Hi All,

First, I'm really sorry for the length of this post!

I am in a real conundrum as to whether Geo this is the way to go for my home.

I've done a bit of research ( and now spent the morning reading a number of the posts here) but am at a loss and need some impartial advice (i.e not people trying to sell me stuff!)

So I would be eternaly gratefule for some feedback/comments/advise - whatever you want to throw at me!

Facts about my Home

1. It is in Caledon, which is in southern Ontario.
2. Its on 3 acres but a lot of it is landscaped/wooded so while i could find a way to get the distance for horizontal i don't much fancy it - so I think I'm settled on a vertical system.
3. Vertical could be installed in front lawn, very close to basement furnace room. no obstacles to get pipes in basement.
4. House was built in 1981, 2x6 construction. Newer windows and doors, appears well insulated (lots of pink insulation stuff in roof and behind wall boards)
5. House is 3,000 sq ft plus 1,200sqft finished basement.
6. Current fuel and furnace is oil (its 1.30 a liter or 4.90 a gallon) so pretty steep. We used about 3,000 liters last year but it was mild. I guess on a harsh winter that might be 3,500 to 4,000 liters.

Anyway, So Ive had 3 contractors quote so far, 2 of them use a next energy system and one uses a water furnace system. I will get another waterfurnace contractor quote.

I will also ask the questions on the stickies for things such as J values etc but i haven't done it yet.

Oh quotes don't include a desuperheater as I have an electric water heater which is the other side of the basement by water pump so I cant see payback for extra $1500 for it inc install.

Anyway, the 2 next energy contractors came in at similar prices of around $29K + Tax. The one guy I though was a complete tool and I wouldn't use him so I will concentrate on the quote from the Contractor I liked.  He is offering a 5 tonne system (a tranquility 22 TZ heat pump model#60). He says he will drill 5 180 ft holes . and provide 1 yr maintenance, 5yr labor and 5 & 10 yr parts from manufacturer.

The other guy from water furnace also said 5 tonne system. he quoted $36K which made my jaw drop to the floor.  When i questioned him he said the reason he is more is because he will drill 3 cores at 300 ft (same overall length of pipe as 5 x 180ft) and that this is very important. He also provides 10 year labor, which is nice but not worth an extra $7k IMO.

So i really thought i was getting somewhere until this guy mixed it up with the core depth argument. I cant seem to find any evidence that this depth is necessary but he told me they always go 300ft deep and that the other guys are cheaper because their drilling rigs cant go that deep. Is this something I need to worry about? Is this guy just trying to justify taking more of my $?

I challenged the $29k about his depths and he said that as long as you have 180ft per tonne which is industry standard the sytem will work well. He says that temp changes about a degree for every 100ft under 30ft i think (cant quite remember what he said). He also said that residential systems are over engineered when it comes to pipe length because the testing they do on big apt blocks to determine exact needs are too expensive and not worth doing for homes.

Anyway , thats where I'm at - I feel a bit lost in woods right now. I want to do this for all sorts of reasons but I really don't want to make a $30k mistake that wont perform or reduce my fuel bill.

Any advice would be fantastic. Or any recommendations for good contractors who operate near Toronto!

Again, thanks for taking the time to read my appallingly long post.

Best Regards,

Adrian

P.S - Ive probably left information you guys may need off - so let me know if there is anything I should be telling you!

What we didn't get was the model of the WF. The Climatemaster mentioned is an entry level model. If you are comparing it to a 7 series that is about a $4,000 difference or a T30 climatemaster is somewhere in between.
Feet of pipe matter more than depth.
You might find horizontal is at least $5K-8K less.

Charting oil use against heating-degree-day data would get you pretty close to the whole-house heat load number at the 99% outside design temp (probably about -19C to -21C in Caledon -more similar to Kitchener than Toronto due to distance from the lakes), with a bit of arithmetic & the nameplate efficiency of your oil-fired beast. 

At 5 tons it seems a bit high- most 2x6 houses with better performance windows and at least R30 in the attic run would around 12BTU/ft (give or take 5) at -20C, and can be cost effectively air-sealed & spot-insulated to bring it under that.  Heat losses out of the basement might be high if it's not insulated, but if that's the case isulating the foundation walls to  R12-R20 would likely peel more than a ton off the size (and make the place a lot more comfortable too!).  Basement losses per square foot would be well under 10 BTU/ft if insulated. 

