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Sanity Check on Heat Loss Calcs/Boiler Size

BoilerQuest Member Posts: 17
I’ve been reading this forum for the past 3 months in preparation of hiring out a boiler replacement job. I’m ready to hire someone, but would appreciate a sanity check on my heat loss and boiler size decision.

30+ year old Crown Aruba 185.5K BTU input.

Two zones of SlantFin baseboard. 1st floor 70 ft, 2nd floor 75 ft.

50 year old, 2-story colonial

2100 sq ft. (1st and 2nd floors only). Not counting basement since it has no heat.

Double pain windows, vinyl siding, R-49 blown into attic, bat insulation in walls from original construction (thin bats)

Partially finished, uninsulated and unheated basement, below grade except for one wall (walk out). I plan on insulating the rim joists this year, but it never gets too terribly cold down there. We use a small electric baseboard periodically if we plan to spend time down there in the dead of winter.

Location: Philadelphia Suburbs

I received replacement boiler quotes from 5 local contractors with varying methods of sizing the boiler. About half way through the process, I decided to do my own heat loss calculations to make some sense of what I was being told. I completed a fuel-use analysis using the last 2 years of gas usage data, completed an estimate using the SlantFin App, and completed an estimate using the Crown boiler heat loss spreadsheet. I calculated the following:

Fuel Use Analysis: 44,100 BTUh
Slant Fin App: 41,400 BTUh
Crown Spreadsheet: 48,200 BTUh

Outside Design Temp: 12.6 (99.6% for Philly)
Interior Temp: 70

*Requested all quotes be for standard efficiency boilers (80-85%)
Contractor 1: No heat loss. Looked at old boiler size.

Contractor 2: No heat loss, didn’t look at anything other than the boiler room. – Said 105K input would work, but he suggests 140K input to be sure.

Contractor 3: No heat loss, didn’t look at anything other than the boiler room. Said houses around here are 40BTUh per sq ft. – Suggested a boiler with 96K input.

Contractor 4: Asked a few basic questions, didn’t look at the house, and came up with heat loss on his IPAD. Suggested 103K input boiler since heat loss was around 80K BTUh. He was using 0 degree design temp. When I had him correct some of his inputs, it was closer to 60 K BTUh loss. NO MEASUREMENTS were incorporated into his IPAD. Said he would install a boiler one size smaller if I wanted, but he was hesitant because he has a “comfort guarantee” and worries it may not keep up when really cold.

Contractor 5: Quickly measured rooms, but no windows/doors. Didn’t ask about insulation, etc. Suggested 105K input boiler. When explained that I don’t agree with the heat loss, they agreed that they calculated 60K BTUh heat loss, but they went up a boiler size to be sure.

Many other variables are going into this decision, but I decided to hire contractor 4. I am going to have a Buderus GC144 installed, but feel strongly about telling him to install the smaller, GC 144/3 (74K input, 61K DOE). Does anyone think I am cutting the numbers too close? I am confident in my numbers, but lack any real-world boiler sizing experience.

Any help is much appreciated!


  • SWEI
    SWEI Member Posts: 7,356
    Assuming you took six inches off the measured length on that baseboard, and assuming it is Fine/Line 30 or equivalent, you won't be able to dissipate more than 72k into the house regardless of how much the boiler puts out. The Slant/Fin app overestimates by at least 20% IME, so you're good to go on the smallest model.

    I don't know what kind of controls the GC144 currently ships with, but I would strongly suggest you employ outdoor rest control. It will improve comfort, increase efficiency, and greatly reduce the probability of noise in the baseboard.
  • Tinman
    Tinman Member Posts: 2,806
    Contractor 4 - Asked a few questions but didn't look at the house?

    How did he do a load calc w/o looking at the house?

    Why do you have confidence in him?

    Based on the information you provided, I'd say your calcs are the closest.
    Steve Minnich
  • ChrisJ
    ChrisJ Member Posts: 15,175
    Interesting to see the hot water guys are oversizing boilers just as much, and as badly as the steam guys.

    Such a shame.

    Things work so well when just a little effort is put in.

    "He was using 0 degree design temp. When I had him correct some of his inputs, it was closer to 60 K BTUh loss. NO MEASUREMENTS were incorporated into his IPAD. Said he would install a boiler one size smaller if I wanted, but he was hesitant because he has a “comfort guarantee” and worries it may not keep up when really cold."

    But, the idea is NOT to keep up when it's very cold, the idea is for it to run 100% and perhaps, fall behind just a hair. That's the entire point. So, you'll be at 68F instead of 70 a few times every 20 years.

