Degree Day Math Question

Constantin

Gentlemen,

Let's say my contractor and I use separate heat loss programs and come to the conclusion that my house will lose about 88kBTU on a -15*F design day (i.e. a 80 degree-day) in Boston, MA (which has 5630 heating DD/year). Below are my calculations. I started by calculating the BTUs lost per degree day:

88kBTU/h x 24h/D / 80*F = 26.4kBTU/DD

Then, I multiply the result by the number of degree days per year:

26.4 kBTU/DD x 5630 DD/year = 148 MBTU/year net heat loss

since we use oil at about 86% efficiency, this turns into

148 MBTU/year / (140kBTU/gallon x 86%) = 1234 Gallons/year

Does this seem reasonable? The reason I ask is that a certain program that allows fuel calculations based on heat loss and HDD data came to a very different conclusion than I did re: the amount of fuel I can expect to use.

I hope that the program is right and that I'm wrong, as it would save me roughly ½ the fuel I expect to need based on my own calculations. As the author of the program has not replied yet, I figured I would run my calcs by the fine folk on the Wall. Perhaps you will see something I missed!

Robert Wright

BTU Puzzler

Constatin

I am struggling with the same problem trying to figure whether to convert my steam system to hot water. I have done some calculations and would like to talk with you. If you are interested please e-mail me a phone number I can can reach you at.

Thank you

Bob

Constantin

Two things...

Your e-mail address bounces. I presume it's because it's missing a .net, .com or .whatever. Please send me an e-mail and I'll reply.

Secondly, the author of the program replied and told me about an ASHRAE correction factor that is usually applied to account for the amount of actual heating and cooling. The correction factor is 0.65 which corresponds with the discrepancy I noted.

This is very exciting news. The thought that we might only need 65% of the fuel I thought we did is making me feel better and better about the insulation, etc. investments we made! Yay! Now, all I have to do is see how much fuel we actually use once the house has been finished...

Carl PE

Your math looks right.

Perhaps they use a different btu value for oil, or a different efficiency?

Constantin

Not quite...

... see further below. Apparently, ASHRAE derates the degree days as far as their fuel calcs is concerned to 65% to account for solar gain, etc. That accounted for the discrepancy.

I'm very excited about the news, as a 35% fuel usage reduction w/o having having to do anything is welcome news indeed!

jp_2

what?

how can that work across the board? we had ONE, yes, ONE sunny day the last 40! also HDD do not acount for wind.

seems you are counting your chickens ......

Mike T., Swampeast MO

Interesting that you said this 35% reduction factor was used for both heating and cooling.

Perhaps such reflects the "average" overstatement of Manual J calculations.

Constantin

Not counting chickens at all...

This is only done as far as the degree day data to actual fuel used calculation is concerned. As with most fudge factors, even officially endorsed ones, you could debate endlessly re: what is right vs. what is wrong.

To clarify: This 35% derating has nothing to do with the size requirements of your heating plant. That plant still has to meet maximum heat loss on design days, or you'll be cold.

Allegedly, this derating only has to do with solar gain and other factors that are expected to reduce your actual fuel requirements. Naturally, the actual factor is a function of house construction, exposure, location, etc.

Only time will tell in our case how much we could derate the actual fuel requirements as a function of Design-Day heat loss / HDD / degree day predictions.

michael_15

linear correlation: DD vs. BTU

Is there really a linear relationship between degree days and BTU used? That's like saying you'd use as much energy if it's 55 degrees every day for a month (300 DDs) as you would for it being 5 degrees for 5 days (also 300 DDs).

If it were that easy, wouldn't people (or perhaps just geeks like me) be regularly quoting their home in terms of degree days? Then, you'd end up with things like,

"Oh, at 85% efficiency and where oil is nowadays, my heating costs are 30 cents per degree day.", or

"Boy, with gas prices having spiked I'm over 35 cents per degree day!"

Of course, I could be wrong -- maybe it really IS that simple. Then, we could come up with approximations like "oh, for a house that size, I'd expect your costs to be 25 cents per degree day" and "you're paying 50 cents per degree day in that tiny house? Something must be wrong with your system, or you don't have any insulation!"

