Heat loss calc vs. Gas CCF purchased

Steve D_5

I did calculate the MBTU/degree day. My heating season averages are around 20 for 2005 and 25 for 2004.

Is this what you were asking me about, Rob?

Steve

SteveD

Heat loss calculation vs. Gas purchases

Using various calculation schemes and levels of rigor, my heat loss comes out to 100000-120000 BTU/hr. I have a 12 year old W-M CG5 boiler which is 140k input. My gas bills for the last 2 years show that during the worst months I hit an average of 45-50 MBTU/hr including about 1600 BTU/h for the hot water heater.

I am considering an upgrade to a Vitodens but am leary of a $10k install.

Questions: What is up with my usage vs. heat loss calculations? What options are there that come close to the fuel savings of Vitodens without the initial outlay? What is the going rate for a plain vanilla CG5 install today?

Unknown

Your load calcs are for peak heat output. Your fuel usage is a monthly average (in the middle). That's a really high peak, and a really high average, so I don't see a huge discrepancy there at first glance (just a heavy need to improve your envelope). Did you compute the heat load and then calculate load by degree day from that? You should be able to get more accurate that way.

Steve D_4

Heat load

Did you compute the heat load and then calculate load by degree day from that? No.

The BTU/hr calculation is the gas usage(CCF)*100000 divided by the billing days*24. That is time averaged consumption. If the boiler consumes 140MBTU/h with 45% duty cycle in a bad month, that would average to around 60.

I left out that I run the boiler at 160* to get longer burn times.

I guess where I am coming from is that I have lowered my water temp, and even though I pay for 60MBTU/h in a bad month, with low efficiency, that is not the heat I get into my living space. Can the heat calc be relied upon in the case of a modern modulating boiler with much longer duty cycles that allow sizing closer to the average load than with a traditional on/off boiler.

I would be interested in any retrofits that would fill the gap between my current setup and a high tech boiler.

You guessed correctly that the envelope needs improvement. 1938 uninsulated colonial. There is insulation in an addition and remodeled areas (20%of wall space). Windows are double pane. Attic has 6" rock wool. Also, outside is aluminum sided. These are quirks that would make unadjusted heatloss calcs come out higher because the parameters are "between" categories for heat loss.

Unknown

I mean did you check your load calc in terms of usage per degree day, and compare that to your actual usage per degree day. That's the comparison you are looking for to doublecheck your load calculations... correcting for efficiency and such, of course.

Lurker_2

another way to measure

Since your "worst month" is probably not a design temperature all day every day of the month, you won't get to your heat loss. Perhaps you might want to try looking at the gas meter itself and measuring only during a prolonged period of the worst cold.

-Michael

Mike T., Swampeast MO

You need an estimate of your actual heat loss during a period. Here's a simple way to estimate based on degree days and a heat loss calculation.

For a finite period of known fuel use, e.g. 31 days and 150 ccf of natural gas.

Find degree days for the same period then divide by the number of days in the period.

Subtract this number from 65. (65°F is the base for most degree day calculations.) 65° is a very high base for the temp at which an occupied house requires heat. 60° is more reasonable so subtract 5 from your result.

This will be a roughly average heat loss basis temp for the period. Use this outdoor temp as the design temperature in your best heat loss calculation.

Heat loss calculations are almost always overstatements. How much depends on how accurately you've calculated and actual conditions inside and outside the structure. The overstatement is frequently considered to be 10% - 20%.

I'd suggest using 85% of the heat loss value calculated with the "average" temp used as design temp.

So, multiply your "average" heat loss by 0.85. Then multiply times 24 (hours in a day) and then times the number of days in the period.

Divide by 100,000 (BTUs in one ccf of gas). This will be a rough estimate of the therms lost in the house over the period. For a rough estimate of what a Vitodens would consume divide your loss estimate by 0.95

Steve D_2

I doublechecked per your suggestion

and got the following results. Using thorough heat loss (from I=B=R H-22)and matching average outside temp to design temp of the heat loss, the heat loss is 36mbtu. Gas consumed is 42mbtu. 36 is about 85% of 42. Can I conclude the heat loss calc is accurate with these numbers? Can I spec a boiler using the design temp difference of 60* (54mbtu)?

***

No one is biting on my hardware question. I want to believe that there is a reliable alternative to the Viessmann. Lochinvar Knight looks interesting. My observation as a homeowner is that many contractors have favorites and discourage the customer from picking something different. I trusted the first boiler guy and am 2X oversized after putting 30% addition to house after the install. This time I want to do my homework.

Unknown

Sounds like you are doing well on the calcs. As for heat sources, they are only as good as their installer. See what the guys around you install. Viessmann is great, but other boilers are good too and if the locals have good track records with other units then so be it.