sizing rads

Bob_36

I want to replace a thin tube c.i. rad with a wide tube one in a hot water system. My question is, does the thin tube rad of equal size emit more heat due to more surface of all the tubes over the wide tube model? A comparison is a glass of water with crushed ice will chill water faster than a glass with ice cubes. On the other hand, The glass with ice cubes will keep the water colder longer. Am I making any sense, or am I all wet(pun intended)?

Mike T., Swampeast MO

Much depends on what you are calling "size".

The size of radiators is measured in EDR (equivalence of direct radiation). Such is related to the actual surface area of the radiator but not perfectly.

If you're talking about physical size a tube-tube radiator will almost always have more output ability (EDR) than a column-type radiator of the same physical dimensions. The last radiators widely manufactured (often called "slenderized" tube) were especially compact with regards to EDR vs physical space (they also used considerably less iron).

Mike T., Swampeast MO

You'll find EDR ratings for most common radiators here.

The good thing is that radiator manufacturers were quite standardized. e.g. For a general style and a given number of tubes/columns and within an inch or two of height, EDR and section width will be the same.

The bad thing is that the general shape of a standing iron rad can significantly affect output. If you are replacing, really such should be done with a rad of the same general proportions.

Short and deep "window radiators" have the greatest output potential per unit of EDR. This is because they are especially good at moving air, e.g. convecting because the moving air has hit near its maximum velocity at the top of the rad. Friction prevents faster convection with taller rads.

Two-column rads (they're always tall) also have relatively high output for a completely different reason--they excel at radiation because so much of their area is directly in "view" of the space. Unfortunately, they could get AWFULLY long...

Standard, tall tube-type radiators (up to four-tube or five-tube depending on particular design) are generally as good as tall two-column rads. The later "slenderized" tube rads fall in this category as well. My personal opinion based on observation is that five-tube U.S. Radiator rads in the "Capitol" design are especially efficient and strike a great balance between radiation and convection.

Standard, tall tube-type rads (more than five-tube or six-tube depending on particular design) will have a less output potential per square foot of EDR. This is because the additional tubes do nothing to add to radiation and instead rely solely on convection.

Tall, column-type rads loose output potential per square foot EDR with each column added beyond two. They could not increase radiation and tried to increase convection with extremely complex castings in the inner columns but such was limited by the height. For the same physical length a four column rad will have little more output than a three-column and it certainly used LOTS more iron...

Bob_36

Mike, thanks for the info. I'll look over the charts before I decide to swap from one type to the other. There is definately some good information in the charts you provided. Thanks