Maximum linear feet of baseboard radiators on a single loop…


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Old 01-07-09, 08:07 AM
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Question Maximum linear feet of baseboard radiators on a single loop…

What is the maximum linear feet of baseboard radiators that should be installed on a single loop? I think I read somewhere that it's about 65 feet. I have over 100 feet on my loop. I've had problems with my system since I bought the house. (see more details below.) The biggest room has the least linear feet of radiator and my smallest rooms have the most. To compensate for this I've tried to "tune" the system by wrapping the fins in the small, over heated rooms with aluminum foil and kept the dampers open in the colder rooms. This has worked pretty well but I wanted to check with the experts here to be sure I'm doing the right thing.

Also, all the baseboard radiators on a loop should be close in temperature, right? The first in the loop might be higher then the last in the loop but the difference should not be huge, right? In the past I noticed BIG differences in temp between radiators (30-40 degree differences) but I think this was related to problems with air in my system.

Thx, Mike D

For additional background, here's a link to pictures of my system, http://dimitroff.com/heater/heater.htm
...and, as I mentioned in another thread "This is a 100+ year old house and the previous owner did a lot of half-a**ed work on it himself. I've owned the house for 5 years and have always had trouble keeping the house warm in the winter but wasn't sure if it was because of the original 100 year old windows and poorly installed storm windows or if the heater wasn't working properly. I recently did a bunch of work on the house and used a dozen tubes of caulk to seal the storm windows and fill a gap that opened up where the floors meets the exterior walls. (The floor and base molding had shrunk and opened up a big gap. When I got down on the floor I could literally feel a breeze.)"
 
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Old 01-07-09, 10:20 AM
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See this link and choose the proper radiation. look under the water heading and max radiation.
http://www.comfort-calc.net/tech_area_index.htm
 
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Old 01-07-09, 10:38 AM
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Max radiation...

Thanks for the thread - what does it mean if there is too much radiation?

For example, this data shows 3/4in cu to carry 40k btu, and limits max radiation at ~68ft. What happens if there is more radiation? (my house has loops with ~35k max heat load each, but they used 140ft of radiation in one of the zones!!). This zone appears to be piped to where some of it feeds in parallel (not one big series loop), but what are the implications and should I change this?

I would think:

too much radiation would mean higher heat flux than needed - no biggie per se (could the water temp be reduced in this case??) but

return temps could be lower than desired (boiler stress??)

large temp differences from beginning to end, resulting in un even heating room to room (I can feel the difference)
thermostat setting not reflective of the actual temps (but I can compensate for this)

what else? should I take the time/effort to replumb or block off some of the radiation?
 
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Old 01-07-09, 03:52 PM
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Lemmee see if I can add info to explain the info at the link site.

FLOW VELOCITY: In general, the VELOCITY of the flow should be between 2 and 4 Feet Per Second. This will assure that the water is moving fast enough to carry away bubbles of air, and slowly enough that you don't hear it moving in the pipes... (and other problems, such as erosion from the inside out, causing pinhole leaks)

FLOW VOLUME: With 2-4 FPS velocity, in 3/4" pipe means that you are pumping appx 4 Gallons Per Minute.

I won't go into the math, but each GPM that you pump can move appx 10K BTU with a 20°F DELTA T (Difference in Temperature).

Most systems are designed around a 20°F DT ...

Since 3/4" pipe should be flowing 4 GPM, that's the reason for the 40K BTU limit... (4 X 10K)

If you take the 590 BTU/FT rating of a copper fin-tube baseboard as an example, and do the math, you will see that 40K BTU / 590 = appx 68 Feet. The water would exit from this circuit appx 20° cooler than it entered.

OK, that's the 'ideal'. But the 20° DT isn't 'cast in stone', it can be more or less. 25 or even 30 isn't uncommon to find. A loop with a 30° DT might have around 100 feet of CFTBB... and it would function just fine.

Rooms toward the end of the loop might need a few extra feet of baseboard due to the fact that the water entering them is going to be cooler than the water at the beginning of the loop.

One can't 'go crazy' with this though, because the longer the loop gets, the more HEAD (friction, resistance) it will present to the pump, at which point the flow will go down, into a vicious spiral... less flow = HIGHER DT ... so the water at the end of the loop is gonna get that much cooler...

Mike what you are doing with the foil and the dampers is good.

The whole thing is a big balancing act... if you have way more radiation than you need, you can run cooler water and still heat the home ... but as mentioned, you do need to watch the return temps to the boiler ... you want to try and keep the water returning to the boiler above the point that it causes flue gas condensation.
 
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Old 01-07-09, 03:56 PM
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One other thing to mention... it always amazes me at how much MORE HEAT comes out of a fin-tube baseboard after the dust is cleaned out from between the fins. I'm always surprised that such a thin layer of dust can cause that much decrease in output. It's well worth the time it takes to properly clean out the baseboard units... ESPECIALLY if you have hairy critters sharing your home.
 
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Old 01-07-09, 05:02 PM
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Any chance that it once ran the other direction so that the baseboard was more proportional to room sizes and the hotter water went the other way first?
 
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Old 01-07-09, 05:36 PM
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wow Trooper

Thanks - an explanation that even I can understand.

Who: the loop does indeed route to the larger rooms first, so a good question. I think one piece of the puzzle is a smaller heat load room (even at the end of the loop) that is pretty over-radiated (33ft baseboard for a 6800 btu/hr heat loss). This room is always warmer than necessary.

I am going to try the aluminum foil trick Mike-D used to cool off this one room relative to the others. My house in general seems over-radiated (253 ft of baseboard for 70-80k total heat loss), and supplying energy to rooms that I dont use doesnt seem frugal.

*note that with this large amount of radiators, and a boiler size of ~245k btu/hr, I get some serious rise/fall times during operation - the on/off control syndrome at its extreme. It warms up fast though!

Thanks Mike-D, for letting me ride this post.
 
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Old 01-07-09, 09:51 PM
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Frastick, on your system something like the Beckett HeatManager or the Intellicon might help... not so much to save fuel per their claims, but to widen the differential to give better cycle times.

I don't wanna drift this thread... it is after all Mike's, but wanted to suggest that. There's plenty info on the HM in other threads...
 
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Old 01-09-09, 10:34 AM
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Thumbs up

Thanks for the responses. My system uses 3/4 inch pipe so the theoretical max is 68 feet, but I've got over 100 feet of baseboard. The baseboard radiators were installed along all external walls but as I mentioned this means that some of my smaller rooms ended up with more radiator then my biggest room. Wrapping some fins in aluminum foil and opening other dampers has allowed my to more or less balance my system.

Thx, Mike
 
 

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