Fuel-use measured heat load on my antique ~2400' house + ~1500' of insulated basement 2x4 framed antique house with known gaps in the insulation and original circa 1923 single-pane double hungs + clear glass storms comes in at about 40,000BTU/hr @ -20C. Your house should perform better than that despite the larger size. If it doesn't, fixing the heat leaks before diving into a heating system as expensive a ground source heat pump.  The incremental cost per ton isn't a simple division by tonnage, but every ton of drilling & ground heat exchanger DOES come with a price, and in rocky-ledgy areas like mine the increment is still substantial.

All successful heating systems start with a heat load analysis, using realistic design conditions. It pays to be very aggressive on minimizing the size (and even reducing that size) when looking at geo.  Converting fuel use against degree-day data to BTUs per hour at any arbitrary temp puts an upper bound on it, but don't begin & end there- there's more to it.  But at LEAST begin there.

In the US many oil delivery contractors stamp a "K-factor" on the billing, which is the same information in a different form. I'm not sure if that's common practice in Ontario(?).

Thanks So far guys.

I'm waiting to find out the exact model # for the water furnace.

I did get the following info from one of the contractors regarding his assessment which lead to a selection of 5 ton.

<!--[if gte mso 9]> Normal 0 false false false EN-AU X-NONE X-NONE MicrosoftInternetExplorer4 indoor design temp  72

Outdoor Design Temp -6.00

above grade wall R 20

Below Grade Wall Factor 0.81

Mean Soil Temp 46.00 Window R 2.78

Soil Conductivity 1.00 Door R 9.08

Temperature Zone Orangeville Roof and Ceiling R 39

Heat Loss Delta T 78.00

Exposed Floor R 34.06

Basement Delta T 26.00

Air Leakage Multiplier 0.07

<!--[if gte mso 9]> Normal 0 false false false EN-AU X-NONE X-NONE MicrosoftInternetExplorer4

Total Net Load 54901.00

Ventilation Heat Loss  8424

Total Home Heat Loss 63325.00

70% Design Criteria 44327.49972

Building requirements - 63325 BTU

Heat pump capacity - 45900 btu

second stage - 17425 btu

electric backup 4.16 kw

I'm afraid this info doesn't really mean much to me but if its relevant - there it is.

Using -6F (-21C) is in the rigth range- maybe a couple degrees cooler than reality, but not 10 degrees cooler.

The window assumptions are U0.36, with no curtains or blinds, etc. factored in.

Not sure what their below grade factor means. If that's a U-factor it means you have NO foundation insulation and they're assuming a basement delta-T of 26F, which would be a HUGE loss through U0.81 walls.

The air leakage factor is simply a WAG, unless they actually did a blower door test and verified where the leakage was. Not sure what units they are using there either, but maybe we can work backard: With a ventilation loss of 8424 at a delta-T of 78F, that's 108 BTU/hr per degree-F. Air has a specific heat of about 0.18 BTU per cubic foot, so at 108 BTU/hr would be 600 cubic feet per hour, or exactly 10 cfm. Looks like it was indeed just an order-of-magnitude "...call it 10 cfm..." WAG, but not a ridiculously overstated guess.

The soil conductivity is also just WAG, but it makes a difference if it's an uninsulated basement.

The heat loss delta of 78F means they think you'll insist that conditioned space be able to maintain 72F (22C) when it's -6F (-21C) outside, which is a bit aggressive.

They made NO adjustments for plug loads. A typical 4 person house in N. America averages over 3000BTU/hr of power use. At night that may drop to 1000BTU/hr or less, but never zero.

They also made no adjustments for occupancy- 4 sleeping humans is another 1000BTU/hr straight off the heat load.

This was definitely not an aggressive heat load calc. If you want to try your own, download a copy of HOT2000 (an NRCan freebie) energy use modeling- it's pretty dumbed down, but gives reasonable results.

http://canmetenergy.nrcan.gc.ca/software-tools/hot2000/84

Also, if your foundation really IS uninsulated, there's no time like the present to get that taken care of- it's probably something on the order of 20-25% of your total heat load if the other R-values an U-factors are correct!

