Im not trying to instigate a disagreement or anything--but I find this discussion really interesting:

1. On one hand I have precise info about how many therms I've burned and what the temp was each day and can calculate BTU's used per degree day per hour;
   
2. On the other hand I know what materials are used in my house and what their thermal characteristics are and can calculate a heat load;

As I noted in a previous post I got 2 substantially different numbers using method 1.

For giggles I downloaded a trial of a heat load application and started putting numbers in for 1 zone (first floor living space) with 4 rooms. Based on this technique I got a head load of roughly 36KBTU (which more or less matches one of the numbers I got using method 1).

Since Im not a professional and have never done a heat load before I have no idea if I used the application correctly.

Having said that I do know that I measured the walls, windows and doors correctly and entered proper R values for various materials. Given that it's interesting that the number produced by this app was really close to one of the numbers generated by calculating past usage.

Lastly, I just want to add that Im doing all this stuff so I can bet a better understanding of the underlying principles involved in designing a radiant system...so I dont get taken to the cleaner when I hire someone to do all or part of this job. I'll hire someone to do an actual load calc. I don't have any confidence in the numbers I've come up with...but if a professional doing these calcs ends up with similar figures I'll know that I've learned something ;)

thx
PP

oh heck, we disagree all the time on all kinds of things, no worries! better than even chance I'm full of a distasteful substance some days as well.

but since you have such a fine resolution with your data PP, I wonder... for a similar time of day with a similar temperature profile, do you see similar usage, or great variance? say a morning sample, afternoon sample, evening sample, mid-night sample?

I'd love to see what happens to numbers on that scale. of course it takes work to dig through the data like that too, just curious.

oh, and the big gotcha with load calcs is usually infiltration, as long as you geometric calcs are good and you are not using some 'mass enhanced' r-value or 'fresh' value for closed cell foam. and usually people dramatically overestimate infiltration.

Aw, that'd be too EASY, now wouldn't it?!

But in all seriousness, having never seen numbers on an existing conventional home that varied anywhere NEAR as wildly as what ppmax came up with I suspect there's something else going on- like bad arithmetic or incorrect data, an unoccupied house heated to only 50F for the low-use month or something along those lines. But if the house indeed has a lot of south facing glass I wouldn't expect a boiler-efficiency measurement methodology to hit nearly as close as a heat load calc.

If weekly data periods are way too short, daily/hourly fuel use numbers are going to have even higher error issues unless the boiler is nearly-perfectly matched mod con and the house temp is kept constant, no setbacks, etc.  High sunshine will only have a significant affect on the heat loss of the south-facing walls, but heat gained through south windows & skylights can be significant during mid-day- enough to make large offsets to measurements on an hourly basis for those hours, but offsetting but a small fraction of the daily heat load in conventional housing.  It would be an interesting thing to instrument though- it's pretty common to have the exterior surface of S-facing siding in full sun hit temps 50F+ above ambient on sunny winter days, effectively nulling the heat loss out of that side of the structure for a few hours.

>>for a similar time of day with a similar temperature profile, do you see similar usage, or great variance?

I dont have access to my data now but I'll post a table for every month in 2009. Just for the record I get a geeky thrill out of doing this stuff ;)

>>High sunshine will only have a significant affect on the heat loss of the south-facing walls...
and
>>But in all seriousness, having never seen numbers on an existing conventional home that varied anywhere NEAR as wildly as what ppmax came up with I suspect there's something else going on- like bad arithmetic or incorrect data, an unoccupied house heated to only 50F for the low-use month or something along those lines. But if the house indeed has a lot of south facing glass I wouldn't expect a boiler-efficiency measurement methodology to hit nearly as close as a heat load calc.

Funny you should mention that: the roofline of our house runs N/S and almost the entire south facing wall of our downstairs gets massive sun on clear days. The south wall is basically 25 feet wide with 4 2.6X7.25Ft double paned glass windows (9Ft ceilings). Im sure that heats things up nice and good during the day. After all, our FHA heat only runs during darkness.

I also want to mention--as you'll see when I post the table--that I got some weird numbers during the summer...so one can't rule out bad arithmetic or bad data.

Lastly, WRT my 38KBTU vs. 60KBTU results...as far as I can tell that's really only a 20% difference if we fuzzy up the numbers a bit due to rounding errors, imprecise data, etc.

