How to account for wind with heat loss calculation?

Robert_25

Hello, I am in the process of collecting information on our just-purchased home to do a heat-loss calculation.  We are considering replacing the oil boiler, but the previous owners always supplemented the heat with wood, and often left for extended periods of time and turned the heat down.  I have no idea if the current boiler is sized correctly, so I want to do a clean-slate heat loss calculation.

I am planning to use Slant-fin's free heat loss program, it seems straightforward to use but I have one concern.  How do you account for the amount of expected wind during the heating season?  Our house is 100 years old, and its in a very open area that gets a lot of wind during the winter.  My previous home heated very easily at 0F with zero wind, but with a 15-20 mph wind the boiler never shut off.  I don't want to neglect the wind factor and end up with an undersized boiler. 

Also, how to you calculate the load from an indirect hot water heater?  We have a 30 gal. indirect that I would use with the new boiler.

-Robert

Jean-David Beyer

calculate the load from an indirect

You have to know how much hot water you will be using, the temperature of the supply water, the temperature of the water to be delivered. From that, you calculate the number of BTUs to heat it. And if you know the firing rate of the boiler, you can compute how long it will take to heat that hot water.



If you are lucky, the time to heat the water is not very much (for me, about 5 to 10 minutes a day, but I use very little hot water). In that case, you will not have to calculate the load, just have a zone from your boiler run at highest priority.



If you are unlucky, and the water heater is going to be running a lot of the time, just treat it as any other zone in computing the heat loss. But I still suggest running it at the highest priority.



My boiler runs the indirect at highest priority, but after 30 minutes it gives control to the next higher priority. Since my recovery rate is so high, I do not expect this to ever happen, but the feature is there.

Robert O'Brien

Factor

The infiltration factor is built in when you select the type widows/doors/construction. The indirect is a non factor in a situation like this, size the boiler to the actual load.It sounds like you should tighten up the envelope!

EricAune

Select the proper infilatraion factor

Slant fin will allow you to select an infiltration rate, this is based on the type of construction and the windows/doors etc. 



Often times a savings in operating costs are better achieved by tightening the envelope of the house.  Consider the amount of money needed to upgrade the boiler system and then estimate the cost of upgrading the insulation, windows and doors.  The true savings will more likely come from the envelope upgrade because they are long term.  These upgrades will realize immediate savings, allowing you to then take those savings (in time) and  replace the boiler system with what will likely be a much smaller system.  An upgrade to a smaller more efficient boiler will be a direct effect of the newly upgraded envelope, and ultimately achieve the savings we are all looking for.

Robert O'Brien

Just

took the BPI Building Analyst class,insulation and weatherization is where it's at,.windows/doors are not the largest ROI by any means.If you're not sure,get an energy audit.The blower door test will give you a real number as well as pinpoint problem areas

Robert_25

Infiltration

Thanks for the responses.  I saw the infiltration factors available in the software, but I think they are based on a 15mph wind.  It is not uncommon to see 25-35 mph winds at our house.  



As for tightening the envelope, I don't think there is a lot I can do right now without really tearing the house apart.  A previous owner insulated the walls and attic floor, and the windows have been replaced with Anderson double-pane units.   I was looking at replacing the boiler more from a reliability perspective than for a gain in efficiency, it runs very well but is 31 yrs old.

Steve Ebels_3

Infiltration factor

While it's true there will be greater infiltration with a 30mph wind blowing than when it is calm. I have never found an instance where I had to use anything other than the factors listed on the software I use for load calcs. The only thing I would say is don't be optimistic when you select that factor even though the place has newer windows and retrofit insulation. 

The Steam Whisperer (Formerly Boilerpro)

A general suggestion

I never have liked the fixed infiltration rates in heat loss programs.  When calculating whole building loads they are fine.  However, if you are calculating the loss of a single room, its orientation and location in the structure is going to effect its loss.  A safe way I approach this problem is to use the fixed infiltration rate for rooms on the leeward side and increase it by 15% on the windward side and somewhere in between for rooms that are in between.  Also, loads on the first floor will be higher than upper floors since this is where cold air will enter the most due to stack effect infiltration.  The leakier the structure the bigger the impact,with a corresponding reduction in load on the upper level.  Also, if the heating system produces mostly radiant heat and less convection or air heating, the variation in load between floors decreases since stack effect infiltration is reduced with radiant sources.



Boilerpro