If you can tolerate 20C rather than 22C interior temps during the coldest hours of the coldest days (usually while you're in bed), that's going to peel off some too. If you look at it carefully, 5% here, 2% there, 7% some other place, reality is going to be quite a bit lower than their calculated heat load. It's OK to size condensing gas furnaces that way, since the up-front cost is at most a couple hundred Loonys. But it's not the way to size a ground source heat pump- you really have to dig down, find out what the REAL heat load is, and even ferret out the low hanging fruit on the building envelope efficiency.

Even when it costs more per ton of displaced load to air seal and insulate, the lifecycle of the insulation and air sealing is longer, and the comfort factor higher than going with more heat pump. And unlike an upsized heat pump, the air sealing and insulation upgrade has no operating cost.

If you have an oil bill and know the exact dates between fill ups, it's possible to look up degree-day data and use the existing heating system as the measuring instrument of the current as-is, as-operated heat load. The ~20BTU per foot of conditioned floor area is probably at least a 25% overstatement of reality of the existing house, and could easily be a 35-40% overstatment of what your house could be with $5000 of building upgrade.

How does 3 tons rather than 5 tons sound to you? Think it might affect the quote? It won't turn a $30K quote into $20K, but under $25K is pretty likely.

A better WAG: Assume in a "normal" year you use 3500 liters of oil The source-fuel BTUs of 3500 liters of oil is about 127.7 million BTUs. With an 85% burner it delivers about 108 MBTU into the house (the rest went up the flue.)

The September through May heating degree days (base 18C/65F) is about 4400C/7920F. (see: http://www.theweathernetwork.com/statistics/degreedays/cl6155790/caon0103 ) Since the contractors heat calc use degrees, F, lets do it that way. That's 108MBTU in 7920 HDD is 13,636 BTU per Fahrenheit HDD. With 24 hours in a day that's about 568 BTU/degree-hour, base 65F.

The base-temp is the outside temp at which both the heating and cooling loads are approximately zero, so the heat load will rise linearly with degrees below that temp. Using -6F for an outside design temp that's 71F heating-degrees.

So at 71F outside heating degrees, at 568 BTU per degree hour, the heat load at -6F is 71F x 568, or 40,328 BTU/hr.

If you have real fuel use against real heating degree days you can come a bit closer, but that 40K number feels pretty real to me, and jives well with real-world experience on that contruction type. But I'm just sitting here 1500km away, with only the data and WAGs you've used about fuel use that see on my screen. If your 4000 liters/year WAG is more realistic that number only rises to about 46K, not the 63K that the contractor came up with.

And the fuel use calc is really an upper bound, since any oil-fired boiler would likely be oversized, and not hit it's real numbers, and air-handler driven infiltration on a hot air furnace can be several times that 8K they calculated for the ventilation loss, and unless you tuned up the burner at least every other year it's not hitting 85% either. Knock at least 5% off the fuel use calculation, 15-20% if you're heating your potable hot water with oil.

That's a big difference- a difference that YOU would be paying for, which is why you really need to nail this down first.

since the up-front cost is at most a couple hundred Loonys

ROTFL. Maybe we should do the upfront costs in toonies and the ongoing costs in loonies. Things might look better for Green Energy that way.

Posted By Dana1 on 06 Mar 2013 03:54 PM
Using -6F (-21C) is in the rigth range- maybe a couple degrees cooler than reality, but not 10 degrees cooler.

The window assumptions are U0.36, with no curtains or blinds, etc. factored in.

Not sure what their below grade factor means. If that's a U-factor it means you have NO foundation insulation and they're assuming a basement delta-T of 26F, which would be a HUGE loss through U0.81 walls.

The air leakage factor is simply a WAG, unless they actually did a blower door test and verified where the leakage was. Not sure what units they are using there either, but maybe we can work backard: With a ventilation loss of 8424 at a delta-T of 78F, that's 108 BTU/hr per degree-F. Air has a specific heat of about 0.18 BTU per cubic foot, so at 108 BTU/hr would be 600 cubic feet per hour, or exactly 10 cfm. Looks like it was indeed just an order-of-magnitude "...call it 10 cfm..." WAG, but not a ridiculously overstated guess.

The soil conductivity is also just WAG, but it makes a difference if it's an uninsulated basement.

The heat loss delta of 78F means they think you'll insist that conditioned space be able to maintain 72F (22C) when it's -6F (-21C) outside, which is a bit aggressive.

