Comfort & Efficiency: Radiators vs Convectors

John Ketterman

Apparently you have outdoor reset. The radiated heat drops off rapidly as the temp differential between the room and the radiator is lowered (I believe it goes as the fourth power of the temp difference). So if you are condensing on a design day, much of the year you are in the 100-120F range and most of your heating is convective. It's not worthwhile or cost-effective to worry about optimizing radiated heat.

Covers that increase convection are worthwhile because they increase net output and allow you to lower the curve. Even a cover that is rated as reducing output 10% may not reduce output for someone with outdoor reset, since you have more convected and less radiated to begin with.

gasfolk

Better Comfort: Radiators or Convectors?

1. In a hot-water system with more than enough cast iron radiators to satisfy design conditions (regardless of any potential reduction in heat output from radiator covers), could certain radiator covers be modified to benefit comfort and economy?

2. Besides raising or lowering heat output, can radiator covers alter the proportions of heat released by radiation or convection?

3. Could greater convection increase heat at the ceiling, potentially increasing energy costs without a commensurate increase in comfort?

4. Could an inch of foam board inserted between the radiator and the inside back panel of the cover substantially decrease wasted heat?

http://www.heatinghelp.com/newsletter.cfm?Id=77

5. Is there enough data to know whether we can improve comfort and secondarily the overall efficency of our system by modifying our radiator covers, leaving them alone, or dumping them?


gf

gasfolk

Better Comfort: Radiators or Convectors?

1. In a hot-water system with more than enough cast iron radiators to satisfy design conditions (regardless of any potential reduction in heat output from radiator covers), could certain radiator covers be modified to benefit comfort and economy?

2. Besides raising or lowering heat output, can radiator covers alter the proportions of heat released by radiation or convection?

3. Could greater convection increase heat at the ceiling, potentially increasing energy costs without a commensurate increase in comfort?

4. Could an inch of foam board inserted between the radiator and the inside back panel of the cover substantially decrease wasted heat?

http://www.heatinghelp.com/newsletter.cfm?Id=77

5. Is there enough data to know whether we can improve comfort and secondarily the overall efficiency of our system by modifying our radiator covers, leaving them alone, or dumping them?


gf

gasfolk

Better Comfort: Radiators or Convectors?

1. In a hot-water system with more than enough cast iron radiators to satisfy design conditions (regardless of any potential reduction in heat output from radiator covers), could certain radiator covers be modified to benefit comfort and economy?

2. Besides raising or lowering heat output, can radiator covers alter the proportions of heat released by radiation or convection?

3. Could greater convection increase heat at the ceiling, potentially increasing energy costs without a commensurate increase in comfort?

4. Could an inch of foam board inserted between the radiator and the inside back panel of the cover substantially decrease wasted heat?

http://www.heatinghelp.com/newsletter.cfm?Id=77

5. Is there enough data to know whether comfort and overall system efficiency can be improved by modifying radiator covers, leaving them alone, or dumping them?

Thanks,

gf

gasfolk

Better Comfort: Radiators or Convectors?

1. In a hot-water system with more than enough cast iron radiators to satisfy design conditions (regardless of any potential reduction in heat output from radiator covers), could certain radiator covers be modified to benefit comfort and economy?

2. Besides raising or lowering heat output, can radiator covers alter the proportions of heat released by radiation or convection?

3. Could greater convection increase heat at the ceiling, potentially increasing energy costs without a commensurate increase in comfort?

4. Could an inch of foam board, inserted between the radiator and the inside back panel of the cover, substantially decrease wasted heat?

http://www.heatinghelp.com/newsletter.cfm?Id=77

5. Is there enough data to know whether comfort and overall system efficiency can be improved by modifying radiator covers, leaving them alone, or dumping them?

Thanks,

gf

gasfolk

Better Comfort: Radiators or Convectors?

1. In a hot-water system with more than enough cast iron radiators to satisfy design conditions (regardless of any potential reduction in heat output from radiator covers), could certain radiator covers be modified to benefit comfort and economy?

2. Besides raising or lowering heat output, can radiator covers alter the proportions of heat released by radiation or convection?

3. Could greater convection increase heat at the ceiling, potentially increasing energy costs without a commensurate increase in comfort?

4. Could an inch of foam board, inserted between the radiator and the inside back panel of the cover, substantially decrease wasted heat?

http://www.heatinghelp.com/newsletter.cfm?Id=77

5. Is there enough data to know whether comfort and overall system efficiency can be improved by modifying radiator covers, leaving them alone, or dumping them?