If it could be simplified like that, I could also determine almost immediately if changes I make meaningful improvements to my energy bill. I could determine the payback and cost coverage rate for all that pipe insulation and caulking I did around the house, for example by staring at my gas meter and determining usage and then normalizing to a per-degree-day basis. Now, that would be really cool.

-Michael

Constantin

It's a simplicifcation, but it's close enough.

OIl companies, gas companies, etc. keep a very close eye on degree days to predict demand at any given time. Things like water heaters are very predictable, there is a spike in the morning and the evening, the rest of the day, they just loaf, a nice base load.

Heating systems, AC, are weather-driven. Any good oil company will figure out your degree-day losses, then schedule deliveries only when they expect your tank to be ¼ full. That saves on deliveries and hence lowers cost for everyone.

Your idea that people could talk in terms of degree days is entirely possible, yet improbable. Most people aren't that technical, they usually simply compare fuel costs between neighbors... If you really wanted to get into this, you wouldn't just account for degree days but sq ft as well. Only then do you have a "objective" comparison between houses.

To get to that magical number, just divide the BTU/DD by the sq ft. of the structure. The resultant number is usually quite low. By my calculations, our house makes it to about 5. Allegedly, very good houses are around 3-4, average construction is around 10. Older, drafty houses can be higher... Considering that we started at something like 17, we seem to have done well.

I would think that monitoring the gas meter on a monthly basis should give you your answers fairly quickly. The only trick is to find good actual degree-day data for your part of the world.

michael_15

An epiphany

Okay, so contrary to my prior post, the relationship between BTU and HDD for my home is much stronger than I initially presumed. I've went and dug up five dated gas meter readings since November 1 of 2004 (I've lived at my place less than a year, so only one winter's worth of data). This allows me to have four time periods for which the total gas usage (therms) is known.

Comparing to my summer usage and making some guesses, I've decided that the hot water heater and gas dryer eat up approximately 0.25 therms/day on average (which sounds high, but oh well). Subtracting this from my general therm usage, I've approximated the "heating therms" used by the house in each time period.

http://www.erh.noaa.gov/box/dailystns.shtml has, for Boston (where I am), a daily degree day listing, so I pulled up the last several months of data to determine the total degree days in each time period.

A simple least squares regression on this left me with the relationship Therms = 0.1242 * Degree Days. Now I only have four points right now, but here's the thing -- I had an R^2 correlation coefficient of 0.9982. I thought that was pretty darn surprising.

In terms of BTU/(DD*SqFt), I'm at around 14. (The heated area is only aroud 900 square feet). I guess that home inspector was right when he commented that I didn't appear to have a speck of insulation anywhere in the house.

No wonder my fuel bills are so darned high.

-Michael

Carl PE

I'm still missing something..

I've reviewed the ASHRAE chapters on heat loads and energy modeling, and I haven't seen any "correction equation" like they're describing. Honestly, I've never heard of it.

Perhaps I'm not looking in the right place.

I understand that it's not as simple as multiplying the max load by DD. There will be adjustments for solar gain that's not included in the heat load, thermal storage in the building mass, etc.., but I'm suspicious of a 35% reduction in usage. That's a big number.

Maybe Mike T is on to something..

Constantin

I don't know myself, Carl...

... I'm simply reporting what the author of the program sent back to me. If you like, I'll ask him for chapter and verse, as any correction factor should be well understood...

Constantin

A mere R² of 99.8%?

Come on Michael, you gotta do better than that! :-) ...I guess it's a pretty good correlation after all.

I'd suggest in-wall foaming, combined with foaming the attic floor (or rafters), and the basement ceiling at the very least. With Icynene, you get a pretty good R-factor, no infiltration and a pretty impermeable vapor barrier to boot. Just be sure to have a good company apply it, i.e. one with a thermal camera to verify their work filling all the bays.

Dale Pickard

Balance Point

Hello Constantin,

I could write a lot on this but I'll try not to.

Degree days are measured used a "base 65". By definition, A degree day is accrued in a given climate when the ambient temperature is 1 degree lower than 65 for a 24 hour period. The "balance point" that is being used is 65. The underlying assumption is that for a particular kind of construction, (with respect to thermal properties) typical solar gains and internal gains in the building will be sufficient to raise the inside temperature at least 5 above ambient to 70.

