Rule of Thumb Q.

Hilly

So the rule of thumb you see printed often for bb fin/tube is approx 25' and 70' for 1/2 and 3/4" respectively. I want to clarify.
this 1/2 RoT is based on Dt20* and bb having about 600btu/ft output which give 1.5gpm which matches 1/2 pipe nicely.

What I want to know is if you had a small zone of 1/2" now but you are running at a lower temp and only getting 330btu/ft output can your element amount increases. So all things being equal on a 15000btu load requirement would tell me that at 180 I get 25' of basement required and at 140 I get 45' required. The load is the same, dt same, gpm same. Just more element required. Is this acceptable? Or am I being to simple with my calculations/observation?

I know I didn't write this in the most clear manner but my brain is in a haze right now and I don't have mental capacity to re-write. I think what I'm trying to ask/clarify makes sense.

Thanks for seeing your way through it. ha

bmwpowere36m3

Have to look at the specs, but more feet of baseboard allows for lower water temps for the same btu. The specs are generally assuming 20* delta at some supply water temp 190 or 180*.

However, IIRC, baseboard output drops significantly when supply temps get down to 120-100*.

bmwpowere36m3

SlantFin lists the BTU output of their baseboards at different water temps, FYI.

Ironman

Also remember this: with a 20* delta (180* supply, 160* return), your AWT is 170*, not 180*. This usually means closer to 500 btus per foot, not 600.

Hilly

valid point on the SWT vs AWT.
So am I to believe that you can get more feet of bb on a 1/2" loop if the supply temperatures are lower.

bmwpowere36m3

Short answer Yes, to a point on 1/2" or 3/4" BB. However it's not directly the supply temp that dictates MAX length of BB. You could install a TON and still run 180* thru it. It would heat the house in a HURRY. The point of increasing the length of the BB is to lower supply temps, take advantage of ODR and lower operating costs.

You're only limited by head loss…

If you stick in the 2-4fps range (for air elimination), then 3/4" copper pipe should handle 32-62k BTUs @ 20* delta. I think I've seen 40k BTU published somewhere for max BB loop (not sure why 40K, other than increasing headloss as your GPMs get higher).

The more BB you install for the same heatloss, the lower the supply temps can be. However there's a limit, as far as loop-length because the headloss get's really high and you start needing BIG circulators.

Maybe it would be helpful if you gave some actual #'s here.

JUGHNE

FWIW, I used 1/2" nom cu for my BB heaters installed 20 years ago. Boiler 140 output for supply water temp. Today with mod con that I want to run at 120 output, the heat is a little slow. Thinking my 2' & 6 ' BB in office may get upgraded to 6' & 8' and while at it probably go to 3/4", still thru a 1/2" ZV though.

We have a quite tight house insulation package.

So in a nutshell, I woulda, coulda, shoulda used 3/4" Cu, not knowing what was in the future. I would have been a little more prepared for the future. In 1995 Mod Cons were scarce around here and not to be trusted.......don't know if they are more trustworthy now but certainly not scarce. (There is one in the basement now)

bmwpowere36m3

120* supply on design day? What's your current system delta or boiler return temps?

Are you running ODR?

One book I read, Modern Hydronics by well respected guy John S IIRC, mentioned BB doesn't do well much below supply temps of ~100*. If you look at BB specs, SF for example doesn't go much below that or even there for that matter.

So if your running ODR bb output could be problamatic at very low temps since it's relying solely on convection, i.e. heating air. The delta between room and bb temp becomes small... thus reducing convection. BB is not like radiant or radiators or radiant panels where you get heat transfer by radiation and/or convection. Those you can run at very low temps and still feel warm since they are solely heating air.

Hilly

My questions were just to understand the ROT more and options around it. But it makes sense if you have it all running in 1/2 that your total loop should be considered and not just the baseboard because of allthe head. Thank you for explaining some of this in a little more detail, you brought my mind where it wouldn't go on its own.

bmwpowere36m3

I took a look at SF ratings... 1/2" and 3/4" output is the same (at same AWT).

If lets say your limited to 40k BTUs in 3/4" BB... then with a 170* AWT (typical spec, supply 180*, delta 20* and return 160*), you would need ~ 80' of BB (@ 170* AWT, 500 BTU/ft). 80' of BB is ~ 2.5' of head.

If head is the "issue", then you'd only be able to run 1/2" BB a MAX of 15' per loop (and only 7.5k BTUs for the same AWT). At 120* you'd only be able to put out 3k BTUs.

Again, a mental exercise... there are a lot of variables. You could increase your delta from 20 to 30* to reduce the GPMs and thus head loss, but also BTU output since your AWT will be lower (unless you raise the supply temp). Or you could put in a bigger circ, etc....

bmwpowere36m3

Only use the 4GPM specs, if you know for sure you're getting that. They recommend using the 1 GPM output and also remember the specs (all of them) are inflated by 15% (heat effect factor).

So many advise to also subtract 15% from what they list as BTU output.