Baseboard feed 1/2" to 3/4" ?


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Old 01-14-10, 08:26 AM
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Baseboard feed 1/2" to 3/4" ?

I recently purchased a 1950s ranch home in need of major renovation. I am going to replace the existing hot water baseboards with new ones. The system is a single 1" copper loop going around the perimeter of the basement. Each baseboard heater then taps off of this line with a monoflo type tee and then returns to the main line via a monoflo type tee.

The branch off of all the tees is 1/2" copper, but all the baseboards have 3/4" elements. Is there a good reason for this or is it likely a mistake? Shouldn't this loop be all 1/2" or all 3/4"? And how do I know which?
 
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Old 01-14-10, 10:33 AM
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1/2" monoflo risers were often used to supply heat emitters rated 10,000 Btu/hr or less, from the basement to the 1st floor - that's with one monoflo tee. 1/2" risers can supply about 7,000 Btu/hr to a second floor. Since this is a ranch-style house, I assume it's single storey, and the higher rating would apply.

If you use two monoflo tees (one installed backwards), you can double those numbers.

All these monoflo numbers (based on 20 deg differential) were developed by Bell & Gossett many decades ago. So, you need to match the riser sizes to the Btu rating of the baseboards, which, in turn, should match the calculated heat loss from each room.

There is no basic reason why 1/2" risers can't be used with 3/4" baseboards. I wouldn't conclude that it was a mistake. Of course, you will need a reducer on each end.

Did the system work satisfactorily with the 1/2" risers?
 

Last edited by Mike Speed 30; 01-14-10 at 10:48 AM.
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Old 01-14-10, 03:34 PM
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A little bit more info added to Mike's good info...

Are the baseboards the copper pipe with aluminum fin type? or are they CAST IRON? If they are cast iron type, I personally would clean them up and keep them.

Another angle to look at this:

In general, 1/2" is good for about 15K BTU, and 3/4" for about 40K BTU.

If you are using fin tube baseboard, those elements put out about 550 BTU/FT at 180 water temp.

If you divide 550 into 15K, you get about 27 feet give or take... so if your baseboard runs are less than that, you are fine with the 1/2" pipe.

Since you are piping a monoflo system, do NOT skimp on the air bleeders on the new baseboards. Set them all up with 'baseboard tees' at the 'downstream' end of each run where they turn to go back into the floor. Install a manual 'coin type' air bleeder at those locations.

READ THE INSTALL MANUAL for the baseboards and understand how to leave room for expansion! If you don't do this, you will be plagued by NOISES! click.... click.... click... bang! ... tink... tink...

See the illustration on page 9 for riser hole sizes, and how to allow for expansion:

http://www.slantfin.com/documents/211.pdf
 
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Old 01-14-10, 05:11 PM
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Originally Posted by NJ Trooper View Post
In general, 1/2" is good for about 15K BTU, and 3/4" for about 40K BTU.
I'm thinking those numbers may be for the differential pressure of a pumped supply across the emitter? For a monoflo system, according to B&G engineering data, the heat capacities (and flow rates) are less, unless two monoflo tees are used, one on the supply and one on the return to the main.

Trooper's suggestion about air bleeders is very important. Do it. Monoflo systems are a bit more difficult to bleed, probably because the flow rates are a bit lower.

Like Trooper, I'm wondering why you are replacing the baseboards. It's hard to imagine what would go wrong with them, besides being clogged with dirt, lint, or cat hair. That can be blown out with compressed air.
 

Last edited by Mike Speed 30; 01-14-10 at 05:27 PM.
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Old 01-15-10, 08:35 AM
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These are the tube and fin type. The enclosures are in poor shape (painted many times, covers missing or damaged) plus they are very large. They seem to function ok but I don't know if I can retrofit new smaller enclosures on them. So I'm prepared to replace the entire units if needed.

I think the 1/2" feeds are large enough but I was unclear why they chose to use 3/4" element instead of keeping it all 1/2".

While looking at the system last night, I found that a heater had been removed from the system at some point. So I now have 2 monoflo tees in the main line with their branches plugged off. I realize that that is not a good thing. Lucky for me I need to re-install a heater at that location anyway, so that restriction will be fixed.

Thanks for the comments guys.
 
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Old 01-15-10, 09:37 AM
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Originally Posted by crowtdog View Post
I think the 1/2" feeds are large enough but I was unclear why they chose to use 3/4" element instead of keeping it all 1/2".
Maybe that was what was readily available? Today, manufactures typically supply 3/4" baseboards, for example, Slant/Fin: Baseboard , Baseboard Heaters , Baseboard Heat , Slantfin Baseboard - PexSupply.com

Also, 3/4" will have a higher Btu rating per foot - so less length of baseboard would be required than if 1/2" were used. Don't get hung up on 3/4" baseboards fed by 1/2" risers. There is nothing necessarily wrong or inconsistent with that.

I have a cast-iron radiator fed by 1/2" monoflo risers, only a few feet above the main. The radiator's connections are 1-1/4", but they have been bushed down to 1/2" - it works fine. But if the risers were going from, say, a basement up to a third floor, the risers would have had to been larger than 1/2", maybe 1", to supply the same, identical radiator at its Btu/hr rating. The larger risers would be needed to overcome the higher flow resistance associated with longer piping runs.

Does this make sense?
 
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Old 01-15-10, 03:17 PM
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I'm thinking those numbers may be for the differential pressure of a pumped supply across the emitter? For a monoflo system, according to B&G engineering data, the heat capacities (and flow rates) are less, unless two monoflo tees are used, one on the supply and one on the return to the main.
Yes... I think... but used as a 'ballpark' to illustrate that there would need to be an awful lot of baseboard running off each pair of monoflo's to make the 1/2" piping inappropriate. ESPECIALLY since there are MF Tees on BOTH ends.

Those figures would typically be used if one were designing a pumped system with flow at 2-4 FPS and a 20 delta T.

3/4" baseboard may in fact be cheaper to buy (guessing)... because nearly every supply house stocks it. MOST systems designed and installed today use series loops... not monoflo... and this doesn't mean there's anything wrong with monoflo... and I don't know why it seems to have fallen out of favor with installers... but with 3/4" one can run about 65' of baseboard on a single loop, and this is probably a handy length for MOST systems installed today.

If you ever do remove rads from monoflo systems, don't cap and stub as someone has done in your home... instead, run a pipe in place of the rad... this way you don't add restriction in the system by 'closing the road'.
 
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Old 01-15-10, 05:34 PM
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Originally Posted by NJ Trooper View Post
I don't know why it seems to have fallen out of favor with installers...
Guessing that it's because monoflow systems require attention be paid to pressure drop and flow rate, have more costly components, and have less room to play with flow rates and heat distribution options due to the aforementioned pressure drop and flow rate considerations.
 
 

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