Manual J Heat Loss Calculation

dkirkwood · November 13

I have a 100+ year old house that has a one pipe gas steam system. It currently has a Weil McClain 250 btu boiler that is about 30 years old...works good, but I am contemplating replacing it, given its age. My municipality requires a heat loss calculation done "according to Manual J standards"...meaning the cooling calculations aren't necessary, but the heat loss is.

My question regards how necessary it is to worry about the Manual J details for windows .. specifically, the window orientation, type of glass, etc. It seems to me, for an old steam system, if I match the heat loss multiplier to the type of window, e.g. single pain or double pane, that should be good enough. I can understand if I was sizing for a heat pump system that perhaps such details are material, but for an old steam system, in an old house, that works fine, it seems the finer points of the calculation are overkill....not saying the calculation is overkill...e.g., calculating the heat loss for all walls, windows, doors, roof and unconditioned floor makes sense, but the minutae of detail regarding window orientation, air infiltration and other details seems like overkill. So does doing a room by room calculation, as opposed to just the exterior envelope boundaries. Does such minute detail really result in a better choice of boiler size for steam?

Curious about thoughts here ...In an old house, with a boiker size that works fine, and has for decades, is the Manual J calculation overkill?

Jamie Hall · November 13

Don't sweat it. Make good general estimates of al that sort of thing — no need to get carried away. Remember too that this Manual J is for the building code folks — since this is steam, you are going to size the boiler to the radiation anyway, not the building heat loss.

mattmia2 · November 13

If it actually did matter, the windows are where most of the loss is although even the best window is pretty bad so to get an accurate calculation you have to calculate the windows correctly. If it is 2 pipe steam you can let the actual heat loss of the building have some effect on the boiler sizing, you can partially close radiators to balance a boiler that is somewhat smaller than the connected radiation. On a 1 pipe system you should size it to the EDR of the connected radiation since that can be very difficult to balance if you can't fill all of the radiation.

EBEBRATT-Ed · November 13

Just do the calc as best you can. Close enough will be ok. I will bet you are in Connecticut??

EdTheHeaterMan · November 13

I would also include the EDR calculation with the boiler manufacturer's recommendation for sizing according to the EDR. Just in case you actually need a 4 section boiler to get it to operate properly without being undersized (and the inspector will only approve a 3 section boiler that more closely matches the building load.) Unlikely but power can be intoxicating.

OR

You can always make the manual J come out to the BTU that you need it to, by increasing the infiltration number to 2 or 3 air changes per hour. You won't believe how that increases the load calculation numbers.

EBEBRATT-Ed · November 13

@EdTheHeaterMan if it is CT they probably will not care about oversizing. Their only concern is the boiler being able to heat the building.

hot_rod · November 13

If you spend the time to do a load calc it would be nice to have an accurate number. If the building has had any upgrades to load will be lower than when it was built

What size home, with a 250K boiler?

dkirkwood · November 13

Thanks all...all comments are appreciated, and spot on... especially paying attention to th EDR. House is 3 floors and 1900 square feet, not including unfinished basement. I think the 250 boiler is overkill, but it works like a champ....but at 32 years I just want to be prepared given how strict the code enforcement has become. I had other houses in this municipality, and 10 years ago they didn't require any calculation for a boiler permit unless it was new construction. Now regulations developed for new construction and modern technology are to be applied to 100+ year old building methods and technology...heat physics don't change, but it seems like unnecessary hoop jumping to require calculations when the exist system performs fine, and all one is doing is swapping a component.

The municipality is NY...prefer not to say which one

AdmiralYoda · November 13

Generally older steam systems were sized for houses with zero insulation and to keep it at 70F+ with the windows open. You'll likely have way more btu's than you need, especially if you've done some home improvements.

The building code people just need to check a box to make sure you have adequate heating capacity.

EDR is the way to go as others have mentioned. Carefully add up the EDR of each radiator and that is the boiler steam capacity you need. Period.

9 times out of 10 non-steam professionals don't run the calculations and get the next size boiler than the existing one "just to be safe". They are just covering their butts. To them more capacity is better than too little and they don't want call backs. But the boiler can be less efficient and cycle on pressure if it is too large.

Slant Fin has a good heat loss calculator and I have an excellent Excel spreadsheet with all sorts of detail. Not sure where I got it from though. They are all over the internet.

DCContrarian · November 13

If you want to know your actual heating load, follow the process in this article to estimate using past fuel usage:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

If you need a piece of paper in order to get a permit, first figure out what number you need, that will be determined by your radiation. During the Manual J process there will be lots of opportunities to put your thumb on the scale to direct the output.

Manual J Heat Loss Calculation

Comments

Categories