Need to upgrade boiler?

Discussion in 'Boiler Forum' started by LizzyBB, Jan 31, 2014.

  1. LizzyBB

    LizzyBB New Member

    Jan 31, 2014
    I'm a regular home owner with very little knowledge of boilers. I've read some of the threads here and you all seem very knowledgeable, so I'm hoping you can help me.

    We had a new gas boiler installed about 4 years ago to replace our very old oil boiler. It is a Triangle Tube Prestige Solo 110. We have hot water heating (baseboards) with 3 zones, and a 4th zone for the also new water tank. It has worked flawlessly for our current 3 bathrooms and 2200 sq ft of house (2 levels).

    We are now getting ready to add a second floor, with an additional 900 sq ft and 1 more bathroom. So the final home will have 4 bathrooms and 3100 sq ft. We live close to Boston MA. We are planning to put foam insulation in the new second floor.

    So here's the question. When the boiler was installed, we told the plumber we may eventually add a second floor, and he felt this boiler would be fine for that. Now that we are getting ready to add the second floor and are getting quotes from plumbers, we are getting conflicting information. One plumber in particular is saying that this boiler is not big enough to handle all that heating, while the others did not seem concerned.

    I have tried to look up websites to calculate what we need, but it's very confusing. I just want to get a very general idea of whether you think this kind of boiler can handle that size house. Or do we need to upgrade?

    Thank you so much for any help you can give me.
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 2010
    The plumber that said its not big enough is an idiot. Its big enough and then some.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    In the Rt 128 region typical heat load for older construction that has been tightened with retrofit insulation & maybe storm windows over single-panes come in at about 15 BTU/hr per square foot of conditioned space. Code-min new construction typically comes in at about 12 BTU/ft-hr. These are really crude rules of thumb with big error bars, but it won't over 20BTU/ft-hr, and it's unlikely to be under 8 BTU/ft-hr (unless it's super-insulated with super-performance windows.)

    So if your new-improved conditioned space is 3100 square feet, it's highly likely that the even smaller Solo-60 would have capacity to spare, and the -110 is probably 2x oversized for the whole-house load. Even at 20BTU/ft it has a huge amount of margin.

    Since it's all chopped up into zones with low-thermal-mass heat emitters, a critical aspect to getting the condensing efficiency out of it without short-cycling it into an early grave. At an average water temp of 120F (say, 130F out, 110F water returning) the boiler can condense the exhaust, hitting the low-mid-90s for combustion efficiency. Fin-tube baseboard emits about 200BTU/hr per foot of baseboard at that temp, and at the 30,000 BTU-in minimum-fire output of the -110 (~28,000 BTU/hr out) it takes 140 feet of baseboard to balance perfectly. That means ideally the smallest zone would have no less than 100 feet of baseboard, or you'll be short-cycling the thing. It'll heat the house just fine, but it'll add a lot of wear & tear on the boiler.

    If your smallest zone has 50 feet of baseboard you can bump up the minimum temperature on the outdoor reset curve to keep it from short-cycling, but at 28,000BTU/hr that's 560 BTU/ft-hr, which means an output temp of 180F or higher, with a return water temp well above the condensing zone, and you'd be lucky to do better than 88% efficiency.

    On the new zone it's worth putting in low-temp panel radiators with an "equivalent baseboard length" of at least 100'. Not only will it be more comfortable, but since the radiators have substantially more water volume there is some thermal mass there to work with to extend the minimum burn times. Even though the heat load of that zone will probably come in under 10,000BTU/hr and could be served with less than 25' of baseboard, if you cheap out and go with low-mass radiation of minimum size it'll only aggravate the short-cycling issues. There's really no point to having a condensing boiler if it can't be operated at a condensing temperature without short-cycling it to death.

    As a sanity-check on the heat load, look at your most recent gas bill- if they report an average outdoor temp and therms/day figure that's enough information place a firm stake in the ground bounding your current whole-house heat load. If that info isn't on the bill, give us the fuel used, the EXACT meter reading dates, and your ZIP code, to be able to re-construct the heating degree-day information by looking it up at a nearby weather station.
  5. LizzyBB

    LizzyBB New Member

    Jan 31, 2014
    Tom Sawyer, that's exactly what my husband said, so I was glad to see you say that!

    Dana, thank you so much for giving me all that detailed information. I need to re-read it all a few times to completely understand. I will be measuring the baseboard length per zone, for starters and looking at my heating bills.

    I got another plumber to come Friday afternoon and he said that boiler is more than adequate, which further confirms what you are both telling me. He thought the new addition with insulation and new windows would add minimally to the requirements. Thanks to both of you for your help. I will be back when I've done my measurements.
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