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New system

dmr1595
dmr1595 Member Posts: 1
I was wondering if anyone had some advice for me. Im replacing one of our Oil fired forced water boilers in a small house 1400 square feet. My heat loss calculation came out to about 50,000 btu's but the btu output of all the old radiators is about 95,000. THe old boiler does not have any info about its output. The house is old and not very well insulated. The house is being converted from oil to gas and im also adding an indirect tank. So my real question is what size boiler do i need for this application? A weil mclain 4 section puts out 74,000 btu's and im not sure if thats enough to handle all the radiators even though it will cover the heat loss and the tank. Being an oil guy myself i would usaully just go ahead and put a 3 section oil fired boiler but with gas i noticed the btu ratings are much lower per section. anyone have any good advice????

Comments

  • icesailor
    icesailor Member Posts: 7,265
    New System:

    Size the boiler to the building heat loss, not the other way around. The building is over radiated and will work fine with lower system temperature water.

    Oil boilers tend to be on the larger output because there is a limit to how low you can effectively fire them.

    A Mod-Con boiler would be a better choice for you. You can do all you want with ease.
  • icesailor
    icesailor Member Posts: 7,265
    New System:

    Size the boiler to the building heat loss, not the other way around. The building is over radiated and will work fine with lower system temperature water.

    Oil boilers tend to be on the larger output because there is a limit to how low you can effectively fire them.

    A Mod-Con boiler would be a better choice for you. You can do all you want with ease.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Size the boiler to the building heat loss, not the other way around.

    I think that is really funny.



    Dear heating Help,



    I have a 75,000 BTU/hr boiler. What size house can I build to use all the heat it provides? etc.    ;-)





    I know that is not what you meant.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Boiler sizing

    Has it always heated well? I always recommend firing the connected load. You said that the house is very poorly insulated, and although we never want to oversize a system, we dont want to undersize it either. It would be a shame to put all that money in a new system, and have it never shut off during the heating season. If you want a system to run more efficiently, install a cold start boiler w/indirect storage, an outdoor reset, max the system temp at 160-170. you can always increase the water temp from there if needed.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Boiler sizing

    Has it always heated well? I always recommend firing the connected load. You said that the house is very poorly insulated, and although we never want to oversize a system, we dont want to undersize it either. It would be a shame to put all that money in a new system, and have it never shut off during the heating season. If you want a system to run more efficiently, install a cold start boiler w/indirect storage, an outdoor reset, max the system temp at 160-170. you can always increase the water temp from there if needed.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Boiler sizing

    Has it always heated well? I always recommend firing the connected load. You said that the house is very poorly insulated, and although we never want to oversize a system, we dont want to undersize it either. It would be a shame to put all that money in a new system, and have it never shut off during the heating season. If you want a system to run more efficiently, install a cold start boiler w/indirect storage, an outdoor reset, max the system temp at 160-170. you can always increase the water temp from there if needed.
  • Firing to connected load.......

    My home had 900 sq ft of gravity water radiation and piping .  Firing to connected load would give you either 900 x150 = 135,000 x 1.33 = 179,000output or 900 x 170 = 153,000 x 1.33 = 204,000 output.  Installed boiler was 270,000 input. 

    Heat load calculation revealed a heat loss of 48,000 btu/hr.  I down fired the boiler to 90,000 btu /hr input and the boiler was still cycling off in subzero weather.  Fuel usage also dropped measurably.   Firing to connected load would result in a boiler that is nearly 4 times the size necessary, greatly reducing efficiency, shortening boiler life  due to cycling wear and cost a lot more to install.  Just as an aside, the home is a 3200 sq ft 1905 frame structure in northern Illinois.   The near twin to my home ( but with no insulation and missing many storm windows) had been running a 140,000 btu/hr input boiler for about 30 years  with no problems.

    Always size to heat loss, and with an over sized system either take advantage of the lower water temps with a condensing boiler or adjust the piping for use with a non condensing boiler. 
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert





    Noisy Radiators are a Cry for Help
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Agree with Boiler Pro

    He is right. The connected load is not the way to size a boiler. The heat loss is the first chapter of the story. From there finish the book.



    If your staying oil I'd be looking at a 3-pass oil boiler, throw outdoor reset on it and start putting those wasted oil dollards back into your pocket.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Rod
    Rod Posts: 2,067
    :)

    LOL- Needed a good laugh!
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    As stated with hot water heating systems

    you size the boiler to the heat loss of the building. If it was Steam that would be different. Also as stated a mod con would be really sweet for this system since it has over sized radiation you can now match the water temp to the heat loss with an out door reset control and run cooler water through the system on calls for heat. This will save fuel dollars.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
    https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating
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