On February 27, 2000 I discussed our solar heating design and in that article I briefly touched on the subject of determining our home’s heat load. At the time, there wasn’t a whole lot of information on the internet about the subject and I used a heat design guide to help determine this load. I came up with about 25,000 Btu/Hr for a worse-case scenario. Worse case being -20°F outside and 70°F inside temperature. A delta of 90°F.

As it ends up, I should have used a “worser-case” scenario of -40°F just to play it safe. The low last night was -28°F and tonight it could be -35°F. One thing is for sure, this house has never experienced it being this cold.

The good news is however is that our house made it through the first night with no issues whatsoever and that was with a less-than-stellar solar day yesterday. Today we have 100% sunshine and we’re pumping in 120°F+ heat right now into the floor. Once the sun sets, we’ll start up the wood stove and that should keep us warm until we go to bed, then kick in the electric boiler for an overnight stint.

We rarely use the electric boiler, but not knowing how things would go with this cold snap, I decided to use it to keep the house warm overnight. Usually we leave the stove burn out and restart it for a morning burn. Temps are typically in the low to mid 60’s when we wake up and putting on the stove for a few hours does the trick. But not wanting to contend with trying to get temps to climb during a -30°F morning, we thought it best to keep things warm 24hrs.

The question is however was the calculated heat load assessment we did 19 years ago accurate? This past weekend I attempted to find out and here’s how I did it.

It’s difficult to determine how much heat you put into a structure when there are many variables. Sunlight can vary, wood heat can vary and outdoor weather varies too. It’s really an educated guess.

This past weekend we really didn’t have any meaningful sun and on weekends, our electric utility allows our off-peak rate boiler to stay on 7 x 24. With an outdoor temperature of about 5°F and an indoor temp of 68°F, could I keep the house warm with just the electric boiler?

Our electric boiler is a 9 kW/hr boiler. A 9 kW/Hr boiler puts out about 31k Btu. Considering that our concrete slab is not 100% efficient, meaning there is some loss to the sand bed underneath and around the perimeter, I would estimate we lose about 20%. This would mean our electric boiler was approximately putting out 25k Btu into our living space.

I originally calculated that we would need about 25k Btu to heat the house if it were -20°F. It would appear more than likely that I miscalculated our heat loss and/or the efficiency of the boiler is not what I had hoped. The temperature dropped slightly overnight. This meant that even at 5°F, I needed more than 25k Btu to heat the house.

Scratching my head, I went on the internet to see if I could find a decent heat load calculator. I found one on Builditsolar.com. This calculator allows you to plug and play to help figure out your heat load. It’s a little involved, but by entering your home’s square footage and putting in R-values for your floor, walls, ceiling, windows and doors, you should be able to get a fairly accurate number for your area.

In our particular case, here are my new numbers for our house:

With a temperature delta of 90°F (70°F in and -20°F out) our heat load is approximately 40,000 Btu/Hr.
With a temperature delta of 110°F (70°F in and -40°F out) our heat load is approximately 49,000 Btu/Hr.

110°F is a HUGE delta, but if you really want to make absolutely sure your house can survive a cold outbreak of epic proportions, it’s better to be safe than sorry.

So how does this stack up to what our space heating systems can provide?

Solar (Active)
After watching our 10-collector panels bring in 125°F heat today with an outdoor temperature of -12°F, I’m convinced that the collectors are producing about 80k to 100k Btu/Hr. at peak. Of course, the sun doesn’t always shine so you can’t rely on solar as your main source of heat. But it sure is great when the sun is shining!

Solar (Passive)
Since our house is almost round in shape, most of our windows face SE, S and SW. They do definitely bring in warmth from the sun. Maybe 10k to 15k Btu/Hr at peak.

(Our active and passive solar heating systems bring in plenty of heat to heat the house and it keeps our house warm for many hours after the sun sets. As to the actual amount of Btu/Hr., it’s a guess for sure.)

Wood Stove
The Hearthstone Phoenix model produces 60k Btu/Hr but that’s a maximum number. Most typically, it’s probably 35k to 40k Btu/Hr. No matter, the wood stove can solely keep our house warm if we were to tend to it throughout the night. We don’t normally do this, but if we needed to keep things warm throughout a 24-hour period we could.

Electric Boiler
The small electric boiler we have was never intended to solely heat our house. It was more for a supplemental heat source that only heats the house at certain times of the day. (I used it quite a bit during winter months when I was away years ago.) Considering now that we are settled in our home and Jo would probably want to keep our purrrfect cats purring without them ever needing an extra blanket if we were away, I am reconsidering this part of our heating system. After redoing a cost and carbon footprint assessment, LP gas looks to be a better option. Stay tuned for another article on this subject.

— Alan Stankevitz