Expert/Advanced Level Question regarding modcon and buffer tank

Discussion in 'Boiler Forum' started by mratner, Oct 13, 2013.

  1. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    Have a new PROPERLY sized Burnham Alpine 150. I have 7 baseboard heating zones of which it does not make sense to "combine" them. Its only been a couple of months since i have had it but because of the micro zones i'm getting average run times of 5-7 minutes. The DHW runs at least 15 minutes so that helps the average a lot.

    So my questions are:
    1. If 5-7 minutes is my average cycle time would you be stressed/concerned?
    2. Would anyone realistically spend $1500 to add a buffer tank to add another 10+ min of runtime or is that just throwing money away?
    3. I have a 10 year/parts and labor warranty on the alpine so "premature" failure isn't going to be that costly since i'll get a new fan/board for free.

    Should i just stop obsessing about it and forget about my boiler! :)
    Thanks.
  2. Dana

    Dana In the trades

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    3,027
    Location:
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    Min- burn times of 5-7 minutes are not an issue for low-mass boilers. The average time doesn't matter as much as the minimum time, and the number of burns per hour/season. If it's chattering away most of the day at 10 burns/hour on zone calls, the 200+ burns per day is a bit on the high side at any length. But if it's no more than 5 burns/hour under the worst-case conditions, and under 100 burns/day, it's not going to suffer much. With sufficient buffer to guarantee overlaps with 3 burns/hour or less you can microzone the place to nth degree without any concerns.

    I'm not sure how you're getting 5+ minute burn times out of a mega-microzoned setup though. The very rare house that really NEEDS an Alpine 150 at Andover's 0F 99% outdoor design temperature would have to be pretty large (7000' +) or fairly leaky/lossy (say, an antique solid masonry building with no insulation). It's min-fire output at condensing temps is about 28,000 BTU/hr, and it takes about 130' of fin tube to fully balance the output with load at 120F average water temp (about the lowest you can go with fin-tube and get consistent results with outdoor reset modeling). If you don't have at least half that on your smallest zones there's some potential for short-cycling. But if the micro-zone calls are always overlapping and you never see 5 burns an hour, it doesn't matter that much.

    To be sure during the beginning of this season to date it's likely wouldn't need output any hotter than 125F to meet the load, and if you haven't tweaked in the outdoor reset curve yet now is the time to get started.
  3. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    Dana i was hoping you would reply. you have a lot of great threads on this forum. couple things...
    My house is about 6k square feet. my heat loss was about 110k btu, so the Alpine 100 was too small.

    I have the maximum modulation set at about 100k for both my heat and hot water.

    My modulation is setup correctly with 125 temp at the current outdoor temp and 170 at the max (may be able to bring that down; will wait and see).

    The reality is my heating calls for a couple of my fringe zones ( colder zones) w 125 temp having run times that are really short (2-3 min) but my IHW calls are bringing up the average. I think as it gets colder out more zones will be running at the same time for longer times which should help the overall run times.

    My question to you is if you were having 5 minute runtimes and you were not getting 100 cycles but instead 200+ (theoretical at this point) when would you bite the bullet and spend the $1500 on the buffer tank? Or would you just accept that the boiler would take a little more of a beating but leave it alone with the 5-7 minute run times.

    Again, its a little theoretical at the moment because it will eventually get colder and the runtimes should get longer with it.

    Also ,remember i have 10 year parts and labor so premature failure for at least the first 10 years will have minimal/no financial impact on me.

    Would love to get your thoughts.
  4. mage182

    mage182 Member

    Messages:
    71
    Location:
    NY
    I'm running the same unit in a much smaller house (Improper design by the installer 3 years ago) but with two larger zones (1 for each floor). I've been able to configure the unit to get ~2 burns an hour of 12-14 minutes each (all cast iron baseboard). A few things I've done are:

    1. Adjusted the +/- min max temp for heating calls. It does allow the swing in temp to be bigger but I find it has no affect on overall comfort.
    2. This is obvious but set the max fan speed lower, less speed means the unit heats the water more slowly and thus longer burns.
    3. I run Honeywell RTH7600D thermostats since I haven't been able to find anything smarter. There is an option in the manual which I found last year to adjust 'Heating Cycle Rate'. It comes from the factory set at 5 'Gas or oil furnace: Standard gas/oil furnace (less than 90% efficiency)'. I changed it to 3 'Hot water or high-efficiency
    furnace: Hot water system or gas furnace (more than 90% efficiency)'. This seemed to help in controlling the number of heating calls per hour but I haven't been able to find out exactly what it does. I assume it will let the temp drop lower than normal before calling for heat.

    Hope some of this helps. I've had the unit for 3 years now with no problems other than that it's WAY bigger than I need.

    One more thing to note, are you doing anything to condition the power on your system? The computers in them are very sensitive which I learned the hard way when a power spike during Sandy blew mine out and I had to wait 6 weeks for a backordered replacement to arrive. I now have a Tripp Lite LC1200 filtering the power on my whole system. Might be something worth looking into depending on where you live.
  5. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    Mage182,

    I too have adj the min max as well with the max +10 and the min -7. That allows my smaller zone to run at least 3-4min, and then have a solid 5-7 min of running and burning off the excess heat before another cycle.

