Fin tube baseboard ratings

Jay Wisth

I am working on a building with fin tube baseboard throughout. Some areas have theromstatic valves, some do not. The energy management company has a reset schedule that has the water at 150 degrees at 50-60 degrees outdoors, ramping up to 190 at 10 degree outdoor. To combat the obvious overheating issues in mild weather, he is cycling the pump off the lowest zone temperature in the building. As you might guess, this is a hit and miss strategy. His statement was that fin tube "loses all convection" below 140 dgrees. I would like to counter that with some hard facts, but the ratings I have found don't go below 140-150. This is on a hot water system heated by municipal steam, so no worries of low water temperatures going back to a boiler.

Uni R

If that were true...

If that were true I'd be freezing right now. It would be nice if the manufacturers would list them down to lower - such as 120* and even 100*. What you could do is make a graph and plot the given points to establish a curve and then extend it to the temperature points you need. It won't be 100% but it should be in the ball park.

Ron Schroeder_2

Fin tube is not linear with the delta T between the air and the hot water. It is more proportional to the square of the temperature differance. For example if the water temperature is 140 degrees and the air temperature at the bottom of the baseboard is 70 degrees, the delta T would be 70 degrees. If you lowered the water temperature to 105 degrees (delta T of 35 degrees), the output would be about 1/4 of what it was at 140 degree water temp.

Ron

Kal Row

but it still works to maintain even temps...

with a primary/secondary/vari-injection with a tekmar control the injection speed would ramp down and maintain temp

- if the baseboard is just 5f warmer than the floor it will convect - which is a real bummer to those that have indirect hot hater and no spring checks and heat traps, the heat from the boiler/indirect hot water convects up the pipes to the baseboard and loads up the ac system -

rembemer a 75 degree pipe will feel cool to the touch as it's cooler than your skin temp!!!!

Uni R

Fin tube baseboard ratings

Ron nailed it 100%. It's not linear, it is a curve and that's what makes it so tough to plot. Also as you go under 140 the desired temperature makes more and more of a difference. At 105* supplied there would be a lot more difference in BTUs per lineal foot if it was trying to heat 68* or 72* air.

JimGPE_3

Compromise....

All other things being equal, Q=UAdT. For a given finned tube configuration, U and A are constant, so the heat output is proportional to the temperature difference between the air and the water.

I just looked in a catalog, and for close average water temperatures (say 140 and 175) it is pretty proportional. 50% more dT gives you 50% more capacity.

I suggest he plot temperature difference (NOT average water temperature, but average water temp minus air temp, ) against capacity and approximate extending the curve, which will not be a straight line except in small ranges.

I disagree that it is proportional to the square of the dT.

hr

Also

the baseboards at the tail end of a string will see a lower temperature and of course a lower BTU/hr input.

hot rod

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[Deleted User]

Why not just

ask him to start @ 120* or lower? He might find out that it works. Down here in the not so deep south, 100* works on those damp 65* days.

Ron Schroeder_3

> All other things being equal, Q=UAdT. For a

> given finned tube configuration, U and A are

> constant, so the heat output is proportional to

> the temperature difference between the air and

> the water.

>

> I just looked in a catalog, and for

> close average water temperatures (say 140 and

> 175) it is pretty proportional. 50% more dT

> gives you 50% more capacity.

>

> I suggest he plot

> temperature difference (NOT average water

> temperature, but average water temp minus air

> temp, ) against capacity and approximate

> extending the curve, which will not be a straight

> line except in small ranges.

>

> I disagree that

> it is proportional to the square of the dT.



Hi Jim,

The reason that it is closer to the square (at least first order aproximation) is the baseboard is mostly convective heat, not purely conductive or radiant.

Ron Schroeder_3

Hi Jim,

The reason that it is closer to the square (at least first order aproximation) is the baseboard is mostly convective heat, not purely conductive or radiant. A greater temperature differance increases the transferance of heat from the fins to the air, but also, the higher temperature air is lighter and flows upward faster which further increases the heat transferance more than just the linear increase in delta T.

It's like with a fan coil when you raise the water temperature and increase the fan speed at the same time.

Ron

Weezbo

Jay.... Hold down a momment...

you have some bad scoop you are steering by at the momment. I have had people tell me things and would tend to belive them were it not for the Reality of the work that i have done. The base board can indeed be producing heat and well away from the 140 mark that you were told. were the room 35 the fluids traveling through the pipe will work were they 50 degrees... how on earth would a place ever aquire btu's were it not true? i will grant you this were you to push 320 degree fluid through the system it might perk up a bit faster some older homes and buildings have "cold spots" in the pipe lay out...an example would be a small piece of convector near an entrance door these things freeze up sometimes..so,the manager of the system thinks I know! i will crank up the heat so it wont....thing is, things like to do what they been doing and while adding more heat convectors might be an answer turning the temperature up seems like an easier fix. hash that around and you will see what i mean by that.the up and down temeratures that cirulate through the system ,improper sizing,poor insulation and a variety of other reasons have an influence on what is going on..low temperature and constant cirulation works fine when someones done the Math sort to speak. You have found a place where you can find some accurate information..that is something you can make some New tabulations of your own. i have convector(baseboard) with low water temps running through them,thing is the temperature modulates. the flow is pretty much concurrently variable and hey it Works so ...eh hemmm....maybe you could find someone in your area that could put an injection loop between the muni heat exchanger and the current "fields" with a take off for Domestic water ahead of the injection loop add some controls and a couple pumps....drift the water temp down to what is needed and save some dollars all year round.then maybe, put in some new boilers and drop the muni heat, call it good:)

bob_25

output

in Modern Hydronic Heating by Siegenthaler (sigi?)pp. 208,209,210 he gives the equations and the results of the IBR tests on reduced temp operation of fin tube baseboard. My math skills ain't up to snuff to really grasp the math but it looks like it doen't vary as the square of delta t but delta t to the 1.4172 th. bob

Ron Schroeder_2

Hi Bob,

That sounds about right, taking in account for 2nd and third order effects like reynolds number etc.

Ron

Robert O'Connor_3

fin tube ratings

having been designing and selling vulcan commercial fin tube for the past 30 years (don't know where the times gone),i can offer the followingthat are available from the Vulcan engineering catalog which include water velocity and IBR correction factors,IBR heating effect factors for various styles and mounting heights, correction factors for average water temps other than standard and entering air temps other than standard and correction factors for steam pressures and entering air temperatures other than standard. In addition the factor for 110 degrees AWT is .19 times the steam rating, 120 deg awt =.25,130 degrees =..31 , 140 degrees =.38 and 150 degrees = .45 and so on.all this data is available from the Vulcan catalog.
t.p. tunstall