass or feeder or hook a hose up to the boiler drain on the boiler from a hose bibb or faucet to provide higher flow and pressure. You put a hose on the purge set up boiler drain and allow water to flow until no air comes out. Sometimes you need to run the purge hose into a bucket so you can see if you are still getting air.

Even if it's not series loops and us a traditional two pipe system where each piece of radiation come directly off a supply main and back to a return main the non working pieces could be air bound due to poor piping installations; lots of ups and downs. They could have too much resistance to flow compared to units nearby. Water takes the path of least resistance. Remember a circulator doesn't pump water it just creates a pressure differential that allows the water to move. That is how hot water boiler systems get air bound because big pockets of air dont allow the pressure differential to be realized across the whole circuit.

If it's a monoflow system one main loop around the perimeter of the basement with lots of tee's coming off it could be a problem with being air bound or poor system design/rework do to additions to system. Monoflow systems work by creating a pressure differential between the two connections for each piece of radiation. This is done with special tee's that have orifices built into them to create some resistance in the main thereby making the radiation the path of less resistance. So a portion of the mains flow goes around the resistence by traveling through the radiation around the resistance and back the the main.

Hope this helps

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