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  1. #1
    Join Date
    Oct 2005
    Posts
    8
    Hi Folks,

    I would like some advice on sizing a new boiler and indirect hot water heater. My local HVAC'ers contradict each other and I'm trying to be as intelligent as possible.

    House is old Cape Cod, 2200 Sq/Ft baseboard hot water heat, 3 zones. Boiler is natural gas 100K btu 81.4% AFUE, hot water is natural gas 40 Gal 40K btu. Two adults - no kids. The house stays warm but feels chilly when its very cold with a wind.

    The house is leaky at sidewall to roof joint and very difficult (expensive) to fix (second opinion validates).

    I would like to install a sealed combustion direct vent boiler and indirect hot water heater. All of the boilers that I am considering are 86-87% AFUE and about 105-112K btu.

    My local HVAC'ers are telling me that I need to have a larger boiler (130-140K btu) due to the indirect HW and after reading this and other sites I'm not conviced I do. I would also like to know if I need a 40 gal indirect with a 40 gal tank or could I go smaller due to the recovery rate?

    I've read comments like it will only take 15-20 minutes to recover hot water and that you will not feel the house change temperature during that recovery. Since recovery is fast wouldn't a smaller sized indirect tank work well?

    Thanks for your help.
    Robo.S


  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    have found the smaller tanks dont seem to work as well
    nor keep up with heavy use
    i am by no means sizing it for you
    best to use the bulls eye at the top of the page and get a load and have a better idea of what size you can use
    most boilers are over sized for the house but in this case with all the leakage and the cost of fuel it may be more to your best interest to seal up the house.more then likely if you do this and use priority on the water heater you might even be able to use the existing boiler.
    do the heat load and see.

  3. #3
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,615
    Why are you going to change the boiler?
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,308
    Some indirects listed as 40, don't hold 40 gals, so don't go smaller.

    Shouldn't need a bigger boiler. But you should weatherize, it will save you more in the long run.
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  5. #5
    Join Date
    Feb 2005
    Posts
    655

    Boiler sizing


    I would concentrate my efforts in making my home as tight as possible, good windows and weather proofing.
    In regards to heating system, if you are thinking of change
    do NOT go with bigger boiler because of indirect hot water heater. you can have new system with Priorty zoning,
    meaning if there is a demand for hot water the total
    105K btu will be prioritized to hot water only,tank would be satisfied in very short time.
    I have used Super-Stor as a brand for my indirects, made in Massachusetts 316 stainless, lifetime warranty, 45 and
    60 Gal. two inch foam insulation, loosing less than 1/2 degree
    per hour.(excellent product)
    The Munchkin gas fired condensing boiler is also an
    excellent product, has modulating gas valve which will
    supply energy needed. Google munchkin boilers.
    If this is the direction you go, get experienced techs
    to install. (This is my opinion) GOOD LUCK

  6. #6
    Join Date
    Dec 2001
    Location
    Chicago, N/W burbs
    Posts
    8,004
    why would you want to heat hot water?
    R2B4BTU

  7. #7
    Join Date
    Feb 2005
    Posts
    655

    otto

    Hot and cold is relative.
    Is 76 degree water hot or cold?
    Is 50 degree water hot or cold?
    At what temp does water become hot or cold?

    Just something to think about.

  8. #8
    Join Date
    Dec 2001
    Location
    Chicago, N/W burbs
    Posts
    8,004
    Hmmmmmmm, i guess you're right. Now my brain hurts.
    R2B4BTU

  9. #9
    Join Date
    Oct 2005
    Posts
    8
    Hi Folks,

    Thanks for the replies. Let me answer a few questions and give you additional info.

    In order to seal my house 4' of the roof needs to be removed at the edges to allow access to the rafters and sidewall. The house has vented overhangs/ridge-vent and you cannot get up into them to repair the sidewall/roof joint. Internal access is worse since its all finished with wallboard and insulation.

    I will be having an insulation contractor add insulation to the 1st floor ceiling joists to see if that helps the situation. I know that the house needs to be sealed but oh the costs.

    The sealed combustion boiler and indirect H/W heater would be one of the steps in keeping the heat in the home. Currently all the windows are double pane, the doors are insulated, everything is well sealed except for the roof to sidewalls.

    Local building code will not permit me to build a sealed externally vented basement room for the existing boiler and H/W heater. The code requires large basement facing vents in case the external ones are blocked. There are also added heat loss effects of a sealed room vented externally. Even with electric dampers its a cooler environment.

    My local HVAC'ers want me to add 10K btu for every 10 gal's of indirect H/W that I purchase. So an additional 40K btu for a 40gal tank. They are saying I should design for the worst case. From everything I've read about boilers, oversizing creates a loss in efficiency.

    I understand its walking a fine line. I assume that I could be paying up to 70% more this winter for my natural gas and I'd like to be as efficient as possible without being on the wrong side of the line.

    With the above in mind do you have additional thoughts or suggestions?

    Thanks S.Robo


  10. #10
    Join Date
    Nov 2003
    Location
    northern mass
    Posts
    411
    This sounds pretty simple. Though I am a forced hot air man, the theory is still the same.

    Priorities are important. That being said, step back and take a look at what your doing/saying.
    You want to remove a heating system to re-install a heating system. You want one that is more efficient to save money.

    Ok, let's prioritise !


    You want to save money.......so why don't you start.

    First off, your house is drafty, which = poor efficiency, and $$ loss.
    Secondly, removing a heating system costs money, buying a heating system costs money, and installing a heating system costs money.
    Thirdly, buying a heating system and putting it in a drafty, money eating house, is both dumb and a waste of money.
    And lastly, you obviously need to find other contractors to help you with your project.

    So, why don't you go up into your attic and rip down the drywall.....which is cheap.....and fix your draft problem. Have a qualified insulating company check your entire envolope and fix what needs to be done to get your home up to speed. Then replace the drywall upstairs, 'cuz it's alot cheaper to buy a few pieces of sheetrock than it is to hire someone to remove half your roof,make repairs on staging,and then re-roof your house.

    Once your house is fixed, then you can do a REAL load calc and see what size heating system you really need. This "going for worse case" theory is a load of C**p !

    When you have actuall FACTS about your needs for heating and hot water, you can then find a contractor that is qualified enough to install it properly. After all, buying the "right" equipment doesn't do you any good if it isn't installed properly.

    Then in the end, you will have a house that is efficient, doesn't waste money or energy, and is comfortable.

    I myself would rather have a home that is comfortable and doesn't waste my money, than a house that (after spending big $$) is still drafty,costs alot of $$, and still isn't comfortable.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,308
    Still no reason to up size the boiler.

    Unless your going to add a hot tub.
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