They made NO adjustments for plug loads. A typical 4 person house in N. America averages over 3000BTU/hr of power use. At night that may drop to 1000BTU/hr or less, but never zero.

They also made no adjustments for occupancy- 4 sleeping humans is another 1000BTU/hr straight off the heat load.

This was definitely not an aggressive heat load calc. If you want to try your own, download a copy of HOT2000 (an NRCan freebie) energy use modeling- it's pretty dumbed down, but gives reasonable results.

http://canmetenergy.nrcan.gc.ca/software-tools/hot2000/84

Also, if your foundation really IS uninsulated, there's no time like the present to get that taken care of- it's probably something on the order of 20-25% of your total heat load if the other R-values an U-factors are correct!

If you can tolerate 20C rather than 22C interior temps during the coldest hours of the coldest days (usually while you're in bed), that's going to peel off some too. If you look at it carefully, 5% here, 2% there, 7% some other place, reality is going to be quite a bit lower than their calculated heat load. It's OK to size condensing gas furnaces that way, since the up-front cost is at most a couple hundred Loonys. But it's not the way to size a ground source heat pump- you really have to dig down, find out what the REAL heat load is, and even ferret out the low hanging fruit on the building envelope efficiency.

Even when it costs more per ton of displaced load to air seal and insulate, the lifecycle of the insulation and air sealing is longer, and the comfort factor higher than going with more heat pump. And unlike an upsized heat pump, the air sealing and insulation upgrade has no operating cost.

If you have an oil bill and know the exact dates between fill ups, it's possible to look up degree-day data and use the existing heating system as the measuring instrument of the current as-is, as-operated heat load. The ~20BTU per foot of conditioned floor area is probably at least a 25% overstatement of reality of the existing house, and could easily be a 35-40% overstatment of what your house could be with $5000 of building upgrade.

How does 3 tons rather than 5 tons sound to you? Think it might affect the quote? It won't turn a $30K quote into $20K, but under $25K is pretty likely.

A better WAG: Assume in a "normal" year you use 3500 liters of oil The source-fuel BTUs of 3500 liters of oil is about 127.7 million BTUs. With an 85% burner it delivers about 108 MBTU into the house (the rest went up the flue.)

The September through May heating degree days (base 18C/65F) is about 4400C/7920F. (see: http://www.theweathernetwork.com/statistics/degreedays/cl6155790/caon0103 ) Since the contractors heat calc use degrees, F, lets do it that way. That's 108MBTU in 7920 HDD is 13,636 BTU per Fahrenheit HDD. With 24 hours in a day that's about 568 BTU/degree-hour, base 65F.

The base-temp is the outside temp at which both the heating and cooling loads are approximately zero, so the heat load will rise linearly with degrees below that temp. Using -6F for an outside design temp that's 71F heating-degrees.

So at 71F outside heating degrees, at 568 BTU per degree hour, the heat load at -6F is 71F x 568, or 40,328 BTU/hr.

If you have real fuel use against real heating degree days you can come a bit closer, but that 40K number feels pretty real to me, and jives well with real-world experience on that contruction type. But I'm just sitting here 1500km away, with only the data and WAGs you've used about fuel use that see on my screen. If your 4000 liters/year WAG is more realistic that number only rises to about 46K, not the 63K that the contractor came up with.

And the fuel use calc is really an upper bound, since any oil-fired boiler would likely be oversized, and not hit it's real numbers, and air-handler driven infiltration on a hot air furnace can be several times that 8K they calculated for the ventilation loss, and unless you tuned up the burner at least every other year it's not hitting 85% either. Knock at least 5% off the fuel use calculation, 15-20% if you're heating your potable hot water with oil.

That's a big difference- a difference that YOU would be paying for, which is why you really need to nail this down first.

4000 liters should be more realistic, last years numbers in the area across the border from us were 32% less the 10 years average, a 4 ton system also sounds reasonable to me in your climate, usage numbers is the best data you have. 4000 liters put you in the high 40KBTU range, close to 50 KBTU (I actually get 50400). Given your relative high electrical rate, you might want to be a bit conservative for supplement heat. Depth does not matter so much, lenght does. 180ft/ton loops should serve you well. You are running not much risk, but your system appears to be oversized by about a ton given your actual usage data. If your consumption data also includes your hot water usage, your actual heating is lower, depending on your hot water usage. How many people in the household? Reconsider the desuperheater. If you can make 60% of your hot water via the geosystem, why wouldn't you. Especially if you are on oil?

