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ModCon buffer tank plumbing vs source/load flow rates

Dave Carpentier
Dave Carpentier Member Posts: 177
Im convinced that I need a buffer tank in my design, for three reasons;
-design load on two small zones are ~2000btu/hr, and in shoulder season get much lighter of course. The boiler min burn is 10k net.
-the min required flow rate for the boiler exceeds the flow rate of most of my loads
-my design temp is -21f, but it can hit -40 here occasionally. Long burns to fill up the buffer is probably preferable to minimize the pre/post venting cycles of cold (!) air ?

From the Nti FTVN110 manual pump curve, I assume it will be flowing near max speed (the tank will be directly under the boiler)


I had originally though to plumb the buffer like this (from a PMMAG article);


But (from same article), this may be a better connection;


My config would also have an air handler (<4 gpm) connected at those short/generous pipes. The radiant system flows <2gpm.

Or is this just overthinking it, and both ways would be similar?


Article <a href="https://www.pmmag.com/articles/96765-alternate-methods-to-pipe-a-buffer-tank">https://pmmag.com/articles/96765-alternate-methods-to-pipe-a-buffer-tank

30+ yrs in telecom outside plant.
Currently in building maintenance.

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,007
    edited May 1
    @hot_rod will comment. He is an expert on these things.

    There are many ways to pipe a buffer tank. Do you get DHW off the boiler??
  • hot_rod
    hot_rod Member Posts: 16,315
    I have built both 2 and 4 pipe buffer tanks. For your application I suggest the two pipe.

    The advantage is direct to load, quicker start up, better stratification, easier to source a two connection tank.

    The boiler circulator will have no problem moving 10 gpm into the tank as you have hardly and flow resistance in the piping from boiler to tank, probably a couple feet of head.

    Buffer specific tanks are available from Boiler Buddy, Lochinvar, NTI, Thermo 2000, most any boiler manufacturer, so shop around. The Buddy is a plain steel tank, low pressure rated, with large side connections, probably the most affordable.

    Electric water heaters that have side connections are another option, and it gives you a duel fuel option with the element. 6, 12, 20 gallon are common sizes.

    Here is a 6 gallon I built. Boiler feeds into side connection, high temperature out of tee branches, low radiant via the mix valve up top. I upsized to a 4500W element for 15,000 BTU/ hr with the element, if the boiler goes down, I could plug it into a 240V dryer receptacle.

    The boiler is on ODR so tank temperature varies. The low temperature is a manual mix valve, so the temperature also floats with the ODR temperature.

    More detail on buffer piping options here:



    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Rich_49
  • Rich_49
    Rich_49 Member Posts: 2,692
    To add to what Bob said , the buffer also sees less flow in the 2 pipe configuration allowing the boiler to see more consistent and lower return water temps .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    edited May 1
    I can see the unwanted high flow-thru mixing with a 4-pipe setup. With my loads flowing between ~0.5 and maybe 6gpm tops, I wonder if it would be beneficial to add some restriction to the boiler loop and adjust it down to match or just above my max load flows ?

    It seems to me that I would want my slab circ (and the AH circ) operating independent of the boiler controls. Just tstat controlling a relay that switches on the respective circ. The loads are just drinking out of the buffer tank by default. Its only as the buffer cools past a setpoint that I would want to trigger the boiler. So, just an aquastat on the buffer tank that triggers the TT on the boiler ? The boiler's control doesnt seem to have a "tank" remote sensor connection for CH. Funny they dont include this, if low-mass source into microzones is gaining popularity then buffer tank installs should be increasing.

    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    @Ed - No DHW yet. It has the tap and 3-way valve for indirect, but I'll wait until my 6 yr old electric gets a bit older before I install that. Well, or until the price of electricity gets even more crazy..
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    some boilers come with an extra sensor, intended to be used in the indirect tank, instead if an aqua stat, did yours? If not you should be able to get an extra and connect it to the supply sensor connection. So the boiler just sees the tank as its control device.  The tank maintains either a set point or can run on the ODR function

    Does that model have a variable speed boiler pump, or ability to add one?  That would allow flow to modulate as burner ramps up and down
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    Yet to be purchased, but it's the boiler Im getting (in stock locally etc).

    The impression I got for the tank sensor is that its incorporated into the DHW side of things. It would be possible to "fool" the boiler into using DHW controls for CH duty, but then afaik you lose ODR, parallel offsets and all those other fancy things.

    Oh btw, thanks again for the Caleffi info. Thats some great material. Some of it is over my head, but each time I read one of those I certainly learn something. :)
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    Just the sensor itself, they are just RTC thermistors, they can be used at any of those connections on the board. Here is an example of one that I think came with an early Lochinvar boiler. It could be inserted into the DHW tank, connected to DHW connections.
    Or it could be connected to the supply sensor connection, the boiler control recognizes it as that function.
    Some boilers, looks like that NTI, allow you to add a system sensor. It would go downstream somewhere, past a separator for example, or in your case in the tank. So the boiler regulates based on that temperature, not the temperature in the boiler.

