Heat Load Calc

[jebatty]

I've been somewhat surprised by heat load estimates that I've seen from some as they plan their heating systems. Electric baseboard was our primary heat until 19 years ago when we put in a free-standing wood stove for our 1500 sq ft house (built in 1956), and with full basement = 3000 sq ft of heated space. Since then we've replaced windows and done remodeling which resulted in replacing some of the original 2x4 with 2x6 walls. Now the electric baseboard is backup heat when we're gone and can't fire the wood stove.

Where we live it is cold in the winter, -10F to -35F for extended stretches are not unusual; it is windy in winter with west and northwest exposure to catch those Alberta clippers storming down from the north; and the heating season is long, starting in Sept and we're still heating this year and likely will continue until early June. The wood stove provides 100% of main level heat, with a little electric supplement in the basement.

Although I've never done a heat load calc, our total baseboard (main level and basement) at 250 watts/ft is 50 feet = 12500 watts = 42,680 btuh equivalent. A little more than 2/3 of this is in the main level and a little less than 1/3 is in the basement. The baseboard keeps the house a very comfortable 70F if used. Our electric dhw works out to and additional 45 btuh (9.6 kwh/mo).

Since we're heating 3000 sq ft total, in a larger house the ratio of outside wall and windows to total sq ft should go down, and it would seem that approximately 90,000 btuh should cover a 6000 sq ft heated space.

So when I see heat load calcs of 100,000 btuh and up, I'm wondering what's happening? what's being heated? or am I making an error in calculating heat load?

 

[in hot water]

Older homes can run in the mid to high 20's of BTU/ square foot range, as a rule of thumb. Some un-insulated one much higher. 1500 square feet of home X 25= 37,5000 btu per hour AT DESIGN.

I've done calcs for well insulated SIP and ICF homes and seen numbers in the 10 btu/ sq. ft range! Almost low enough to heat with just internal gains from lights and appliances.

Yes windows take that load number up, as even the best struggle to meet R-3. A far cry from a 6" wall.

Infiltration is the other big number and hard to guesstimate, especially in older homes. To really nail down that number have a blower door test performed and use the number it gives you. Otherwise it is a big guess and most designers error on the high side. Play around with the ACH (air changes per hour) input on your heat loss design program and you will see that difference.

This is why commercial buildings with large overhead door that open a lot in cold weather work so well with radiant slab heat. The recovery is almost instant when the door closes, as radiant energy moves at the speed of light. Just like when the sun comes out. You feel the warmth instantly even though the sun is 93 million miles away.

hr

 

[Como]

Our calc worked out at about 80 per sq ft.

But this is our worst case scenario.

I can quite believe we are 8x worse than modern construction.

 

[stee6043]

The recovery is almost instant when the door closes, as radiant energy moves at the speed of light. Just like when the sun comes out. You feel the warmth instantly even though the sun is 93 million miles away.

hr

It actually takes roughly eight (8) minutes for light to travel from the sun to earth.....ha....sorry, I had to!

 

[Gooserider]

One thing to consider is that while I'm not sure just when the requirements kicked in, it has LONG been a code requirement for electric heat houses to have more insulation and be built tighter than fossil fuel heated houses, so your house may be tighter than average for it's vintage JE... Also you probably have better insulation by virtue of where you are than those of us in slightly warmer climates...

I know that our house (built in 1980) is mostly pretty good, except for the living room that accounts for 15+K out of the house's 87KBTU/hr design load, and the laundry room that's almost another 9K. Splitting it up by floors, our basement is also pretty bad - 39K, or getting close to 1/2 the load...

The living room has a fairly small square footage, but it's got a couple of really big windows and a peaked ceiling that goes up to 24'....

The laundry room connects the house to the garage, so it has two long exposed walls and a third wall that faces unheated space, with lots of doors and windows...

The basement has poorly insulated walls that are 50% above grade, and no floor insulation.

So I could probably tighten up the house a huge amount just by addressing those three areas... Probably wouldn't get it down into your range, but I'd expect it would be possible to get into the high 50's w/o that much of a challenge.

Gooserider

 

[jebatty]

Goose - your comment raised what may be one important energy efficiency point, as people consider buying or building a different house from what they have -- the exterior wall layout. Our house is virtually square, 40' x 38', and also only standard 8' ceilings. Maybe it's the square layout that makes a contribution to energy efficiency. As to the electric heat, the house originally had an oil burner/forced air furnace that was removed sometime prior to 1990 when we bought the house and replaced with baseboard electric. Insulation was poor, and I have done just about everything possible to improve the insulation in an older home.

 

[Gooserider]

Goose - your comment raised what may be one important energy efficiency point, as people consider buying or building a different house from what they have -- the exterior wall layout. Our house is virtually square, 40' x 38', and also only standard 8' ceilings. Maybe it's the square layout that makes a contribution to energy efficiency. As to the electric heat, the house originally had an oil burner/forced air furnace that was removed sometime prior to 1990 when we bought the house and replaced with baseboard electric. Insulation was poor, and I have done just about everything possible to improve the insulation in an older home.

I totally agree - the GF bought the house shortly before I met her, it would not have been my first choice if I'd have been making the decision. It's a beautiful place, but very non-practical, and I have described it as the "house from hell" to work on... For it's footprint, it has a fairly low square footage as well.

It's a contemporary w/ a 12/12 pitched roof - first floor has the living room that is completely open on one end, a combined kitchen / dining area in the middle, and two bedrooms and the main floor bath on the other end. The second floor is much smaller because of the roof line, and has the master suite centered over the kitchen and bedroom areas, with a small attic area on each side, plus one above. The mostly finished basement more or less matches the first floor layout. There is also a screen porch on the north wall (facing the non-heated in-ground pool) with yet another attic space above it... Sticking out to the south is a laundry room that connects the house to the attached two car garage (one more attic space above those two...) Aside from the living room, ceilings are all in the close to 8' height range. Insulation varies, but isn't really good anywhere, needs a lot of improvement...

Gooserider