    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
  • BoilerQuest
    BoilerQuest Member Posts: 17
    To be honest with you, I don't have confidence in any of the contractors when it comes to there heat loss calcs. Contractor 4 was a salesman from a larger local company and I could tell he wasn't exactly prepared to answer detailed design questions. He essentially entered my house square footage, design temp, and desired interior temp into his IPAD program and it spit out a number. I spoke to some references and their installs look good based on pictures provided. I'm comfortable with this company except for the heat loss.

    The GC 144 comes with the Aquasmart controller. I was going to add the wireless outdoor temp sensor to the package. It will provide outdoor reset but not constant circulation if I understand its function correctly.
  • BoilerQuest
    BoilerQuest Member Posts: 17

    When I say he didn't look at the house, I mean he walked into the front door and immediately into the basement boiler room as most of the other contractors did. He had a general sense of the house, but asked me the square footage, insulation, etc. type of questions.
  • Rich_49
    Rich_49 Member Posts: 2,763
    edited August 2016
    The 3 will be more than sufficient . Both zones will probably be calling simultaneously about 65% of the time or less and the
    GC 144/3 can supply that demand . Might even look into one of these , as far as cast iron boilers go it stacks up quite nicely , better price point than the Buderus ( I use Buderus) . I will be installing some of these in the near future as a few guys I know and have faith in have used them .


    Will this boiler ever be considered to make your hot water ?
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • BoilerQuest
    BoilerQuest Member Posts: 17
    Perhaps, but I am going to stick with a direct-fired, chimney vented HW heater for now. I briefly considered doing an indirect set-up, simply to get a tank that would last longer, however I hear that it may not be the case. Efficiency aside, having to replace an expensive indirect at roughly the same frequency as a cheapy direct fired doesn't appeal to me.
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    edited August 2016
    Is there a particular reason you've selected a cast iron boiler?
  • BoilerQuest
    BoilerQuest Member Posts: 17
    Two companies provided quotes that were nearly double the cost of a cast iron boiler and direct fired HW heater. The numbers didn't seem to work. Contractor 5 provided a proposal for a Lochinvar Knight (KBH) and indirect DHW that I was considering. However the proposal from the Buderus contractor comes with a 10 year parts a labor warranty and is a much simpler system. The Lochinvar complexity worries me a bit to be honest. I'm having a hard enough time getting someone to do an honest heat load calc, let alone a modcon install.
  • BoilerQuest
    BoilerQuest Member Posts: 17
    I meant Lochinvar KBN
  • BoilerQuest
    BoilerQuest Member Posts: 17
    Ok, bear with me here. I have some questions related to modcons now. Between the cost of a chimney liner making the CI boiler price within spitting distance of a modcon, and the fact the buderus salesman doesn't seem to understand outdoor reset/constant circ. (and doesn't understand why I want it), I'm back to looking over all the quotes again. I also have a 6th contractor coming next week to provide an estimate. However, contractor 5 was the only one who measured and calculated heat loss and is the only one I met who actually installs the equipment himself. The rest were salesman. I think a discussion with him may clear things up. He was pushing the Lochinvar KBN 106, so I will call him to discuss.

    If anyone can clear-up two of my questions before I call I would appreciate it.

    1. How does a modcon handle two zones (70ft + 75 ft)? Do they both run constant circulation?

    2. Based on what I have read, I want to stay away from the Giovanni hx in the KBN. I am going to ask if he would install the WHN or the new KHN with the fire tube HX. My question it this – Based on my heat loss (45-50 BTU/ha at design), can go with a WHN55 (55 input, 51 net, 44 ibr). Or would I be better suited with the KHN85 (85 input, 79 net, 69 ibr). I will plan to use a squire 40 gal indirect with this set-up as well. While the KHN 85 seems oversized, the 10:1 turndown allows it run at a slightly lower output than the smaller WHN55. Which would you choose?

  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited September 2016
    A good approach would be to use the "Find a Contractor" function (about half way down the page) and get someone knowledgeable out to your location...

    Contractor #3 stated 40BTU/sq-ft heatloss? ..... maybe with every door and window wide open 24/7 during the coldest weeks of the year. Your own analysis based on actual usage indicates 21BTU/sq-ft heatloss which seems pretty reasonable.
  • keyote
    keyote Member Posts: 659
    edited September 2016
    I would take a look at lochinvars newest modcon a small 13"x 16" floor mount the KHN 085, which from what you say is large enough but modulates down to 10% [8000 kbtu]for shoulder season or single zone calls.mod cons are most efficient at lowest modulating down. so the 085 has you covered best either way if your loss is accurate you will get best efficiency if its low you will have the capacity and can add a large indirect DHW later if you want.