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Help sizing a Gas modulating condensing boiler

robjmc1
robjmc1 Member Posts: 7
I need help sizing my new boiler. I am a licensed electrician that has wired many boilers of all types for many plumbers and every one of them think I need around 100k btu but my math tell me otherwise. My house is 1500sq ft on Long Island that was completely gutted and remodeled r15 wall and 3/4” tuff r on the outside with a unfinished basement that will eventually be finished. I have 3 zones of cast iron recessed sunrad’s hooked up to radiant manifolds running at 150f max water temp and a 40gal weil-mclain aqua 45 indirect. The current boiler is way over sized a peerless 3 or 4 section oil that is under fired with a 1 GPH nozzle.
1st floor radiators
207sqft x .8 = 165.6sq ft x 110BTU =18,216 btu
2nd floor
261sqft x .8 = 208.8sq ft x 110btu = 22,968btu
(the .8 is for recessed)
Basement (temp)
1 old cast iron maybe 20sqft that turns on when ant zone calls for heat
20 x 110 = 2,200btu
Total 43,384btu @ 150f of radiators and once the base meant is finished the total will be around 50,000btu @ 150f for the house
With 150f water and these radiators I hade no problem with heat and I think the boiler did not run half the time on the coldest day and the temperature over shoots the set point by 2 degrees every heating cycle. I also used a program called HVAC-Cal manual J and came up with 30k to 40k heat loss depending on how generous I was when putting in the house info.
I like the Buderus gb142 but they discontinued it this year so now I think I will use a Bosch Greenstar 57 output (52 MBH DOE) input (57.2 MBH max 12.9 MBH min) or a Bosch Greenstar 79 output (79.2 MBH max 24.6 MBH min). I am concerned the 57 will be to small and run 24/7 when it is really cold out but the 24.6 MBH minimum on the 79 will be to much when it is warmer and will short cycle. I want to size the boiler correctly not like my AC which is way over sized. I wanted a 2.5 ton and the HVAC company insisted on a 3.5 and now on the hottest heat wave the unit cycles on 10min off 10min.
Adolfo2

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,170
    Sounds like the 50,000 do it easily.
  • lchmb
    lchmb Member Posts: 2,997
    just one other thought, depending on the age of the HW tank.. could look at a combi unit, or put in a combi now for when the tank fails down the road..
  • robjmc1
    robjmc1 Member Posts: 7
    The water heater is new and the only combi units I can find are to large for the heating. That was what I originally wanted to do.
    ZmanBrewbeer
  • Zman
    Zman Member Posts: 7,435
    Pay more attention to the minimum turndown and less to the max output. You actually want the boiler to never turn of on the coldest day. Just purring along and modulating based on load is the ideal arrangement from an efficiency point of view.
    Check out Lochinvar and HTP. They both have 10-1 boilers which will turn down to ~8k.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    DZoroAdolfo2Rich_49
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,170
    @robjmc1

    Bosh/Buderus are good but most on here (don't want to speak for others) would I think prefer a stainless steel hx versus aluminum. Depends on water quality
    ZmanAdolfo2
  • robjmc1
    robjmc1 Member Posts: 7
    I really like the Lochinvar Knight with the 10:1 turndown and the ECM variable speed pump but I am concerned with my venting system and that boiler. My vent system 636 cpvc and it runs from the basement straight up through the 1st floor 2nd floor attic and out the roof their is clearance around most of it except where it passes through floors. System 636 is approved for 0” clearance to combustibles and Bosch green star is approved for 0” to combustibles as per their manuals. The lochinvar Knight has a 1” clearance to combustibles on the vents and also state not to insulate the vent do to excessive temperatures may damage of to the pipe. Bosch list all their max vent temps I think around 175f but Lochinvar does not and their tech support does not know ether so I am concerned that the vent max temp might be higher than the CPVC rating. Even though all manufacturers list PVC as a vent pipe I used CPVC do to the higher temperature rating I did not like 175f vent gases in a pipe that is only rated for 140f. Does anyone have any info on Lochinvar venting or issues with any venting in contact with wood.
  • Zman
    Zman Member Posts: 7,435
    I would be very surprised if the Bosch had lower exhaust temps than the Lochinvar. I think Lochinvar is more conservative in their installation specifications because, well, they are a better company.
    I would be far more concerned with vent clearances on the Bosch as the heat aluminum exchanges tend to plug up. When this happens, the vent gets very hot and creates a significant hazard, especially with PVC venting.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    DZoroSolid_Fuel_Man
  • DZoro
    DZoro Member Posts: 1,048
    I would seriously try very hard to stay away from the aluminum. They just don't last, condensate is so aggressive that it eats the H/E up in a few years. Not to even mention the water side.

