BTU/h need/estimated (NCB-240E)

[Recovering Engineer]

I am one of those new millennial homeowners and I realize now just how little I know/understand about some of these basics so I apologize for any stupid questions here. This is my first winter in my house and this all started based on trying to figure out if my propane consumption is low/normal/excessive. I thought it would be relatively easy, figure out the BTU/hour of my appliances and compare to how much fuel I’ve consumed and estimate how long things are running. Then I looked at my boiler and it says 18,000 to 199,900 BTU/h and then I fell down a rabbit hole trying to determine how many BTU’s it should take to heat my house and found this forum.

So some details. We have a ~3,800 square foot house in the greater NYC area so we experience decent winters but not crazy. Everything but the lights basically are on propane (gas range, gas oven, gas dryer, combined heat/hot water boiler). We are bad millennials, along with owning a home we cook our own meals, so the stove gets used every day but that isn’t much consumption. The dryer probably uses a decent amount but I haven’t looked at the numbers yet. I figure the boiler is really going to dominate our propane usage and I’m not sure how to estimate it’s useage with that wide BTU range.

We have an NCB-240E combined boiler. Our HVAC system has 3 zones, 1 for the entire first floor, 1 for master bedroom and 1 for the rest of 2nd floor. We have programmable thermostats but my wife is at home with a toddler so zone 1 basically runs all day from 6:30am-10pm and with a 2-story entryway I’m sure some of that is drifting to the upstairs as well. Master bedroom zone runs only a couple hours a day since it is empty all day. The 2nd upstairs zone runs quite a bit to keep baby’s room warm for naptime, etc.

So is there a good way to figure out how much the boiler should be running to keep the house warm? And then to figure out from our actual propane consumption how much the boiler is running? I’m trying to both benchmark our consumption and try to plan/budget for propane for the next year. On a semi-related question, how much hot water output is reasonable for things like concurrent showers/sinks should I get from the NCB-240E?

 

[Dana]

Propane use by the dryer, water heater, etc are all dwarfed in size by the amount used for space heating during the winter months.

To put some basic arithmetic to it, run a fuel use heat load calculation tracking propane use per heating degree-day data from a nearby weather station on degreedays.net, using the EXACT fill-up dates and amounts. If it's a 2x4 framed house use 65F as the degree day base temperature, if 2x6 use 60F. Thermostat settings will affect this too. If it's a 2x4 house you normally keep the house no warmer than 65F, use base 60F.

A reasonably tight 2x4 framed 3800' house with clear glass double panes in most of Westchester would come in with a design load of about 70-75,000 BTU/hr @ +10F if there is no foundation insulation, maybe 50-55,000 BTU/hr if the basement walls are insulated and the windows are a low-E type. On Long Island it would be about 10% lower (due to the warmer +15F outside design temperature.) But there are exceptions on both ends of those ranges.

Wintertime incoming water temperatures in Westchester county are a bit cooler than Long Island & NYC, but let's assume it's 40F. A shower temp runs about 105F, which would be a 65F rise, a tub fill is about 110F for a 70F rise. The maximum burner rate in hot water mode is about 200,000 BTU/hr-in, which at 95% efficiency is 19o,000 BTU/hr out.

So for showering you would be maxed out at 190,000BTU/65F=~2923 lbs/hr (=48.7lbs/minute, = 5.8 gallons per minute, or two simltaneous 2.5gpm showers, three 1.9gpm showers.)

For tub filling your max rate would be 190,000/70F=2714lbs/hr (= 45.2 lbs/minute, = 5.4 gpm.)

Even at your high local electricity pricing, heating with cold climate ductless heat pumps is less expensive heat than condensing propane, especially when it's above freezing out (not so much when it's below 0F.)

 

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[Recovering Engineer]

Thanks, that was helpful. One challenge is the fact that being my first year in the house I have limited data. I’ve only had 2 propane deliveries and 1 basically covered the summer/fall. I had a recent delivery that covered Dec/Jan so using those numbers I can run through the math you gave.

Used ~380 gal of propane * 91,500 = ~34.6M BTUs
degreedays.net closest station shows 1,740 degree days so (34.6M/1,740) = 19,800 BTU/Fdegree day
Given 10F base that means (19,800/24hrs * 55F) = ~45,500 BTU/hr

Does all that look right? That doesn’t include the efficiency loses of the boiler. Considering that includes propane used for hot water/dryer/cooking in addition to heating that’s pretty good for the size of the house right? The boiler is in an unfinished basement (I didn’t include that sq.ft. in my 3,800) but we do have the spray foam insulation in the basement walls at least.

Is there any way to go from those numbers to figuring out annual propane use? Just use the average degree days for my location and multiple by the 19,800? The strange thing is if I use the delivery from the fall which used much less propane per day obviously it says I used more propane per degree day.

