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Buffer Tank Control Strategy

Since the buffer can perform the role of closly spaced t's (P/S) it makes the supplied P/S t redundant it also requires a extra energy wasting circ.

Saw some GB job photos posted recently using a hydro separator, the supplied t's had been removed. Not sure if Budarus would permit this or not, the supplied p/s is from (what I can tell) not required in the netherlands, there the boiler has an inboard circ. and a diverting solenoid for DHW much like the vito dens 200. For whatever reason this did was not approved in the NA models. To bad. I feel that grafting these euro boilers on to typical american on/off thermostatic systems is not always the best application. I like the RC-10 control but this control best suited for continuous circ. single zone systems with trvs. in outlying rooms, Not typical multi zone systems.


  • van_2
    van_2 Member Posts: 20
    Buffer Tank Control

    I'm looking for a control strategy for a buffer tank when I add it to a GB142 which is running ODR. The loads connected woould be a combination of radiant (taco mixing block with ODR), panel/runtal BB and hydro air as a 2nd stage for both areas. Currently the control is a Taco SR 503 and GB is piped with factory header. I would be adding buffer tank and repiping so that buffer tank supply is directly from GB and buffer returns directly to GB. In this way the GB gets colder water as a return to promote condensing. Heat emitters would draw directly from buffer tank.

    How do I control the boiler to supply the buffer tank? If I keep the Taco panel to control the circs or the RMB for the individual hi and low temp zones, the X1 and X2 contacts on the Taco would fire the boiler which would then supply hot water to the buffer tank as per the boiler's ODR curve. However the radiant zone runs almost all the time so it would fire the boiler all the time. Even though the boiler modulates I am concerned it may reach a "steady state" with the buffer tank and the return temp may rise above the condensing range before the boiler turns itself off even though it is still getting a call for heat from the Taco (since the RMB is still satisfying the room Tstat).

    I feel the system would be more efficient if the boiler runs for a while to charge the buffer tank when a zone is calling but then shuts down when the return temp exceeds a certain value (no longer condensing). Then if any of the zones is still calling for heat, the boiler fires again when the buffer tank cools by a certain amount.

    Any ideas on how to achieve this control strategy? I need something to cycle the GB and keep it condensing for as long as one of the zones is calling for heat.
  • Eric_6
    Eric_6 Member Posts: 25
    re: GB142 & Buffer control

    I believe you could use the safety limit (EV) on the Gb142 low voltage terminal strip. Use a setpoint control, such as Johnson A419, on return back to boiler. Would need to use the normally closed contact and have it open up as you approached 140 degrees.


  • hot_rod
    hot_rod Member Posts: 19,256
    A setpoint

    control with a sensor mounted in, or strapped to the tank would work.

    How will you pipe the tank?

    Ideally find a tank with four connections, like the "Boiler Buddy" brand. This provides a hydraulic separator, buffer, air elimination, all in a nicely foamed tank. HTP also offers a nice 20 gallon buffer tank with 4 large connections and air vent tap on top.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • van_2
    van_2 Member Posts: 20
    Boiler Buddy

    I planned to use a Boiler Buddy. I have used it in the past and it is a great way to go. I was going to pipe the GB heat supply and return directly into the tank and then have all of the circs and the RMB draw from and return to the other side of the tank.

    Sensing the return to the boiler and using that control in series with the X contacts from the Taco SR503 to fire the AM-10 on the GB seems like a great way to go. That way the boiler fires only when there is a call from the 504 AND the return to the boiler from the tank is in the condensing range. Since the circ on the GB runs for a while after the call for heat is satisfied, sensing the return line should give an accurate measurement of the actual return temp.
  • hot_rod
    hot_rod Member Posts: 19,256
    If the heating zones call

    and the tank temperature is pulled down, the boiler needs to fire back up. I think the setpoint sensor should be on the tank somehow to fire the boiler back on when the tank drops. It may or may not do that just on the return pipe, as thye return pipe would only be warm if the boiler circ is running.

    The setpoint control should be the boiler call for heat. It could tie into the Taco so the call for boiler only happens when a call for heat. No sense in heating the buffer in the summer, unless it doubles as a DHW tank. An TurboMax for example.

    We may be saying the same thing??

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • van_2
    van_2 Member Posts: 20
    I think we are saying the same thing

    in terms of control. The boiler only fires if there is a call for heat (from the Taco SR503) AND the return line from the buffer tank is below a fixed temp (135* to 140* to insure condensing). The only thing we differ on is where to read the return temp. You suggest the buffer tank itself (BTW, how do you get a sensor on the Boiler Buddy tank?) and I am thinking the return line to the boiler. I feel that reading the temp of the tank may be misleading because the tank may read high but the actual return to the boiler is low enough to promote condensing.

