Heat loss Boiler sizing question....

Discussion in 'Boiler Forum' started by Maxadine1, Mar 20, 2012.

  1. Maxadine1

    Maxadine1 New Member

    Mar 14, 2012
    Weymouth, MA
    I have a question about a heat loss calculation done on our house and boiler sizing. We got a heat loss done on our home and it came to just under 50000. Now we were told by other companies that it would take a few hours and cost about 300$ to have it done. The company we went with, it took them about 15 minutes? Does that sound about right? Not sure about the cost. I'm sure they just used a computer program to estimate? About our home. We have a mid 80's garrison south of Boston. It is about 2500 sq feet. Windows are fairly new and it is very well insulated (we were told after an energy audit). We have 4 zones of heat. 2 on the main floor. 1 upstairs and 1 in the basement. The company suggested a Bosch 100 boiler and indirect water heater. Right now we have an old Burnham boiler 154000 btu and a 40 gallon water tank. We use gas and baseboard heating. Any input would be awesome.

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    There are LOTS of similar questions and answers in this section, you may want to read some of them and come back with more questions. Basically, the old boiler is probably about 5x oversized, and your heating requirements are probably closer to 30K if you've tightened the place up and have good insulation and windows.

    If you have your gas bills, you can look up the heating degree-days for your zip code, then figure out fairly closely what you need. Not familiar with the Bosch stuff, so don't know if that 100 means 100K BTU, or something else, but check that. The last thing you want is to match the old one.

    On a super cold day, under ideal conditions, the boiler would be running full blast all of the time. Each time it shuts off then restarts, you're wasting energy during the cool-down/purge cycle.

    A good heat loss would take longer than that. Depending on the program they used (probably one of the more generic ones), they can be 2-3x oversized. Some are within 30-40%, and if you use your actual gas useage, you should be able to come up with a figure within 10-20%.
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  4. tk03

    tk03 New Member

    Jan 31, 2011
    Harrisburg, pa
    Not sure how they did the heat loss. Just don't confuse a heat loss for measuring radiation. The amount of raidation has nothing to do with size of the boiler and heat loss.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    50K seems high for a house that size & condition with a outside design temperature of +11F (the 99th percentile design temp for Weymouth, based on 25 year binned-hourly weather data.) Don't know much about the Bosch 100, but it has 2x the output you'd actually need.

    It has a min output of 21KBTU/hr, which could be in short-cycling territory if you try to run it at condensing temps with it cut up into zones like that.
  6. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
  7. Dana

    Dana In the trades

    Jan 14, 2009
    Sorta like driving a Ferrari in-town to get to those stop signs at every block as quickly as possible... :)

    If this house has a sub-50K design load cut up into 4 zones, the smallest zone has to be smaller than 12.5K peak, and and may well under that average. If you're not going to add sufficient radiation or thermal mass to eliminate short-cycling it might make more sense to build a combi around a condensing hot water heater (Polaris, Vertex, etc), letting the thermal mass of the tank guarantee long burns, low numbers of cycles rather than going with a low-mass combi heater like the Bosch.

    Every boiler-swap needs to be treated as a system-design update, since it truly is. This is ESPECIALLY true when swapping in a low mass condensing boiler. Low mass modulating-condensing boilers can provide a huge improvement in efficiency, but only if the system design is sufficient to let it to run in condensing mode without wearing itself out in short years.

    Find a pro to do a real heat load calculation and system analysis- from what we've read so far none of the proposals appear even remotely appropriate.

    To do your own napkin-analysis of the system (not the the heat load), start by measuring the total number of feet of baseboard in each zone, since that will determine the lowest temperature at which the boiler can run at minimum fire without short-cycling.

    Adding radiation to the smaller zones can often be the cheapest/easiest/best way to deal with it, but picking a boiler with the lowest possible min-fire is key. There are any number of good condensing boilers out there with minimum-fire output below 15KBTU/hr, with max output over 50K, but even those might not be able to fire low enough to run in condensing mode on the smaller zones unless you add more baseboard or radiator to those zones.
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