Calculating heat usage

[Jigsaw]

Not to hijack this thread, but how would someone go about calculating btu's needed to heat their home based on past energy usage bills? I use a gas fired boiler w/baseboard heaters and would like to know the answer to my previous usage so I have an idea if my new wood insert will only augment my heating or if it can be a stand alone heat source if need be. I know there are many variables that go into the heating calculation, but I need a place to start. Thanks ahead of time for any help.

BG Mod Edit: Moved to its own thread. It's been a while since we've had this discussion.

 

[stoveliker]

Propane has 91,500 BTUs per gallon. Get your old bills and add the total usage in a year.
Then try to find the efficiency of your boiler and multiply. That is a decent measure for the amount of BTUs put into your home.

 

[Jigsaw]

I use natural gas. What are the btu's for a gollon of that? Or is it pretty close?

 

[stoveliker]

Oops I'm sorry.
What unit is your bill in? (I know it has 1037 BTU per cubic foot but your bill might be not in cubic foot...)

 

[Jigsaw]

Mcf is what my bill has for each month. For 2019 it was 80.9 Mcf for the year for natural gas usage.

 

[stoveliker]

Ok. Mcf is 1000 cubic feet. 80.9 Mcf natural gas is about 84 million BTUs.

Now you need to know the efficiency with which your boiler gets BTUs into the hot water.

 

[ABMax24]

Although using past utility bills for energy consumption gives an average of heat required per hour, its not exactly precise at showing peak usage to determine is a wood stove/insert can keep up during extreme cold temperatures.

A few years back we had an extended stretch of -40, over a period of 3 days my house consumed 3000cuft of natural gas, which works out to 41,000 btu/hr @ 95% efficiency of my condensing furnace. Which I used to help size my woodstove, I deliberately sized it a touch small to avoid smoldering and for better shoulder season performance with the understanding that in -40 it would be pushed hard to keep up or the furnace would still run occasionally.

Another way heat load can be calculated is through furnace/boiler on time. If you know the heat output of you appliance and can time (many modern thermostats show heat cycle on time) how often the furnace runs you can easily determine heat load.

 

[DickRussell]

Here is an excellent writeup on doing what you want: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

 

[Woodsplitter67]

Back when I got into wood heating I figured out my basic wood consumption by the size of the firebox and how many time Id load it.
for example.. my box size is 2.3 cuft.. I estimated that ill burn 120 overnight fires or 2.3 x 120 = 276 cuft or 2.16 cords for overnight fires... same thing for figuring out your day burns.. it was pretty accurate

 

[Jigsaw]

Propane has 91,500 BTUs per gallon. Get your old bills and add the total usage in a year.
Then try to find the efficiency of your boiler and multiply. That is a decent measure for the amount of BTUs put into your home.

My boiler efficiency is claimed to be 84.1%

 

[stoveliker]

I agree with the peak demand vs average energy need notes above. But then I note he sized the stove a bit small (from peak needs, I think), and thus average use seems a fair starting point as long as you have the natural gas system to add for the coldest week of the year.

At 84 percent, you have pumped about 71 million BTUs into your home over the year.
(Is part of the gas consumption for hot water...?)

How long is your heating season? I'd divide the 71 million BTUs by the number of heating days, and then divide by 24 hours for a BTU heat output number. This, however, is not enough for winter, because the shoulder season will drag down that average. So I'd take that number and multiply it by 1.5 for average winter heating needs(per hour). Yes, this is a fudge factor. ...

Note that some stoves are easy to dial down (operate efficiently and safely at low outputs) and some are less so.
But the number you reach following the above will be a fair number for the hourly output you want in winter. I.e. when the stove is nicely cruising.