So we replaced our ~25 year old boiler with a sleek new 92% efficient one last November, and finally the gas company has sent us a REAL (not estimated) gas bill that is 100% the new boiler. But I still don’t know how much less we’re using.
December vs. January: I’m an idiot and didn’t read my own frigging meter on December 13th. I figured January would make the difference obvious… Except that January was 13 degrees F warmer than last year, and to help us purge the lines faster our boiler installer recommended we keep the temperature in the house as warm as we can stand it (65 degrees F – we can’t even sleep with our lovely fluffy comforter and the one dog acts like it’s summer he pants so much). So this bill reflects 7.1 MCF “actual” usage (estimated for december was 7.2 MCF, based on the previous 12 months and the temperature), but probably more than that was used in January than December (when we kept the house cooler, and it was warmer outside). Last year, January’s “actual” (including whatever leftovers from December’s estimate) was 14.9… But there’s that 13 degrees difference in average temperature. And we’re not running any heat at all through an entire room that used to receive heat, but we did actually insulate that room and install doors (but no handles…).
Spreadsheet to the rescue. I found December’s bill, and noticed that last year the same months were estimated and “actual”. So I can compare an average of 2010′s December+January, and an average of 2011′s December+January. Using the average external temperature, and the average internal temperatures, I can find the average temperature change I used the boilers to induce in the house. Stay with me, it’s not that bad. In December/January 2010 I used our old boiler to make the house an average of 29.5 degrees F warmer than the outside. In 2011 it was 21 degrees F. Thus in 2010, for those two months, the old boiler used an average of 0.53 MCF per degree to raise the temperature of the house (assuming the load and loss are both linear); in 2011 the new boiler used an average of 0.35 MCF per degree to raise the temperature of the house. This is not taking in to account the poorly insulated (2010) and unheated but better insulated (2011) back room that is still under construction. Still, that’s a pretty decent difference, about 33% less energy used (per degree F).
My math is solid, my assumptions are suspect, but it entertains me. Ultimately I do trust that the new 92% efficient boiler IS more efficient than the old one. I believe that it is due to the change in design and construction more than I believe that it is true due to my “back of the envelope” calculations, but I am an engineer.
I like to learn things, so I think the lesson I’ll take away from this is: it’s not just what you use, it’s how you use it. Using the least efficient boiler on the planet to keep the house just warm enough to avoid a plumbing disaster will likely take less energy (and be cheaper) than using a 98% ultra efficient boiler to create a tropical paradise complete with tropically heated breezes from open windows. But since we can turn the thermostat down with the efficient boiler too, I know eventually we will see a conclusive difference. Our annual MCF usage is down from ~64 to ~52 already (I think that’s all the new water heater, but this next year will be the boiler + water heater).
If I were a betting woman, I’d have tried to sell the house before replacing the boiler, but I’m conservative. I figured I’d have to replace it anyway, it adds value to the house, and I might as well get to enjoy it. I’m just a little peeved that my enjoyment isn’t immediately quantified on my gas bill.
You certainly make your father and grandfathers proud!
Oh, I’m just waiting for dad to leave me a message asking about therms or for a rather handy/experienced friend who regularly reads to poke a myriad of holes in my process. I claim no abilities beyond self-entertainment, and some better than average spreadsheet skills!
And the Heat Capacity of the house is….
See, I knew you would ask. I’d normally be able to say identical for both years, so as a constant it’s not important, but it’s not exactly identical. The back room, off the diningroom, has become better insulated and acquired doors, but it also isn’t heated like it was last year. So I’m not sure whether it’s more, less, or a wash.
This is why I claim nothing beyond self entertainment! And I can make pretty graphs to prove it.
Your MCF per degree F (typical engineering units!!!) is the heat capacity of the house assuming that the MCF all goes to heating and not up the chimney. If your new boiler is 92% efficient and that means that 92% of the MCF ends up as heat in the house. Then the heat capacity is now 0.92 x 0.35 = 0.32 MCF/F. Switching to centigrade and changing MCF to Joules (NOT BTU) would give you a meaningful metric number if you ever wanted to air condition the place!!!
Air condition! Yee gads, that’s blasphemy!! (and I’m pretty sure I could air condition using the English system, otherwise nobody would live in Florida)
The MCF usage is the boiler use (plus hot water heater use, but as a constant that doesn’t matter) for the time period, so the way I used it, it can’t be associated with capacity (unless it’s capacity of the boiler). Otherwise the capacity of the house would change simply by my shutting off the pilot light when the boiler isn’t running (something the new boiler does), which doesn’t make sense.
I divided the gas usage by the difference in temperature between inside thermostat setting (not the bulk temperature) and outside average temperature (not local temperature) to eliminate the issues associated with the difference in temperatures, almost like a dimensionless analysis, rather than measuring an actual value associated with any temperatures. I wanted to remove the temperature difference from the numbers to compare, not use it. Otherwise it would be like looking at miles per gallon efficiency for varying values AND gallon sizes at the same time – not so useful.
And that assumption of the chimney not being involved is directly related to the difference in boiler efficiency – indeed it may be the majority of the difference. The old boiler, quite a bit of heat went out the flu to the point that the vent would have burned you if you’d touched it (yes, I know from experience). The new boiler uses PVC (boo!) to vent outside, and is always cool to the touch. So by assuming that none of the heat escapes through the chimney, you’ve eliminated most of the problem description. Silly Physicist!
In order for me to measure the heat capacity of the house, I’d have to NOT change boilers if I were to use that MCF number. Then, holding the efficiency of the boiler identical, I could measure the difference insulating the back room makes. The fact that I changed boilers (known improvement, just currently unmeasurable) AND the heat capacity (unknown status and unmeasurable right now) of the house at the same time makes most of this simply an exercise in spreadsheet entertainment.