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Direct-vent gas boiler?

I would like to swap our old heating oil boiler for a direct-vent condensing gas boiler. I'd like the new boiler to provide domestic hot water & heat for my cast iron water radiators. The attached building has two floors plus the basement. My contractor says it'll take two condensing boilers to do the job. His reasoning was that 1) The 3000 square feet building needs more than 165 MBH 2) Direct vent gas boilers cannot produce more than 165 MBH without requiring more complicated venting. Is he trying to sell me an extra boiler or is this true? I'd prefer to have just one unit. Thanks in advance for your input.


  • Greg Swob
    Greg Swob Member Posts: 167
    Condensing boilers come in many shapes and sizes...

    Perhaps he needs to look at other manufacturer options for gas fueled condensing boilers. Munchkin and Trinity for example, both make units capable of that Btu requirement. I am certain Viessman and many other makers have models easily capable of your needs using only one boiler- the first two are ones I am personally familiar with.

    Now, you may want to back up one step and verify if the home does indeed require 165 MBtu. I am a little confused as to what Btu's have to do with causing complications to venting. Maybe a decrease the overall run length or an increase in piping diameter size, but not necessarily more complicated.

    Was a proper Manual J or similar heat loss calc done? Or was a rule of thumb applied to determine the needs? If it is square footage, SWAG or some other rule of thumb, you cannot afford to continue your relationship with this firm. There is a no-cost heat loss program attached in this forum for your use. If you feel uncomfortable doing this yourself, ask the contractor for a copy of their calculations. If we then back up one more step, it is possible that simple and reasonable modifications to the home exist to help reduce the heating load in the first place. Greg
  • Floyd_7
    Floyd_7 Member Posts: 136
    Is that 3000 sq. ft. /floor???

    If it is 3000 total, he is really off his rocker, unless you are heating a barn......
    better get a second opinion....

    WM makes a 310,000 BTU mod. boiler and will have a 1 million with a 10 to 1 turn down out in the next year or so....

    many other options out there, don't let one guy BS you....

  • DaveGateway
    DaveGateway Member Posts: 568
    At least a half dozen out there

    Wall hungs from Buderus and Viessmann. WM Ultra, Dunkirk-Quantum. Laars has a few models along with the ones listed above. Time to talk to more contractors.
  • Kenneth Young
    Kenneth Young Member Posts: 6

    The buildings is 3000 sq ft total. It's an attached row house with three units. 1000 sq ft per floor and a bathroom on each floor. I thought the two boilers sounded a little strange, I'll get another oppinion for sure now.

  • Get a proper heat loss calculation done. The answer is in the math, and it is not that hard to figure out.

    Do not buy more boiler than you need. Row houses are great because they are not exposed on all sides. You can probably use way less boiler than your "contractor" thinks.

    Ask him for his heat loss calculations on paper. If he balks, call someone else.

    Where are you located? Someone here may be close to you and could help you out.
  • Kenneth Young
    Kenneth Young Member Posts: 6
    heat loss

    I'll definetly ask for the heat loss calc. I think he just used a rule of thumb. I'm in Brooklyn, NY. It's been tough finding someone who wants to install a condensing unit, help is great.
  • Constantin
    Constantin Member Posts: 3,796
    Despite being a mere homeowner...

    ... I thought I'd chip in also. I ran a heat loss calculation on my detached home when I was in the planning stages to see what my options were. Yet, I insisted my potential HVAC installers do the same, so we could compare results. In my case, we came within 10%.

    With an attached townhouse, you only have two sides and a roof through which you will lose significant amounts of heat. I would be surprised if you needed much more than 25BTU/sq ft. However, I haven't seen the structure and don't know how well insulated it is... For all I know, there are huge infiltration issues or whatnot.

    So try "Find a Professional" on this site... and get a proper heat loss done before someone slaps a heating system into the place that may or may not be warranted. Even a modulating boiler benefits from not being oversized.
  • c.t.kay
    c.t.kay Member Posts: 85

    how is the system piped? 1 pipe or 2 pipe? what is the existing equptment? send a reply to me , possibly i can stop by. approxmatly where in brooklyn are you located?
  • jerry scharf_2
    jerry scharf_2 Member Posts: 414
    do your own heat loss


    If you look at the pipe on the left, there is a tab for "free heat loss calcs." Slant-fin has generously offered it's heat loss calculator free to everyone. With attached walls (1 or 2) , 3 floors of 1000 sqft, the size numbers seem too high.

    Also as others have mentioned, there are lots of condensing gas boilers that run up to 400MBH that vent into dual 3" pipes or similar.

    I'm also nervous about your comment that contractors don't like to install these type of units. You need to find a contrator who will give you several references to customers that have condensing boilers installed. If they can't do that, you don't want them anyway. These aren't so new anymore that you need to be the first one.

    Pay to get it done right the first time, it will cost much more and be extremely aggrevating otherwise. This is not some pop the unions, slide in the new unit and fill job.


  • Kenneth Young_2
    Kenneth Young_2 Member Posts: 1
    Much thanks and updated email address

    Much thanks to you all. I really appreciate all the time and information you've given me.

    Obviously I'm going to be needing a new contractor; if anybody is convenient to South Brooklyn (Sunset Park) and wants to come take a look, please contact me.

    Note that my originally provided email address had a typo. It should be [email protected]

    Thanks again, all!
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