Radiation and convection often confused in heating

Posted Dec. 03, 2010, at 9:36 p.m.
Last modified Jan. 29, 2011, at 12:47 p.m.

Physics and technology are often not well understood by the media. There recently was a program on TV about heating. The presenter referred to hot water heating baseboards as radiators. This is a simple mistake, but there is very little heat that comes off a baseboard by radiation. These heating units deliver heat into a room by convection. Convection is the movement of heat by warm air rising and cold air falling.

If you look inside a baseboard heater, you will see aluminum fins that are pressed onto a copper pipe. The hot water that flows through the pipe heats the fins. The space in between the fins allows the air in that space to get warmed. As the warmed air rises, it exits the baseboard housing and heats the room.

All this heating is predicated on the fact that the fins are clean and the space on top and bottom of the housing that holds the fins is open and cleared of debris.

Baseboard heaters should be cleaned once a year. The covers can be simply popped off and a vacuum cleaner can make short work of this task. Most people never do this. It must be done to ensure proper room comfort and heating system efficiency.

Another little secret of baseboard heaters is that there is a louver on the top end of the enclosure. This can be rotated to allow more or less heat flow from the baseboard. You can fine-tune the room temperature if several rooms are on the same loop of heat by adjusting these louvers.

If you want to completely cut off the heat flow and the louver is missing, the baseboard can be covered up with a blanket or be taped off. Most people inadvertently do this with dirty clothes or by blocking the baseboard by stacking stuff right against the baseboard. The good news is that when this is done to a hot water baseboard, it is not a fire hazard. If you do this with a conventional electric heater, you will set your house on fire.

Hot water and steam radiators function somewhat similarly. They actually do radiate heat into the room. Radiant heat is the heat that you can feel coming off a wood stove or when the sun shines on you.

Radiators will deliver heat into a room by radiation, but most of them will also deliver a reasonable amount of heat into the room by convection. If you look at a radiator, you can see that they have open areas in between the sections of cast iron that are pressed together. Air is heated in between the sections, and the heat will rise off them. Most radiators in older homes will work like the day they were installed. Many homeowners and heating people prefer the comfortable heat that they can deliver.

They usually do require a little updating. The control valves that are on old radiators are usually stuck open — hopefully! — or they are very hard to turn. These can be easily replaced with new valves. New radiator valves can be fitted with simple thermostatic heads that can control each radiator as its own heating zone. They are completely passive and can make the radiator perform more efficiently. The better efficiency comes from the fact that the room will not overheat from the valve that’s stuck open.

Radiators should also be cleaned a couple times a year. They are easier to deal with because of the big open spaces and the fact that you can see the dirt in between the large sections.

Over the years, large old cast iron radiators have been covered with housings that disguise them as heaters. Unfortunately, many of these units are architectural treasures that really can be fine accent pieces in a vintage home.

Old radiators can be removed, sandblasted and repainted with automotive finishes. I am too lazy to want to remove a several hundred-pound radiator and get it refinished. This usually got done in place in the house.

The last house we did had 17 radiators, and the prospect of having a professional refinishing of 17 radiators did not balance against the joy of moving them.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329.

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