    Some brands use these 6mm sensors. Nice about that is the fit into a 1/4" compression adapter if you want to put them in to a pipe. Or get a 6mm sensor well.
    Most tanks designed as buffer tanks have a dry well for a sensor. It may be a larger diameter well, so add some copper to the smaller sensor to get a tight fit into the well with some sensor paste. My newer Knight has screw in sensors with molex plugs, so it varies.

    You need to decide how you want to control the buffer tank temperature. If the boiler ODR is enabled, and you insert a sensor connected to that system sensor connection, the tank will maintain at whatever that temperature is at any given time. Hotter as you close in on design conditions. This helps keep return to the boiler low, and keeps efficiency up.

    OR, run the boiler/ tank at a fixed setpoint, no ODR. This gets you more drawdown before the boiler fires, but you loose boiler efficiency if you maintain the tank above 130F or so.

    If you run a hot tank via a setpoint temperature, the loads could pull off the tank with a mixing valve controlled by ODR like the Taco electronic 3 port valves with ODR built in, for instance. So your distribution is on ODR, but the boiler runs hotter.

    Most times hot tanks are used with wood or bio boilers as you want to run those hot and long cycles for best efficiencies. Drive them to 180- 190F for example.

    If you run the tank on ODR temperature, I'd go with a larger tank, since your goal is to lessen boiler cycles on a small load zone. The more you store in the buffer, the longer both the boiler on and boiler off cycle.

    Use this formula for sizing a buffer with a modulating boiler.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    Ah.. so basically just disconnect the on-board sensor (HeatFlow 1 in this case) and extend a compatible 5v one out to the tank ?


    In the case where heat calls are done, the boiler pump would continue to run until the tank is at set temp (plus delta and ODR etc), then shut down.

    In the case where any/all heat calls are current, it would still hit set point because my boiler pump outpaces the CH systems pumps. There would never be a case where the tank sensor is not sensing boiler operation (like where the CH is eating all of the flow, so none of it is flowing into the buffer tank, so the boiler just keeps burning).

    For control I would strap the TT, and use the nice assortment of tweaks including odr to manage the buffer tank temperature.

    Currently, I run my oil boiler always hot (130-160) and just set the slab mixer to 85 or so. Id love to be able to push hotter into my very long garage loop (400ft), but I also have laminate floors in some bedrooms so I have to keep the temp reasonable. Unsure if I want/need an ODR-able mixer.

    I did the buffer tank calc with a 20 minute burn at min rate (10k) and my smallest load at min demand (a spring day), and the buffer tank doing a 30deg swing. It works out to 13 gallons. The thing Im unsure of is - will the boiler stay at min rate for long enough to allow a 20 min burn, or will it step up it's burn ?


    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    I’m not up to speed on that boiler control.  On mine I can lock it at a fixed output, but also ramp delay. So I can keep it at all the firing rates, for a programmed time and choose what temperature it steps to the next. Six steps, maybe. It also has boost function to ramp up if it doesn’t catch the load in a specified time.

    Read through the manual to see what is available on that control I think it has a cold slab reduced firing option? So it must be able to step somehow.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    edited May 2
    Other than a setting I see for capping the max power %, everything seems to be temperature related. Fixed, stepped over time, odr sloped target, boost, delta range, min/max temps, etc.

    I think Im just having a hard time with the concept of modulating. I get the basics of how 10:1 means 10% burner (11k btu/hr on this one) to 100% burner (110k btu/hr), but how does it know to auto-select 10% for 20 minutes or 100% for 2 minutes or something in between. Is it based on "whatever it takes" to bring the return water up to target temp before it exits the boiler ?

    For the "hot tank" approach (TT strapped), it's going to think the heat call is continuous and leave the boiler's circ pump on continuously. If I leave the CH sensor in the boiler, it would be valid since the circulation is constant between the boiler and buffer. Its an ECM pump (fairly low power cost) and any waste heat would be inside the building envelope anyhow (not wasted).
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    No reason why the circulator couldn’t stop when the tank reaches temperature?  It should do that whether you add a sensor to the tank or not. Running that circulator constantly would cool the tank and boiler after the burner is off, somewhat.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    Ok. How about this ? The boiler pump should only run when either system calls for heat.



    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315

    Ok. How about this ? The boiler pump should only run when either system calls for heat.



    Yes, and the boiler pump needs to run to charge the buffer, under a no heat call condition. So treat that as a call to TT also.
    The boiler can handle some of that control logic.

    If you prefer a couple Caleffi relay boxes could also manage all that you need. So just one connection from the relay boxes to TT.
    It comes down to how you want to build the control logic. I prefer all in one box as opposed to mixing boiler, AH, zone 24V from 3 different sources or transformers.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    "Yes, and the boiler pump needs to run to charge the buffer, under a no heat call condition. So treat that as a call to TT also."