What is important about the particular kind of construction is it's thermal performance in terms of it's insulation values, it's air infiltration values and it's ability to see and utilize solar gains. Calculations based on base 65 DD data may ring true for typical 1950's style frame construction, Rectangular buildings, 4" - R11 walls, R 20 ceilings, medium high air infiltration, single glazing, little or no solar exposure, little or no provision for solar gains.

However, calculations based on base 65 DD data for modern construction will wildly overestimate fuel use.
Why? Because modern buildings do not incur a heating load at 65 outside air temp. Their balance point is much lower.

Why? Because the same internal gains and solar gains now account for a larger fraction of a reduced annual heating load.

A modern well insulated building may have an actual balance point as low as 40 deg!! When you recalculate the annual heat loss based on the adjusted degree day base, you end up with a much smaller number.

This concept is really important when evaluating energy production/conservation options. For example, if one were to size the annual output of an active solar space heating system based on base 65 calculations, you will find yourself up against an impossible task. It will be real too, if the house really has that kind of thermal performance the solar system won't be as economically feasible. That's why we don't put solar space heating systems on leaky log homes, for example.

As you tighten up the envelop and lower the design load, you also lower the balance point, lowering annual losses and conservation options like solar systems etc start to reveal themselves, economically and practically.

How do you calculate the new balance point? Difficult; you have to evaluate the internal gains and solar gains and apply the average energy gains against the calculated AU (btu-hr/deg dt) of the building. There are algorithms that will re-calculate measured degree days based on a new balance point. Software like F-Chart uses these algorithms.

I doubt that is what your software is doing. I suspect the author is aware of all of this and is applying a gross correction factor. That's the problem with SW that you don't write yourself.

Another way to do it is to drop DD altogether and perform annual heat loss calculations on an hourly basis using TMY data (typical meteorological year) for one of the 239 TMY climates for which data is available. The annual hourly calcs will have to credit the building envelope with solar and internal gains on an hourly basis, which gives geeks a real opportunity to play SIMHEATING with the Sim family. They can bake bread and take showers and leave the door open and pay power bills........
Even if one were to do all this, the real world is a far more complicated place.

Later,

Dale

Kal Row

quick reality check...

right now with steam, your heating style has to be, to warm up or set back, the whole house at once, if you want to keep that style, then dont change a "thang"!!,

but if you want to zone it, STAT it, and time it to death, then you have to convert - i fail to see how dday calcs affect your decision -

"shteem" is one of the best and fastest BTU delivery systems we have, liquid water cant compete - however "zer iz noh vay to cohn-trrrohl zat shh-tuff"

- therein lies your choice - eff or control

-and yes i am sticking my neck out and saying that if you have a really good working steam system with all the radiation in plain sight (ie infra-red visible to all in room) - then you cant beat it's eff - except on a grade level slab - where in slab radiant wins the eff war - you guys can bang the math around till your fingers fall off - the reality is still the same –
someone i know converted to HW because of multi-apartment control, and now he is cold all the time – and his new boiler has more btu, but he did not increase the room radiation and thus cant deliver the BTUs (no i did not do the job, i walked away, cause i know what would happen) – if you convert to HW – i strongly suggest large panel radiators, like those from myson, runtal etc

another advantage of steam, is less freeze failure prone in a power outage, and if it’s gas, it draws so little power (just the gas valve), you can run it on a 600va computer UPS for a day or more

Dale Pickard

Balance Point for Boston

I've attached a pdf file that shows monthly base 65 dd and base 55 degree days. Check out the difference. A well insulated house will not call for heat at 55 deg in Boston. I'm sure that is our experience, eh?

Dale

Constantin

Thanks Dale!

Your insightful replies always bring a smile to my face. Your extensive solar experience shines through in a most obvious manner. The best I have currently to offer is looking at the results from the cooling side of things and making some guesstimates re: solar gain.

