question for dan h.

bruce smith

hey dan, we're installing old cast-iron hot water radiators
in an old house. the home owners picked out all the radiators (after we told them what size and configuration). some of the radiators are tapped high and low, some are low on both sides and some are high and low on the same side. while these various configurations probably worked fine with a gravity system are we looking at potential problems by using a circulator? we're going to pipe a reverse return system. the owners spent a bunch of money to have the radiators sand blasted and re-painted so we're reluctant to pull the bungs out and change the tapping locations. thanks in advance for any help or advice you can give us.

Radiator Ranger

Fitting Radiators

Hi -

I'm no Dan Holohan but I've got some interesting thermal images of cast iron radiators plumbed in a manner similar to what you are describing. These are from a water system that serves over a hundred buildings. There are many aspects of this system that are likely different from a system that serves a single house but the radiators themselves are the most common specimens.

Included are 3 images.

1) This first is of an old radiator installed winter 2008. The radiator was flushed and the interior clear. I plumbed it bottom to bottom with the bleeder valves in on the top, outgoing port. This radiator has large fins i.e. the tube the water moves through is ~2.5".

2) This is a picture of a radiator installed 25 or fewer years ago. It is plumbed bottom to top with a bleeder valve in the top port on the side the water enters the radiator. It appears that water does not flow through the entire radiator. The fin cavities are about 3/4-1" in diameter.

3) This is a picture of a radiator installed 25 or fewer years ago. It is also plumbed bottom to top with a bleeder valve in the top port on the side the water enters the radiator. From the image and temperature differences between the top, bottom and middle of the radiator, I don't think water is flowing through the middle of the radiator and the heat present there is due to conduction. The fin cavities are about 3/4-1" in diameter.

My conclusion based on these images and assessment of other radiators pulled from this system is that bottom to bottom plumbing is the best for water systems and larger fin tubes offer less resistance overall. That being said, it took 20+ year of presence in a system that has definite flaws for these radiators to develop this way (not to mention we know nothing of the system the radiators were removed from).

The issues I would consider are: Do you have good ways to eliminate air from the system? Are the radiators sized for the boiler and room they will be installed in or are they oversized? How long do you want this system to heat the structure is serves 20+ years, 50+ years etc.? Did you pressure test the radiators before prepping them and painting them? Is there someone with an aptitude for changing the fittings - it's 'easy' but requires significant effort.

Radiators are forgiving and adaptive; in the pictures, the radiators are all oversized for the space so they are providing adequate heat BUT they are reducing overall system performance and are on the list for replacement....

I hope this helps in some way; may your system be a comfort!

Warmly,

Radiator Ranger - Gwen Healy