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Boiler Sizing Help

Brent H.
Brent H. Member Posts: 161
Any thoughts?


  • Brent H.
    Brent H. Member Posts: 161
    Boiler Sizing Help

    I'm currently adding a 680 sq. foot addition on my house and considering replacing/upgrading my 12 year old New Yorker boiler. The current boiler has a tankless coil and is rated at Net Output of 76000 BTU. I'm considering converting to a Buderus G115 Boiler with a SuperStorUltra tank. The one thing I'm unsure of is what size boiler I should get. My current house has 130 ft. of baseboard which, using 600 BTU/ft., would mean 78000 BTU. I'm currently estimating approx 20000 BTU for the addition. So here are the questions:

    1. Would a 4 section boiler with a Net Output of 95000 BTU/Hr be enough? The 5 section with a Net Output of 119000 BTU seems huge since I've lived with a 78000 BTU boiler for the last 12 years. I don't want to end up oversized.
    2. Should I figure in any extra BTU for the indirect hot water or just ignore it since it can be run as a priority zone?
    3. Any estimates on the savings achieved with the Logamatic controller?

    I've been tossing around ideas with my contractor but would like some other opinions.

  • ChasMan
    ChasMan Member Posts: 462

    You could have 1000 feet of baseboard and a 20,000 BTU boiler and still be overfired. You need to do a heat loss calculation. You want ot install the smallest boiler possible.
  • Brent H.
    Brent H. Member Posts: 161
    Heat Loss Numbers

    So I bought HVAC-Calc. and have had a fun time experimenting with it. I computed my whole house plus addition and it only shows about 55,000 BTUH of heat loss. This just seems really low...... The house is 12 years old, typical 2x4 construction, and will be approximately 2600 sq. ft. with the addition. I've had 2 contractors tell me that I need a 119,000 BTUH boiler. Are many people just oversizing boilers? How does the indirect hot water heater I plan on adding figure into the sizing of a boiler? Thanks in advance.
  • Nathan_6
    Nathan_6 Member Posts: 40
    heat loss numbers


    Many contractors oversize the boilers because they never preform a heat-loss calc on the home and when they finally do try it they reject the numbers because in their mind the house could never heat on less then a 6 section boiler. If you entered the information correctly then by all means go by the numbers. I just recently upgraded the boiler in my 30+ year-old home and the heat-loss numbers told me the old boiler was almost twice the size need so I put in the boiler that's IBR output closest matched the heat-loss and it has been operating for two years now saving me lots of money and with no difference in how the home heats.

    How the DHW tank plays into the heat load? It doesn't if you put in on a priority circuit. Which means when there is a call for DHW the heating circuits are closed and all of the boilers BTU's go directly into making hot water.

    As far as choosing a heating contractor goes if they don't do heat-losses you don't want them sizing your boiler, especially with heating oil prices these days.

    I would also look at using the horizontal Buderus DHW tank as the boiler sits on top of it taking up less floor space and getting the boilers burner up off the floor and making it easier to service. Plus these tanks have a real low standby loss. Also if you can find a qualified heating contractor who can purchase and install the new G125BE Low-NOx Oil Burner form Buderus then by all means go that route as I've just gotten back from training on this boiler and burner and it is super nice and efficient. Plus it comes with The Logamatic controller which has a lot of real nice energy saving features on it as well.

  • Supply House Rick
    Supply House Rick Member Posts: 1,404

    You need 65,000 btu max. Don't let them put in a boiler that large, ask to see their heat load!
  • mark ransley
    mark ransley Member Posts: 155

    Oversizing is and was common. I dont think many companies know or ever do a load calculation, I heat 1800 sq in zone 5, I had 115000 btu and lowered it to a 2 stage Ng 47000 on low fire and thats all I ever use. Later after I super insulated I had a load calc done it was about 40000 btu. Oversize and get uneven jumpy heat, oversize AC and you stay humid all summer. A heating system needs only be big enough to heat on the coldest days running full time, but then any set back in temp can be tough to make up.
  • Brent H.
    Brent H. Member Posts: 161

    Thanks for the feedback. A couple of followup questions.. While I may want to keep the boiler size small to better match the heat load, don't I also need to insure it will be big enough to provide the necessary recovery time for the indirect hot water to meet my hot water needs? Also, the house has far much more baseboard than really necessary. Will this cause any problem when going to a smaller boiler?

    As for Nathan's comments about the new Buderus System/Burner, it looks quite interesting. Ideally, I'd love to find someone willing to install the boiler/burner and DHW tank, and allow me to take care of all the zone circulator connections and wiring. This would save me enough to probably step up to a little nicer system.

    Thanks again.
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