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Thread: Hydronic heat buffer tank

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  1. #1

    Default Hydronic heat buffer tank

    I've recently complete an addition to our home and now have two different type of radiators low mass and high mass.

    System Description:
    1) Low mass boiler w/outdoor reset
    2) existing system (2/3 of the house) has large cast iron radiators (high mass) - 2nd floor rads have thermostatic radiator valves (TRV)
    3) 1st floor of the old system does not have TRV and is where the main thermostat is located
    4) new radiators are runtal baseboards (low mass) and runtal towel (low mass) radiators all with TRV
    5) new zone pump is a taco variable speed
    6) new zone has outdoor reset mixing valve - to allow a lower temp than boiler min.
    7) new zone is set up using primary/secondary loop configuration

    1) Run the new zone as continuous circulation, low water temp, letting the variable speed pump control the flow
    2) prevent boiler flashing when most of the runtal TRV are closed - old slant fins had this problem
    3) Use main thermostat and 1st floor loop as the reference zone - area of greatest heat loss in house
    4) fire the boiler using reference zone only
    5) I believe I should add a buffer tank to the mix to both prevent the flashing when only a couple of runtal TRVs are open - low pump flow / speed
    6) A buffer would also allow for a degree of decoupling with the TRVs decide to open when the main zone is still open

    1) Could an electric water tank (unwired) be used as the buffer tank?
    - stainless buffer tanks are rather pricey and 30+ gallon in size
    - I would like to use a 10 -20 gal tank
    - since it's a closed system I would think a glass lined tank would hold up just fine
    2) Should the sacrificial anode still be left in the tank?
    3) Could the buffer tank be placed in series with the secondary loop
    - avoid the cost and energy use of another circulator, controls etc.

    Thanks for any input

  2. #2
    In the trades Dana's Avatar
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    Jan 2009


    Without doing the math on the actual heat loads and thermal masses of each zone and the modulation range of the boiler it's hard to know if any of it makes any sense. I haven't measured the thermal mass and couldn't find volume specs of the Runtals, but I suspect they're considerably more massive (think "water volume", not dry-weight) than fin-tube (cood b rong offen am.)

    Electric HW tanks can be used as buffer tanks, but whether you need one at all isn't really obvious, but since it's microzoned with TRVs that are each agnostic of the state of the others it may be necessary to prevent short-cycling (or not- it depends). It need not be plumbed as a hydraulic separator- you can plumb it in series, but if your system is already primary/secondary using the buffer as the point of hydraulic separation the mass of the tank is involved with every burn.

    Whether 10 gallons would be enough also depends. eg: Assuming all TRV are off and there's ~5 gallons of water in the bypass plumbing & boiler, 10 in the buffer, you're talking ~125lbs of water. If the boiler's min-mod output is 15KBTU/hr (250BTU/min) and they boiler's hysteresis is 5F you get a 2.5 minute minimum burn, which is on the short side but still fine for a smallest in class low mass boiler. If min/mod is 25K (416BTU/min) and the hysteresis is 3F the min burn is reduced to less than a minute- it's a short cycle, and even 20 gallons would fall short. (Ideally every burn would be 5+ minutes, but 2.5-ish isn't going to short cycle it into low efficiency and an early grave the way it would a cast iron boiler.)

    The buffer doesn't need a sacrificial anode since it's a closed system, but SFAIK it's not a problem to leave it in.

  3. #3
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
    Join Date
    Mar 2011


    Lots of speculation.

    I design mixed radiation hydronic heating systems every day for new well insulated homes and old leaky ones but have only used a buffer tank in two of them in the last several thousand jobs. They are an expensive energy-wasting crutch normally used when big mistakes have been made and no other viable solutions exist. (Note the most recent "short cycling string in this blog". Solar heating would be the notable exception.

    A proper hydronic heating system design starts with a room-by-room heat load which, dictates the size of the radiation within design temp parameters. In your case the old cast iron radiators would be sized after the room load to determine the design temperature and the Runtal - if you must - sized to the original design temp if possible. The control strategy naturally follows. Of course the boiler is sized from the block heat load so, will not be too big.

    A little professional design goes a long way.

  4. #4
    In the trades Dana's Avatar
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    Jan 2009


    What BadgerBoilerMN said. If pay a pro to design it (even if you're doing the DIY sweat-equity) it'll probably go a lot smoother, with less back-tracking and fewer unnecessary components.

    Short of that, at least do the napkin-math before diving in. WAGs just don't cut it.