    Another control feature I will put in is a minimum run time for the boiler. This simply insures that the boiler always runs for a minimum time (say at least 5 or 6 minutes) on a call for heat. This prevents short cycling, which although unlikely with a buffer tank, might occur none the less. The minimum run time is basically a contact in parallel with the AM-10 contacts that need to close to fire the boiler. Upon the initial call to fire the boiler it also fires this minimum run time relay whose contact closes and stays closed for the minimum run time so the boiler fires and charges the buffer tank for that minimum run time even if the call to fire the boiler from the Taco is satisfied.
  • Dobber
    Dobber Member Posts: 91

    I been looking at adding a buffer tank to my GB installs also.Where can I find these Boiler Buddys? Also do you have a piping diagram.Also,Hot Rod,I'm looking forward to meeting you at the CMX show.Already signed up for your solar seminar.

    If you don't have time to do it right, what makes you think you have time to do it over?
  • Paul Rohrs_7
    Paul Rohrs_7 Member Posts: 173
    Might re-think the minimum run time

    Van, maybe try setting the boiler for a minimum off time as opposed to minimim on time. On higher temp systems, if it overshoots the target temp, setpoint, or offset (as with the Knight boiler), you run the risk of tripping the high-limit, popping the relief valve, etc.

    Even though the you are trying to prevent short-cycling, try to achieve it with prolonged off times rather than higher than necessary temps.


  • EtienneHancock
    EtienneHancock Member Posts: 18
    GB requires P/S?

    Are you getting a 'buy' from Buderus to skip the required primary secondary piping?

    I'm a homeowner researching a proposal to use a GB for my install and having lurked here a long time, I think a buffer tank is the way to go in order to maintain largest dT to the boiler. Problem is that Buderus REQUIRES p/s piping.

    Has this come up? I presume they could void your warranty...

  • scott markle_2
    scott markle_2 Member Posts: 611
    Some thoughts

    Why the second stage air? I would consider building envelope improvements before a heating system that requires so much output.

    I recognize the issues surrounding low mass boilers and multi zone, multi temperature systems. I hate short cycling and I think it reflects design and control shortcomings.

    However I find the requirement of adding mass to a low mass device to be a inelegant work around. Much of the efficiency of a low mass boiler comes from the fact that it is not holding and dissipating heat in a undesired location (up the flu, in a closet, basement etc.) This is especially true in the summer when DHW is the only use. Think of the heat left behind (and lost) in a high mass boiler (summer) every time there is a dhw cycle.

    Why invest in this technology and essentially ask it to work in a way that does not best suit it's design? Buffers are required when controls fail to properly structure firing patterns under low load conditions. This happens when minimum modulation falls significantly below actual loads. In a typical American multi zone system if only one small zone is calling this can happen under many outdoor conditions. Sizing the boiler close to the bone is also important to avoid this.

    Question that second stage air ,What makes you think you need it?, Is this for recovery purposes? if so reconsider the deep setback, it may not be any more efficient to have a deep setback and second stage air recovery than to just maintain a more constant indoor temp.

    I struggled with short cycle in my own house which has design loads that are at or slightly below my boilers minimum modulation. Even though the house is a one zone system there were conditions at which the bioler was incapable of delivering the very low circulating temperatures required without cycling.

    My solution was to use a timer (interval) type control. (Tekmar 269). My mod con fires (heat only) at it's lowest modulation(fixed). The system is constant circ. but instead of modulating water temp, this control modulates cycle length (simplified) based on ODR. Works very well, no short cycle even delivery, high comfort.

    I would try to avoid a design that will require a buffer tank, If the delivery system and controls can not be structured to avoid the low mass pitfalls than why not just use a conventional boiler.

    If the tank is your chosen route, I would say size the AH coils such that they can operate at the same temp as the panel rads. at near design operating conditions. Remember it is unlikely there would be a second stage call when odr temps were at the low end of the curve. Consider placing the supply sensor in buffer tank and modulating tank temp based on odr, This will permit accurate constant circulation modulation of panel rads. It will also require a third party control as the gb's supply sensor is internal, However, this control will not require DHW functions as the Gb has a built in DHW sensor and relay contacts that can override the third party heating control. Your radiant can have it's own mix-down from the buffer (separate tapping). If the panel rads and/or second stage air serve the same indoor space as the radiant it may be sufficient to run the radiant on a simple fixed temperature (seasonally adjusted) mix valve (floor warming) as opposed to another expensive ODR curve.

  • Ron Schroeder_5
    Ron Schroeder_5 Member Posts: 3

    "However I find the requirement of adding mass to a low mass device to be a inelegant work around."

