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Optimal flow rate for indirect WH circulator?

Hi all,

I'm planning on using a 80 gallon Bradford White indirect tank for DHW. The manual has a section "AHRI certified water heater ratings" which indicates the minimum flow & boiler output for the rated first hour & continuous ratings as 13.7gpm and 157,000Btuh. My system is using a large storage tank wood boiler (GARN) however, so the output capacity is not an issue (they say 325KBtu/h based on loading every 3 hours), and more importantly, return temperatures are not important. I can design for the widest possible delta to minimize the pump size. So I'm trying to think theoretically what the best flow rate for the DHW circulator would be. My figuring is as follows; can someone check me here?

They don't have published numbers on the heat transfer coefficient of the HX, so I'm going with u = 80 btu/ft^2*hr*degf based on this page. The area of the coil is A = 14.2 ft^2. Assume boiler water temp = Tb = 180*F, temp of water in indirect tank = Tt. Then:

F = heat flux through coil for given tank temperature (Btu/h) = A * u * dT = 14.2 * 80 * (Tb - Tt)
flow rate to deliver that amount of heat with the widest possible delta = F / 500 / (Tb - Tt) =
14.2 * 80 * (Tb - Tt) / 500 / (Tb - Tt) =
14.2 * 80 / 500 =~ 2.3 gpm.

This is quite different from the manual's suggestion of 13.7 gpm, but the Btu's work out nearly the same. My thought is that their flow rate is based on limiting the delta T for a conventional boiler, since if you rearrange the equation, their specs would give a delta T of 157000/500/13.7 = 22.9*F.

Am I on the right track here?

Thanks,
Jacob

Comments

  • hot_rod
    hot_rod Member Posts: 16,286
    I think that tank has an 1-1/2" diameter, .030 wall thickness, glass coated coil, so it can easily handle flow rates in the 14 gpm range.

    If it is the same coil as used in a few other brands it should be a 36' coil. So at a 14 GPM flow rate a mere 3.2ft pressure drop. I believe the factory output tests were with 200F supply to the coil, by the way.

    The more flow the better the output as the average temperature across the coil will be higher. Higher coil temperature = more DHW recovery and output.

    You could easily move 20 gpm or more thru that large bore coil. I believe they limited the rating to that 14 GPM flow rate to stay under the ASME 199,000 BTU/hr requirement which would required a certified $$ tank.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Zman
  • icesailor
    icesailor Member Posts: 7,265
    GAGO: Garbage In, Garbage Out. old dead dry wood is garbage and so are the ashes after you burn them. Alternative fueled systems only work when they are way over engineered and oversized. Then, maybe you can store enough water. The designing isn't much more advanced than reading tea leaves or taro cards. If it is big enough. If not, and you provide sufficient back up (which is 100% consistent), you will never know how poorly the system works under marginal conditions. Then, there's all those dead trees. A gallon of #2 oil is 139,000+ BTU's per gallon, LP/Propane is 91,000 BTU's per gallon. A Cord of wood is whatever someone decides it is.

    When it doesn't work as the customer expects, the customer expects you to come back and make it work to their expectations. At YOUR cost. How many dead and abandoned systems have I seen in my career, paid for with Government tax credits over 20 years that the system didn't work from the beginning.
  • RobG
    RobG Member Posts: 1,850
    Have you considered a reverse indirect? That would give you some storage / buffer as well as an endless supply of hot water.

    Rob
  • hot_rod
    hot_rod Member Posts: 16,286
    use a solar tank with an upper electric element, unless you plan on keeping the 1800 gallon Garn hot all summer?

    The reverse indirects are another nice option, but require fairly high tank temperatures to get full rated output. So you would need to keep the Garn running at top end.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • icesailor
    icesailor Member Posts: 7,265
    The first Solar system I installed in 1980, was a Rudd drain back system with an air pump and a 1500 watt electric element back up. It worked for years. Never had a complaint. Even when the air pump died once. Then, the electric element died once. No hot water. That electric back up element had been providing hot water for years. The customer never complained of running out of hot water. The next Solar system I installed had no electric back up. They never had enough hot water. With the outrageous cost, and transfer loss, I never installed another one. Neither did anyone else. Mostly I disconnected panels from hot water tanks or put in new tanks that were electric back-ups that did fine as primary heaters.

    If you want to have Solar, you better have good back up.
  • hot_rod
    hot_rod Member Posts: 16,286
    icesailor said:

    The first Solar system I installed in 1980, was a Rudd drain back system with an air pump and a 1500 watt electric element back up. It worked for years. Never had a complaint. Even when the air pump died once. Then, the electric element died once. No hot water. That electric back up element had been providing hot water for years. The customer never complained of running out of hot water. The next Solar system I installed had no electric back up. They never had enough hot water. With the outrageous cost, and transfer loss, I never installed another one. Neither did anyone else. Mostly I disconnected panels from hot water tanks or put in new tanks that were electric back-ups that did fine as primary heaters.

    If you want to have Solar, you better have good back up.

    The suggestion was to use the solar type indirect, wood heat from the Garn into the coil, electric element for summertime when the wood boiler is not used.

    Modern solar SDHW generally are two tank system, the solar heats or pre-heats the first tank, that feeds to the second standard tank, tankless, HP, whatever your DHW choice is.

    Another "single tank" option included the upper element for non, or low solar days. Current controllers have a provision to lock out the element until the homeowner returns home, or whatever schedule you chose. This gives solar priority.

    Or buy a $59 electric water heater timer from the box store to lock out the element, programed to the owners schedule, not unlike a setback t-stat.

    If the electric was heating the solar tank the entire time, sounds like a control problem, or operater/ installer error :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • varadhammo
    varadhammo Member Posts: 27
    I like the configuration I've seen elsewhere of an indirect with an electric on-demand unit in series to "finish" if the boiler can't keep up (or isn't being fired during the summer), but the problem is that our electrical service in that building is only 100A.

    So what about using a regular indirect tank (no electric element), with a small EOD on an aquastat triggered recirculation loop? I could tee off from the output of the EOD with a small circulator to cycle tank water through the EOD if the tank falls below a setpoint. I could also turn that circulator loop off during periods of very low DHW usage (when there aren't many visitors using the bath house) so that the tank doesn't have to sit there wasting energy, and just let the couple gpm from the 12kW tankless satisfy 1 shower at a time, etc.

    Has anybody done something like this? Would this work? I realize the usage scenario might be a bit unique.

    Thanks