Thanks

Brad White_185

In Order

of your questions, here is my take:

1. As the posted diagram says, if you have an enclosure with a top outlet and toe-kick inlet (making a very effective chimney) you will increase the output of a radiator by about 5% or as the diagram states, you can deduct 5% for the same potential output. The increase in convective output outweighs the reduction in radiant output. The other cover styles as you can see, require more radiation to compensate for the output loss.

2. Yes on all counts. All covers will reduce radiant output because they block "line of sight" transmission of IR radiation. Anything between you and your radiator, just like anything between Brooke Shields and her Calvins? Not good.

3. The noted convective cover does exactly that, increase convection beyond the loss of radiant output.

4. Yes, particularly in that the delta-T behind the radiator and through that outside wall will be greater than any other place in the house. Higher delta-T, higher heat loss.

5. That posted diagram has it all really. Personally? Give me as much radiant effect as possible. It will warm me effectively even before the room is up to temperature. Bathed in radiant flux, physiologically I will feel warmer than if in a convective environment, even at the same temperature.

To paraphrase Patrick Henry, "Give me Radiant, or give me, well, a beer. Or both."

gasfolk

Thanks, Brad.

If the covers in the diagram were numbered from left to right, and given the above,

6. Would the second cover (low, solid shelf with open front) increase the ratio of radiant to convective output?

7. Could that ratio be much different from the fifth cover, which overall appears to emit 120/130 = 92.3% of the second cover?

8. Since radiation drops with the square of the distance, do radiators (as opposed to radiant floors) depend more on convection to evenly heat a room?

9. If an abundance of existing radiation could make up for overall reduced output, can you imagine an application for a ratio of radiation to conduction higher or lower than occurs with bare radiators?

10. Taken to the extreme, would a pure radiator in the corner of a room would leave the far corner cool? A pure convector?

11. If radiator covers allow some modification of the radiation-to-convection ratio, is it always better to eliminate the covers, or could it sometimes be useful to change from a poor cover to a better cover depending on conditions in the room?

Our radiator covers most resemble the fifth--solid shelf low over the radiator with fully fenestrated grills.

Thanks

gasfolk

Thanks, Brad.

If the covers in the diagram were numbered from left to right, and given the above,

6. Would the second cover (low, solid shelf with open front) increase the ratio of radiant to convective output?

7. Could that ratio be much different from the fifth cover, which overall appears to emit 120/130 = 92.3% of the second cover? (Our radiator covers most resemble these--solid shelf low over the radiator with fully fenestrated grill.)

8. Since radiation drops with the square of the distance, do radiators (as opposed to radiant floors) depend more on convection to evenly heat a room?

9. If an abundance of existing radiation could make up for overall reduced output, can you imagine an application for a ratio of radiation to conduction higher or lower than occurs with bare radiators?

10. Taken to the extreme, would a pure radiator in the corner of a room leave the far corner cool? A pure convector?

11. If radiator covers allow some modification of the radiation-to-convection ratio, is it always better to eliminate the covers, or could it sometimes be useful to change from a poor cover to a better cover depending on conditions in the room?

Thanks

Brad White_185

More

The diagram is, if you look at it, self-explanatory. The "deduct 10%" and "add 20%" in the first and second examples means that the configurations allow you a credit of 10% and a deficit of 20% if you use those radiators.

In other words, say you have a room heat loss of 8,000 BTUH. You could install a 7200 BTU radiator using the first example, because the output would increase by the difference, giving you the full 8,000. Conversely, the second example requires that you install a 9,600 BTU radiator to get the 8,000 you need.

From this, to your questions:

6. The radiant output is about the same, with some loss from the top, but you are not hanging on the ceiling, I suppose. (You may in fact be a bat or a sloth, so no judgments are made here The convective portion is certainly diminished so between the two, the loss is about 20%.

7. These types of enclosures tend toward ambivalent airflow and mixed radiant performance. The presumption is that the face cover is stamped metal of some sort and takes on some temperature above ambient, so I suppose that restores some radiant function; it has to. Just not so effective given the lower temperature.

8. Yes, radiation does drop off by the square of the distance but still has an effect. Naturally placing a couch against the radiator will defeat this but you know where to find the cat. I do not sweat the loss by square of distance thing too much. After 93 million miles, the sunlight makes me happy

9. Not quite sure I understand what you are asking.

10. Regardless of the types and sizes of the radiators you have, you still have to deal with the mean radiant temperature (MRT), that average of wall, glass and roof surface temperatures versus your body's ability to emit heat and how it responds to cooler surfaces. Specifically, your radiator location is everything. If on an interior wall and your exterior walls are not well-insulated, you will feel colder on the side of your body facing the exterior. To compensate, you would have to overheat your interior temperature. Convective or not, you would be in a pickle, but I suppose at least a convector would stand a better chance against a shielded true "radiator", because it would turn over the room air, eventually.

