Heat loss calc VS fuel usage history

I tried your calculation and come up with slightly different numbers but basically the same:

I used 138,700 Btu/gallon home heating oil time 8 hours burn in a 24 hour day = 46,233 btu/hr. For 10 gallons, 138,700 X10/24= 58,000 Btu/hr.

If you are asking why is the heating oil boiler recommendation so much larger than your load? I think it is because of the desire to have a fast heat recovery. In a wood boiler the concept is different, the desire is to minimize idle time, hence the boiler is sized closer to the size of the load.

(1) I'd say it is hard to argue w/ the usage information, but certainly compare that w/ averages over decades. Too, heat loss calcs that someone may make(at least as far as I've ever seen) are based on the design temp for your area. I believe that number is a temp above which 99% of your expected temp recordings will be. Ours is -50. Your local city/county engineering dept, etc, should have that. (2) When you use 135K btu's, or whatever, for the heat in a gallon of oil, don't forget to account for your boiler inefficiency. Most run about 87%, some 95%; dependent, of course, on boiler age, tuning, etc. Have you done a complete heat loss analysis of your entire house? (3) When you say "boiler size" are you referring to the btu's it can generate, or its physical size?

I have pondered heat loss calculations to try and get a handle on how much wood I need for next year.

This year was my first burn season with the EKO and I estimated low. I had 8 cords of log length wood delivered in Sept 2008. People around here say that equals 6 bush cords when its cut and split. I added 2 cords by foraging fence-rows. So I had 8 bush cords.

At the end of December I had about 3 cords left so I started pulling more standing dead trees out of fence-rows and last week I built a road to the "back-forty". There is lots of wood there (standing dead and fallen dead) and it is (now) accessible if the snows are light.

When I calculated heat loads I used my previous year's oil bill as a guide:
I used 950 gallons
140000*950=130,000,000 BTUs
Assuming my oil furnace was 87% efficient; the house required:
130,000,000*.87=115,710,000
Assuming a 6 month heating season, the average hourly heat load would have been:
11571000/6/30/24=26,784 BTUs

The house is 2700 sq/ft (basement 1300 sq.ft) so 4000 sq.ft heated last year with oil.

Heating the garage and 1500 sq.ft in the barn are the new heating additions this year and of course it is with wood now.

It is taking 6 Front-end loader (FEL) buckets of wood per week. Those FEL buckets are not carefully stacked and split wood they are just bucked unsplit pieces but I use nearly the whole tree. The small limbs that many users discard are my favorite pieces of wood. The gasser runs nice on the small stuff and I don't have to split it. I do quarter the bucked wood. The combination of quartered and small limbs is adequate to keep gasification; although some improvements in efficiency may happen with smaller splits, storage and less idling.

The Front-end loader buckets are 2' by 2' by 7'or 28 cu.ft and a cord is 128 cu ft. I estimate that when I split the bucket load and stack it, the bucket load equates to roughly half its volume. By these "guesstimates", I am using 1/2 cord per week to heat.

Since ash, elm and oak predominate, I am estimating 20 million btu/cord.

Thus, I am burning 10 million BTUs per week and if I assume an 80% efficiency with the EKO and I am getting 8 million btus of heat.

The hourly consumption would be 8000000/7/24=47,619 btus

So, last year heating 4000 sq.ft with oil used 26784 or 6.696 btu/sq.ft/hr (26784/4000)
This year heating exclusively with wood is 5800 sq.ft 47619/5800=8.21 btu/sq.ft/hr.

Accounting for the difference :
The garage and barn are subjected to a greater wind effect, and are less well sealed and insulated. The thermostat is around 70 degrees in the house versus 63 last year when we were on oil exclusively.

I am going to need a lot of wood. If I am using 10MM/week for 24 weeks that is 240mmbtus/season or 10-12 cords.

Would you agree?

