Replacement boiler sizing based on past usage
I’m looking for some reassurance and validation before I pull the trigger on an expensive purchase.
I currently have a 40+ year old, 137,000 BTUh oil boiler that is getting long in the tooth due to substantial corrosion. I have done many efficiency upgrades to my home. I am very interested in replacing the boiler with an electric boiler as the boiler I have now short cycles since I have improved the efficiency of the home so much.
The estimates I have received for oil or propane were in the 100,000 BTU range and the electric one was approximately 60k BTU. These estimates were either done rule of thumb, or with a heat load calc and home dimensions and insulation value estimates. It seems to me I should be able to size the new system based on past usage rather than guesstimates of air infiltration rates and effective R values but so far I have not found any contractors willing to look at my data.
At the end of this heating season will have 2 years worth of seasonal oil usage data to build a sizing profile. I’m looking for some validation on how to appropriately size the new boiler. If I’ve done my work correctly, I think I should be able to work with a boiler dramatically smaller than the size of my existing unit…maybe as little as 1/5 the size and still cover my worst case design day.
During the most recent servicing, I was told the oil boiler is firing at about 80% efficiency, I think based on stack flue temps. Using this efficiency value, which I think should be bounding, my total number of gallons delivered and the energy density of oil, I think I get the number of BTUs put into the house in a season.
For the 2015-2016 season I used 348.2 gallons. At 140,000 but/gal, I input 38,998,400 BTU into the house for the heating season.
I then divide this value by the number of heating degree days and hours in a day (24 🙂 to determine my hourly required BTU input for the home? If I divide the above value by 3639 for Lancaster, PA and 24, I get 447 BTU/hr.
Multiplying this by my design reference temperature (60F, we like the house cooler and the internal loads do the work when it’s more than 60F outside) minus the design heating day (0F) gives me a little over 26k BTU (7.72 kW) as my design boiler size.
Did I do this right? Is there a reason I couldn’t size my new boiler this way?
If I am on the right track, I should get an electric boiler of at least 8 kW. I have been very impressed with the specs on the Thermo 2000 Mini Ultra 9kW boiler. It appears to meet my needs, has built in connection points to control my three zones via zone valves, a 120VAC pump connection and an outdoor reset sensor all included to improve efficiency in the shoulder seasons. Does anyone have experience with this brand? Long term durability would be my primary concern, I think.
It’s a Canadian manufacture and not locally sold here in Pennsylvania, but I think it’s quite impressive.
Long first posting. I am really enjoying this site and have learned a lot, so I hope this community can help me settle on my new and improved boiler purchase.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Here is a link to an article by Dana Dorsett that explains how to use old heating bills to size heating equipment: Out With The Old, In With The New.
Your decision to purchase an electric-resistance boiler is unusual. Electricity is usually an expensive fuel, unless you are using heat pumps (for example, ductless minisplits).
-- Martin Holladay
The reference temperature is what you should use when you look up heating degree days. The temperature difference you should multiply by is the difference between interior design temperature and outdoor design temperature. 68-72F would be typical design interior temperatures. The reference temperature will generally be 5-15 degrees less than your thermostat set point due to internal gains. (The low end for a poorly insulated house with little solar gain and the high end for a well insulated house with plenty of solar gain.)
If you are on a regular fill-up service and have a "K-factor" stamped on the billing slips, what are the K-factors on mid-to late winter fill-ups?
The heating/cooling balance point on most tightened up 1960s vintage homes is about 5F below the indoor temperature, but could be as much as 10F below, as Reid suggests. If you really keep the place at 60F, use base-55F and base-50 heating degree-days for running the fuel-use based. For sizing the boiler, to meet code you still need to extrapolate the size it would take to hit a code-min 68F, even if you don't keep it that warm.
Do NOT use a whole year's fuel use for making these calculations, since that would introduce a large distortion, overstating the actual load. Using only mid-winter oil use would give you the least amount of distortion from solar gains, warmer days, etc. But to use your whole-season numbers, if the HDD were base 55F (and not the typically reported 65F):
348.2 gallons of 140K/gallon fuel burned at 85% efficiency delivers 348.2 x 0.85 x 140,000 = 41,435,800 BTU, over 3639 HDD would be 11,387 BTU/HDD, or /24= 474 BTU per degree-hour.
