In the Name of Science (Heat Loss Calculation)

Chester

Maybe we should do an experiment that would be fun and make the world a better place.

Let's test the hypothesis that an easy way and accurate way estimate heat load (for the purpose of sizing a replacement boiler) is to use weather-adjusted actual energy use. Nothing against Manual J, etc. but this is something a homeowner or professional can do at no cost and in about 10 minutes. If nothing else it's a reality check. And certainly way better than any rule of thumb.

My suggestion is to have a little contest where you-all provide me some basic information and I estimate a heat loss. Then we compare my number with your number, which could be based on whatever method you choose. Even better if you already have an accurate number.

Here's what I need:

1. Historic fuel use over a 2- or 3-month period winter period. (Convert oil, LPG, natgas to BTUs.) I'll need specific dates. For example, your bills show the gas company read your meter on Dec 2, Jan 4 and Feb 5. I want to know how many BTUs you used between Dec 2 and Feb 5.

2. The relevant Zip Code.

3. Characteristics of existing boiler. Is it an old, oversized dog or reasonably efficient and well maintained (i.e. relative thermal efficiency -- probably a number somewhere between 60 and 85.)

4. Heavy or light DHW demand?

5. Average indoor temp kept at 68 or above or 65 or below?

Any takers?

bmwpowere36m3

I checked my S/F calc vs. actual CCF/HDD and they were within 5k (21-27k BTUs/hr). However its only 1 months of data, more interesting will be next month's when average temps will actually be low.

JUGHNE

I have years worth of data for some buildings. I have come up with CCF per HDD factor to gage how effective upgrades to the heating system have been.....(feathers in my cap ).

Jan 06 to Jan 08 the factor went from 1.48 down to .91; that was the low hanging fruit of upgrades for 1 pipe steam system.

This year will be without setbacks (someone figured out how to unlock the T-stat and found the "hold" function) and may reveal some info about that controversial subject.

The monthly HDD are from the weather station and the NG company furnished the CCF.

One factor that is not in the calculations is wind speed.
The wind chill is available, it is not a constant but varies all day long. Not that the buildings "feel" wind chill as a person would. But winds up to 30-40 MPH greatly contribute to air infiltration as we all know. So is there a simple factor that could be applied to HDD??

Steamhead

JUGHNE said:

Jan 06 to Jan 08 the factor went from 1.48 down to .91; that was the low hanging fruit of upgrades for 1 pipe steam system.

That's roughly a 1/3 reduction- same as we've gotten on some jobs. Nice work!

JUGHNE

Steamhead, thank you. I did inquire about this project years ago and followed your advice on some matters. Since 2008, more venting and 1" FG was added to mains & returns. There had never been insulation on those pipes.

SWEI

The number we use for overall building performance is BTU per square foot per degree-day. With variable DHW demands, varying incoming (potable) water temps, and the simplistic model used for degree-day calculations, we are still working with guesstimates.

Jamie Hall

You're on -- I have a raft of data -- some of which may save ou some time. However, I can't get at any of it right now since my "real" computer died and a tech. is trying to resurrect it. When I get it back...

Chester

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"One factor that is not in the calculations is wind speed.
The wind chill is available, it is not a constant but varies all day long. Not that the buildings "feel" wind chill as a person would. But winds up to 30-40 MPH greatly contribute to air infiltration as we all know. So is there a simple factor that could be applied to HDD??

Wind speed and air infiltration is already factored in to some extent. Even though the HDD data doesn't include wind, the BTU usage will reflect the combined impact of cold, wind, solar gain, etc. over the measuring period. So your BTU/HDD ratio includes an imputed estimate of wind impact. Of course it's only an average. If you want to design for peak 1 in 10 year conditions with cold and wind then you upsize accordingly.

Chester

"Your steps need a bit of refining.

The DHW demand must be quantified. "Heavy" or "Light" won't cut it. You will need demand values over several months when there is no CH load to determine annual usage for DHW. This can be subtracted on an annual (or monthly) basis.

The average indoor temp is elusive."

If you can get more granular data on DHW that's great, but the question is how accurate is accurate enough. Boilers come in lumpy size increments and for most residential applications the available choices are something like whether your customer needs 50,000, 80,000, 110,000, 120,000 or 150,000.

My observational experience is that DHW is around 10-15% of my winter fuel use (I'd characterize that as pretty light -- a couple showers, a little laundry and a load of dishes). Might not even be that high. It takes 15-20 minutes for my indirect to fully recover from a decent shower. I doubt my boiler runs for more than 1 hr/day for DHW. And then there's cooking, but that's relatively insignificant except maybe around the holidays, or pizza night.

My guess is the efficiency of the existing boiler is more of a wildcard than DHW because anything based on the label on the boiler assumes as-built steady state operation and doesn't factor in standby losses, etc.

The thinking behind my question regarding avg. indoor temp is simply to determine whether it would be more appropriate to use Base-60 or Base-65 HDD data.

Chester

bmwpowere36m3 said:

I checked my S/F calc vs. actual CCF/HDD and they were within 5k (21-27k BTUs/hr). However its only 1 months of data, more interesting will be next month's when average temps will actually be low.

Similar experience for me. I had used this method (credit for the idea to Dana who posts on another site) and came up with approx. 30,000 BTU/hr for my house. I compared that with SlantFin (and spent a lot of time measuring everything) and came up with 38,000. Then I had an energy audit with a professional Manual J and he came up with 29,000.

BTW the energy auditor estimated I could reduce my heat loss to around 22,000 if I implemented their recommendations for air sealing, some extra cellulose in the attic, and doing my knee-walls correctly. I did what they suggested and just did a year-over-year comparison using Dec and Jan fuel use. It came in at 23,000.