CI Radiator placement

michael_15

michael_15

Distance from wall

I'm comfortable with the theory of the radiators under the window and on the coldest wall of the house, but I have a question about the distance of such radiator from the wall.

I've heard that the ideal location for a cast-iron radiator is two inches from the wall. (In addition to the under-window, north wall, etc.) Why is this the case, and how did someone test this theory?

-Michael

Big Ed

Distance

They measured the thickness of a paint brush

thfurnitureguy_4

In the library thay said 1.5" and 3 for a spray can. (Ok I made that up)

Mike T., Swampeast MO

2½" from the wall. No, that's not a "rule of thumb", it's a true rule.

Yes, there were numerous old tests. Most however, were concerned with steam radiators as they are much easier to measure output-wise.

The placement on outside walls under windows tends to maximize the convective output of a radiator in particular and the output in general.

It also tends to maximize temperature stratification.

With reasonable modern insulation and infiltration control however interior wall placement in full view of windows/outside walls works extremely well--particularly if reset and/or TRVs are used. Such placement tends to maximize radiation while minimizing convection. Overall output is reduced slightly, but most cast iron systems are very generously sized. When the dead men placed rads this way they often added in some extra iron for what they considered a less-than-ideal placement. This only makes those rads run even cooler with even more radiation, less convection and higher comfort. If you cover such a radiator you completely loose the benefit of the placement!

At least one modern study of modern panel radiators finds that interior walls with the best "view" of windows/exterior walls is ideal for both comfort and efficiency. Such panels should be tall and narrow as opposed to short and wide.

michael_15

Interior placement

I suppose that, you're right, if you've got plenty of radiator, you can trade off the convective heat for radiant heat by placing it in a less-than-ideal-convection spot.

One thing I'd worry about -- doesn't placing the radiator on an interior wall potentially create an issue of "dragging" cold air across one's feet? I wouldn't know if this would be noticeable, though.

-Michael

Mike T., Swampeast MO

All of the upstairs rads (and about half the downstairs) in my house are installed on interior walls. 1903 house with reasonable, modern insulation and decent infiltration control. TRVs on all the rads with full reset. All but one room in the house has two exposed walls and windows (particularly N-facing) are large and abundant.

I've honestly never noticed any draft across the floor anywhere upstairs. The rads are only above 100° in the coldest weather--they're typically much lower in temp.

Only place I've noticed a draft across the floor is downstairs in the big NW facing family room. Three rads in the space, all on outside walls. Two of these are very large twin low "window radiators" underneath a huge bank of N-facing casement windows. Very good interlocking weatherstripping, but no storm glass on these as such would interfere with operation. One of those window rads is typically the warmest (by far) in the house and has MUCH more convection than the rest. The 2nd of the pair only gets warm on RARE occasion. The 3rd rad in the room (other outide wall) is "typical" in temp. I still haven't figured out a way to adjust the TRVs (or piping) to have both of the window rads share the load equally.

Unless raising space temp, the interior wall placed rads have so little convection that you can only detect it with the flimsiest of tissue paper. Convection can easily be felt by hand above the window rad.

Tall radiators tend to be self-limiting when it comes to convection. The air can only get so warm and only move so fast. The air rising through the rad hits its max temp and speed well before it reaches the top of the rad.

Short rads have always been noted for very high convective output.

Modern panel rads are similar. That's why tall, narrow rads are recommended with interior placement. I believe that a 2 to 1 aspect ratio is considered ideal.