The 99% outside design temp in Lancaster, PA is +8F (not 0F, even though it gets to 0F or colder most years: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf ), so you are looking at (55F-8F=) 47F heating degrees above your balance point to keep the place at 60F. But the code-min 68F adds another 8f heating degrees (up from your 60F indoor temp) so you really have (47F+8F=) 55F heating degrees, for an implied heat load of:
55F x 474 BTU per degree-hour= 26,070 BTU/hr, which is (/3412 BTU/kwh=) 7.64 kw.
Mind you there are far fewer base 55F heating degree days than 65F heating degree days, so the BTU per degree-hour constant is going to be higher, with a correspondingly higher heat load if you were using 65F for a base temp.
Run the real numbers on the real quantities between fill up dates, using both base 50F and base 55F, you probably really need something in the 10-12kw (34K - 41K BTU/hr) range to be able to meet a code-min 68F indoor temp @ +8F outdoors, even if it doesn't need that much to maintain 60F indoors.
BTW: Even though it's more expensive than an electric boiler up front, going with ductless air source heat pumps rather than an electric boiler would likely pay for itself in fairly short years.
41,435,800 BTU/year is (/3412=) 12,144 kwh/year. At PA's average residential retail electricity price of 13.8 cents/kwh (https://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a ) that's $1676/year for heating. With an optimally sized ductless system you'd be looking at less than $550/year.
With $1100-1200/year in operational cost savings it takes well under a decade to recover the installed cost of 2-3 tons of ductless (assuming the ductless was competitively bid- which would run $3-3.5K/ton in my neighborhood for first-tier vendors.)
Thank you for all of the responses. It is much appreciated.
Dana.
My home is actually a 1930s home that was originally renovated in 1998 and I again upgraded things since 2012 (attic increased to R55 and air sealed, walls all covered with 1" polyiso and air sealed, new R-5 triple paned windows throughout.)
In my calcs, I used 60F as my HDD design reference point to determine my heating degree day values.
I didn't know I shouldn't use the the whole year's oil usage. We are on auto refill but looking at my bill I see no K-factor for the refill cycle. I will do a reanalysis based on a single cold month to see how that would turn out for sizing.
In my original sizing review I used 0F temperature design low and a 60F inside temperature to calculate my BTU design temperature thinking that represented my condition (internal loads being sufficient to keep the house comfy on a 60F day). I was misusing the formula. We normally keep the house at about 68F during the day and the thermostats set back to 60F at night. This was my attempt to stop some of the short cycling of the oil boiler, plus we all sleep better in a cooler room. Since our official design day in Lancaster is 8F that happened to meet the 60F temperature drop (from 68-8F) for the code minimum for Lancaster. Sometimes the blind squirrel still finds a nut. :-)
Regarding the mini-splits, my wife and I both greatly appreciate the smooth and breeze free nature of hydronic (radiator) based heat. While the mini-splits are likely to be cost effective in the long run, our somewhat chopped up home (more smaller rooms) would have a hard time being warmed evenly without many head units. Aside from our preference for hydronic heating, I was looking at electric boilers because at our low apparent BTU requirement, there are no oil boilers that fire that low and it actually starts to be difficult to find propane boilers sized that small even with a a mod con. Electric boilers, however, don't have efficiency hits from short cycling and should work well in our situation. My estimates show a price difference of $100-200 per year for electric versus oil costs. I have a couple estimates and it looks like I can get an electric boiler installed for $3-4k less than oil or propane respectively, so it would be a good long time to pay back the fuel cost savings. Also, by going electric, I free up the chimney the oil boiler uses (it's been relined) and I can put in a small wood stove to either cut the electric back a little on the coldest days or keep the pipes from freezing if we ever have a power outage.
I will rerun the cold month load estimate and update in a but.
Thanks again for all the help!