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  1. #1

    High Efficiency Boiler with Gravity Fed Hot Water System

    We are in the process of replacing a failing 1.2 million BTU gas fired boiler in a gravity fed hot water system. To get a sense of the amount of water in the system, the pipes exiting the boiler up to the attic are 3 inches. Pumps were added at some point in the past, so I guess it isn't technically a gravity fed system anymore. There are two main zones and one small zone. Several contractors have proposed replacing the boiler with two Lochinvar Knight boilers or one Knight XL. Has anyone had any experience in replacing a large cast iron boiler in a gravity fed system with a high efficiency boiler? I am concerned that the boiler will have to run constantly to heat the large volume of water, thereby stressing the boiler and losing the efficiency gains. Any feedback would be greatly appreciated. Thanks.

  2. #2
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,196

    High Efficiency Boiler with Gravity Fed Hot Water System

    Welcome to the modern age replacing your boiler with modern condensing boilers makes a lot if sense but you will certainly need a very good contractor. A few things first......

    The piping design (or lack of) is very critical. Often times the systems are piped with a main supply and return. Each individual radiators will tee off such that the 1st radiator will be the 1st supply and 1st return. The 2nd will be the 2nd supply and 2nd return..... And so forth. The issue is that the 1st radiator can have significantly Less total feet of piping than the last and the flow rates and heating ability of each radiator will be different. This can be changed to a "reverse return" piping arrangement.

    Condensing boilers only condense with 'cold' water generally less than 140 degrees. Most systems need to operate at 180 degrees or more and will result in paying a lot of money to get very little efficiency gains...

    It sounds like you have a very large home or a very poorly insulated/air sealed home. An absolute must is to have an energy audit done on your home to try and reduce your heat load. Spending thousands in insulation can save you far more money per dollar spent than most any investment you can have. It can also reduce the needed water temperature which will drive up the boiler efficiency. An accurate heat loss in your home is an absolute must!! You at find that by insulating, air sealing, and other improvements will allow you to install 1 boiler and you will have a more comfortable home saving you a fortune.

    As for the unit running continuous,,, and modern condensing boiler worth it's weight will fully modulate and run continuous by design. It should run continuously. If you have 2 boilers it will in all likelihood run both boilers simultaneously and modulate with each other. It is not uncommon for your boiler to run 24/7 below say 50 degrees. Since you have zones that may skew the runtime however.
    ...

  3. #3
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,196
    How many square feet is your home? 1-10 how would you rate how well insulated and sealed your home is?
    ...

  4. #4
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
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    394
    Mod/con's work great on converted gravity systems. The buffer created by the large piping can actually be a benefit if they're properly insulated in unconditioned spaces.

    Like a car engine, 95% of the wear on it happens at startup - so with a mod/con. It's designed for long smooth run cycles at the lowest modulation. This produces the highest efficiency and greatest comfort.

    They key lies in getting a KNOWLEDGEABLE installer - he's 98% of the equation.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  5. #5
    Join Date
    Jan 2004
    Location
    PA
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    68,753
    You have cast iron rads. A mod/con will work great. Those rads don't have to get real hot to heat your house. make sure they size the boiler/boilers to the heat loss of the house, and not to the over sized radiation it has.
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  6. #6
    Thanks for all of the information. The house was built in 1925 and is approximately 9,000 sqft. It is brick and terra-cotta with a slate roof and very drafty steel casement windows. The attic is currently uninsulated, but I am contracting to have insulation put in the rafters shortly. The walls are currently uninsulated, but eventually I will blow as much insulation in as possible between the plaster and the terra-cotta, but it will not be much. Interior storm windows will go in eventually, but the cost is prohibitive to do all at once. Sealing the house beyond these measures would be impracticable. I guess on a 1 - 10 scale, I would say a 5. I did have an energy audit done and they said I had the leakiest house they had ever seen. Sealing the house beyond these measures would be impracticable. Even these steps will be a process that won't be completed all at once.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
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    You might find that some caulking of the holes in the plates in the attic and basement where wires go through will help a lot. Along with putting in foam seals on your receptacles and switches. Replacing any worn door seals also if you have any. All of those things are inexpensive to do, and they pay for themselves very quickly.

    If your boilers steady state efficiency is 80%, then you BTU per sq ft is 106, if its only 70% your BTU to sq ft is 93. Shouldn't be hard to get the heat loss down that 2 Lochinvar Knights can handler the load. I'm thinking two KB-286 boilers could probably do your house. that would be just a tad over 541,000 BTUs of output. That would come out to 60 BTUs per sq ft with both boilers at high fire.
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  8. #8
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
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    I'd recommend two Knight WHNs over the KB model. Much better heat exchanger in it than the KB and not as touchy to maintain. JMHO.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  9. #9
    All of the installers had proposed the KBN units. Looking at the WHN units they seem completely different. Is there a consensus that the wall mounted design is superior? I need to talk to the installer about whether a wall mount is even feasible in my house.

  10. #10
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,589
    Wall mount should always work, we usually build a wall with a 4x8 sheet of plywood, it leaves lots of room for piping. Do you have any Weil McLain dealers? Wm97 is a nice boiler, stainless heat exchanger. Cast rads and condensing boiler is a no brainer like said.

  11. #11
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
    Posts
    394
    There's no comparison.
    To me, comparing the Gianonni heat exchanger in the KB to the fire tube heat ex. in the WHN is like comparing a Yugo to a Mercedes diesel.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

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