Calculating heat loss based on average winter fuel oil consumption

Discussion in 'General Discussions' started by centipede16, Sep 10, 2011.

  1. centipede16

    centipede16 New Member

    I am starting to look into Geo, and I'm trying to sanity check the size of the system for Heating purposes, based on present fuel oil average usage. For reference, my pre-build room-by-room radiant heat load total was computed at approx. 35k BTUs based on a 6 degree F. outside temp., and if basement zone is added in (currently it is not hooked up), then add another 15k for a total of 50k-55k BTUs/HR (paper world). Here's how I did my calculation, please let me know if this approach is sound and also a reasonable way to sanity check paper calcs. My assumption here is that average fuel consumption per hour equates to a number of BTUs/hour, which is the heat loss. I observedusage of 150 gal per month during Dec 2010-Mar 2011, = approx. 5 gallons per day. Foroil-based systems, each gallon of fuel oil contains 139,000 BTU of energy. Myboiler has an 0.85 AFUE (efficiency) rating. 139,000BTU * 0.85 = 118,150 BTU per gallon. 5gallons * 118,150 = 590,750 BTU per day. 590,750BTU / 24 hours = 24,615 BTU (average) per hour.

    If the above is a reasonable way to compute the ballpark heat loss (25k BTUs based on my usage calc, 35k-55k room-by-room specs), then I'm expecting the Geo system to be sized according to the 25k-35k or worst case, max of 55k BTU loss, or in that vicinity. I'm not clear on how 25k BTUs equates to "tons", but in my first quote I was given a heat load of 5.4 tons - does that size square with my usage or the paper calcs? Thanks for any inputs including correcting my assumptions.
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I understand the reasoning, but there are several flaws

    1) 85% efficiency is steady state best case for oil furnaces. Actual efficiency is a bit lower owing to cycling and standby losses.

    2) A system sized for average heat loss will fail miserably. A month in which average fuel oil consumption was 5 gal / day probably had many days during which daily consumption was 7+ gallons. Then there is the variation of load throughout the day. You are gong to want a system able to meet load during bitter cold windy nights.

    3) I'd be suprised at a basement adding so much more load relative to the whole house.

    A design load of 5.4 tons might reasonably be met with somewhere between 3 and 4 tons of geo heat pump supplemented by electric strips.

    Best solution may be a 3 ton geo + strips, and spend some of the savings on steps to bring down design load - insulation and air sealing.

    If not already done, consider a blower door test.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Usage vs. Peak

    You can not size for a peak demand in this manner.
  4. centipede16

    centipede16 New Member

    Ok, so you can't size for the peak demand based on averages, understood. However, this usage over time does vet the paper calcs to a degree. The paper design of 35k BTU/hr (again, w/o basement) based on a 6 degree outside temp for my area seems to be a design for the "peak". So, in looking at my usage, which seems to average 25k/hr, it sounds like the paper calcs of 35k would be what the Geo system should be sized to, or thereabouts, unless sizing additional for the basement. My understanding is that 1 ton equates to 12k BTU, so a 5.4 ton system equates to 64,800 BTU. Given the 25k usage average, and the 35k paper design load, why would the Geo system need to be sized at 5.4 tons, just trying to understand how the sizing is determined and whether additional headroom is needed in geo designs, thanks.
  5. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    The trouble is that a 5 ton heat pump will not give you 5 tons of heating. It will give you five tons of cooling.

    There is a saying that is popular here, it goes like this: "you just don't know what you don't know"

    Not trying to be rude, but there is a lot you don't know about sizing geo systems...yet.
  6. Designer_Mike

    Designer_Mike Member

    If you go to the manufacturer site and pull up their brochure (like the tranquility series of climatemaster) they will define the BTU/hr output in stage 1 and stage 2 with the requirement that your ground loop provides entering water temperatures and flow rates to the specs.

    I personally used the same method you have to size mine, although I know it was NOT the correct way to do it.
    I have secondary heat sources (propane fireplaces) for the cold nights plus I did my absolute best to ensure my loop was not undersized. I undoubtedly paid a bit more for the loop than if I had shaved the budget and minimized it. On the flip side, a slightly oversized loop can make significant differences in operating efficiency along with increased BTU output.
    The tax incentives are based on INSTALLATION costs and I want to minimize my operating costs in the future and will take a larger incentive check at the installation.

    My secondary reason for choosing the size of my unit was that the ductwork would have needed to be re-worked for the larger unit (increase air flow) and I just didn't feel like going through that effort (I know, that's contradictory to the incentive comment above).

