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How do you know when to use a buffer tank?

RayWohlfarth Member Posts: 1,321
A customer asked me the other day if a buffer tank was required on a large commercial hydronic system. I told him I would get back to him. I have never seen a buffer tank on large commercial projects and was wondering how you know when one is required. I see them on residential projects with mod con boilers. Is there a rule of thumb or formula for this?
Thanks for your help
Ray Wohlfarth
Boiler Lessons


  • pecmsg
    pecmsg Member Posts: 4,201
    Buffer tanks are used to prevent short cycling. On commercial jobs the equipment is staged so with the exception of slow times buffer tanks are not needed. Now if the equipment cycles under a load load situation then a tank can help that. 

    Honeywell Gray Manual has pages on buffer tanks. 
  • GroundUp
    GroundUp Member Posts: 1,656
    Long story short: when the smallest load is lower than the lowest firing rate, a buffer tank starts to become necessary. Commercial can be very tough to dictate, often having many very small loads and a considerable amount of distribution loss. With that said, I don't recall ever having installed or even seeing a buffer in a commercial heating system as most are now cascaded with multiple boilers or have a massive amount of onboard storage to soak up the extra BTU and prevent short cycling.
    EBEBRATT-Ed Member Posts: 13,844
    I used one on a commercial job it was a boiler replacement. The boiler was a Smith 19 can't remember the output. But the job had like 8 3/4" zones. The burner was oil fired about 2.5 gph so no modulation.

    I had visions about the burner short cycling if one zone was on say in the spring or fall. So we talked them into a buffer tank.

    We used an ASME air compressor tank as a buffer tank as a buffer tank that large was special order long delivery. One pipe going into the tank we used a double tapped bushing and an internal pipe to get better mixing in the tank
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,321
    @pecmsg Thanks I will look it up in Honeywell Manual
    @GroundUp That makes sense I wonder if system water volume has to do with it
    @EBEBRATT-Ed Wow that was an expensive adder I know how much those tanks run
    Thank you all
    Ray Wohlfarth
    Boiler Lessons
    EBEBRATT-Ed Member Posts: 13,844
    edited April 2021

    Yes on the system volume. Zones that are zoned off don't exist. Most boilers especially cast iron are more tolerant of this, mod cons with lesser water volume not so much. Where system volume really gets exposed is on chillers. because they work at only a 10-12 degrees TD you need 3 gallons of system water / ton for comfort cooling and 6 gallons of system water /ton for process cooling. That's what Carrier calls for. Trane does a different calculation but with the same result.

    I know that's a little off track but it still applies. We had a Carrier chiller once that was installed without enough water volume. 5 years old it had like 10,000 starts or some redicuolus number. It ended up self distructing before it's time

    They call it the flywheel effect or having no flywheel
  • Tinman
    Tinman Member Posts: 2,806

    My March PHC News column answered this very question.
    Steve Minnich
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,321
    @EBEBRATT-Ed Thanks for your expertise
    @Steve Minnich Thanks that answered my question Good article by the way
    Ray Wohlfarth
    Boiler Lessons
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    Maybe if piped right, the tank could take the place of the hydraulic separator, although an air remover and magnetic filter would still be needed.—NBC
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,321
    Thanks NBC
    Ray Wohlfarth
    Boiler Lessons
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,436
    The one caveat with the calculation in Steve's article is that the smallest zone's calculated design load will almost never be the actual load of that zone - short cycling would be correlated with warmer outdoor temperatures and lower loads plus load calcs seem to be overestimated. The buffer tank size calculation can probably be simplified by just considering the minimum output.
  • hot_rod
    hot_rod Member Posts: 19,958
    I don't know of any hard and fast rules for determining when to add some buffering. Perhaps some boiler manufacturers have minimum firing periods they recommend? That may help determine the need for a buffer tank. A 10 minute minimum run time has been one industry suggestion or rule of thumb.

    For me, just hearing a heating device banging on and off every several minutes helps with the decision.
    Todays mod cons with all the control functions for limiting firing rates and step modulation goes a long way towards eliminating short cycling. The fixed speed boiler, especially the over sized ones, may benefit most from some extra capacity.

    As you alluded to @RayWohlfarth, the buffer could be in the system volume and thermal mass of the boiler, piping and radiators.
    Some bucket time on a mild heating day is the best way to observe the cycling.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,321
    Thanks @Hot_water_fan and @hot_rod Spent many an hour on the bucket watching
    Ray Wohlfarth
    Boiler Lessons