    I think I found the setting for that.. "Boiler Pump Post Circulation Time (After CH)". Can set from 0-15 mins. That would have to be set by trial and error. Im thinking at ~10gpm, 2 mins would be a good starting point for a 20 gallon buffer tank.

    Less wiring would be nice, but Im starting to warm up to the idea of using the TT1 and TT2. In the config of the boiler, those two zones can be configured completely independently. I could use a more aggressive heat strategy for the TT2 air handler loop (it eats BTU faster), and a concurrent call on the slab side (TT1) wouldnt care because it's just a mixer set to 90f or so. The AH doesnt come on much until it gets really cold (upper floor of the house), so normally the TT-1 (slab) settings could be operated with a slightly cooler buffer tank.

    Actually, I havent found yet (in the manual) how the boiler handles a concurrent call of TT1 and TT2.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    I’m not sure you want to run a post purge pump on a mod con connected to a buffer tank  It will just bust up any stratification you have in the buffer, lower the usefulness of the stored energy. The term is exergy, and when you blend hot and cooler water you lose exergy or usefulness. The math on that concept is in Idronics 17.

    Notice that Boiler Buddy tanks tend to be skinny and tall, that encourages and maximizes stratification.

    Even a fire tube design only holds a few gallons, and not much mass in the weight of the boiler HX, nothing to purge out, really

    When the tank reaches setpoint, and no heat calls are active, stop pumping😗
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    edited May 3
    Drat. Just when I think I've figured it out.. lol
    Well, the exergy thing seemed to indicate that it was better to keep the slab and AH pumps doing their own thing (eating up the buffer) *without* triggering the boiler pump on.. until it needs btu of course.
    So.. Im back to needing a sensor in the buffer tank that talks to the boiler.
    You mentioned a few posts ago about the System Sensor.
    In the separate FTVN110 manual for cascading, I found this..


    So the boiler is certainly able to use a 5v sensor remotely, as you said, simply plugged into the SYS connection.
    And now I see (from the cascade instruction manual) that TT1 takes priority over TT2. So in my case, since I want the higher heat on an air handler call, I would connect my AH to TT1 and the slab to TT2.
    I wonder why they left some of this info out of the normal IO manual ?

    Sorry for not paying better attention to you earlier. Im learning though ! :)
    (and feeling much more confident about this upcoming project)
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    For what you are asking of the buffer, you need an aquastat, setpoint control or NTI sensor in the tank. The boiler just sees the tank as another load to heat up.

    If you are not comfortable with configuring the boiler control just use a setpoint control like this, sensor into the tank, wired to TT. These have quite a bit of adjustability for on/off, differential, etc.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    I think there's a basic operation that Im unsure of.
    If you short the TT terminal on a boiler with an integrated pump, does the pump;
    -run continuously , or
    -only run if the boiler temp sensor (or in my case, buffer tank sensor) is below setpoint


    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    I don't know of a mod con that could operate with the fire on and no flow? Variable speed and post purge are options. Check with the manufacturers on that.
    I know they trip out on limit very quickly when a pump fails.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave Carpentier
    Dave Carpentier Member Posts: 177
    Well ya, for sure the pump is on when the fire is on ( else it would just overheat and trip out).
    But if the TT is strapped, then the pump keeps on running (and the heat cycles between low and high setpoint range) all the time, right ?
    It seems to me that my buffer tank is going to need both a thermal sensor *and* something like the Ranco.

    I was thinking of this for wire control. (the buffer tank Sys sensor isnt shown)


    I have one of those Ranco units (I use it now to expand the delta on my oil boiler), as well as the DPST relay (that I use now for circ pump control)

    Im thinking of setting the Ranco to 100-130.
    As long as there is a zone heat call;
    -the boiler pump will run until the Ranco hits 130
    -the burner will run until it hits high setpoint, OR the Ranco hits 130 (and releases the TT2)


    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 16,315
    Typically all the controls and limits are in series, any one of them that opens will drop the TT call, stop the pump and burner. Unless you enable post purge.
    Regardless if you use the NTI sensor to control the buffer, or add a setpoint (aquastat) into the tank, the end result is the same. In some cases installers may yank the boiler sensor out of the boiler and put it into the tank. However some sensors now have 4 wires and are doing multiple functions. You want to keep the high limit or overheat sensor in the boiler.

    You would want to confirm with NTI how their particular model operates. Contact a knowledgable rep, or factory tech support directly.`

    The main difference between these two options is with the NTI sensor, the boilers onboard control and display shows the operation. It may also offer more options like boost, etc.
    With a Ranco, or other, you program in your temperature and differential.
    I prefer letting the boiler control run the tank/boiler operating condition.

    Or buy a higher mass boiler like the Viessmann 300 or HTP tank style and skip all the confusion :) Since that is what you are trying to accomplish. I think.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream