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When to use a buffer tank and sizing it

bob eck
bob eck Member Posts: 930
I have a few questions on buffer tanks.
When installing condensing gas boilers or combi boilers when is it best to install a buffer tank?
I know they should be installed when there are many small zones are there any other advantages to installing a buffer tank?
When installing a 30 gallon buffer tank with a good air vent on the tank is there still a need to install a air eliminator on the system before the buffer tank?
I know there are air eliminators, air and dirt eliminators, air - dirt eliminators with magnets and hydrolic seaporators that do air and dirt eliminators and have a magnet and these units give you P/S piping. How does a contractor know when and what one of these to use on the boiler jobs they install.
How do you size the buffer tank? I see they go from 10 gallon to 120 gallon.
Hot Rod does Caleffie address this in one of their Idronics books?
How does the contractor explain to the home owner why in his quote of a condensing gas boiler he has included a buffer tank and a good air - dirt eliminator with magnet and his competition on this job has bid a condensing boiler less the buffer tank and is using just a regular air eliminator.
Are there any short videos that show the condensing boiler plus air-dirt eliminator with magnet and buffer tank and the video tells why these items are needed.


  • NY_Rob
    NY_Rob Member Posts: 1,370
    When I was considering buffer tanks... Boiler Buddy had a lot of resources on their site including videos...

  • hot_rod
    hot_rod Member Posts: 22,256
    Here is the Idronics 17 that talks about buffer tanks, with formulas.


    I've done 3 systems now with the two pipe connection method at my house and shop and they work as explained in the I-17.

    At the house for the past few years I had a 50 gallon buffer on a Lochinvar Cadet. Just this summer I replaced that with a Lochinvar Nobel and went down to a 6 gallon buffer as it has 10-1 turndown.

    At the very least a micro bubble type air elimination is required with any system, new or old in my opinion. They do a much better job at quick, 100% air elimination right down to the bubbles smaller that the eye can see.

    As for dirt and magnetic it would be a requirement on older system renovation, iron pipe upgrades, cast iron radiators, etc.

    On a brand new system, all new piping it really serves as a start up filtration device, grabs any debris left over from the installation like wood chips, solder balls, shavings from copper or threading, etc. There should be no reason to expect a newly piped system to continue to shed dirt and crud into the piping.

    The magnetic function "fine tunes" particle removal by grabbing the smallest of ferrous particle, magnetite can be a .05 micron size particle. It is a good insurance policy for ECM circulator use.

    As for the need to have a buffer tank, it is mostly for reducing cycling of the boiler. With multi and especially micro zoned systems they really add value. Even if the boiler has a turndown to the lowest zone BTU requirement, that is at design conditions. Most, 60% or more of the heating season is below design condition, so buffering smooths out that low load condition over the entire load conditions.

    Secondary it can be storage from which to pull a lower temperature via an ODR function. With conventional boilers you could charge the tank to 180 or so, and pull the mix or various mixed temperatures from the tank. Wood boiler installers or jobs with oversized non con boilers use buffers for multiple functions like that.

    Sure you get some dirt removal in the buffer tank due to the low velocity zone established and the space at the tank bottom to collect.

    If you get a buffer with additional ports, add a magnet into the spare port. Caleffi has the brass wells and segmented magnet assemblies to build in your own mag function.
    #49684A 49685A.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bob eck
    bob eck Member Posts: 930
    Hot Rod how do you like the Lochinvar Noble combi boiler. Anything special or hard with the installation?
    I will be installing one in my father in-laws home in September.
    Taking out a 1976 steel oil boiler with domestic coil and installing a natural gas Noble combi boiler. He has one zone with all large cast iron radiators.
    Using the Webstone P/S piping kit and their tankless valve kit for the domestic water hook up plus a Caleffi mixing valve and Caleffi air - dirt removal with magnet.
  • hot_rod
    hot_rod Member Posts: 22,256
    I had a bit of a hassle getting the air out of the boiler. I was running DHW mode so the Discal air purger downstream of the buffer wasn't involved. I ended up holding the relief valve open just a crack for about a minute.

    There is an air vent built into the composite Grundfos pump inside the boiler. I also added a air vent on a tee up where they have you place the relief valve, strapped the air vent tube to the relief valve discharge tube down to the floor sink.

    I think the vent up high on the tee would have worked better than the one inside the boiler circulator.

    Despite the small BTU if does a fine job of DHW, even with a fast fill valve on the tub that Ellen uses.

    I do have 80 gallons of solar pre-heat however, on some days the Nobel never fires, the solar just flows through and keeps the boiler from firing. We had a week of rain, no sun last month and DHW supply kept up, about 58F well water.

    You learn to live with tankless type DHW. We do dishes and clothes via there delay timers, in the evening after all the showers or tubs are done.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream