Help sizing a gas modulating condensing boiler

[Robert McCallum]

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.

 

[Dana]

Heat loads are not a function of square feet area of conditions space- it's about the exterior surface areas and U-factors of different exterior surface types (windows, walls roofs, foundations) and the temperature differences.

The right place to start is to run a Manual-J type heat load calculation , but you need to be AGGRESSIVE rather than conservative assumptions (as per the instructions in the Manual) on R-values/U-factors/air leakage etc. A reasonable but not perfect freebie online version to get a sense is loadcalc. Most other free or low cost versions often deliver paradoxical results. Even loadcalc overshoots (by a double-digit percentage) the numbers that a professional tool such as Wrightsoft delivers with proper inputs, so there is no need to upsize from there.

Another approach for sizing the boiler would be using a classic I=B=R approach, which is simpler but similar to Manual-J.

With a oil fill up history with exact dates & quantities for (wintertime only) fill-ups it's possible to accurately infer the whole house heat load of the "before upgrades" picture using these methods. A fuel use load calculation will serve as an important sanity check on any other load calculation.

That said, after building upgrades the load of a 1500' house at +15F at Long Island's +15F 99% outside design temp) is likely to come in around 18,000 BTU/hr or a bit less (not 30-40K) , assuming the basement gets insulated to the current code-minimum R10 continuous insulation. ASHRAE recommends a 1.4x oversize factor from the load at the 99th percentile temperature bin to cover the Polar Vortex disturbance cold snaps, etc., which means a ~25,000 BTU/hr boiler would be more than enough.

The ideal is to dial in the outdoor reset curves on these things closely enough that they DO run 24/7 for days or weeks at a time, modulating with load at the lowest possible water temperature without losing ground or cycling excessively. The minimum fire output of the Greenstar 57 is too high for loads that low. It would only modulate with load during the coldest weeks. Find a boiler that can modulate down to ~8000 BTU/hr or less- the Navien NHB-055, HTP UFT-080W, or Lochinvar KHB-055, would all be a better fit.

You have enough radiation and thermal mass in the system to run these boilers at very low max-efficiency water temperatures without short cycling, so use it! Some napkin math on modulating condensing boiler sizing how much radiation you would need to install in the basement zone to keep from short cycling can be found in this bit o' bloggery.

For reference on load- I live in a cellulose insulated 2x4 framed oddly shaped 1.5 story antique with clear glass storms over the original single panes. There is 2400' of above-grade fully conditioned space, and 1600' of insulated basement (~R17 continuous polyiso) that never drops below 65F in winter. The fuel use numbers put my house at under 30K @ +15F, about 40K @ 0F. With 14 corners to the foundation footprint I have to have more surface area per square foot of floor area than most homes on L.I., and it's considerably more floor area. I'm radiation-limited to about 44-45K at the 125F water temperatures I'm running, yet it sails through -10F outdoor temps without comfort issues. (My local 99% design temp is +5F.)

 

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[Robert McCallum]

Heat loads are not a function of square feet area of conditions space- it's about the exterior surface areas and U-factors of different exterior surface types (windows, walls roofs, foundations) and the temperature differences.

The right place to start is to run a Manual-J type heat load calculation , but you need to be AGGRESSIVE rather than conservative assumptions (as per the instructions in the Manual) on R-values/U-factors/air leakage etc. A reasonable but not perfect freebie online version to get a sense is loadcalc. Most other free or low cost versions often deliver paradoxical results. Even loadcalc overshoots (by a double-digit percentage) the numbers that a professional tool such as Wrightsoft delivers with proper inputs, so there is no need to upsize from there.

Another approach for sizing the boiler would be using a classic I=B=R approach, which is simpler but similar to Manual-J.

With a oil fill up history with exact dates & quantities for (wintertime only) fill-ups it's possible to accurately infer the whole house heat load of the "before upgrades" picture using these methods. A fuel use load calculation will serve as an important sanity check on any other load calculation.

That said, after building upgrades the load of a 1500' house at +15F at Long Island's +15F 99% outside design temp) is likely to come in around 18,000 BTU/hr or a bit less (not 30-40K) , assuming the basement gets insulated to the current code-minimum R10 continuous insulation. ASHRAE recommends a 1.4x oversize factor from the load at the 99th percentile temperature bin to cover the Polar Vortex disturbance cold snaps, etc., which means a ~25,000 BTU/hr boiler would be more than enough.

The ideal is to dial in the outdoor reset curves on these things closely enough that they DO run 24/7 for days or weeks at a time, modulating with load at the lowest possible water temperature without losing ground or cycling excessively. The minimum fire output of the Greenstar 57 is too high for loads that low. It would only modulate with load during the coldest weeks. Find a boiler that can modulate down to ~8000 BTU/hr or less- the Navien NHB-055, HTP UFT-080W, or Lochinvar KHB-055, would all be a better fit.