Thanks again for all the discussion on this--it really is helpful to understanding the magic behind all this stuff.

PP

Any large thermal imputs would other than the boiler would have to be factored into that sort of calc, so a house with ~75-80 square feet of S-facing glass a location with big variations in month-to-month sune could throw it off a bit, but I suspect there's more to it than that. My house has less than half that much S-facing glass ( including the skylights) and post-2:30PM winter shading to boot. But other houses I've analyzed the fuel consumption with glazing comparable to or even greater than yours don't measure out to a 40KBTU/hr boiler-calculated design-day heat load vs. 60KBTU/hr from one month to the next. (60 is 150% of 40. 40 is 67% of 60, so either way you cut it it's more than a 30% difference.)

Got a wood stove?

>>Lastly, WRT my 38KBTU vs. 60KBTU results...as far as I can tell that's really only a 20% difference if we fuzzy up the numbers a bit due to rounding errors, imprecise data, etc.

>>40KBTU/hr boiler-calculated design-day heat load vs. 60KBTU/hr from one month to the next. (60 is 150% of 40. 40 is 67% of 60, so either way you cut it it's more than a 30% difference.)

Clonk! Im an idiot...I dont know what I was thinking...it's indeed a 50% diff.

No wood stove on the premises. We have a nat gas fired fireplace (sealed unit) with fan that we seldom/never use.

Will hopefully post the data later tonight...

PP

On closer inspection of the results it seems like something is wonky with the formula I used. Here's the formula (based upon an example posted previously):
therms used(month)/degree-days(total for month)*100,000*heater efficiency/24(hours)

...Upon 2nd look shouldn't I be dividing by (days in a month)*24(hours)??


Substituting real numbers for Jan 09:
168therms(month)/794degree-days(month)*100,000*.91(FHA furnace)/24 = 802BTU/degree-hour
Ratio of therms to degree-days: 168/794 = .21
For 75 degree-days this = 60150BTU's

Here's Dec 09:
152therms(month)/1146degree-days(month)*100,000*.91(FHA furnace)/24 = 502BTU/degree-hour
Ratio of therms to degree-days: 152/1146 = .13
For 75 degree-days this = 37650BTU's

The weird part comes during June 09:
34therms(month)/95degree-days(month)*100,000*.91(FHA furnace)/24 = 1357BTU/degree-hour
Ratio of therms to degree-days: 34/95 = .35
For 75 degree-days this = 103125BTU's

The ratio of therms to degree-days ultimately drives these results--and as the above show it's crazy that the ratio of therms to degree days is *higher* during a month with fewer degree-days....and shows that therms to degree days doesnt scale linearly.

One minor source of error is that the therms number is actually therms per billing period. In Jan 09 there were 31 days of degree days but 35 billing days...so there is some mismatch here but it doesn't matter much.

Posting data at this point would probably be useless; my formula needs work to account for the non-linear relationship between therms and degree-days. Hiring someone to do a heat load calc based upon Ft^2 and materials is probably the way to go.

PP

Now you have the answer!

Fasinating speculation. But the math was done a few decades ago. See Manual "J".

Takes me about an hour to get within 3% for an envelope; longer for a room by room with a program that accounts for the lower heat loads common when floors are used as emitters.

ppmax: You'd need to match the HDDs of the exact reading dates of the billing period to come anywhere near to reality. A 4 day cold snap or warm spell missed by your calendar shift (or calendar dilation) could be huge. 35 days vs. 31 days of billing posted against a 30-31 day month is more than a 12% error even if the weather every day were IDENTICAL. Even an early morning meter-reading vs. a late afternoon reading results in a ~1% error over a 31 day period.

And if it's a 2-stage condensing furnace, fuggedaboudit- the actual efficiency will vary hugely depending on load, setbacks, etc.

And June numbers? At the trailing beginnings/ends of the heating season the error would be large, and fuel use might be dominated with hot water use (assuming the same fuel is used.)

Ya gotta have reasonable methods if yer gonna measure stuff.

Morgan: An hour? It takes me about 3 minutes looking at my gas bill (which conveniently posts the average daily temp and average daily fuel use for the billing period), and that's using lipstick on the bathroom mirror, not a calculator. :-)

Lipstick?

I'm in the wrong bathroom...hehehee

A proper heat load takes into account weather-both mean and extreme- and allows the designer to match building contruction with weather to come to an outdoor design temperature. From there radiation can be chosen and from this information a design water temperature derived.

