Time to replace the hulk in the basement

Discussion in 'Boiler Forum' started by Skeptical_thinker, Dec 22, 2013.

  1. Skeptical_thinker

    Skeptical_thinker New Member

    Massachusetts Metrowest
    tl;dr: want to replace huge, old, oil fired boiler to supply ~75Mbtu/hr to over-radiated, soon to be micro-zoned finned tube system. Looking for advise, opinions and comments.

    The hulk:

    Burnham V34 SN 7551110 with a Frankenburner that might have started life out as a Carlin, but only the original frame has not been replaced. The burner has 1.0GPH nozzle and the boiler was last measured at 70% efficient, which matches the tag. That may be an overstatement. It has a tankless coil. Converted to cold start two years ago, no leaks. This beast has had a long life. It was here when I got here more than 18 years ago.

    3200 sq ft "cottage" built in 1919 in Metrowest of Boston which is not all that well insulated, but has new windows all around. Design temperature 0F. Project planned to insulate the attic floor.

    Three heating zones with roughly equivalent loads, two with 1/6 HP circulators, one with 1/25 hp. All circulators on the return. Total radiation is ~140Mbtu/hr.

    Potable water circulator through tankless coil and 50G electric water heater (no electrical hookup to elements) as a "booster" tank. RIB relay system to call for heat with top thermostat, end call when bottom thermostat is satisfied. This keeps a single guy in hot water by firing for less than 1/2 hour every two days. Under 75 gallons of oil from mid May to early October.

    Honeywell AQ-2000 control with Honeywell communicating thermostats and ODR controlling all four circulators and boiler via aquastat on boiler. Aquastat is really just functioning as a combination burner relay and high limit safety.

    Recently nature gave me a chance to compute the building's heat loss. There was a heavy overcast with snow and near constant 13F outside temperature. I set all of the t-stats to 66 and waited for the building temp to stabilize. Over the course of the next 190 minutes the burner was on 94 minutes and off 96. The average boiler temperature when the burner turned off was 146. The ODR was setting the high limit to 155-157, but it never got there, 151 was the highest I saw. By the end of the test the AQ had synchronized the heating zones so that all three zone circulators started at the same time.

    Oil burned at 1GPH with 70% efficiency means the output of the boiler is ~98Mbtu/hr as I figure it. The On/Off ratio of 94:96 means the burner was on 49.5% of the time for an average loss of 48.5Mbtu/hr. That was a 53 degree rise. Normalizing that to a 65 degree rise I get a building loss of 59Mbtu/hr. Please correct me if I got the arithmetic (or concepts) wrong.

    I want to split each zone in two to get to six zones. The house lies east/west with a large number of windows to the south and north. Each floor of the original house is one zone (one feed on the north, one feed on the south, with each floor having a common return. A large addition has two distinct areas, an open kitchen/family room and a small laundry/mudroom. In addition, I would like to have radiant heat in the two bathrooms (6'x8' stacked, one on each floor) and maybe in the 13'x16' kitchen.

    Here is what I think I want. A high efficiency boiler (condensing: MPI? Firebird? non-condensing: Buderus? Burnham? Biasi?) with an output ~75Mbtu/hr (Ideally able to be down fired later) using either a buffer tank and flat plate HE for the booster or a reverse indirect (which seem to be disappearing) for DWH and buffer. I would use a mixing valve controlled by the AQ2000 to supply the building zones from the buffer with the low limit set to 100F and high at 180. The buffer/reverse indirect would be the load on the boiler. Both the boiler circulator and the system circulator will be VDT controlled.

    If I go the reverse indirect route, I would probably use something under 50 gallons as that should supply plenty of hot water for a five bedroom house. I don't know how I would size a buffer tank, but I would guess it would be smaller.

    Any advice, comments, corrections, jokes, expressions of sympathy etc will be most welcome.
  2. Tom Sawyer

    Tom Sawyer In the Trades

    I like the firebird boiler. Impressive performance and it can run either in condensing or non-condensing mode. Perfect for fin tube, zoned delivery. I would go with a super Stor SS40 for domestic hot water.
  3. Skeptical_thinker

    Skeptical_thinker New Member

    Massachusetts Metrowest
    I would like to avoid a two tank solution if I can, which is why I was thinking a reverse indirect. Can the SS40 be used as both an indirect and a buffer?
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