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boiler sizing dilemma

gschallert Member Posts: 170
getting conflicting recommendations for boiler replacement size - would like other opinions at this point.

this is for CI chimney vented standard efficiency hot water boiler with 40 gallon indirect DHW on priority zone

first contractor is saying weil mclain CGa3 with 70 MBH input, 59 DOE/MBH and 51 Net IBR

second contractor says we need the CGa4 with 105 MBH input, 88 DOE/MBH and 77 Net IBR

Both were given the same independent heat loss calc using manual J (done by energy audit company - a very detailed accounting room by room for windows/doors/construction etc)

The heat loss for DTD was 61k and they (the energy company) said there was some fudge factor even in the manual J calc so the 61K was probably still a bit higher than actual HL especially when we "winterize" the envelope with temporary measures every year.

-House is 1960 two story colonial with below grade unfinished/unheated (stays around 55-60 degrees year round) block basement
-Approx 1450 sq ft living space above
-Two copper fin baseboard zones ~90 ft each (finned) - total linear feet of radiating baseboard 180 feet

I understand why the second contractor is saying CGa4/105, because of the 77 net IBR, right? boiler in basement=unconditioned space, the net IBR is supposed to be equal or higher than the heat loss. 77 > 61

The first one is saying that the CGa3/70 is more efficient because the HL is DTD which accounts for less than two weeks of the heating season here, and that the DOE is is more appropriate to size on than the IBR. So he says 59 DOE is close enough to actual heat loss to be sufficient.

We're actually fine with a lower indoor temp on the design days if it means the boiler is more efficient year round.
Right sizing the boiler for 97% of the year is more important in my mind. Price difference of the two boilers isn't the issue as it's negligible anyway, sizing and efficiency is. What do you think?


  • ChrisJ
    ChrisJ Member Posts: 12,819
    edited October 2016
    This isn't totally helpful, but I have an 1860s two story balloon framed house with little insulation in the walls and attic. It suffers very much from infiltration etc. Most of my windows are from the 1860s as well.

    It's approx 1600 sqft and the absolute most I've ever needed was around 70,000 btu/h output and that was at -8F.

    65,000 btu/h in my house would likely cut it for 98% of the time maintaining 72F indoor and 60,000 would likely be usable for me but it'd get chilly in the house on rare occasions.

    Your house is slightly smaller, much newer and will likely have far better windows and insulation. I'm betting 51,000 output would work fantastic, especially if you insulate the piping in the unconditioned space.

    The bigger boiler is unnecessary and will only cost you money the entire time you have it.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • gschallert
    gschallert Member Posts: 170
    2x4 walls with R8 ~R11 insulation and lots of single pane glass surface area. Lots of single pane glass. Uninsulated floors. Part of second floor living area is over unheated poorly insulated garage. Very drafty house in general. The audit was very thorough and they used thermal imaging where visual inspection alone wouldn't cut it. I would not be at all surprised if the actual heat loss was in the mid 50k range on DTD (which was -5) and with a 10% fudge factor built into their manual J that's what it would be.

    We're slowly working on tightening up the areas the audit revealed as the biggest problems and eventually I feel I could get it down to 50k or a little less which is another reason I don't want to oversize now.

    "The real question is, "what are you doing for DHW"?

    I don't understand your question....as I mentioned in my first post DHW is a 40g indirect on a priority zone. I've been told not to factor the indirect into the boiler size, size to heat loss. Hot water supply not a big issue with just two of us empty nesters now. Only 1.5 bath and no hot tubs, unfortunately.
  • gschallert
    gschallert Member Posts: 170
    ChrisJ said:

    Your house is slightly smaller, much newer and will likely have far better windows and insulation. I'm betting 51,000 output would work fantastic, especially if you insulate the piping in the unconditioned space.

    Lol, not sure about "far better" but probably a bit. I was thinking that insulating the exposed copper of the first floor zone could offset the IBR "loss" so I could use DOE rating to size with some confidence.
    Thanks for the feedback guys, I'm feeling better about not going bigger.

    EBEBRATT-Ed Member Posts: 11,987
    Go with the smaller boiler. The net rating already includes a 15% pick up factor.
  • gschallert
    gschallert Member Posts: 170
    smaller boiler it is, thanks guys! :)