    In six months I will know if my choice was a wise one. I also plan on improving the insulation and windows to tighten the envelope a bit but one project at a time....and spread out the tax incentives too.
    Yes...I took the risk but I know the possible problems I may see and it will be MY problem if it's small!
  7. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    D. Mike, not just your potential problem.

    Developers like to skimp on installation costs as well before selling the house to someone else. Lately, I've been involved in a few home inspections by potential purchasers. Somewhere along the line, someone pays if the system isn't adequate.
  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Gotta throw a yellow flag on this comment as well.
    Slight oversizing makes for slight improvement.
    Only significant oversizing can make "significant" differences.

    What is significant oversizing? at least 25% more though 50-100% extra is where the numbers start to matter.

    I've told the story before of a man who wanted to convert his 20 yr old CM to closed loop. In reading the original owners manual I noticed minimum EWT was ~45*. I ran the calcs and to assure 45* EWT with horizontal loops in MI for a 5 ton we needed about 5 miles of pipe. IOWs to get a 15* jump required 25,000 extra feet of loop. That's a "significant" EWT improvement though it came with a price so significant it was far cheaper to buy a new heat pump and smaller loop field.

    Centipede 16:
    If someone suggests a 5 ton for your home run away.
    With geo, the sweet spot is to hit most of the average load and augment with another fuel.
    You need to solicit contractor bids and have them show you computer models of op costs.
  9. Designer_Mike

    Designer_Mike Member

    I don't have access to, or the expertise to crunch the numbers for EWT changes on a loop.
    I was simply looking at the performance charts.
    From my old notes, it looks like BTU/hr went up by 13% in stage2 with a 9 degree EWT increase and stage 1 production increased by about 7%. (from 32 to 41 degrees per my notes)....I can't remember which model or manufacturer that was.

    I do not know how much more loop that would require. As usual, every site would be different anyway.
    I have read enough posts to know how detrimental an undersized or poorly installed loop can be for sure!

    BUT as you said, the computer models of op costs are really what everyone should take a careful look at for good analysis.

    Personally, I might prefer to spend an extra $1000 in 2011 (30% from Uncle Sam) and save 7% of my operating costs over the next thirty years.
    On the flip side, quoting a job with that extra cost may put you out of the running and some homeowners may not see the value to improved efficiency.

    OK, now I'm curious, can you run the numbers Joe? You can use those cold sandy soils of Michigan for the calcs:) ...I'm sure you have done it hundreds if not thousands of times!
  10. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Ok a job I'm on now:
    extra thousand would buy ~10% more loop. All heating cooling and hot water production plunges from $1,406/yr to $1389/yr. so times thirty your $700 after rebate saves you $510.00. Not a bargain.
    of course all depends on soil, cost/KW etc.

    No one suggests short loops. Just right-sized.

    More than once pros on this and other forums have suggested to one another too much emphasis on the wrong things. While loops are important, the suggestion that "a little extra loop" without designed in benifit (such as doc's saving labor on compaction) has very little value.

    BTW a 9* EWT increase on the same project would require 3 times the loop field or loosely $10,000 more.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You can do a reasonable job on using previous usage data to check your sizing calcs.

    You would need efficiency of your current boiler, and you can match your usage with the BIN data for your area.

    Give me the total gallons used for your entire season (not just Dec-Mar), and your location, and your current boiler efficiency, and I run the numbers for you.

    It will be better than the "guestimate" of a manual J, where you have to guess wall R values or window R-values.
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Little late when I posted this. A revisit makes sense.
    The case study I used would have to change flow center size when I added loop which ate up a chunk of our $1000 budget.
    In fairness I added $1,000 to the loop budget on other jobs and found the impact to be from 1 to 3* on EWT and 1 to 3% on the operating cost (regardless of cost/kwh).

    Working in reverse I tried to pick a value that would save $100/yr in my area's higher electric rate (12 cents/kwh) and found it required almost twice the loops in most instances.

    Don't forget pumping cost goes up with loop length too.

    This is why I am (bordering on) militant against the notion that a little extra loop is a good thing. While it obviously is not a bad thing, folks rightfully believe "good" things are worth paying for, but I would suggest that seldom is a $20 dollar annual savings worth paying a thousand dollars for.

    Why do I say seldom instead of never? Doc, charges the same for extra loop that he would charge to get good compaction in the clay in his area. By trading loop for labor and letting nature handle compaction, his customers win satisfactory numbers in year one and superior numbers thereafter.
  13. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Joe is right, it is usually not worth it to add extra loop for efficiency purposes. Just to clarify,
    we have a standardized approach where we add more loop, especially in the colder southtown climate. Before we do all the analyses, calculate the soil conductivity and worry about compaction, we simply install 15% more loop.

