How do I model a ducted wood burning furnace?

[Will Puddephatt]

I audited a house that uses a wood burning furnace, which is connected to the duct system, as its main/only heating source. The furnace is also used to heat water during the heating season. The manufacturer’s label gives its capacity as 120,000 Btu/hr. How should I enter this so that my OM file most accurately represents the situation?

[Gamaliel Lodge]

EDIT: Wood, pellets, and coal are now available as options in the Fuel control for heating systems.  Unfortunately we do not yet have direct support for sidearm water heaters, so most of this work around is still necessary.

 

This will require two workarounds and probably a little research on your part.  The workaround is for using wood as a fuel.  We hope to build direct support for wood, but for now you will have to hijack another fuel.

Using wood fuel:

1. Set heating system fuel to a type not used by any other system
2. Choose a system type with the appropriate delivery method (Boiler, Furnace, Direct Heater)
3. Set the efficiency to the known efficiency of the system.  If unknown look up system in http://www.epa.gov/compliance/resources/publications/monitoring/programs/woodstoves/certifiedwood.pdf or choose appropriate defaults from RESENT standards http://www.resnet.us/standards/RESNET_Mortgage_Industry_National_HERS_Standards.pdf.
4. Go to the Envelope/Building tab and click the More button to the right of Electricity Prices
5. Set the units on the selected fuel price to $/MBtu.
6. Set the fuel cost to that of the wood source using the conversion 1 cord = 20 MBtu or 1lb = .008 MBtu

The second workaround will be for the water heater.  You didn’t give me any details on that, but assuming that it works like a typical side-arm water heater, you can use the workaround posted in this thread from our old knowledge base system http://help.optimiserenergy.com/article.aspx?cid=4997&aid=12773.  The only problem that I foresee with that workaround is that the AFUE of the furnace is based on the efficiency of the air-based heat exchanger and the recovery efficiency of the water heater would be based on a separate hydronic heat exchanger.  I don’t know how to estimate that difference.

You will want to use the same fuel that you select for the furnace and you will need to estimate the annual load split between this system and the backup system that operates during non-heating seasons.

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