DIY fintube "convectors"

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Old 03-04-14, 01:25 PM
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DIY fintube "convectors"

In a previous thread about heat for my greenhouse NJTrooper mentioned that fin tube elements not in their usual baseboard enclosures will not provide their rated outputs because of the lack of convection caused by airflow through the enclosure.

This got me thinking about a situation I have in my kitchen dining room. Several years ago I did some renovations there that included removal of a 10 section cast iron radiator, relocation of a CI convector, and installation of fin tube elements in recessed wall niches (to offset the removal of the CI radiator.)

Before I converted to gas heat last spring the heating in that area was a little light compared to the rest of the house but it was tolerable because my old oil boiler was blasting out water at high temperature all the time. After I converted to gas and tried heating the house with lower temperature water the kitchen/dining area was not getting warm enough. I figured that was because the rest of the house on CI could generate enough heat at lower temp but the fin tube could not. I was thinking that in the long run I might have to replace the fin tube with CI convectors to get enough heat in the space.

When NJTrooper made the comment about convection through the enclosure I thought I would try to recreate a convector effect in the wall niches to see if there would be improvement. The pictures below show the niches with shutter baffles in front, the fin tube elements inside, and the simple wood and cardboard baffles that I installed. With the baffles in place the air flow is definitely noticeable (in at the bottom, out at the top) and overall the temperature in the kitchen/dining room space seems higher than before. I may replace the current baffles with sheet metal to gain any addition radiation from the heated metal that might occur.

Thanks Troop.
 
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Old 03-04-14, 01:29 PM
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I think I can see another issue though... but might be my tired eyes?

Are the 'folded' sections of the fins on the top and bottom of those elements?

They should be front and back!
 
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Old 03-04-14, 01:54 PM
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Trooper is correct, those elements are turned 90 degrees from what they need for proper air flow. Also, they appear to have quite a coating of normal household dust on the fins that will further impede the heat transfer. But the biggest problem is that the cover does not properly direct the airflow. Having the elements stacked in that manner reduces the output of the higher elements significantly.

Honestly, I would rather see a panel radiator in this location.
 
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Old 03-04-14, 02:59 PM
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Having the elements stacked in that manner reduces the output of the higher elements significantly.
Maybe so, but isn't this one of the methods that the higher output baseboards use? Stacked elements?

From what I've seen in the S/F spec sheets, if a single tier element is used, let's say the output is 600 BTU / LF , add one tier it goes roughly to 900, add a third tier and it's at 1200.

So yes, you won't double for two tiers, or triple for three, and in that sense the upper elements DO lose some output, but the overall output per foot is increased.

I think that's your point Furd?

In any case, to maximize heat output, they should be fed with the hottest water to the top I think. This gives the maximum delta T between the air passing through and the hot element.

There's another point to be made about fin-tube output... a TALLER cabinet will output more BTU for a given element because the VELOCITY of the air across the elements will be increased due to the increased 'draft' that develops.

John, if you've got room behind those elements, it might be possible to twist those fins 90. I don't know if you'll lose any conductivity of the heat in the pipe to the fins because they will inevitably not be as tight a fit after twisting them, but the increase in the airflow would surely more than make up for that, if it even occurs.

You might have to come up with a way to twist a complete section 'en masse'... I'm envisioning a plywood 'trough' that fits snugly around the fins, slide on and twist... I don't know if it's even possible, but one way or another, those need to be turned 90, and the other option is to unsolder them... I don't think you want to go there! ... and BE CAREFUL! Count your fingers before and after to make sure you have as many when yer done!
 
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Old 03-04-14, 04:14 PM
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Fortunately for me...It's yer eyes!!!

I knew the dust/dirt would be an issue but I was hoping the low res photos would get me a pass. I am recovering from the flu and just did not have the energy to drag out the vacuum when I was taking the photos and saw the dust.

On the other hand, the photos do look like the fins are not aligned correctly but they are. The smaller unit has fins that are only slightly bent at the edges so they don't look so different from the ones that are not bent. Those bent edges are definitely at the front and back so the air flow up from the bottom through the fins is unimpeded. The fin tubes in the larger unit have folded edges that intersect and those too are at front and back visible in the photos.

If I do decide to replace the fin tubes in the future how would a panel radiator compare with a CI convector? Say for water operating in the 140 to 150 degree range.

Thanks again for your feedback.
 
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Old 03-04-14, 05:06 PM
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I didn't see the dust! no worries there... not too concerned with some minor bending of the fins as long as they aren't 'mashed'.

What I thought I could see was the typical 'folded' edges of Slant-Fin elements.

You're saying that your elements do NOT have the folded interlocking edges as shown in this (correctly oriented) photo? Or if they do have the folds, they aren't top and bottom?


image courtesy wallingtonplumbingsupply.com

But rather are like this, without the folds so air can pass through no matter the install orientation?


image courtesy pexsupply.com

OK then, not a problem.

If I do decide to replace the fin tubes in the future how would a panel radiator compare with a CI convector? Say for water operating in the 140 to 150 degree range.
Thing about cast iron is that it retains a lot of heat that continues to radiate after the heat call is over. Mixing CI and F-T is a no-no typically because of the rate that they cool. Modern panel rads will cool fairly quickly also, they don't have much mass and don't hold much water.

Finish with your experiment, see how you feel when done. I think you said you still have some CI in that room so it might be OK!

You might try blocking the SIDES as well so that ALL the air has to enter at the bottom and exit the top. Without the sides blocked, air can enter there as well.
 
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Old 03-04-14, 05:14 PM
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The larger unit has the fins with interlocking folds, the smaller one does not. In both cases they are oriented as shown in the pictures you provided.

Will probably update in a year or so if I make any changes.
 
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Old 03-04-14, 05:19 PM
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OK, great... now go get a glass of OJ, and get back to bed!

Feel better soon!
 
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Old 03-04-14, 11:06 PM
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I see those pics and I thought pure boiler room porn... imagine frenched in panel rads with TRV clearance but a clean back there look! Instead of where you have the stack of convectors...

Shoot it with a temp gun and see how even it is. If it is fairly even you are just adding more surface, and the convection isn't doing much. If you aim an inch above the bottom one and an inch above the top one and it isn't more than 10 degree... some is just cooler ground effect.
 
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Old 03-05-14, 05:38 AM
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Who, you been in Cali too long I think! Ummmm, what did you say?
 
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Old 03-05-14, 09:06 AM
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Huh!

Wheeew, Who.

With talk like that I wouldn't even need hot water to add heat to the space.

Currently the system is in its daytime "coasting" mode--no calls for heat, solar gain doing a nice job, ODR in play and 30* outside. I'll check the temps as you suggested later when it cranks up again.
 
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