PS: the talk about drill hole depth by the WF sales guy is BS, the TZ22 is a substandard unit as mentioned above, which I would not put in.

Given our cheap electricity here, TZ22's are quite popular as they are inexpensive (relatively) and only cost (in my AO) ~$30-$50/yr more to operate depending on the home's requirement. I would rather that folks that are stretching their budget picked one of those over propane. Where folks are paying double digits for kwh's it begins to make sense to buy a more expensive unit.
My customers are offered higher efficiency models as well, not all choose to pay thousands extra to save $50/yr.

Wow, thanks for the input.

I really appreciate it. I am going to challenge my Contractor.

I'm pretty sure these guys oversize these systems to cover their own asses. To be honest as I am nervous about it all, i don't mind paying a couple thousand extra loonies for peace of mind - yes i know i will probably get beaten over saying that.... Its just that in my nightmares I end up paying $$$$$$ for something that just wont pass muster.

Anyway, lets see what the Contractor says......

Regarding my basement - it is finished- so for the most part there is drywall with pink insulation behind it and a vapour barrier.. however in the furnace room and the water pump room it is just exposed blocks on the outside. I don't have heat blowing into those rooms however. I quite like them exposed so i can see if there are any leaks etc. but if i'm hemorrhaging heat through these spots i may reconsider.

Adrian

Doc has mentioned before that within 60 miles of his shop there are huge differences in temps for certain areas vs closest major city weather average. Local guys know this, free on line advisors don't.
I was grateful this year a DIY'er informed me that he had significantly colder weater than the closest major city (Traverse City). Indeed I found his HDD's were closer to Kinloch AF base in Michigans U.P.

Caledon weather patterns are similar to Albany, NY. -6F design temp, but I guess your deep ground is not 46F anymore (old data), but more closer to 50F.

I regards to the TT22, Joe is right, different markets for electricity dictate different strategies. The TT22 3ton has a COP of 4.0/3.6 versus the 4.8/4.4, which to me means that the TT22 is about 20% less efficient. In our area, that translates to $200 annually (roughly), certainly different scenario in Joe's market. But I guess when a customer spends around $30K to increase efficiency, like our Candian friend here, would I render the system 20% less efficient to save a couple thousand $$$? Again, it depends on the electricity rates, and upfront costs versus ROI.

docjenser writes: "4000 liters put you in the high 40KBTU range, close to 50 KBTU (I actually get 50400)."

Show your math. 

#2 fuel oil runs between 136-140,000BTU/gallon I usually pick 138K, and 85% efficiency for estimation purposes, which delivers a net-heat of  117,300 BTU//gallon.  (Some d.o.e. net estimates use 115K as a "typ" number, but let's assume this is a tuned up fairly-new burner, and there is NO duct leakage, assumptions which already add some margin.) 

4000 liters is about 1056 gallons. 

At 7920 heating degree-days is .133 gallons/degree-day, or 15,640 BTU/degree-day. 

Per degree hour that's 652 BTU/F-hr.

Times 71F degrees (base 65F outdoor temp) you get 46,292 BTU/hr heat load @ -6F.

The heating cooling/balance point really IS going to be about 65F, not the indoor conditioned temp, if that's what you were using. (72F - -6= 78F, 652 x 78F= 50,856.)  You have to work from the balance point delta, not the delta between inside and outside temps.  With a 72F interior the heating/cooling outdoor balance temp might move up a degree, maybe even two, but not all the way up to 72F unless the power is off an nobody is at home. If the balance point is assumed to be some other temp, you have to adjust your heating degree days to that temp, and not use 65F at all or your math will be off.

If you use base 65F for the heating degree days, then turn around and use the conditioned indoor temp against the 99% outdoor design temp on a fuel-use/HDD calc, youhave about a 10% pinky pushing up the heat load scale.   (...10% here, 10% there... DON'T DO THAT!)

The heat load at an outdoor temp 68F is NEGATIVE with a 72F indoor temp in any REAL house, and is usually negative or zero at 66F outdoor temps in 2x6 w/double-pane houses like his.

