Reverse Buffer Tank Setup

Discussion in 'Boiler Forum' started by RJAD, Dec 18, 2009.

  1. RJAD

    RJAD New Member

    Messages:
    4
    Location:
    Thunder Bay ON
    Hello, new guy here.
    I am currently installing my second Hydronic Heating System in a new house I am building, (first system I installed in a garage a few years back).
    I have thought and read a lot about how to best incorporate the latest in industry accepted practices and equipment into this new system.
    The question I have is in regards to the use of a buffer tank with a small thermal mass condensing boiler, in my case a TT SOLO 110.
    I understand the concept of the Buffer tank is to act as a thermal flywheel and to store energy and smooth out rapid temperature fluctuations / short cycling etc.
    I have an idea of using a small indirect water heater (TT Mini Smart) piped into the boiler supply(return) using closely spaced tees with a check valve between. The idea being that the domestic cold water would first flow into the buffer tank, get heated to whatever temperature the remaining heat in the boiler supply/return water has left in it, and then flow into another larger DHW tank that is connected in standard fashion to the hydronic system. This (in my mind) would allow the larger DHW tank to require less input from the boiler (which in itself does not make much economical sense, only increaded capacity) however, it would also allow the return water to the Mod/Con boiler to be lowered, increasing the efficiency of the condensing aspect of the unit. I have not come accross any info or posts on this idea (although I am sure others have thought of some form of this), but would be interested in an industry professional's opinion regarding it.
    I realize that the DHW usage would be inconsistent, and that return temps would vary, but they would always be lower than if the return buffer was not there.
    The concept is still the same, storage of available energy. Just the components are moved around a little to affect an efficiency improvement for the boiler.
    Any thoughts pro or con would be greatly appreciated.
  2. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    I think it's a lot of extra equipment and expense not to mention time when you could just install a Buderus GB or a Viessmann boiler in the first place.
  3. RJAD

    RJAD New Member

    Messages:
    4
    Location:
    Thunder Bay ON
    Re. Buffer

    The cost is less than 400 bucks for the Mini smart, and would be no more than a stand alone buffer on the load side. I realize a standard IDHWH could be piped into the load side and act as the buffer, but the return temps would not be reduced.
    I have not set this up yet, or even purchased the unit, just added the tees and valves in the return in case it is a good idea.
    As for the other boilers you have mentioned, I have not looked into their potential or features, so cannot comment on them. (Also the TT is hanging on the wall, so I got what I got, and thats what I will work with.
    Thanks for the info all the same.
  4. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    If you have a micro zone that requires less than the lowest turn down firing rate of a modcon boiler whether it be a Triangle Tube, Lochinvar, Viessmann or a Buderus, it will react all the same, manufacturer makes no difference, but oversizing doesnt help.
    The purpose of a buffer tank is to add mass for those small-load, low flow smaller heating circuits.

    A proper SS steel buffer tank is still your best bet.
  5. cattledog

    cattledog New Member

    Messages:
    42
    Location:
    Portland, Oregon
    re: reverse bufer tank

    RJAD--

    In my opinion, the best way to lower the return water temperature and improve the condensing efficiency is in the design, sizing, and flow rates for the radiation. Using outdoor reset to lower the supply water temperature when less heat is required is also important.

    The percentage of water vapor condensed at any given return temperature will vary with the boiler design. The efficiency gain may be on the order of a few percent if you are already in the condensing range. I have seen charts where efficiency rises from 86% to 96% with a drop from 140 to 60 degree return temperatures. As you say, the performance of the reverse buffer in lowering the return temperature will be a function of the dhw usage.

    One drawback I see is that there is an offsetting system efficiency loss with storage losses from the reverse buffer tank. Any heat extracted from the return water by the tank will have to be added back in by the boiler to achieve the outlet water temperatures. Theoretically, that will balance the gain on the dhw side, but storage losses will mean that you will have to put more back in than you get out.

    I would ask what else could be done with the money you might spend on a TT minismart. Are there upgrades to better controls, high efficiency pumps, zoning, insulation, etc. which might be able to achieve a small percentage gain in system efficiency in a more predictable manner.
  6. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    How many zones, how massive is the radiation, and how perfectly-sized is the Solo110 to the heat load?