  5. #5


    Thanks for the input.. The new part of the system was designed and installed by a professional plumber. The new half of the system has 1/2 pex piping and between the piping and the runtals it contains 7.8 gallons of water max. There are no rooms with both old radiators and runtals. The old radiators heat the old part of the house just fine. Heating the addition is the case in point. The primary issue here is that when only a single radiator valve is open we could have less than a gallon of water to play with.

    It's hard to do an analytical design since we are working with fairly incomplete data. The boiler in question is also fairly old (late 80's) Teledyne Modutherm. It has a peak output of 130kBTU (I don't recall a lower BTU rating being listed) perhaps calling it a modulating boiler is a bit of an over statement - it states that it can modulate the output or something like that. It resembles a swimming pool boiler with 1 1/2" piping and negligible internal water capacity. The new part of the home is very well insulated and the older section has has some upgrades but is certainly the leakiest part of the house.

    Our indirect water heater works just fine with the boiler when it is the only zone, but it's got a lot of water to dump the heat. I know from past experience that a low flow / low temp drop situation will cause a "moaning sound" the best guess by the pro's has been that it is flash to steam before the flame shuts off.

    It will be replace, relocated and we will be re-piping the old parts of the system a couple of years down the road. I would have loved to start with a clean sheet but the budget could not stand it at this point. Since the plumber did not have first hand experience with the problems of the old system, he could not say for certain if it would be a problem or not.

    My simple logic is that if low flow was a problem in the past, it's likely to still be one and adding some additional water to the system might be a good idea. We are both scratching our heads and don't want to have to come up with a fix when it is -10 outside.

  6. #6
    In the trades Dana's Avatar
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    Jan 2009


    Low flow won't be fixed with a buffer tank, a buffer will only fix a low MASS problem.

    Low flow through the boiler might be fixed by setting it up primary/secondary, but without sufficient mass in the primary loop you may still have issues at low flow on the secondary, but it won't be a steam-flash, more likely it would be a short-cycle.

    A primary/secondary configuration using a buffer tank as the hydraulic separator is pretty foolproof, but you still have to do the math on something real to know if it'll short cycle, but the mass of the tank and the boiler's output at low-mod, and the hysteresis in the boiler's controls will determine the minimum burn cycle, and that min-burn time number is independent of the state/flow of any of the secondary flow and valves- it's a hard-defined minimum:


    (With a HW heater used as a buffering hydraulic separator you'd be using Tees at the cold & hot water feeds, since it doesn't have two ports for each.)

    A professional plumber isn't the same as a professional hydronic heating designer, even if it says "plumbing and heating" on the truck.

    How much water is in the Runtal baseboard? IIRC it's not just a pipe-thru the way fin-tube is, and wouldn't be considered "low mass", but knowing the actual volume is useful. Fin-tube convectors like the SlantFin are indeed low mass, with extremely low water volumes, but the Runtals are really low height thin radiators. Cast iron baseboards are radiators too, feel a bit heftier, but their thermal mass is probably a bit lower than the Runtals due to iron's much lower specific heat- about 1/9th that of water. (One gallon,or 8.34 of water has the same thermal mass of ~76lbs of iron.) Runtals are convecting radiators, and while it's probably an order of magnitude less than the water volume of an equivalent-emittance finned radiator, I suspect it's still about an order of magnitude more than that of it's fin-tube equivalent. (Just a WAG- I've never ogled one right out of the box, couldn't find spec online.) Even if it's only 2 gallons of water in an 6' section (and I suspect it's more) that's the same thermal mass as 100' of 3/4" fin-tube.

    Most of the old water-tube copper fin HX boilers of the '80s had a 4:1 turn down, so if 130K was max, min is probably in the 35-40K range, but again we're into the realm of WAG. Water tube boilers were popular in low temp systems (including pool heating) because they're tolerant of entering water temps much lower than that of cast iron, but most had an 80-82% combustion efficiency on day-1 and after 2+ decades of service between liming on the water tubes and corrosion on the fins I'd be surprised if you're running better than 75% steady-state efficiency on the sucker.

    Hopefully there's an appropriately-sized condensing boiler in your mid-term plans, but please find a hydronic designer (not a plumber) to review it now before diving in any deeper. I'm not sure going with outdoor reset circulators for the low mass zone was all that smart, since it would only add to an insufficient boiler flow issue and shorten up the short-cycles (even if it didn't flash-boil on you.) But maybe the goal is to short-cycle the thing to death in short years so you can move on? ;-)
    Last edited by Dana; 11-11-2011 at 01:45 PM.

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