As for weather swings etc., condensing boilers with outdoor reset: all done.

What does all the long math do for us when it has been done by ASHRAE years ago, built into the software (specific to radiant floors), practiced in the feel and proven by experience?

DIY design; always the first mistake.

...or maybe it's taking an hour because you have to set up the blower door to MEASURE the air infiltration in order to get it TRULY within 3%??

If you have the software, use it.

But if you don't, and want a sanity check on the "professional" giving you a quote on a replacement boiler/system, using the old system to measure the load is remarkably close if you use reasonable assumptions and do the arithmetic correctly. In my small and non-scientific survey of fairly recently installed replacement boilers near me, most were over 2x oversized, and at least 3 were over 3x.

A couple of years ago I heard/watched a number of local "heating & plumbing" guys eyeball a place and say something along the lines of, "OK, that's about 2200 square feet, times 35BTUs per square foot, you need at least 77K of output, so you'll need a 100K boiler to be on the safe side", on a condo with an actual design-day heat load of around 30K- manual-J done by your's-truly, verified with the fuel bills on the prior system.

I couldn't agree more.

Thus I tell my clients to look for a contractor that can produce a computor generated heat load, based on Manual 'J' for residential. This simple standard eliminates 9 out of 10 "professionals". Fuels bills are good for verification but lack enough information to design radiant heating systems.

Actually 2% on the last one, if I take the blower door test for granted hehee.

Fair enough about the data I used. I hear you both about hiring a pro to run a load using software based upon Manual-J--which I'll do before any purchases are made.

However, I'd at least like to make sure that Im running the numbers in the same way that Dana1 does. Please let me know if there is anything wrong with this procedure:

• I'll use 1 month therms and avg temp plucked from my gas bill;
• I'll calculate HDD during this month by multiplying number of billing days * (65 - avg. daily temp)
• I'll use these figs in this formula: therms/HDD*100,000*.91/24 = BTU/HDD-Hour
• BTU/HDD-Hour * 75degree-days (97% coldest day) = BTU/Hour output required to heat the place on the coldest day

Does that sound right? Thanks for the discussion and help guys--

thx
PP

ppmax- that's the right arithmetic, so long as you're confident of the average daily temp data for the exact days between meter readings so that you have good correlation with HDD and fuel use. If you had many days during the billing period with high temps over 65F you can also be skewing the calc. November-March data is dead-consistent for me, but October& April data less so (but still well-within 10% of mid-winter numbers). May-September I don't even bother, since hot water use during that period becomes a much larger fraction of the fuel bill (if not the whole shebang.)

Morgan- this was never intended to be used to design radiant heating, just a quick & dirty method of getting a reasonably accurate measure of the whole-house heat loss to put a stake in the ground for boiler sizing. For that it seems to work very well in my area. (In my nieces' condo I made 'em drop to a 60K mid-efficiency boiler, & 30 gallon indirect against their dire warnings. So far she's yet to get cold or run out of hot water. I would have gone smaller, but there were local availability issues on 2-plate boilers at the time.)

I understand your method, concur on the conclusion and applaud your personal success as your neice will surely save money and be more comfortable. Oversizing equipment is one of my greatest frustrations concerning the HVAC business.

I fear "down and dirty" is as close as our hapless novice will get, but if it's good enough for him...

I'll accept being labeled a "hapless novice..."

... but I'm not going to base a decision off this number. Im doing this so I can get a down and dirty number and understand how it was derived. FWIW Im also reading up on Manual J and have downloaded some sample spreadsheets and load calc software (RadiantWorks Pro). I dont imagine I'll be able to calculate an accurate number--but I'll be familiar with the process of using these tools and hope to know enough to call BS on someone who wants to sell me an oversized boiler.

Present company excepted, my two gripes when hiring contractors are:

• Cant keep appointments or stick to schedules
• Cant reasonably defend how they arrived at a particular decision or answer

When I do hire someone to do a heat load calc I'll know what questions to ask and will know the strengths/weaknesses of each particular method of arriving at this number. Basically I dont want to be oversold and I dont want to end up with a new heating system that's worse than the one I have today. It's tough trying to be an educated consumer--especially with service providers who treat their trade as a dark art. [EDIT: Just wanted to clarify Im not lumping you nice folks in this category--you've been really helpful.]

PP

I'm proud of you already.