Sizing for a new boiler in Watertown, MA

Discussion in 'Boiler Forum' started by mrjohneel, Mar 8, 2014.

  1. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
  2. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    If properly sized and controlled and always expensive in residential applications, more especially if your contractor knows how to use them.
  3. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    Looks like the cost of one of those exceeds what I can buy another boiler for and just swap it out.
  4. Dana

    Dana In the trades

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    2,924
    Location:
    01609
    A boiler swap out isn't going to necessarily address the short-cycling. The amount of radiation on the smaller zones has to be able to emit at least the majority of the BTU/hr rating of the boiler at the temperature (and modulation level if it's a modulating boiler) to stop the short cycle. For example, you can't dump 40,000 BTU/hr into a 15' stick of fin tube- even at 200F average water temp (which is a realistic upper bound for most cast iron boilers, and higher temp than most modulating boilers will deliver.)

    eg: At 200F AWT you're lucky if you can get 750BTU/hr per foot of baseboard, which means it won't emit more than 15' x 750BTU/ft-hr= 11,250 BTU/hr, which means the other 28,750 BTU/hr is going into raising the temperature of the boiler until it hit's it's safety hi-limit, which doesn't take very long at all. The lower the temp, the lower the emittance, and thus the greater the excess BTUs going into the system.

    Heat purging boiler controls combat this by maximally increasing the difference between the high-limit and low-limit, utilizing the boiler's thermal mass to maximize the burn time, but the thermal mass of a modern cast-iron boiler is not infinite either, but it can mean the difference between 1-minute burns and 3-minute burns when serving ultra-small zones. If a boiler has 250 lbs of iron and 3 gallons of water in it, that's about 50 water-equivalent lbs of thermal mass, and if you let it swing from 140F to 200F you get a 60F differential, which takes 3000 BTU. In the scenario above with ~28,750BTU/hr of excess going into the system, that pushes the burn time out to 3000/28,750= 0.1 hours, or 6 minutes, which isn't an efficiency disaster. If the differential on the boiler controls is set for only 10F, the burn time is 1/6 of that, or about 1 minute, which really IS an efficiency ( and boiler wear & tear) disaster.

    There are also variations on the theme of adding mass than can work. A 40 gallon tank of water has 334lbs of water in it, and if you allow the temperature to swing 25F between a low & high limit control, that takes about 334lbs x 25F= 8350 BTU. At 40,000 BTU/hr that represents a minimum burn time of 8350/40,000 = 0.2 hours which is 12 minutes. If the boiler's minimum output is 80,000 BTU/hr the minimum burn time is 6 minutes. If the heating system is set up to draw from the tank, and the boiler is agnostic of the state of the zone calls, and slaved only to the tank, it simply can't short-cycle, whether just one micro-zone is calling for heat or all zones are calling for heat. A cheap 40 gallon electric hot water heater (not wired up as a hot water heater), and some aquastat controls is a common band-aid for micro-zoned systems with low-mass heat emitters and oversized boilers. You have to do at least the napkin math on pump sizing for the boiler-tank loop, but it's not rocket science. As a plumbing-competent DIY it's well under a grand in materials (sometimes under $500).

    [​IMG]

    But if a $200 heat purge control retrofit and combining some of the smaller zones boosts your min burn times to over 3 minutes, and your average burn times to over 5 minutes there's no point to the buffer-tank band-aid.

    Another approach to adding thermal mass is to add it to the radiation on the deficient zones using high-volume (=high mass) radiators instead of fin-tube, but there are aesthetic & other issues that often make that approach less than desirable. Replacing fin-tube with medium-mass flat panel radiators sized to be able to emit a large fraction of the boiler output can also work. While that approach can be pretty expensive, it's also a pretty big comfort uptick over fin-tube baseboard.
    Last edited: Jun 11, 2014
  5. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    I have a cape w/ 2 story extension off the back. Many years ago I split the first floor of the cape into 2 zones. Neither of them are large and one is rather small. But I can wire the 2 zone valves off of one thermostat and find out what the boiler run time becomes on a typical heating call. Important also will be finding out how this effects BTU usage per HDD. I certainly appreciate all of the helpful information I've gotten from everyone here and do apologize to the OP for hijacking his thread. Most of this is copied and pasted elsewhere, the heat is off, A/C is on. Thank you all again.
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