You have enough radiation and thermal mass in the system to run these boilers at very low max-efficiency water temperatures without short cycling, so use it! Some napkin math on modulating condensing boiler sizing how much radiation you would need to install in the basement zone to keep from short cycling can be found in this bit o' bloggery.

For reference on load- I live in a cellulose insulated 2x4 framed oddly shaped 1.5 story antique with clear glass storms over the original single panes. There is 2400' of above-grade fully conditioned space, and 1600' of insulated basement (~R17 continuous polyiso) that never drops below 65F in winter. The fuel use numbers put my house at under 30K @ +15F, about 40K @ 0F. With 14 corners to the foundation footprint I have to have more surface area per square foot of floor area than most homes on L.I., and it's considerably more floor area. I'm radiation-limited to about 44-45K at the 125F water temperatures I'm running, yet it sails through -10F outdoor temps without comfort issues. (My local 99% design temp is +5F.)

Heat loads are not a function of square feet area of conditions space- it's about the exterior surface areas and U-factors of different exterior surface types (windows, walls roofs, foundations) and the temperature differences.

The right place to start is to run a Manual-J type heat load calculation , but you need to be AGGRESSIVE rather than conservative assumptions (as per the instructions in the Manual) on R-values/U-factors/air leakage etc. A reasonable but not perfect freebie online version to get a sense is loadcalc. Most other free or low cost versions often deliver paradoxical results. Even loadcalc overshoots (by a double-digit percentage) the numbers that a professional tool such as Wrightsoft delivers with proper inputs, so there is no need to upsize from there.

Another approach for sizing the boiler would be using a classic I=B=R approach, which is simpler but similar to Manual-J.

With a oil fill up history with exact dates & quantities for (wintertime only) fill-ups it's possible to accurately infer the whole house heat load of the "before upgrades" picture using these methods. A fuel use load calculation will serve as an important sanity check on any other load calculation.

That said, after building upgrades the load of a 1500' house at +15F at Long Island's +15F 99% outside design temp) is likely to come in around 18,000 BTU/hr or a bit less (not 30-40K) , assuming the basement gets insulated to the current code-minimum R10 continuous insulation. ASHRAE recommends a 1.4x oversize factor from the load at the 99th percentile temperature bin to cover the Polar Vortex disturbance cold snaps, etc., which means a ~25,000 BTU/hr boiler would be more than enough.

The ideal is to dial in the outdoor reset curves on these things closely enough that they DO run 24/7 for days or weeks at a time, modulating with load at the lowest possible water temperature without losing ground or cycling excessively. The minimum fire output of the Greenstar 57 is too high for loads that low. It would only modulate with load during the coldest weeks. Find a boiler that can modulate down to ~8000 BTU/hr or less- the Navien NHB-055, HTP UFT-080W, or Lochinvar KHB-055, would all be a better fit.

You have enough radiation and thermal mass in the system to run these boilers at very low max-efficiency water temperatures without short cycling, so use it! Some napkin math on modulating condensing boiler sizing how much radiation you would need to install in the basement zone to keep from short cycling can be found in this bit o' bloggery.

For reference on load- I live in a cellulose insulated 2x4 framed oddly shaped 1.5 story antique with clear glass storms over the original single panes. There is 2400' of above-grade fully conditioned space, and 1600' of insulated basement (~R17 continuous polyiso) that never drops below 65F in winter. The fuel use numbers put my house at under 30K @ +15F, about 40K @ 0F. With 14 corners to the foundation footprint I have to have more surface area per square foot of floor area than most homes on L.I., and it's considerably more floor area. I'm radiation-limited to about 44-45K at the 125F water temperatures I'm running, yet it sails through -10F outdoor temps without comfort issues. (My local 99% design temp is +5F.)

Thanks this is helpful

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. 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 and I did not like 175f vent gases in a pipe that is only rated for 140f. Do you have any info on Lochinvar venting.

 

[Dana]

It's highly unlikely that you would ever need water temps anywhere near what would be delivering 175F exhaust temperatures.

Is there a reason you MUST run the vent all the way through the roof, rather than side venting it?

Rather than pick a solution and try to make it fit, get better numbers on the heat load, from which you should be able to derive the water temperature that would emit that much heat with your existing radiation. Not counting the rad in the basement your radiation emits about 41,000 BTU/hr a water temp of 150F. The same rads would emit about 23,000 BTU/hr at a water temp of 120F which is probably going to be enough, so your peak exhaust temps will be related to where you set the indirect water heater temperature.

The Lochinvars are nice, but have "feature overkill" for the simpler heating systems found in most 1500' homes. The UFT-080W is also a 10:1 turn down fire-tube heat exchanger type boiler, simpler to install- it has a pre-plumbed additional port and controls to support an indirect water heater, is designed to be able to pump-direct on most systems ( no primary/secondary, saving a pump) and is less expensive than a KHB. (The Westinghouse WBRUNG-080W is exactly the same boiler under the nameplate, and sometimes less expensive than the HTP branded version. )