Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Gas Boiler Sizing

britm3 Member Posts: 2
OK team, I have a recently installed Buderus GB142/45 with a max output of 141k BTUs. The house is 1,950 sq ft, 1960's construction, older windows, no basement, single floor ranch on slab, 500 sq ft over crawl space. I realize the boiler is oversized at the start, however we anticipate adding to the house at some point.

My very rough heat loss calc shows 75,000 BTUs required. There is 147 feet of baseboard which can output a max of 86k BTU with 180 degree water. I also have an indirect 45 gallon superstor tank.

Now my issue is cycling based on my zone sizes. The GB142/45 can modulate down to 42600 BTU however my zones baseboard lengths and output at 170 degrees are as follows:

Zone 1 Living Room, Playroom - 56 feet, 28k BTU

Zone 2 Front Room, 35 feet, 17.5k BTU

Zone 3 Bedrooms, 54 feet of baseboard, 26.6k BTU

As you can see, with the boilers lowest output, if just one zone calls, it will cycle with return temps too high, followed by an interval time out.

Outside of moving the boiler down in size, or doing a buffer tank, I am strongly considering combining all of the zones and running single one to drive boiler efficiency, as we literally use most of this house, most of the day (4 kids).

The baseboard is distributed evenly, so I do not anticipate any major hot or cold spots. Thoughts on a single floor zone approach? Thermostat with remote sensors for a blended average temp target on the single zone?


  • Paul48
    Paul48 Member Posts: 4,469
    My Opinion

    Copied and pasted from another thread.High Efficiency

    Mod/cons work best, when they see a large load. The more you chop the home up into zones, the more likely you are going to have trouble keeping the boiler condensing, and that's when it's most efficient. The best way is to use an outdoor reset and let the boiler handle the whole structure.It is a totally different type of heat than conventional heat.The days of standing by a piping hot radiator to chase away a chill, after coming inside, would be gone. But the house would be warm and comfortable.Mod/cons in general do not work well with night setbacks. Keep in mind, they are attempting to provide just enough heat to offset what is being lost. If you turn the temp down to 62* at night and let the whole structure get that cold, it will take a long time to recover.These are my opinions, and some will disagree.Some will say that a boiler that they use provides answers for these situations. Just discuss your habits with the contractor, if you choose to go with a mod/con. Sizing of a mod/con is absolutely critical
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Single Zone Approach

    With constant circulation will work great. I would add thermostatic mixing valves to the baseboards to control room comfort. Would also make sure I used a Taco Bumble Bee or Grundfos Alpha as a system pump.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.