Boiler Facts

By George Carey

Can water

move too fast?

“I think the room is under-heating because thewater is moving too fast through the baseboard. If Iuse a smaller pump or at least throttle down on thepump’s flow rate, the baseboard will start heatingbetter.” I have heard this statement numeroustimes from a variety of heating technicians thatwere having problems with their customer’s heatingsystem. I guess you could reason that with thewater moving so fast, it doesn’t have enough timewhile inside the baseboard to “give up” its heat.Unfortunately, you are giving the BTUs too muchcredit…they aren’t that smart!

So how do “BTUs” in hot water give off their heat?There are three methods that govern heat transfer:thermal radiant, conduction and convection. With-out getting too deep or technical, the mode of heattransfer that we want to focus on with a baseboardsystem is convection. Most people are aware of theconvective nature of the air surrounding a base-board. The hotter, lighter air wants to rise andfloat into the room while the colder, heavier airwants to drop to the floor and move along towardsthe baseboard to replace the hotter air that has justfloated up. In the process, the hotter air gives upits BTUs to the cooler surroundings.

But before this even takes place,there is another convective occur-rence that must take place…andthat is the hot water (fluid) flowingthrough the tubing has to give off itsheat to the tubing wall. Before thebaseboard can emit heat into thespace, the heated stream of watermust transfer its heat to the base-board’s inner pipe wall by convection.

Now, at the risk of getting too deep,for convection to occur, there arethree factors to consider: 1) surfacecontact area, 2) temperature differ-ence between the water and theinside wall of the tubing, and 3) theconvection coefficient which iscalculated based upon the propertiesof the fluid, the surface area’s shapeand the velocity of the fluid.

Instead of spending too much time

with the math formulas, I think if you use yourmind’s eye, you can visualize the following: as thestream of water flows through the baseboard, theouter edge of this stream is in direct contact withthe tube’s inner wall. This “rubbing” against thewall creates drag, which means the water that istouching the inner wall of the tubing is movingslower than the “core” or inner stream. Because ofthis, the temperature of this outer layer of waterbecomes cooler than the inner stream. This drop intemperature impacts the rate of heat transfer—itslows it down.

Remember, one of the factors that affect convec-tion is temperature difference! In fact, a good visualis to think of this outer layer as an insulator thatimpacts the rate of heat transfer from the hotterinner stream of water to the tubing’s inner wall.This is especially true when the speed of the waterapproaches laminar flow instead of turbulent flow.So in effect, the faster the water flows through thetubing, the thinner the outer boundary layer orinsulator becomes, thus increasing the rate of heattransfer from the hotter inner “core” water to thetubing’s inner wall.

You can confirm this by taking a look at any

baseboard manufacturer’s literature and check outtheir capacity charts. They typically will publishtheir BTU output per linear foot based upon twoflow rates: 1gpm and 4gpm. The BTU output isALWAYS higher in the 4gpm column.

Here is another way of looking at this concept.More speed (faster flow rate) equals more heattransfer (higher output). Consider the size of a hotwater coil used in an air handler and the amount ofBTUs it can provide. Now think about how muchfin-tube baseboard you would need to install toprovide the equivalent amount of BTUs. Obviouslythe difference is the speed at which the fan blowsthe air across the coil, which is much faster thanthe air that flows naturally across the baseboard.

Everyone experiences this phenomenon eachwinter. The weatherman refers to it as the “windchill” factor. He will tell you what the actualtemperature is outside, but because of the windchill, it will feel X degrees cooler. Why? Because thecold air is moving across our body much faster,which in turn takes heat away from us muchfaster; so it feels colder than it really is.

A trick of the trade that has been passed on tofellow technicians over the years has been to turnup the aquastat setting on the boiler to increase thewater temperature. The reason the guys will do thisis if a room or a zone isn’t quite heating up to thethermostat’s setting, they find that by increasingthe water temperature, they can increase theamount of BTUs per linear foot available from thebaseboard. (Of course, this will only work if theboiler is big enough to offset the home’s actual heatloss.)

Knowing that baseboards can provide more heatwith hotter water, follow this next example to seewhy moving the water faster and not slowing itdown will provide more heat. Let’s use the averagedesign conditions when sizing for the amount ofbaseboard you would need to offset a room’s heatloss. Typically, we design a hot water heating

system for a residential home at a 20°F tempera-ture drop. That means the water would enter theradiation at 180°F and exit 20°F cooler at 160°F.The average water temperature in the radiationwould then be considered 170°F.

You would then check the BTU/h rating of thebaseboard at this average water temperature anddetermine how many feet of baseboard the roomneeds to offset the heat loss.

What happens if we increase the flow rate so thatthe water only takes a 10°F temperature drop? If itentered the radiation at 180°F and came out at170°F, then the average water temperature wouldbe 175°F. At this higher average water tempera-ture, the baseboard would be capable of providingmore BTU/h. output. Taking it one step further, ifthe system only took a 5°F temperature drop, theaverage water temperature would be 177.5°F in theradiation, yielding even more BTU/h output.

So now, you can clearly see that water can not bemoving too fast through your baseboard to preventthe BTUs from jumping off where needed. Ofcourse, to achieve these tighter temperature dropswhile delivering the right amount of BTU/h, theflow rate has to increase accordingly. And herecomes the trade-off…with higher flow rates througha given pipe size, the pressure drop increases quitedramatically. The result can be very large, requir-ing high headed circulators that cost more and usemore electricity. Also the higher flow rates increasethe velocity of the water—and at some point thevelocity will create noise issues. The point is not todesign a system around these high flow rate/smalltemperature drops, but rather to recognize that thenext time someone tells you they think the water ismoving too fast and that is why the room is under-heating, you’ll know that’s not the cause of theproblem.

If you have any questions or comments please callme at 1-800-423-7187 or email me atgcarey@fiainc.com.

Can water

move too fast?

from the May 2009 issue of