Mushroom Man said:

...
I am going to need a lot of wood. If I am using 10MM/week for 24 weeks that is 240mmbtus/season or 10-12 cords.

Would you agree?

Click to expand...

Sounds like a valid estimate. Ignoring your actual use this year I come up with similar numbers.

Taking the house by itself, you used 950 gallons oil for one season. Since your average outside temperature is about 28F from 15 Oct to 15 Apr, changing the setpoint from 63 to 70 would require about (70-28)/(63-28)*950 gallon=1140 gallon. Converting to wood btus by adjusting for efficiency we have (1140*140000 btu)*(87/80)=173.6 Mbtu. At 20 Mbtu / cord = 8.7 cord, on paper, for just the house, with the higher setpoint.

You're adding 45% more floor space, offset by presumably a lower setpoint, but worsened by higher heat losses. Call it an additional 20% to 40% and it comes out ten and a half to twelve cord.

So looks like you'll be pretty busy this year scrounging up 24 cord of dry wood for 2010-2011 and 2011-2012, plus 12 more cord of green wood for 2012-2013. The worst that can happen is you end up with more wood than you needed and you don't have to put up as much down the road.

All for $3000 to $6000 after-tax income per year depending on the price of oil. Sometimes hardly seems worth it to risk loss of life and limb cutting wood, but I sure do enjoy being as warm as I have ambition to be, and it sure is delightful to participate in legally nontaxable economic activity for my own direct benefit.


--ewd

Mushroom man, 24 cords is an awfull lot of wood for 5500 sq. ft. I asume the garage will be set at 55* or so. I don't think your house and garage are very well insulated. I have 2200 sq. ft and a basement that gets some heat and I went through 2 1/2 cords max. this year. My house is at 65* at night and at 4pm we get home and get it to 70*, then back down to 65* at night. My basement is always 62*. I have a new super insulated house and it helps. I think you are low on insulation.

All for $3000 to $6000 after-tax income per year depending on the price of oil. Sometimes hardly seems worth it to risk loss of life and limb cutting wood, but I sure do enjoy being as warm as I have ambition to be, and it sure is delightful to participate in legally nontaxable economic activity for my own direct benefit.

Click to expand...

I would be at the higher end of that cost range here in Canada. I think it is going to be much higher than that within a few years. Believe me, some days I wonder whether it is worth the effort and risk, but I have a belief that oil prices will rise.

I have numerous things planned to reduce the load and improve the efficiency. I hope to get the load down a bit each year.

Mushroom man, 24 cords is an awfull lot of wood for 5500 sq. ft. I asume the garage will be set at 55* or so. I don’t think your house and garage are very well insulated.

Click to expand...

It is not 24 it is 10-12 now. Bad enough! You were reading too fast.

I believe these changes can help to improve the heating situation:
1. thermal drapes or blinds. There are presently no window coverings. We sit atop a windy hill, with a nice view and we did not want to impair the view. Privacy isn't an issue because the neighbors are far enough away. Thermal window coverings can save from 15 to 25% of heat loss where careful installation is applied, according to my initial research.
2. a south facing sun-room. The passive solar heat can dramatically lower daytime loads by exchanging the higher temperature air in the sun-room with the cooler air in the house. At night the steel door will be closed to trap the heat. I got this idea from hearth.com. According to some contributors, on a sunny day, the furnace may not come on at all in the daytime. Wouldn't that be nice.
3. finish connecting the storage. This will help in 3 ways: a) reduce idling. This can be 50% of the time when temperatures are not extreme. b) allow greater control of temperatures in the daytime. With the boiler located in the garage over the family room, we have a situation where the family room is 6 degrees hotter than the rest of the house, even in the daytime when nobody is in it. It gets too hot sometimes up there in the evening and we strip down to be comfortable. With the 1500 gallon storage in the basement, heat will be "leaked" to the cooler part of the house and the thermostat can be turned down. We'll wear sweaters like last year but still keep it around 68-70
c) "leakage from the basement storage tank will heat the basement and I'll close vents to the basement.
4. I made a design mistake in the barn when I connected the pumps so I am getting less than ideal flows. That change in the spring and the addition of a thermostat will help in that loop.
5. One of the MushRooms was insulated too fast. There are air holes in the insulation around the headers that need filling on the windward side of the barn. I am losing a lot of heat there.
6. Use dry wood. Our wood this season is lightly seasoned. It is split just before we use it. It is scrounged standing dead wood, so it is not green, but it could be improved upon over a summer.
7. 3 outside doors need weather stripping and there are some places where spray foam could seal some air leaks. Almost any house could use more insulation and I see a need for it around the headers in the basement. It is a 13 year old house and the insulation situation is not bad.

