# BTU sizing based off oil boiler?



## SIERRADMAX (Feb 23, 2012)

I will probably be installing a wood boiler over the course of the off-season (spring/summer). My house currently has an oil fired 144,800 BTU/Hr boiler providing HW to an indirect domestic HW tank and two hydro-air AHU's. This serves heat to a 2600s.f., 2 story 2.5 bath colonial with (I would say) moderate insulation values. 

I've been monitoring the oil boiler operation and degree loss on 32 degree nights. On average, the boiler kicks on twice an hour, the upstairs hydro-air unit operates once and the downstairs unit operates twice with temperature swings between 62-65 degrees. 

My first question is, if they rate a boiler at 144,800 BTU/Hr., does the output produce that heating capacity if in operation for a full hour? Additionally, would a lower rated wood boiler at 100,000 BTU be able to keep up demand/idle less? Or should I size the wood boiler to equal the capacity of the oil boiler and have it idle more frequently?

If I opt to go with the 100,000 BTU boiler, does this mean my wood consumption would be greater?


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## muncybob (Feb 23, 2012)

Folks here and my boiler's rep all basically advised me to not oversize the boiler, esp if going w/o storage. My old oil boiler was rated at 120k and my wood boiler is rated at 100k. I don't currently have storage and so far it has worked out well for us.

....in the meantime, get out there and cut some wood


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## henfruit (Feb 23, 2012)

Do you plan on having storage?


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## Jeff S (Feb 24, 2012)

Since many installers have the bigger is better attitude chances are your current system is over sized and therefore less efficient,you really need to do a heat loss calculation on your house,I believe this site has one you can use.Also remember that heat loss calculation lets you know what your requirements are on the coldest day of the year as you may have guessed happens once a year. The use of a storage(buffer) tank it can help your system ride out temperature fluctuations regardless of boiler size.

In my own house a heat loss calculation let me know that my heat requirements for an inside temp of 70*F and a outside temp of 0*F would be 30,000 BTU yet the furnace in my house was rated a 90,000 BTU.At the time the smallest gasification boiler I could find was the EKO 25 rated at 85,000 BTU so I installed it.Since I have a storage system and the winter has been mild I have been able to run my boiler for approx. 16 hours every other day.You see even though my boiler is over sized I run it at full capacity(high efficiency) for 16 hrs then coast for the next 32.Even when we have a colder than usual winter like last year I rarely ran the boiler more than 12 hours during a 24 hr period.


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## SIERRADMAX (Feb 24, 2012)

I will probably plumb for future storage but for now, I want to plumb without and see how it will perform.

I did a quick Heat loss calculation from here. http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
How do they compute heat loss without interior design temperature? 

I punched in my location with a 20 degree design, 6000 heating degree days and all the appropriate s.f., etc. I came up with a 45,600 BTU/Hr. design loss. This seems low for the size of a house I have but all the numbers I punched in are modest. So it leaves me to believe the oil boiler is oversized for my house and a 100,000 BTU wood boiler would be sufficient. Even with a -15 degree day (we do see this a couple nights during the winter), BTU/hr. loss is only 80,000.

Also, I would like to add a heater in my insulated garage which would probably add another 40,000 BTU, but with a much lower interior temperature.


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## ALASKAPF185 (Feb 24, 2012)

That is not a low number. 99% of older boilers are oversized. You have input-output- and DOE ratings. In fact you HL might be on the high side for 2600 sqft. If you go with storage you can go with a smaller boiler. You want long burn times, short cycles are less efficient. Just like highway driving or city driving.  If you have low water volume then its hard to keep up with the wood boiler. Depending on how much you system volume is compared to what the boiler holds , will determine how often it will have to fire. And yes doing a heat loss for the coldest day of your area oversizes the boiler for 95% of the heating season, plus there is already a 10% fudge factor built into the heat loss calculation. Look up sizing by DOE versus net boiler output and there is more fudge factor.  Proper system design will maximize your efficiency and fuel usage. I have seen 80 KBTU with 1000 gal storage heat 4000+ sqft and have 8-10 hour burn times at 10 degrees.  If you have multiple zones they would all have to be calling at the same time for you to need that 45.600 btu on your design temp. Change your design temp to what your average temp is just for kicks and see what your load is then.  Good Luck


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## StuckInTheMuck (Feb 25, 2012)

Don't forget that your current system at 144,800 BTU per hour was designed to heat your house AND at the same time provide enough excess BTUs to heat your domestic hot water.  The size and design of your new system should take into account whether or not you will use it for DHW and how much you use.  Many wood stoves are rated around 40k BTUs per hour and do just fine heating 2000 square foot houses that have decent insulation.  Not an expert, just an observation..


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## SIERRADMAX (Feb 27, 2012)

Thanks for everyone's input. I didn't take into consideration the BTU output of my Harman Oakwood at 40,000 (specified). It does keep the whole house warm at 65-70 on a 32 degree night.


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## EffectaBoilerUser (USA) (Feb 27, 2012)

The 144,000 BTU on the boiler could be one of several values : Input BTU, output BTU and DOE BTU.

The tag on the boiler should clearly state these values.

With regards to boiler size and tank storage, you definitely do not want to purchase a gasification boiler that is too large. 

Without knowing the exact heat loss of your home (and making the assumption it is not a 100 year old farm house with single pane windows and no/minimal insulation in the walls or attic), I would recommend a 35-40kw boiler and 1,000 gallons of storage.

This setup would allow you to do one complete burn (firebox full of wood) and raise the temperature in the 1,000 gallons of storage by approx. 35-45F depending on the outside temperature.

My house/garage set up is approx. 4,000 sq. ft and I am also heating my DHW and a 350 gallon jacuzzi hot tub at 104F. If the outside temperature is in the mid 20's I am able to let the tanks get down to 130/140F  nd in one burn (approx. 95 lbs of 15-18% MC wood) provide heat to the house/garage, DHW and hot tub while also rising the 1000 gallons 35-45F.

In layman's terms I am able to make (1) ONE (5) FIVE hour burn every 24 hours and keep the house at 73F and the wife happy! 

Hope this helps!

Brian


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