For example, our heat gain in the summertime is driven almost exclusively by the windows in our home - 42kBTU/h out of 66kBTU/h. The next biggest line item is people and walls... If I assume that just 1/2 of that energy is available in the wintertime, and that it'll benefit us just 6 hours of the day, that turns to about 126kBTU of free heat per day... almost a gallon of oil.

Even a gallon a day won't save us 35% in fuel costs, but it's a big first step in that direction. I need to educate myself re: F-chart, et. al. as it would be fun to compare the theoretical results to reality. Why else would I buy HDS, HVAC-Calc, etc.?

Lastly, IIRC in one of my Taunton books there was a description of a house in NH that is heated primarily via solar gain. Big tank in the basement, no heat required in the wintertime, etc. The coldest the house ever got was 40°F on the coldest of nights when it was howling -15°F outside. It can be done...

David_5

my results with your math

Worked out to 25% less fuel use than predicted by DD, and I use fuel for hot water also.

David

Dale Pickard

Thanks, Actually, I made all that up. ; -)

My business partner of 25 years, Bob Knebel PE, helped eluciate those relationships in a Master's thesis in ...... 1982, I think.

Dale

Mike T., Swampeast MO

When I suggested a 35% Manual-J overstatement, notice that I qualified it with "average".

As you know, when we calculate heat loss we don't include any sort of occupancy (human, appliance, lighting, etc.)gain, yet when we calculate cooling requirements we include significant occupancy gain.

The sun has nothing to do with either.

The wind however is built-in to Manual-J. I don't recall the number, but was told it assumes some fairly high level of wind arriving from all directions.

Even with very little in the way of occupancy, my house rarely needs any heat above 55° or so outside. It's old with windows similar to yours but with less effective (e.g. fiberglass and cellulose) insulation. While I do keep much of the house at a lower than "normal" temperature due to ongoing renovation, I rather suspect that the added occupancy load when everything is done will come rather close to offsetting the current lowered temps.

Right now I'm rather warm in my south-facing office. After numerous days of cloudy, cool weather with the space temp at 67° ±1½° it's now (at 6:10 p.m.) 73°. Outdoor temp (solar invluenced) peaked at about 65.5° with a true north high temp VERY spikey peak of around 55°. The ground floor that had been at 60° ±1° for days peaked at 65° and is still 64° @ 6:18 p.m. My worker and I both have the flu and we haven't worked down there all week so there is zero occupancy gain save the 3 6-watt compact fluorescents that light the stairway at night.

Leo G_99

C,

in plain english, how is a degree day described?

Leo G

Constantin

A heating degree-day is...

... usually defined as a 24 hour period over which the exterior temperature is 1 degree lower than 65°F. That temperature was chosen to reflect the balance point that Dale pointed out... the assumption being that home occupants are comfortable when their homes are at 70°F and that most homes only need heating once it's colder than 65°F on the outside.

However, a well-insulated home may have a lower balance point. Hence, the PDF that Dale included above that shows the difference in the number of heating degree-days for Boston whether you start at a 65°F or 55°F balance point. Obviously, a house that doesn't need to be heated until the exterior temperatures hit 55°F will require less heat than a house that has to start heating at 65°F exterior temperatures.

Thus, one day at 0°F is typically equivalent to 65 degree-days, as are two days at 32.5°F, etc.

Leo G_99

Thanx,

C, the murk has lifted a bit now.

Leo G

John Cockerill

BTU delivery

There is a lot to think about here. Are you heating with steam or Hot water, or hot air.???
If it is hot water what will your temperature ranges be??
If steam what are your pressure limits???
All of these factors will determin the deliverd BTU value to the thermostat. If you are not operating the system for smooth delivery of comfort, how do measure the waste and higher settings on the thermostat to make up for bad distribution. That is why most guy go a little higher on the powerof the heating unit. It is much easier to tame the slightly oversized boiler that beat the heat out of it when it falls short. Think about what you present boiler is doing and how much it runs in units of fuel per hour, on a cold day. That will give you some idea as to how oversized it is. Think then that it can hit zero every so often. Of course you can give the system "Exquisite heat" control when you finally decide your size, to keep it tame and comfy.

