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  1. #1
    Join Date
    Jan 2005
    Posts
    152

    Hmm

    HO looking to learn....

    I'd posted before & learned that the rate at which water moves thru my baseboard affects the BTUs it radiates. --Water moves too slowly, and the BTUs decrease - push it too fast and you get condensation in the boiler. I have question about this condenstation. Plus, isn't there an inefficiency if the water's moving too fast to stay in the exchanger long enough to be heated properly? I guess there's a proper ratio between how big the exchanger is, versus how fast water can move thru it & still be heated properly.

    I'm guessing that the condensation will occur if the water in the heat exchanger is cold enough, it will cause water to condense on the inside of the exchanger, causing it to rust in some cases. Morale of the story is that you don't want water to be that cold when it's entering the exchanger(?) However, my furnace doesn't do potable hot water, so if heat's not called for, for a long time, the boiler will go down to room-temp. When the burner fires up, you've got heat in excess of ~800degrees - that 65degree room-temp air is below the dewpoint inside the exchanger at that point, and condensation should immediatly start occuring.

    What's the greater evil? Having a contractor install a more powerful circ, to squeeze out more BTUs, or the condensation that occurs when the burner fires up inside a 65-degree heat-exchanger?

  2. #2
    Join Date
    Jan 2002
    Location
    New Hampshire
    Posts
    1,285

    If it's all fin tube baseboard,

    Don't worry about it a bit. Faster is better in the boiler, too. Same reasons.

    Baseboard comes above the dew point (near 130°) of the gasses pretty fast, then things dry out and get better.

    That's acceptable, and how it's done today. Cast Iron radiation is a different story, because of the lower operating temperature and the higher volume of water it contains. A system bypass at the boiler can be used to blend supply water with the return to the boiler and bump up the return temperature.

    A REAL high water volume system would possibly need more flow than you want through the boiler at low temps, so you might build a boiler bypass to keep some of the pumped system flow out of the boiler and at low temps.

    The hotter boiler water could blend with this bypassing water and keep the system temperatures where they need to be. That's how the primitive Levittown radiant systems were mixed, using a tankless coil boiler. Most of the water bypassed around the boiler and back into the floor.

    Watch that the return water to the boiler gets above 130° in the first few minutes of a run cycle, and you'll be OK.

    Noel

  3. #3
    Join Date
    Jan 2005
    Posts
    152
    It is all aluminum fin-tube. You've answered my question about condensation & stated that it's OK to have water moving thru the boiler at a good clip. Now my question turns into, "Can I get more BTUs by having a higher GPM circ installed??"

    I've got ~400ft of 3/4in copper, 20% of which is larger 1-in supply trunks. From Taco's website, the 007 cartrige circ gives Flow Range: 0-20 GPM Head Range: 0-11 Feet. Do I need more, or should I have a few contractors come out & do the math..?

  4. #4
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    How many heating loops (zones) do you have? If you have 3 or 4 loops with 400 total feet of pipe, a Taco 007 should do just fine.

    You want to have 3.25 to 4 gpm going through the 3/4" pipe. If it is much higher than 4 gpm, it will be noisy. If it is less than 3.25 gpm the water won't be able to move entrapped air down the pipe, and you may get air noise or flow problems.

    If you are having heating problems, you may not have enough baseboard register. Most contractors think you can get 600 Btus per foot out of typical residential baseboard if the boiler's high limit is set to 180 °F. But when you adjust for the actual average water temperature running through the baseboad, deduct the 15% fudge factor that the IBR ratings include, and deduct for eventual dust and dirt in the fins, you probably will get only 350 to 400 Btus per foot.

  5. #5
    Join Date
    Jan 2005
    Posts
    152
    I've got two zones, one upstairs, one downstairs. The downstairs has 3 loops, the upstairs has 2, for a total of 5. Boiler is a 1999 SlantFin (GG-175) 145,000BTU DOE, 126,000BTU IBR. "Net Rating" is 840 sq. feet, according to http://www.slantfin.com.

