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buffer tank?

MikeJMikeJ Member Posts: 103
Putting in a knight boiler 80,000 BTU, with indirect water heater. A coupler of the zones will be small, staple up for one and wall heat for other, maybe towel warmer. Wanted to see what some of you have done. Would a Buffer tank be good? I seen the boiler buddy, does this work? Or is there some other ways to keep the boiler from short cycling.


Mike J


  • KBP&H_3KBP&H_3 Member Posts: 67
    edited December 2009
    Buffer tank

    Apparently buffer tanks are avoided post. My previous one also received no response..   My thoughts are since the buffer tank has minimal loss, and aren"t that expensive, they will save  a great deal of cycling of the boiler and the pre and post purge that happens every time. Especially  with multiple zones.. Most supply houses don't stock them and alot of people look at you strangely when you mention them. The big old ( 700 pound ) boilers didn't need them and the new mod-con only need a seperator. I think that they would help out alot with the overall eff. of a standard gea or smaller oil boiler where there is the pre and post purge..By next year I'll have one installed. KBP&H    ---------- Ergo max's new tank also has a domestic coil  using a outlet mixing valve. . Thus also serving as a domestic water source also,,, all in one tank.
  • NRT_RobNRT_Rob Member Posts: 1,009

    cycling can be a bigger deal with cast iron than with mod/cons. the penalty for cycling is much higher. Reset controls can help because by actually targeting a water temperature and giving you a floating differential, you can use the boiler itself as a bit of a buffer device, but the firing time on a cast iron boiler isn't that long without load and when the demand ends, you just lose all that heat.

    It's a difficult question to answer with buffer tanks. We use them a lot to guarantee a minimum run time of 3.5 minutes on any boiler. We got this from some white papers on cycling, but the papers are about big commercial boilers and some indications are that with modern purge controls and low mass boilers that cycling concerns may be more about lifespan than efficiency... hard to say at this time.

    pre and post purges of the fans though can lose a couple percent efficiency by the themselves in extreme cycling situations. If you can document any other losses of efficiency from cycling, cost justifying a boiler buddy isn't that hard.

    I would likely use one in this case. I'm not sure you need 30 gallons, but good luck finding a good, affordable, small volume (10-15 gallon) buffer tanks. If you do, please share! the 30 gallon boiler buddy works well and is relatively affordable for everything it does.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • digger_2digger_2 Member Posts: 39
    homemade buffer tank

    This 17 gal tank is homemade. It was built mostly to save a friend some $. Available tanks were nice but too big and expensive.
  • SpeyFitterSpeyFitter Member Posts: 422
    What about a larger hydraulic seperator?

    There are some large hydraulic seperators out that could add a bit of a buffer to short cycling.
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • L'town Radiant_2L'town Radiant_2 Member Posts: 39

    That's a great looking install. I'm impressed with the custom tank. I wish some manufacturers would take note and start making smaller buffer tanks. I run into this problem often. A 15gal would be perfect in many circumstances.

    P.S.  ....where is the expansion tank in this picture?
    A warm floor warms my heart!
  • digger_2digger_2 Member Posts: 39
    Good observation.

     There is a standard compression tank with a B&G filltrol hanging between two floor joists overhead. It was already there. Once again this was a low budget job. The line to the tank is incorrectly piped from the make up water line in this photo. The connection  was moved to the air seperator after the job was compleated.
  • shoudabeenaplumbashoudabeenaplumba Member Posts: 74
    I want one

    I started checking out buffer tanks for my own house.  I've got a heat load of about 90k split between 5 zones and an indirect.  One zone pulls about 30k on the coldest day of the year, and you can bet if its that cold I have the wood stove fired up, so most of the time my boiler is well under 25% load. 

    I like the low mass concept, and keeping the heat in an well-insulated tank instead of an appliance with an open flue makes a ton of sense to me.  I also agree with the idea there has to be a certain amount of mass somewhere in your system when you're friggin' boiler doesn't modulate.  The Boiler Buddy has large tappings and comes in at around $700, which is about 2x what I'd be willing to spend.
  • KBP&H_3KBP&H_3 Member Posts: 67
    Indirect w/ 3/4" tappings>\?

    I have an extra ss 40 gallon indirect I'm thinking of cutting the coil out of,,  Any one have an idea if you can stuff 80,000 btu of water thru a 3/4 inlet about 3" long??  It would be fed w/ 1" pipe...
  • shoudabeenaplumbashoudabeenaplumba Member Posts: 74
    I was wondering

    If there's an easy way to use an indirect as a tempering tank for a boiler.  I'd like to get a B4 biasi and use the current indirect w/ my tekmar 260 and pipe as a buffer tank.
  • hot_rodhot_rod Member Posts: 13,499
    why remove the coil?

    Use it for a solar input. If you keep that 3/4" connection as short as possible you could move 8 gpm or so through without excessive pressure drop.

    I have found 3/4 MIP by 1" sweat adapters to increase water heater connections right at the tank connection.

    Or a close nipple and a 1X 3/4" reducing coupling to increase as soon as possible.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • MikeJMikeJ Member Posts: 103

    If a person was to use a buffer tank. It seems as if it would be beneficial for small micro zones. How to size a buffer tank seems a little tricky. The larger the temperature rise the smaller the tank size. If you have a 75,000 BTU boiler with a 2000 micro zone and a temperature rise of 60 degrees, a 25 gallon tank would do. If you had a temperature rise of 20 then you would need 75 gallon tank. This is if you want a min run time of 10 minutes. But with a mod con boiler it would start to modulate as it approach set point temperature. So you would think, you could get longer then 10 minutes? Any ideas on this? I want to keep the temperature rise as small possible to get maximum efficiency.
  • CC.RobCC.Rob Member Posts: 128
    siggy's formula

    Vbt = t(Qh -qload) / (500 *dT)

    Vbt size of buffer (gal)

    t desired on-cycle (min)

    Qh energy source output (BTU/hr)

    qload rate of heat extraction from buffer (BTU/hr)

    dT temp rise of tank between on/off cycles (F)
  • shoudabeenaplumbashoudabeenaplumba Member Posts: 74
    I came up with

    a value of 75 gallons, which is in line with the Boiler Buddy calculator estimate of 80 gallons.  If I add a buffer tank, I'd really be pushing it as far as space requirements in my utility room.  Closer to a closet, actually.   

    Now, what's the formula for expansion tank volume?
  • MikeJMikeJ Member Posts: 103
    use siggy equatiion

    to come up with the buffer tank size, that equation does not compestate for the boiler modulating. I would think if any one had a equation to compensate for the modulating. It would be siggy. I have some his books but could not fine a equation for it. What I want to do it calls for a 80 gallon tank according to the equation. But by gut feeling (plus NRT_Rob post) is saying a 30 would suffice,

    As far a buffer tank size, too big shouldn’t be a problem.

    Plus with the Ramp delay, gradient limiting. You would think you could get a min 10 minute run time with a 30 gallon buffer tank.
  • shoudabeenaplumbashoudabeenaplumba Member Posts: 74
    An outdoor reset

    should stretch the use of a 30 gallon.
This discussion has been closed.


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