The convention to use 65F is based on the measured average balance point of typical 1950s insulation levels and a 70F interior temp.  Better insulated houses can use a slightly lower base (superinsulated houses often balance at 45-50F depending on occupancy and plug loads), but for this house you'd only do that if you're trying to narrow it down to the last 1-2%. A presumption of linear-loading with temperature below 65F is going to be good enough, and if anything, may shoot 1% to the high side at the 99% condition.

Without anything better than WAG on the fuel use/HDD it's somewhat academic, still using a methodology that won't skew the result is important.

And even if it's starting out at ~46K, rare is the house where whittleing that down a half ton or more to under 40K wouldn't be cost-effective against the up-front costs of bigger geo.  (If it's been blower door tested and all the insulation gaps & easy-to-fix air leakage taken care of, maybe.)   Insulating an uninsulated foundation would take about a ton (sometimes more) off the heat load of 46KBTU/hr house, and make the place less mold-prone and more comfortable in the process.

Indeed I used the indoor conditioned temp of 72F. The reason for that is the fact that it took 1056 gallons to heat the house up to 72F, not 65F. If you are going of usage data, the internal and solar gains are already included and accounted for. It took an additional 1056 gallons (in addition to internal gains) to get to 72F, not 65F. You don't have the internal gain BTUs to add to the ones you put in the house, they are already included.

A substantial extra margin likely exists in the 85% efficiency assumed for the oil burner. That is a typical value for a well-tuned burner, but in actual operation is only reached after 10+ minutes continuous firing - that's when the efficiency is calculated during annual tune-ups. There are many many hours per day when the furnace just sits and loses residual heat up the stack, heat that has to be made up during the next firing. The situation is similar to comparing a car's rated highway mileage to real world driving results.

Then there is the fact that the oil burner uses combustion air that likely originates in the home.

Definitely consider insulating any exposed basement walls.

As others have said, invest in a comprehensive energy audit including blower door and infrared thermography and go after the low hanging fruit, particularly with respect to air sealing. A good energy auditor / rater should be able to do a good load calc based on local conditions (micro climate) That individual should be able to identify the better contractors as well. Auditors / raters see a lot of poor installation work and know the town clowns.

Reconsider the desuper - place its dedicated preheat tank near the geo unit and pipe to the existing water heater. Ignore worrywarts concerned about desuper stealing needed system capacity.

Knocking a ton off the load should result in a system cheaper to install and more efficient / quiet to operate. In any product line, the 3-4 ton models tend to be more efficient than the 5, 6 ton models.

Don't be afraid of 50-100 hours of strip heat use in lieu of buying an extra ton of capacity...do the math or have it done by a disinterested party. Contractors whose interests lie in selling extra tonnage may not be the best source of such a comparison.

Doc, Joe, and Dana are better at heating than I, and their quibbling over minor disagreements is enlightening and entertaining, free of charge!

The problem with using the actual usage data is that it has no safety margins anymore, like a manual J would have. So the 85% efficiency is my safety margin. It also does not forgive that the homeowner forgot about one of the fuel deliveries, or has kept his thermostat at 68F and now puts it at 72F. Or his successor in the house now keeps it at 72F, or 74F. So common sense is always needed as a reality check.
So a 4200 sqf house, 32 years old, in 7900 heating degree climate with -6F design temp is not a 3 ton house. 4 ton I can see if the house is very efficient, supported by the usage data.
Glad to entertain Curt a bit!

Posted By docjenser on 08 Mar 2013 01:24 AM
Indeed I used the indoor conditioned temp of 72F. The reason for that is the fact that it took 1056 gallons to heat the house up to 72F, not 65F. If you are going of usage data, the internal and solar gains are already included and accounted for. It took an additional 1056 gallons (in addition to internal gains) to get to 72F, not 65F. You don't have the internal gain BTUs to add to the ones you put in the house, they are already included.

That's just bad modeling.

If you're using 65F as the zero point for one linear interpolation (the HDD base), then move it to 72F, you've completely skewed the model.  It didn't take an 1056 gallons to get to 72F- it took 1056 gallons to reach the base-temp balance point where heat load=0 with an interior temp of 72F. That temp in most houses will be within a degree or so of 65F, which is why it's become the standard base temp for heating & cooling energy use in single-family houses.  (In larger mult-unit buildings it's usually more appropriate to use 55F as a base temp.)