  6. #6
    Join Date
    Jan 2002
    Location
    New Hampshire
    Posts
    1,285

    All you need

    All you need to move is the DOE output of the boiler, the 126,000 BTUH.

    To get all of that heat out of the boiler efficiently, you only need to size for that. At 20° delta (difference), one GPM moves 10,000 BTUH. If you move 12.6 GPM through the boiler and system, you'll get your efficiency. Some zones may be able to take a little more, by virtue of pipe size, but why buy bigger pumps for each zone, if one will be as efficient?

    Now, how long is the longest loop, from the boiler, all the way around to the boiler again? Not a total of all loops, just the longest loop?

    Noel

  7. #7
    Join Date
    Jan 2005
    Posts
    152
    Longest loop is ~118 feet. 30ft of that is 1-in. pipe, and the rest is 3/4-in. A sizing application I downloaded from Taco's website requires that I know the 'head' value of my system, given the size of pipe, legnth, and number of fittings/bends.

  8. #8
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Pray you don't have Honeywell 8043 zone valves. If you do, you won't get enough flow through your double- and triple-loop zones. If you have full-flow zone valves such as White Rodgers 1311s, you won't have a problem with adequate flow. Without figuring in a zone valve, your longest loop needs about 4 or 5 feet of head to move 3.25 gpm. At 16 gpm (5 loops at 3.2 gpm each), a Taco 007 will produce about 6 feet of head.

    Do you have 300 feet of finned tube residential baseboard radiator? That's about how much you will need to put out 120,000 Btus per hour with water having an average temperature of 165 °F.




  9. #9
    Join Date
    Jan 2005
    Posts
    152
    My zone valves are Taco brand, (model 572-B, vintage 1963)--This is the valve itself, not the PowerHead. I'd only counted the number of liniar feet of tubing so far, I hadn't yet counted just the fin-tube.

    Mr. Murdough wrote that I need to get 12.6 GPM thru the *boiler* to move 126,000BTUH out of it. I think now I see that the GPM figures of all the loops needs to add up to 12.6 GPM at the boiler, correct?

    In sizing pumps, I thought the pump must be powerful enough to push water thru the tubes given a certain 'resistance' or Head Pressure. What I always suspected, is that the 007 doesn't have the capability to pull 12.6 GPM thru the boiler, and push it thru the ~400ft of tubing I have.

    The 007 was installed with a new boiler in 1999, before that for the last 30 years, a Taco 110 circ was in place.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,765
    Are just looking to learn alittle or are you having a problem.

    A 007 may not be enough for your longest zone, and a 0010, may be too much for your shortest zone.

    Thats 1 of the reasons I like zoning systems with circs, instead of zone valves. You can match the circ to the individual zone.(A personal prefernce) Not trying to start that debate.

    You might need a 008.

    As Noel said you need to move aprox. 12.6 gpm to get the full rating of your boiler, but seldom will all your zones be calling at once, so don't get too stuck on being alittle under 12.6, be more concerned about going over by too much.
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  11. #11
    Join Date
    Sep 2002
    Posts
    3,708
    Oh come on been you know we all like a debate,all tho my
    prefernce is the same as yours.

    Alex,you need to find the operating point of the circulator
    you have in place now.
    you have to try to get into the guys head that design your
    system from the get go.

    Being that all your walls are in place you can only guess at how long and how many ellows,or other fitting in the circuit that create head,then your appoach should be finding
    your operation point.

    To do this will require some repiping near the circ and a
    testing device that can read the pressure differential across the pump,only then will you get the answer that you
    seek.




  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,765
    LOL...

    Your right about debates simpleman.


    But, he could use temp rise to find the flow rate of each individual zone, and then determine if he needs a different circ, or stay with the 007.
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  13. #13
    Join Date
    Sep 2002
    Posts
    3,708
    Temp rise are not constant,nor is flow rate when you got
    to many zones valve.

    He needs something more to help him find the sweet spot.



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