    I see this happening all the time, one major company says they don't want you to clean their H/E for 5 years, then you can clean it. Apparently they put a substance on the aluminum and don't want it scraped off. After 5 years guess they don't care anymore and go ahead scrub away.
    Hmm warranty is also 5 years....Stainless pay now or pay much more later.
    IMO
    D
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    My house near Red Bank, NJ must be about the same latitude as Long Island, and not all that far away. It is an 1150 square foot Cape Cod type house. Design temperature around here is 14F. I got the smallest mod-con I could get from my heating contractor, an 80K BTU/hour input with a 5:1 turn-down, so it will go down to 16K BTU/hour. This is too big, so I had to do some fun and games to the upstairs baseboard zone that usually takes only 6000 BTU/hour, and that only on very cold days, so it would not rapid cycle. I have never needed the full output of that boiler, even when both the upstairs and downstairs were both running at once.
  • Zman
    Zman Member Posts: 7,435

    My house near Red Bank, NJ must be about the same latitude as Long Island, and not all that far away. It is an 1150 square foot Cape Cod type house. Design temperature around here is 14F. I got the smallest mod-con I could get from my heating contractor, an 80K BTU/hour input with a 5:1 turn-down, so it will go down to 16K BTU/hour. This is too big, so I had to do some fun and games to the upstairs baseboard zone that usually takes only 6000 BTU/hour, and that only on very cold days, so it would not rapid cycle. I have never needed the full output of that boiler, even when both the upstairs and downstairs were both running at once.

    Quite a few 10-1 modcons have come out in the last few years. You should be able to get close to 8,000 Btu/hr
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • NY_Rob
    NY_Rob Member Posts: 1,370
    The HTP UFT-80W would be a perfect fit, but the problem here on LI is that HTP has virtually no representation, and even less support. You can special order one through Blackman and if you find someone willing to install it... pray they never go out of business, retire or move away because finding someone else willing (and competent) to service it would be quite difficult here.

    Lochinvar on the other hand has good representation and support here and it shouldn't be difficult to have repairs made. It's 2x the cost of the HTP, but it's got more features, excellent build quality and local support... so you pay for that.
    Rich_49
  • robjmc1
    robjmc1 Member Posts: 7
    That’s good to know about the HTP I will probably go with the lichinvar Knight. I only need the WHB055N but I think I might go tith the WHB085N and lock the max btu out at 50btu do to they both have similar minimum fire rates but the 85 has more surface area in the heat exchanger so I would think the efficiency would stay up better at lower fire rates and the stack temperature will also stay lower. Does anyone have any info or thoughts with doing this?
    Adolfo2
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,542
    There is a large company where I am that installed hundreds of the Bosch boilers. I know a couple of the guys that work there and they even get to travel to Germany because they'd sold so many boilers in their branch. Now they're failing by the dozens. Aluminum does not fare well with a condensate side. Lots of plugged up condensate traps and then plugged up heat exchangers aluminum powder everywhere. Bosch went so far as to have a lifetime heat exchanger warranty to ease everyone's Minds. But now they're still getting replaced left and right.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!