    Hi Scott,

    That depends on where the added mass is. If it has a hole in it (flue going outside), then the added mass is bad. If the thermal energy "leaks" into the conditioned space then it is good.
  • scott markle_2
    scott markle_2 Member Posts: 611

    While heat "leeked" in the envelop is certainly better than the kind that leaves directly we can not claim that these btus are fully recovered. What is the point of keeping an unoccupied mechanical room at or above the comfort level of the occupied space . Warmer basements will reduce loads for other parts of the structure but at a cost. While hot air rises, heat travels in all direction equally. A warm basement looses more energy than it saves. This reminds me a bit of the M.E heated window thing. Just because a pathway has been altered does not mean that energy is saved (in my assessment).

    I do get your point that a buffer is better than a high mass boiler because of where heat will be lost. Part of my objection is because I really appreciate the compactness of low mass equipment and the buffer largely defeats that.
  • Ted_9
    Ted_9 Member Posts: 1,718

    Why would it be a problem? The boiler just needs the right GPM through it.
  • Ted_9
    Ted_9 Member Posts: 1,718
    Boiler Buddy

    www.hotwaterproducts.com might have them.

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Must agree with Scott. Your system seems overly complicated and possibly redundant. Why add more complexity just to achieve greater redundancy?

    With radiant floors, panels and baseboards I must question if the hydro-air would act as a "second stage" or an adjunct. When used this way, IMHO it's better to use a separate heat source as well. Electric resistance, while expensive to operate is inexpensive to install and when used only occasionally as a short-term adjunct is reasonable. By using a separate heat source--even another boiler--you avoid significant cost for controls and equipment yet to still be without heat in the event of breakdown.

    BTW...a well-sized/designed hydro-air unit or unit heater is likely the "closest to the test" with regards to AFUE...
  • EtienneHancock
    EtienneHancock Member Posts: 18
    GB p/s

    It's just that the applications and installations manuals both use the word "required" and give no other piping description other than p/s. If other pro's are skipping it then likely Buderus has ok'd it. I should probably ask them directly.

    My system will be constant circulation with trv's on cast iron rads with the buffer tank slaved to the GB's ODR. I'm hoping my contractor can score a Wilo eco for the system side (Is the June release real?) since it's a primary goal of mine to keep electrical usage low in addition to that of propane. I also have a wood stove going 24/7 so average heat loads will always be quite low relative to the boiler's min modulation rate.
  • Ted_9
    Ted_9 Member Posts: 1,718
    Good Idea

    Buck, give them a call.
  • jp_2
    jp_2 Member Posts: 1,935
    simplier solution

    constant circulation is great. but in the shoulder season where it will cause constant short cycling, why not just use the typical tstat control until you reach minimum modulation? let the house temp fluctuate a bit more.

    shoulder season is already using minimal fuel, losses in efficiency and comfort should be minimal too.

    buffers also drop efficiency by way of tank loses. the more you use it, the more you loses it!

    hard to believe that your entire system needs buffering anyway?
  • UniR_2
    UniR_2 Member Posts: 3
    Buffer losses?

    "buffers also drop efficiency by way of tank loses. the more you use it, the more you loses it!"

    I installed my buffer tank as a branch off the return leg using a diverter tee to create the ΔP to get some bypass flow through the tank. I normally divert about a third of the flow through the buffer tank (my former electric water heater fed through the top, returned through the bottom - 3 to 4 GPM works fine with ¾" tappings). What it does is it averages down the return temps. No sense returning all the cooler buffer water at the same time as all the cooler distribution water comes back at the start of a heat call. Ideally, flow would be delayed a few minutes, but any type of control that would allow this would never pay for itself. The overall difference is buffer tank return temperature will vary by about 30° (115° max 85° min) on an average winter day due to the heating cycles and some very slight 1-2° setback.

    Given that the main part of my system is large 1¼" diverter tee, the steady state ΔT was pretty dismal. This widened it by 2° from 4-6° to 6-8°. The extra volume allows the return waters to be slightly lower. Any leaked heat typically gets leaked right into the heating system itself - it isn't isolated. And tank losses are going into conditioned space and given that the tank only ever reaches about 120° at its peak the shell losses are nothing.

    Still, I doubt the minor efficiency gains will pay for the fittings, let alone the piping or the extra ET-30 needed to cover the added water volume, but I can't see how a buffer could ever cost a condensing boiler system to lose efficiency unless it had electrical controls that exceeded the marginal gains in efficiency or it was leaking BTUs outside the envelope. And because of ODR, it doesn't even get to strand many BTUs at the end of the season.
  • jp_2
    jp_2 Member Posts: 1,935
    leaking heat

    UniR, if the heatloss from tank is not where you want it, its waste heat......

    though I agree this loss is quite minor, still a loss.

    depends on size and control
This discussion has been closed.