    In a low-mass system even if the mod-con is perfectly sized for design-day heat load, it's oversized for every micro-zone on the system. If you're going to add a buffer (with or without a DHW heat exchanger) your better off placing it between the boiler output & the radiation and control it to define a minimum burn length. A short-cycling condensing boiler still isn't going to live anywhere near it's potential, and even a perfectly sized boiler for the whole-house load on design day is going to short cycle on low-mass radiation zones a large fraction of the time. There's only so much you can do with modulation and flow rates- the minimum modulation is a hard minimum, and usually well over the heat load of smaller zones 100% of the time. Mass, and minimum burn lengths are the answer. See:

    http://www.patkelco.com/uploads/files/4bf77cff8bc8411bb00c67e962d0ef7d.pdf

    If you want to use a buffer as pre-heat for the DHW, a reverse-indirect (TurboMax, Everhot EA series, or ErgoMax) plumbed as the point of hydraulic seperation for the system is a better choice. During high-temp heating water times it'll deliver 100% of the DHW load and buy you some condensing time. But the rest of the time it's there to lengthen the burns. It may cost you a few percent in condensing some of the time, but it'll save you several percent by reliably boosting the burn lengths while at part-load. If you have sufficiently low-temp radiation (like radiant floors), you may want to do it differently (say, plumbing it in series with the output rather than as a hydraulic seperator) to be able to keep the combustion efficiencies above 95% most of the time.
  7. RJAD

    RJAD New Member

    Messages:
    4
    Location:
    Thunder Bay ON
    Buffer

    Thanks for all your replys.
    Good info for sure.
    I have just fired my system up (2 days ago) and the slabzones are begining to show signs of warmth.
    I will take your advice and opt against the reverse buffer. The idea was probably a result of thinking too much about a problem I don't seem to have.
    My system consists of the following loads:
    1 - 30x30 4" slab 1 zone (I have a room beneath my garage and this is the floor)
    2 - 50x40 4" slab 3 zones (basement)
    3 - 110000 Btu Air Handler

    My plumbing exits the boiler as 1", flows into a TACO Hydro-SEP (which is also where the 5 gal expansion tank is connected to), out to a Tee where the Air handler zone taps off for full heat. After the tee, an UPONOR motorized mixing valve and then into a 2" vertical Low Loss Header, where 3 - 1" supplys are pumped by Grundfos 15-58 pumps into 3 seperate Oventrop Manifolds (the Air Handler is also pumped by a 15-58 pump).

    All zones return to another 2" Low Loss Header, this time with 5 inlets, 1 extra for the indirect water heater I have not yet purchased.
    All controlled by an Uponor Climate Control Multifunction controller.

    So far, things seem to be working very well. The system may take a few more days to bring the slab up to temp before the boiler will make full supply heat. It is supplying at 134 (slowly increasing daily) and returning at 105.
    I am aiming for a supply of 180 for the AH, mixed to 130 for the slab, and return at 120 or so.
    The TT 110 produces about 4 gal. of condensate every 8 hours, but I am sure it will slow down once the house stabilizes.
    BTW, my heat loss calcs which were done via the house designer on profesional software came in at 108000. I added 3/4" rigid insulation around the entire exposed exterior of the house, and 2" rigid around the basement. Slab has 2" beneath. (actual HL should be around 100000)
    It would be nice to see return temps below 100, but I think my boiler will fault if the difference exceeds 72 degrees.
    I think the system will work well without any buffer.
    Thanks again,

    P.S. I will try and add a photo of the system.

    Rob Dawe

    Attached Files:

  8. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    If the slabs are concrete, you don't want temp's higher than about 95.
  9. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    And, if the AH is rated 110K @ 180F , that's more than your calculated design-day heat load (which is likely 15-25% higher than reality, with most software packages). That's also more heat than a TT110 can put out at 180F. With 180F out with 150F returns you're well under 90% combustion efficiency, and you'll be lucky to pull more than 95KBTU out the boiler. (Which, of course means, it can never actually HIT 180F at the air handler, even when it's the only zone calling for heat.)

    You should be able to get roughly half the 110K out of the air handler with 125-130F water and still have quite comfortable (105-110F) exit-air temps, which will keep the boiler running in a condensing mode while serving that zone. Most air handler manufacturers will give BTU ratings at multiple water temps (at least two, but some will supply curves.) 140F ratings for air handlers are common enough. 180F is usually a nominal for rating purposes, not an operational requirement. Run it with 140F water for 10 minutes (or a day, to see if it keeps up) and measure the exit air temps- if the exit air is over 110F and the zone is cycling, bump it down. The air handler likely has an aquastat to inhibit the blower below some water temp which may have to be adjusted. If it doesn't go below 120F, you should probably replace it with one that does. (There are some that can be strapped onto supply pipe to the coil.) I've run the air handler zone at my place (the "hail Mary" second stage for when my staple-up can't keep up) as low as 120F on the coil, yielding ~105F exit air at the furthest air register, and it's not bad. It's 180F rating is on the order of 100K (don't have the specs in front of me), and I'm pulling ~55K out at ~125-130F input water (which is where I'm running the staple-up radiant). YMMV.