It is for this reason (MRT) that the best practice is to install the radiators along exterior walls.

11. I would eliminate covers wherever possible and enjoy whatever radiant benefit there is. Second choice would be to cut a top grille and leave a bottom inlet (or choose figures 1 or 3 in that diagram) to increase my heat output.

This of course is done only where there are surpluses and deficits which need correction, or of the goal is to use a lower overall water temperature to heat the house.

That all said, the goal is not to increase heat output so much as to match the radiator output to the heat loss of the room and to afford a balance of heating input to loss throughout the structure.

gasfolk

Interesting...

Your insights are quite helpful.

Even with radiator covers, our system heats well at design-day system temps well within condensing range. We may try foam board between the radiators and the rear wall of the radiator covers.

We would probably benefit from completely removing the cover in a persistently, mildly cool room (but the Boss says no to any removals--she likes the shelf space and the look). Your other answers have addressed question #9.

Thanks again for your help.

gasfolk

That is very interesting...

Hi J. Cricket,

101. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Has testing been done at various temperatures?
http://www.heatinghelp.com/newsletter.cfm?Id=77

102. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar, geothermal as opposed to oil and gas)?

103. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

104. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

Thanks

gasfolk

That is very interesting...

Hi J. Cricket,

101. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Have enclosures been tested at various temperatures?
http://www.heatinghelp.com/newsletter.cfm?Id=77

102. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar, geothermal as opposed to oil and gas)?

103. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

104. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

Thanks

gasfolk

That is very interesting...

Hi J. Cricket,

101. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar, geothermal as opposed to oil and gas)?

102. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

103. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

104. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Have enclosures been tested at various temperatures, or can the performance be extrapolated?
http://www.heatinghelp.com/newsletter.cfm?Id=77

Thanks

gasfolk

That is very interesting...

Hi J. Cricket,

101. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar and geothermal as opposed to oil and gas)?

102. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

103. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

104. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Have enclosures been tested at various temperatures, or can the performance be extrapolated?
http://www.heatinghelp.com/newsletter.cfm?Id=77

Thanks

gasfolk

That is very interesting...

Hi J. Cricket,

101. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar and geothermal as opposed to oil and gas)?

102. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

103. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

104. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Have enclosures been tested at various temperatures, or can the performance be reliably extrapolated?
http://www.heatinghelp.com/newsletter.cfm?Id=77

Thanks

gasfolk

Trying to understand the implications...

Hi J. Cricket,

101. Are there benefits to increasing net output and lowering the curve beyond a) reducing losses elsewhere in the system, b) possibly allowing condensing operation, and c) possibly allowing greater use of renewable, "low quality" energy sources (e.g. solar and geothermal as opposed to oil and gas)?

102. Is one trade-off of lowering the curve a drop in comfort while standing near a window or poorly insulated outside wall? Does a radiant floor have the same drawback?

103. Could insulation inside a radiator enclosure (between the radiator and the enclosure's rear wall) increase net output to the living space by 20% or 30% using an R-value that would fit within the enclosure? Will one inch of polystyrene do anything useful?

104. If testing conditions affect relative performance of radiator enclosures, are the conditions known for the diagram? Have enclosures been tested at various temperatures, or can the performance be reliably extrapolated?
http://www.heatinghelp.com/newsletter.cfm?Id=77

Thanks

John Ketterman

101. The main thing is increasing efficiency, using a condensing boiler. I don't know the technical details of alternative energy sources.

102. Outdoor reset (lower water temperature) is better suited to comfort near windows. With "bang bang" heating there is a lot of heat from the radiators when they are on, and the rest of the time you feel the cold from the windows. With outdoor reset and a lowered curve, the system runs much longer (ideally all the time), leading to a more even and steady heat distribution.

103. Insulation prevents conduction, but that is not a major factor anyway; radiators lose heat mostly by convection and radiation. Even the thinnest sheet of any material stops convection, and thin reflecting sheets turn radiation around. In any case, there is no practical way to create a temperature difference between the back and front of a radiator that is large enough to save any measurable energy. The heat evens out soon enough through air circulation; you can't choose to heat just one part of a room or one part of a house. (I know many people believe otherwise but they are wrong.)

104. Don't know.

gasfolk

Thanks.