Heat loads are approximately linear below the balance point (the zero-load outdoor temp) which is where the internal gains are autmotically included.   If the presumption is that the loads grow linearly below 72F you are explicitly NOT including the internal gains, and setting the zero at 72F.  This is not an insignificant error when projected down to -6F.  Using 72F as the zero point is correct if doing a Manual-J that also accounts for the internal gains as separate entries in the model, but is just bonkers when deriving it from fuel use against 65F HDD. Use a different base temp if you think it's dramatically different from 65F, if you like, but don't mix zero points in the mode or you will get paradoxical results and a crossover point.

The linearity is only approximate, since the R-value of the pink fluff in the walls and attic changes with outdoor temp. In a -6F attic the effective R-value will be slightly lower it's rated performance, but in the walls it will be higher.

Posted By docjenser on 10 Mar 2013 11:10 PM
The problem with using the actual usage data is that it has no safety margins anymore, like a manual J would have. So the 85% efficiency is my safety margin. It also does not forgive that the homeowner forgot about one of the fuel deliveries, or has kept his thermostat at 68F and now puts it at 72F. Or his successor in the house now keeps it at 72F, or 74F. So common sense is always needed as a reality check.
So a 4200 sqf house, 32 years old, in 7900 heating degree climate with -6F design temp is not a 3 ton house. 4 ton I can see if the house is very efficient, supported by the usage data.
Glad to entertain Curt a bit!

There is inherent margin built into the assumptions that the oil burner really runs at it's steady-state thermal efficiency while burning the entire 1056 gallons of fuel, which it very clearly does not.  As-installed, oil boilers are typically 3x oversized or more for houses that size, and anything over 1.7x oversizing pushes it a bit over the left side cliff on the regression curves.  With typical dumb-control high mass boilers, even at 2x oversizing the won't quite make 80% efficiency, and ~78% would be a more appropriate "as used" AFUE to assign for a mid-80s AFUE boiler, according to people who know how to measure & model stuff correctly.  (< see table 3.  The regression curves for each system can be found in the appendices at the end, if you want to estimate what 5x oversizing does.)

Hot air furnaces have somewhat different, flatter curves on the furnaces themselves, but unlike hydronic boilers they take huge efficiency hits on distribution, from duct leakage and air-handler driven infiltration factors. Even with reasonably sealed & insulated ducts those factors add up to ~10% of the total, and in ducts with out mastic (or FSK tape in good condition) sealed duct joints and seams, with NO duct insulation, the hit is 15%+.  (I can dig up credible studies for those numbers too, if you like.  A lot of good work was behind duct-leakage and oversizing limits in the California Title 24 regulations.)

Common sense? Really? Common sense says that you pay the big bucks to get the efficiency for how YOU are going to run it, not your successor.  If the next owner wants to keep it 78F in there, let them! They may pay for the additional power used in the auxilliary heating, but it's insane folly to buy the future owner and extra ton of GSHP on the theory that they just MIGHT run it a lot warmer than you do. (In my book even 72F feels too warm for comfort in a tight house with 30-35%RH interior humidity, but mi esposa begs to differ. :-) )

The only concession I make on design set points is if someone says this will be their forever house we look at slightly higher temps for the days of blood thinners and fixed income.

Posted By joe.ami on 11 Mar 2013 06:03 PM
The only concession I make on design set points is if someone says this will be their forever house we look at slightly higher temps for the days of blood thinners and fixed income.

Moving to a warmer, cheaper country for retirement is the tried & true solution to those problems, eh?    (As long as the health care situation isn't compromised, that is.)

The whole oversizing loss of efficiency for fossil-burners is why I prefer to look at mid-winter billing (when the duty cycle is at it's highest) rather than annualized fuel use.  Having the K-factor stamped on the oil billing with a few oil-fills to compare to one another makes it dead-easy.  Mid-winter gas bills that also have the binned hourly mean temperature and a daily fuel use is similarly simple, and quite consistent in houses that are continuously occupied. 

But everything is subject to gross errors if you're asleep at the wheel.  Fuel-use analysis methods fail pretty badly when the occupants had set the temp way back while they took a mid-winter 2-week trip to the Amazon or Aspen, and neglect to mention that teeny little fact.   Factoring in multiple systems can also be tough to chase down with any accuracy: "Oh yeah, we burned about 5 cords in the woodstove last year."  I=B=R & Manual-J methods will always have a place at the system-sizing table, if used properly.