John Cockerill

conversion

If you were to be able to tame the steam system, you could be comfortable and save money. Then there is no need for the repiping. There are two ways to do that. First install an ounce pressuretrol, so you can control the cycling of the system and the limit of output. With the proper adjustment of pressure and differential, along with thermostats on radiators in newly insulated areas, you can drive it like a stick shift truck. Orrr you can give it "Exquisite Heat" control by varying the fire cycle times according actual thermostat demand. Tame your steam system and you can save a fortune and be very cozy.

Constantin

Allow me to quibble...

... in todays day and age, there is no good reason to oversize. We have fantastic contraptions at our hands to help us determine heat loss, flow requirements, etc. in ways that would make the dead men's head spin. They relied on large tables and simplifications that got as close as one could get to designing systems efficiently. To their credit, most steam systems keep chugging along much longer than the water systems that replaced them.

IMHO, we're just scraping at the tip of the ice-berg as people start to think about heating systems more holistically. If you look at the charts at ACEEE, it makes my head spin that a Vitodens can run on roughly 9x less electrical energy than a Burnham Opus, or 6x less than a Weil-Mc Ultra.

With any hope, we'll see a progression towards low-loss systems that are fed by TRVs and hence minimize waste while maximizing comfort. The europeans figured this out a long time ago, let's see how long it'll take to educate the US consumers to the virtues of efficient heat...

Also, Mr. Cockerill, you really ought to disclose that you manufacturer the "Exquisite Heat" product before gushing about it on the Wall. That way, the rest of us can apply the proper grain of salt when they read your posts. It may do everything you claim it does, yet, you should disclose your financial interests whenever you post about your product. Thanks.

Bob Morrison_3

Bin hours versus degree-days

Constantin,

I would estimate the annual fuel usage using the TMY-2 temperature bin weather data for Boston, and not use the heating degree days equation. The correction factor for the degree days equation (see ASHRAE Fundamentals 1989, p. 28.2) varies widely. HDD certainly is proportional to annual fuel usage, but individual cases differ widely.

The bin weather data approach allows the effects of temperature setback during unoccupied periods and is the best approach for a simple, steady-state UAdT type calculation. The bigger difficulty will be estimating expected infiltration, unless you've got blower-door data or are ventilation mechanically.

Let me know if you want a sample spreadsheet for this calculation.

Bob Morrison

Dale Pickard

Yes

Yes,

These days the hourly calculations are where it's at. With today's computing power, hourly calcs are straightforward.
Nothing will give you absolutely correct answers, These calculations are best used to provide answers to bracketed input variables.

Dale

Mike T., Swampeast MO

Constantin

What made you decide against FHVs? They should have been ideal with your Warmboard and very good with your oil-fired Viessmann. Probably two space heating pumps--maybe only one. Lots less whirring, purring and vibrating...

Constantin

I would love to see that

Please feel free to e-mail it to me. I'd love to compare the two results.

We will have blower door results sometime this spring. In the meantime, I'm trying to be vigilant about infiltration as new windows are installed throughout the house.

The house has two HRV/ERVs, both by Lifebreath (300TRV and 195TRV). The speed setting on the HRV/ERVs will be highly dependent on the natural infiltration we experience. Assuming neglible infiltration, the lowest speed setting would give us the 8 air changes per day that are recommended.

Constantin

I hang my head in shame...

... but it was, to an extent, driven by aesthetics. When I explained the possibility of having large white bumps in every room for exquisite comfort, the look on my wife's face nixed that possibility in the bud. I know that sounds hypocritical, given that the mantra in this home for the most part has been function over form... but there you go.

At the moment, the home is heated in five zones with the possibility of going to individual zone valves for many rooms in the future. However, the possibility of doing so seems somewhat remote right now, considering how comfortable the place is. I'm simply planning ahead for the possibilty, given the impact that irrational urges may have on any offspring I have yet to produce.

No Warmboard either... too hard to integrate into the rest of the home... with 3/8" ID Quicktrack throughout the place and balanced manifolds, there is very little jiggling required to get even heat distribution everywhere. The only place where larger ID tubing found a home was in the basement. One Zone...

Constantin

Dear Mr. Cockerill,

This thread has nothing to do with steam heat, nor with piping, nor with control. This thread is about extrapolating fuel usage based on degree-day data and calculated heat loss.