I believe I can get this down to a manageable level, say 6.5-8 cords but it is all with a cost. There is no free heating for me.

I presently scrounge around my own property for wood. There is 40 acres of woodlot on the farm. I plan to continue that scrounging as it is good exercise. If I can't in the future, for any reason; at the level of 6.5-8 cords, I could buy log length and process it myself or with a helper.

btuser said:

This is an oil question, but I do burn wood as well.

I've done several heat loss calculations that give me an average between 70k and 90k btus. I also put an hour meter on my oil burner, and after 5 months my maximum daily usage was 10gph, no wood burning. So far on the coldest days with only moderate fire burning I'm using 8 gallons of oil. 8 gallons = 45k btu/hr. e

So even after a pretty carefull heat loss calculation I'm still being reccomended a boiler 2x the size of actual use. What's got me scared, however, is my house is 4300sqft in Southern NH, and even the reccomended boiler is 1/2 the size of the previous one.

Click to expand...

There are several factors, all of which tend to push heat loss calc answers towards the high side compared to actual use....

1. As mentioned, the calcs are based on "design day" which is the COLDEST day recorded in your area - nearly all days are warmer than that, so you will use less. It is even possible that one might go several years without actually getting a "design day" load, but it still makes sense to look at the "worst case" scenario.

2. The heat load calcs are based on having 100% of the heating input coming from the heating system - no allowance for heat from cooking, lights, electronics, or even people, (If alive your body puts out several hundred BTU/hr) but in a tight house, these sources can supply a good part of the heating demand....

3. The formula is an ESTIMATE... Like many estimates, it tends to be conservative - as the industry has figured out that the number of complaints about being to hot, or having an oversized unit is nothing compared to the grief they get from putting in a system that is to small to meet the demand...

4. The default indoor temp chosen is often fairly high - and these days most of us try to conserve (especially after we get a bill) by turning down the stat and putting on an extra sweater.... This lowers consumption, but will make the estimate off unless you carefully chose the indoor temp to match what you actually run.

5. Related to #3 - don't know how you did the calc, but most folks are using a program like the Slantfin one, or a web-based equivalent, rather than the actual "Manual J" which is MUCh more complex - and again because people don't object to being to warm, most of those programs tend to give answers that are on the high side compared to what a true "Manual J" would give - and that in turn tends to be on the high side.

In terms of the difference between what was recommended and what you currently have, again there were a lot of factors - many installers used some rough estimate "rule of thumb" calcs (i.e. so many BTU/sq. ft) rather than a proper calculation, which tended to give oversize answers, and tended to oversize from that. In addition, many of us have done things to improve our insulation values, tighten up the house, and so on - thus today's answer to the same calculation would likely be lower than the one that would have been proper when the house was built. Lastly, modern equipment is more efficient at getting heat from the fuel into the house, so you don't need as big a boiler to start with.

Gooserider

btuser said:

I feel that 1 cord is pretty darn close to 100 gallons.

Click to expand...

Depends on the wood and what you're burning it in... I've seen numbers thrown around ranging from 100 to 150 gallons of oil per cord...

Gooserider