Does metal radiator cover affect performance? Absorbs heat or passes it along?

well it certainly blocks a portion of the radiant heat transfer. For convection, you need a slot at the bottom and top for the warmed air to pass out.

Probably not as restrictive as draping a blanket over the radiator but un encumbered would give it the best opportunity to move its output, out😗

I’m not sure it is being eaten? It just doesn’t make it into the room space.

Good news, the next radiator down the line has more heat energy available.

if the top is left open it probably doesn't affect the convective heat output but some of the radiant heat being reflected back to it by the enclosure will reduce the radiant heat output.

Expanded metal like that is too restrictive. You need opening at the bottom and the top (not the front) to get full capacity.

It acts like a chimney.

The primary mode of heat transfer here is via convection. With the open lid and slotted bottom, the radiator cover is fine.

This is a VERY interesting subject, and it encompasses most everything about heat transfer known to mankind, and a few we don't know, "see" or acknowledge. Kudos to Dan and Frank for their contribution, as well as the Dead Men.

First off, remember that thermal energy can't be created or destroyed, only converted, used and dissipated. The actual net output of a radiator, as shown by Dans old charts can be "enhanced" and increased by forcing one of the means of heat transfer to be increased or suppressed , that being the chimney effect of convection.

You can also increase the radiant output, but that requires significantly more manipulation of the variables, i.e. the MRT within the room. If you have a cold room, whereby all the surfaces surrounding the radiator (MRT) are cold, and immediately dump steam into the emitter, once it has its own mass heated up (conduction) and it's emitting surfaces are up to its maximum, its' radiant output is significantly higher than would be under "normal" considerations. Of course, its convective outputs should also be higher at that point due to the same variable, that being differential temperature of the "fluid" (air) being in contact with the emitters (radiator) exposed surfaces.

Covering the emitter changes all of those dynamics. It' can't "destroy" them, it can only change them, and in my minds eye, those changes are numerous and can only SLOW the flow of heat (radiant and convective) but all the heat the radiator is capable of producing should still end up in the room. For example, radiant heat from the radiator will be absorbed by the metal cover. Through conduction (minus the convectionocurring inside the cover) the outer surface of the cover will get hot, and will then re-radiate heat to the surrounding surfaces (minus the convection of being in contact with a cooler air, which then rises up, thereby creating additional convective extraction). The difference between radiant and convective on these outer surfaces are a function of many additional things that most people don't think about, like color.

Not only the color of the emitting surface, but also the color of the absorbing surfaces surrounding the radiator. It is a known fact that the dead men painted the radiators a metallic color to suppress their output. I've tested copper fin tube in joist bays, and by painting the fin tube a flat elm leaf green, and painting the joist bay flat black, increased the radiant output of the fin tube significantly, like by 25% if memory serves. So flat black radiator in a dark painted room will output more radiant than convective energy. This is one of those factors that is tough to "see" in ones minds eye. Copper and shiny aluminum have a different "emmisivity" than do flat colors. I also once tested a ceiling radiator the I had built out of copper and aluminum. Just by painting the metal a flat white color, the delta T on the fluid side went from 5 degrees F to 10 degrees F, and you could "feel" the difference in radiant heat being output.

The convective capacity is a function of surface temperature, surface orientation, chimney affect and fluid (air) flow resistance. Within some radiator enclosures I've seen, there are curved surfaces at the back bottom and back top to enhance air flow and lessen resistance to air flow. Sharp surfaces create eddy currents that cause the flow to slow, and hence reduce output. Surface area and orientation should be fairly obvious. So as with any question regarding Hydronics, the only correct answer is "It depends…"

As Dan always says, keep an open mind and a bright flashlight on hand at all times. It's necessary to "see" all of the players in the game to determine the net results, and even those will change as the room and emitter dynamics change. As the differential in air temperature and surrounding surface temperatures change, so do the active dynamics.

Hope this helped to open your eyes. :-)

ME

Assume any changes made to the radiator will change output.

However adding a fan behind or below will "force" some convection and add output.

The air space between the radiator and the sheetmetal cover will prevent the cover from reaching the temperature of the cast iron, so radiation transfer is reduced.

Any change to the air flow design through the radiator will change the output.

Now you have a constipated radiator.

Any heat that cannot be "emitted" to the room by any changes, goes back to the boiler.

We have radiator enclosures like the pictures below on all of the radiators in our house. The first picture shows our living room radiator enclosure with a 3/4" piece of plywood on its face which creates a very thin opening between the lid and the front grill. The second picture shows the plywood on edge to create a large opening. The underside of the hinged top also has a large piece of reflective insulation for some reason. I did some calculations last season and replaced all the air vents with the correct size Gorton, and insulated as much piping in the basement as I could (that wasn't covered in asbestos insulation). This season the radiators heat up fast and are dead silent. This is where the positives stop.

All the radiators are covered in chipping lead paint. The previous home owners removed radiators on 1/2 of the first floor, leaving the living room radiator to perform a ton of heavy lifting. Our cat likes to sit on the radiator so I almost never put the plywood on edge because it's not that stable.

I want to increase the output of this radiator specifically because our living room and first floor usually run about 2 - 3 degrees colder than the thermostat's set point on the second floor. I want to modify my enclosure to take it from the #1 diagram below to #2. Does anyone have any tips or design references to help with this undertaking?

I sense the cat would complain.

You might be surprised!

My ultimate goal is to have all the radiators media blasted and painted to remove the enclosures in all the rooms except a baby room for obvious reasons. The cost associated with that effort is not feasible right now, so I figure why not improve what I have now and get the immediate return for extremely low cost.

Our cat would adjust, but I just get nervous she would jump up and get burned. The living room radiator gets extremely hot. If I put my hand on the front grille I can't hold it there for more than a few seconds. The reflective insulation under the lid is what makes it possible for the cat to sit on the lid I think. We have a one pipe steam system and I've used my FLIR imager to see that the surface temp gets to around 190. Our boiler room is directly below the living room and I have the pipe wrapped all the way up to the floor joists, so it is screaming hot.

@dabrakeman thanks for the reference material. My first thought was to cut 2 rectangles on either end of the lid and leave some of the lid as is, for my cat to sit on. I would then place some aluminum to create the grilles. For the second option, I'm leaning towards cutting the lid but leaving the hinges intact. Then I could add a mechanism to prop up zone "A" or zone "B". This is probably way less work than option A.

Concerning the mid shield, I could probably get some plexiglass and block the mid section, thus creating an inlet at the bottom, a solid barrier in the middle, and an outlet at the top of the front grille and the lid opening.

Any concerns or comments?

Cutting the lid where you marked it seems an easy immediate remedy for cat and masters. Cats are finicky though. He may like to walk the plank to find his optimal temperature zone😁. Obviously which end you leave him will impact the propensity for a warm seat.

Shield you could probably hook on from the inside. I would lean toward sheet metal or aluminum over an insulating material. Trying to enhance convection, not seriously reduce conduction and radiation.

Cant find any examples of an enclosure that blocks some part of the front but with a wide open top. Guess that is because it is not about aesthetics, rather it's all about the cat.

radiators like that are about 80% convection, 20% convection

Conduction transfer when the cat is touching the cast iron

The amount of heat output depends on the square footage of the radiator

But don’t discount the heat effect of the radiant as you stand in front of a warm surface radiator that is the selling feature if radiant systems

You may need to play around with grill size on top to make sure you cover the heat load in coldest conditions if you go that way

In older homes, windy days can be high load days also