While your "Exquisite Heat" product may or may not work as described, I prefer my truck in automatic, which is something that the Vitoronic 200 on my Vitola can achieve. Besides adjusting the firing cycle, it also controls the 4-way valve on the radiant manifold, etc., a level of control that your controller is unlikely to achieve.

Please disclose in future threads that you are the manufacturer of the product you're promoting. It's good manners and the ethical thing to do. For example, the fine folk at Burnham, Buderus, Kitec, HTP, Viessmann, etc. can do this, so please follow their lead and add a note to every post.

jp_2

constantin???

you said:
For example, our heat gain in the summertime is driven almost exclusively by the windows in our home - 42kBTU/h out of 66kBTU/h. The next biggest line item is people and walls.

what output of "people" are you using here? show me some numbers

Constantin

This is where it gets fun...

... I'm using the HVAC-Calc defaults for bedrooms, assuming 2 people per bedroom. 5 bedrooms x 2 people = something like 5000BTUs/hr, right?

The infiltration may be too low, only a blower door test will tell us the whole picture in future. Ditto for HRV, etc. performance. Let me know what you think. The wrightsoft results from my contractor were very close, BTW.

Leo G_99

C,

I see there is no heatloss for people. What about if you've been out in the snow for a couple of hours, making snow angels?

~


Leo G

Constantin

I have to say...

... this is one of the idiosyncracies of heat loss estimation. On the one hand, BTU-wise we count all the people for the worst case scenario for cooling, but then don't add them back in when it comes to heating the place. Ditto WRT the free cooling that some homes experience via the earth around the basement.

No wonder that heat-loss programs usually overstate actual needs. As if it's relevant that the place doesn't hit 70°F on the coldest of nights when no-one is home to notice. Kinda like trees falling in the woods and all that.

Perhaps this is why someone on the Wall recommended subtracting 10% from the HVAC-Calc results as a starting point. Meanwhile, the author of the program recommends adhering to the cooling results as mantra while adding 15% as a safety margin to the heating needs.

Given that my Vitola is currently set up for 135kBTU input but can apparently be downfired to 0.75GPH (i.e. to have an output similar to that of the VB2-22 model), I think I'll have that nozzle put in once the house has been finished (the nozzle was delivered with the Vitola). That should bring our on-off system closer to an optimum operating condition, hopefully without creating waterfalls in the chimney due to condensation.

jp_2

wow 8 kids?

you have ten people in your house,wow!

I considered the effects of one person on a 2,000cal. diet.

so my numbers are more like 8,000 btu's/person/day. thats if you never leave the house. or 330btuh per person. so my estimate is 2000btuh less than yours. if my numbers are correct this puts you and your nine members on a 3,200cal diet.

are your lights phorescent? if you consider 60watt incandecscents, it looks like 2 burning lights equals one of my persons. estimate using 80% light efficience.

it makes perfect sense why NOT to consider heat gain from people in the heat loss of the house, they are variables not constantants.
what would you tell people if they say the house is cold, "well its cold because you aren't home enough"?

Constantin

.. no I don't have 8 kids, at least not yet.

Like I said, I just went with the default settings as far as bedrooms are concerned. At the time, I was more concerned with getting the ductwork right than the absolute BTU's produced by big or small offspring. As an aside, I think my wife would shoot me if I demanded 8 kids.

As for lights, you raise another good point, which is the heat load created by lighting. The Halogens uprights I used to use in dorm rooms always gave the place a warm glow besides lighting it up brightly enough for an aircraft to land nearby. However, nowadays I not only pay the bills but know better, so we will use CF lighting wherever we can.

IIRC, the BTU load for a person resting is about 400BTU/hr. I guess the higher number comes from assuming that people will do more than just lie in bed. Also, other considerations include how the air system will handle 12 guests showing up for dinner... While I don't like installing a plant according to peak loads, the 2-speed compressor of the HSX19 should reduce noise and energy requirements somewhat in non-peak times.

Bob Morrison_3

please provide your email addresss

Mine is robert.l.morrison@comcast.net

Bob

Constantin

An e-mail is on its way to you...

So it's probably easiest to to reply to that. Many thanks again!