    2. I have lowered the rpm from 5000 to the mid/upper 3000 effectively maxing it out at about 90k BTU. This should work for 99% of the weather in MA.
    3. I also lowered the Lightoff RPM to 2800 from 4000 which is really important so the flame is not too hot the first 10 seconds or so. This was a real good one to avoid unnecessary short cyles.
    4. I have the same type of thermostat and also initially changed its to 3 cycles per hour when i originally installed it. My opinion is it still cycles a bit too much.

    I will check out the LC1200. Thanks for the tip
  6. mage182

    mage182 Member

    Messages:
    71
    Location:
    NY
    I originally started out with 4 smaller zones (2 on each floor) for my first winter with the unit and was getting times similar to what you are getting. For last winter I combined the zones so I only have 2 and burn times have increased significantly. Are you running fin tube baseboard or cast iron? When I bought my house it had fin tube. I spent quite a bit of time hunting down enough cast iron on CraigsList and other places to redo all my rads, but in the end I think it makes a big difference.

    You have a lightoff RPM in your config? I don't think I have that. Maybe you have a newer firmware version? If you get a chance could you send me the version number on the sticker on the top of your computer? I wonder if we can order new versions of that.
  7. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    My darn house is wierd from a layout perspective. i could probably merge 2 of my 6 zones, but that's about it based on the room locations/heat losses.
    As per the baseboard, it would be really costly to go that route and replace the hundreds of feet throughout the house. THat's where a $500 vaughn buffer tank saves the day with a lot less work/aggravation/cost.

    The lightoff is a newer feature. Got the boiler 3 months ago. you can always replace the board (would require a new display too) but definitely not worth the ROI. Sounds like you're in awesome shape with your runtimes and off times. I think you can move on to a new obsession!

    I have to debate whether or not its worth the $1000 for the buffer tank and parts to add another 10 min of runtime. i don't think 6 or 7 is the end of the world, and it will probably get better when it gets colder out and there's is a higher ODR temperature and more zones are running simultaneously longer.

    I'll see if i can be patient! :)
  8. Dana

    Dana In the trades

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    I'm not sure if that boiler has a cycle counter (some do), but logging the number (not the length) of burns over the shoulder season months would give you a good idea of where your annual numbers will end up.

    The focus on the warranty period with the notion that somebody else will pay the cost of abusing it (if that's what's happening) is not only cynical, it's a bit silly. The thing SHOULD last for a good 20 years or more, but if you give it 25 years of ignition cycles in the first 11 years and it fails, you're kinda screwed, compared to having stuff crap out in the 11th month of year 9. Track what the thing is really doing, and if it looks like you're cycling it into an early grave, deal with it now, not at 5AM on the coldest night of 2021.

    Sometimes it's easy to plumb a bargain-basement $200 electric hot water heater as a massive hydraulic separator on micro-zoned systems, but whether that would be a suitable approach in your system would require doing some of the design math. Most commercial buffer tanks are ported with gia-normous fittings for massive flows unseen in residential apps. It's unlikely you need anything bigger than the 3/4" taps on a HW tank if you had to go that route. But if your micro-zone calls always overlap and the cycles/hour are limited to low single-digits in number, you'll be just fine as-is.
  9. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,311
    Location:
    Maine
    Where are you getting 200 dollar electric heaters LOL
  10. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    Hey Dana, thanks for the reply. The burnham alpine does have cycle count and burn hours. Over the last month, the number of cycles per day is about 25 to 35 including the 3 or 4 calls for DHW per day. not a huge number even though the heating is really only the night time during the fall in New England in Sep/Oct. I'm hoping as it gets colder, more zones will run simultaneously keeping the number of cycles per day down to an acceptable number that you have mentioned. I think i'll know more as we go through november and there is more of a full day heat demand (albeit modest) compared to Jan/Feb.

    I agree with you regarding not wanting it to break immaturely. The hard part is justifying a $1500 buffer tank install ROI.

    Thanks.
  11. mratner

    mratner New Member

    Messages:
    6
    Location:
    Andove, MA
    Buffer Tank Price

    I have 7 zones. if each are running at same time, even 5 or 6 are running, i really need the larger ports on the buffer. 3/4 would really be a bottleneck.

    From all the buffer tanks i priced, Vaughn is substantially cheaper. at least from my research.
  12. Dana

    Dana In the trades

    Messages:
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    Location:
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    If you're averaging less than 2 burns an hour you won't have anything to worry about. Keep an eye on it as the outdoor temps drop, but I'd be surprised if the cycles/hour pick up. With the outdoor reset dialed the burns/day counts will likely drop as you head into the real heating season. Sounds like you're going to be just fine, but log it (keep a notebook, write it down, with the dates) and do the arithmetic at at least a weekly basis for a few months. The weather has been SO temperate so far that there is room for other operating modes to crop up.
  13. solarclub

    solarclub New Member

    Messages:
    1
    Location:
    MA
    I have a new modcon (1 week old!), I'm seeing similar cycle times and frequency (~20 per day). I'm counting the cycles per day and will watch this closely as the cold season starts here in central MA. I'm also eyeballing the condensate each evening to just monitor the output there.

    I'm extremely impressed with the expertise and patience in the forums here! Thanks all, I'm learing soo much!

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