Manual "J" Calculation

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sfav8r

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San Francisco Bay Area
We have a client that has been told by the city that their heat may be inadaquate (it's a rental unit). The unit is about 700-800 square feet and there are three electric wall heaters that are each 1500 watts.

The citation says that the heaters must be able to heat the unit to 68 degrees. Well, It seems to me that this isn't enough information. If it is 65 outside, I'm sure the heaters will easily heat to 68. It also says nothing about how long it can take to get to 68 degrees.

Is there some standard calcualtion for this? I found something on-line called a manual "J" calculation but I have no idea how to do it.

Any help would be appreciated. I basically need to caclulate the required heat for this unit then determine if the existing units are OK or if we need to upside.

Thanks
 
For electric heat, unless you want to do a real heat loss calculation according to Manual J, the old rule of thumb "one watt per cubic foot" works very well in most cases.

In your example, assuming 8 foot lids, you'd need about 6400 watts of heat. You're a bit shy, in that regard.
 
benaround said:
Marc, in that rule of thumb, is it based on wall & ceiling insulation and vapor barrier required for electric heat ?
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No, that rule of thumb was developed long ago. If you're applying it to modern construction required to meet the IRC insulation requirements for your insulation zone, it may result in slightly oversized heat. This is not a problem, since there is absolutely nothing wrong with short cycling electric heat. The requirements you speak of is relatively new to the enforcement scene, in the grand scheme of things. The rule of thumb is pretty much right in the middle between the flimsiest and the tightest construction methods. Give me a break, anyhow. That's why it's called a rule of thumb. You can puzzle on the manual J calc and come up with darned near the same numbers, I feel sure. Dont' take my word for it however. Do those calcs and see with your own calculator and see if it isn't darned close to one watt per heated cubic foot. Even if you came up with a smaller number, given the sizes of heat available, you might end up with the exact same amount of installed heat anyhow.
 
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sfav8r said:
We have a client that has been told by the city that their heat may be inadaquate (it's a rental unit). The unit is about 700-800 square feet and there are three electric wall heaters that are each 1500 watts.


Any help would be appreciated. I basically need to caclulate the required heat for this unit then determine if the existing units are OK or if we need to upside.

Thanks
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Heating is based on the Climate Zone and structure insulation. (I.e. The BTU/sf of the living area is related to the new envelope design.)
The permitted job usually includes State or local energy compliance verification that is based on Glazing to floor ratio, insulaton, etc. The energy baseline is either by design or prescription catagories from the bldg. dept. and should be in the plan specs. If not, get the designer's feet to the fire.

rbj
 
Lot of things involved here,insulation,air leaks,location.I would wait for tenant complaint first.However 68 woul never be enough for me and the wife.Just turned ours on and set for 76
 
Jim W in Tampa said:
Lot of things involved here,insulation,air leaks,location.I would wait for tenant complaint first.However 68 woul never be enough for me and the wife.Just turned ours on and set for 76
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68 is the minimum requirement of the International Property Maintenance Code (aka "rental code"), among others. It wouldn't work for me, either. THis is a recent change. It was 65 degrees
 
In support of Marc's rule of thumb, IMO basic room heat will be 5 to 8 watts per sq. ft. in the S.F. area. Again, that is dependant on what the AHJ's call with permit calc's in hand. Ceiling heights are a factor also.
rbj
 
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You'lll have to also check the exact city in which the IPMC was adopted, since IPMC 602.3 has to be amended because it has a beginning and ending month in the code text that just says [DATE] and [DATE]. When adopted, they ususally put in October through April as the months where you need to maintain at least 68 degrees. That section also contains an exception for an unusually cold day, below the design temperature for the area, as not needing to maintain 68 degrees.
 
If those heaters are the original equipment, they are grandfathered in unless additional living space has been added to the existing structure. I believe that the local S.F. regulations do not allow the use of resistance heaters on new construction.
 
I wonder how the rental inspector (I assume?) determined they were too small anyhow? Was this a complaint inspection that they did? or a wild guess?
 
FWIW, I have noticed recently that the HVAC contractors are leaving a copy of their Manual J calcs for the inspectors. I guess that is a new requirement of the jurisdictions around here.
 
Here in northern New York, by the Canadian border, many of the electricians have, for many years, used 12 watts per square foot or 1.5 watts per cubic foot. Never had call backs from anyone complaining of it being too cold. :smile:
 
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