    So if it is sandy and dry, we have the right loop, and if it is wet clay, we are slightly oversized.

    That way we are always covered for all occasions, it benefits our operation since we have the loops standardized and we do not have to compact it, and it benefits the customer in the long run since he has an (only slightly) oversized loop.

    However, there is, within a certain range, no right or wrong. It can even be a good thing to make the loop smaller in order to save upfront cost, with only minimal penalty to the operational costs.

    Be aware, you should only touch those safety margins when you know what you are doing.

    Otherwise stick to the standards established by IGSHPA
  14. centipede16

    centipede16 New Member

    "Give me the total gallons used for your entire season (not just Dec-Mar), and your location, and your current boiler efficiency, and I run the numbers for you."

    Sure. 600 gal. from Dec-Mar, and 400 gal. for rest of the year. Boiler efficiency is 85%. Boiler is feeding an Indirect for DHW (hence the reason in runs throughout the year). Location is RI.

    Other followups - I'm the homeowner, not a designer, I'm just trying to sanity-check the amount of money I'm looking at for certain sized geo systems, starting to get quotes in now. House is approx 3400 sq. ft. and tightly sealed (closed cell foam). Radiant/Warmboard for heat distribution, air ducts for AC. I plan to keep the boiler as a backup system if I go with geo, so arguably, it could supplement on persistent cold snaps. Thanks again for the comments.
  15. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    All right, I took the BIN weather data from Quonset Point AFBin RI, and matched it with your 1000 gallons usage. If this would all be for heating you are looking at 47500 BTU/H at 5F design temp outside and 70F inside (97.5%), with 82% efficient boiler.

    Since I do not know your water usage, I would estimate about 200 gallon of that, then your heatloss comes down to 38KBTU. You can do your own adjustments.

    IF you have a DSH and 2 tank setup, your geosystem will generate about 60% of your hot water. You need to account for this, since this is not a Manual J anymore with safety margins, this is real data. Your hot water generation will take heat away from the heatpump and the loop.

    It appears that a 3 ton would run the most economical, A 4 ton (all CM data here) would run even $29 less per year, but the 2nd pump for the flowcenter would cost you about $60 annually. That is for forced air, where heat strips bail you out.

    If you are sticking to radiant, you have no choice but to design for your total load. Unless you use an electric tank as a buffer tank and the electric strip heaters as your supplement heat.

    Your basement should not add more than 5000 btu/H.

    A 5 ton CM is rated at 48.500 BTU/H at 32F EWT, looks like the right choice for radiant.

    Are you sticking to radiant or are you considering forced air?
    Hope this helps......
    Last edited: Sep 15, 2011
  16. centipede16

    centipede16 New Member

    It helps a lot - thanks docjenser for running those numbers. If there was a abnormal cold snap, I'd want the boiler to kick in as the supplementary heat unit, since it is already in place. At this point, I'm leaning towards the geo supplying radiant for heat with no forced air for heat. The complication is that I have 3 AC zones with separate condensers/ducts. So, one option would be to put in the geo and replace just one AC zone in the location of the geo system, then leave the other 2 AC zones/condensers as they are. In this case, the AC zone that would get replaced is a 2 ton condenser, so plenty of headroom there. The geo then would need to be a hybrid - hydronic for the radiant heat and then air ducts in cooling mode. Next option would be to graft in the 2nd basement AC zone to the geo system (not sure what the logistics of this are). The 3rd AC zone handler is in the attic, so that would be more difficult to graft in I suspect, and I probably would not want to look at 3 separate geo systems just to get the cooling converted.
  17. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    This would be an application where you could use 3 hydronic air handlers for auxiliary heat and chilled water cooling when connected to a WTW geo.
    If duct work is adequate it may save you the expense of converting radiation.

    This would not be an application where i'd use a hybrid.
  18. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I was thinking the same: 3 hydronic air handlers, a buffer tank, and a 5 ton reversible w-w heatpump would do a nice job.
  19. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I got here late, but...

    ....I too vote water to water.

    Google tool bar or my fat finger cost me 30 mins of typing. I know how to do this home. I do not feel like re-typing, sorry.
  20. geome

    geome Member Forum Leader

    Mark, next time that happens, try holding down Ctrl and then pressing Z (before doing anything else.) It may save the day, occasionally.

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