    If the zone keeps up at 140F supply, you'll be bumping close to 90% combustion efficiency. If the air handler zone is lossier than 55K (and it may well be) you'll have to bump it up to KEEP up, but start as low as is comfortable from an exit-air temp point of view, and bump it up 5-10F at a time. The lower you can keep the temp, the cooler the return, & higher the efficiency. At 180F out/150F return you'll doing no better than 86% raw combustion efficiency, but with say, 120F supply, 105F return, you'll be pulling low 90s:

    [​IMG]

    The high mass of slab radiation means no buffer needed there. The hysteresis of the thermostats on the air handler zone can be adjusted if the burns end up being shorter than 5 minutes at half-fire on the boiler. If the exit air is still comfy with 115F water ad the zone keeps up there, you can probably make it a single-temp system (even though the slabs won't ever NEED 115F water, and mixing it down to sub-100 will give you cooler return water for higher efficiency.) Getting it to 90F on the return gets you into the 95%+ range.

    If there's an easy way to configure/control it to be able to use outdoor reset function for the slabs &/or the air handler you may be able to get efficiency averages into the 90s, but not so likely if you're starting with 170-180F an mixing down. If the air handler is the main zone, and you can run it at 140F or less most of the time maybe it'll average 90% efficiency. Don't run any temps higher than you absolutely have to, if you can help it. Below-grade basement slabs under conditioned spaces aren't likely to ever need more than 90F water. I'm less sure what the under-garage heat loss looks like, but it may need a bit more on design-day.
  10. cattledog

    cattledog New Member

    Messages:
    42
    Location:
    Portland, Oregon
    Rob--

    Dana's comments are right on the money.

    How are you controlling the slabs? In my experience with high mass slabs, there is better control by using slab temperature alone with an embedded sensor, and tuning it for the desired air temperature in the conditioned space rather than by trying to control room air temperature with slab min/max settings.

    I concur with the two previous posts that you should not require water temperatures higher than 90 in a concrete slab.
  11. RJAD

    RJAD New Member

    Messages:
    4
    Location:
    Thunder Bay ON
    Great Info

    I must say that I really appreciate all of your info / help.
    The internet and more specifically forums like these make the DIY-ers ability to educate themselves on many subjects very easy and enjoyable. That in itself can be both good and bad. I just want to let you guys know that the reason I wanted to do the plumbing and control portion of this heating system myself was because of the great interest in an efficient and reliable heating system I have. That, and the fact that a hydronic heating system (because of the labour and skill involved with pipe layout etc.) is truly a work of art once installed and functioning as designed. Also, it is my home.
    I will say that I did hire a professional sheet metal installer to build the duct work, and his work was also very impressive.
    I know that professional installers / contractors may feel that DIY-ers are removing a % of market from their access, but for me it is more about what I want, and how it is done, than what it will cost. So thanks for your time.

    Now, to answer the questions.
    The slabs are controlled by Uponor Radiant thermostats, (4 in total, all in the basement) which use a form of Pulse Width Modulation to monitor zone temps. The main floor has an RCS TR-40 RS-485 networked thermostat which is wired to the air handler, and then to the Climate Control Multifunction. This is obviously the primary zone. The slab zones are more for comfort, but as with any high mass heat sink, also provide a stabilizing effect for the whole house.
    I thank you for your info regarding slab zone and AH supply temps. I was going by info I took from manufacturers lit. and the WEB. (Slabs should never receive more than 140 degrees, and I made the assumption that AH max temp to deliver max heat.)
    The system, currently at 135 to the AH provides a comfortable feel from the Main Floor ducts. I would surmise that another 5 or 10 degrees will be more than sufficient.
    Balancing in progress.

    Merry Christmas Guys,

    Rob

    By the way, I am in Thunder Bay Ontario Canada, just a few hundred miles north of Minneapolis MN. We do get the occasional -40 winter temps, but probably average between 10 and -10 deg. F.
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