 

[Dana]

Assuming 95% efficiency (condensing temperature operation <<130F output) you're looking at about 87,000 BTU/gallon and 33,060,000 BTU, for (/1,740HDD=) 19,000 BTU/HDD, which is (/24=) 792 BTU/degree-hour, x 55F heating degrees is 43,530 BTU/hr

At 87% efficiency (high temp operation >140F output) it would be about 77,000 BTU/gallon and 29,600,000 BTU which is =(/1,740HDD=) 16,816 BTU/HDD, or (/24=) 700 BTU/degree hour. x 55F heating degrees is 38,500 BTU/hr.

Those are indeed better than average numbers. A load/conditioned space ratio of ~10-12 BTU/hr per square foot @ 10F outside, 68F inside would usually indicate tighter than average 2x6 construction and an insulated foundation. You can sometimes get there in a simple shaped (rectangle footprint is good) 2x4 framed house with a lower than average glazing/floor area ratio, or if the glazing size, type, & orientation is optimized for maximizing solar gain in winter.

During the winter months the error from solar gains pretty much offsets the other uses. In the spring the solar gain error can exceed the other uses error and in the fall the higher incoming water temps lower the hot water energy use, and when combined with solar gains and fewer HDD introduces more error. So it's right to use wintertime fuel use only on this time of calculation, a time of year when there are fewer daylight hours and lower error from solar gain.

You can extrapolate the annual heating fuel use from this type of calculation, but there's no way to break out the other-uses error. Using summertime propane use to adjust the number isn't too accurate either, since the warmer incoming water temperatures and dryer skimpier clothes results in less hot water use, and fewer BTU/gallon for the hot water. But for budgetary purposes Annual HDD x propane gallons/HDD + 20% is usually in the right range.

If you don't already have air conditioning DO consider installing one cold climate ductless mini-split per floor, not a cooling-only solution. A single modulating 3/4 ton Fujitsu 9RLS3H "rated" at 9000 BTU/hr for cooling delivers 12,000 BTU/hr of cooling at max speed, but in heating mode delivers 16,000 BTU/hr @ +17F, about 15K @ +5F. Set up to modulate during the shoulder seasons it would be substantially cheaper heat than condensing propane in most near-NYC markets. They are nearly silent when cruising along at low to mid-speed (quieter than your refrigerator.) The Mitsubishi FH series ductless mini-splits are somewhat comparable to the Fujitsu, and also very good bets, albeit with lower heating capacity per cooling-rated ton than the Fujitsus.

 

[Recovering Engineer]

Those are indeed better than average numbers. A load/conditioned space ratio of ~10-12 BTU/hr per square foot @ 10F outside, 68F inside would usually indicate tighter than average 2x6 construction and an insulated foundation. You can sometimes get there in a simple shaped (rectangle footprint is good) 2x4 framed house with a lower than average glazing/floor area ratio, or if the glazing size, type, & orientation is optimized for maximizing solar gain in winter.

The house is a new build so I guess this is some confirmation that he used quality insulation and windows in the design. We also have a lot of West/Southwest facing thermopane windows which I assume helps with the solar gain during the afternoons.

If you don't already have air conditioning DO consider installing one cold climate ductless mini-split per floor, not a cooling-only solution. A single modulating 3/4 ton Fujitsu 9RLS3H "rated" at 9000 BTU/hr for cooling delivers 12,000 BTU/hr of cooling at max speed, but in heating mode delivers 16,000 BTU/hr @ +17F, about 15K @ +5F. Set up to modulate during the shoulder seasons it would be substantially cheaper heat than condensing propane in most near-NYC markets. They are nearly silent when cruising along at low to mid-speed (quieter than your refrigerator.) The Mitsubishi FH series ductless mini-splits are somewhat comparable to the Fujitsu, and also very good bets, albeit with lower heating capacity per cooling-rated ton than the Fujitsus.

We have central air with 3 AC units outdoors and they share the ductwork, zones, and programmable thermostats with the heating system. The ductless systems seem so popular in Europe/Asia but everybody seems to default to central air in the US. I wonder if that's because most people are used to giant/noisy window units and don't realize what a new ductless system is actually like.

 

[Dana]

. The ductless systems seem so popular in Europe/Asia but everybody seems to default to central air in the US. I wonder if that's because most people are used to giant/noisy window units and don't realize what a new ductless system is actually like.

Ignorance is truly the biggest hurdle regarding ductless AC in the US.

Even in Europe (as in the US) most people are clueless as to how well the vapor injection scroll compressors in cold climate ductless systems perform (in both capacity and efficiency terms) for h eating even at outdoor temps of -20C to -25C. In Europe hydronic heating is the paradigm and they are reluctant to give that up for any system that delivers the heat via air. A few cold climate air source hydronic output systems exist, but they are predominantly from Chinese vendors, and selling into a market suspicious of the level of quality and support from less-known companies.