The BTU

Britain abandoned its measurement units in favor of the metric system long ago, leaving the US as the only industrialized country still using them.  There was a moment of opportunity in the seventies for us to join the modern world, and a plan was in place and had started to do so, but then the newly elected president’s wife panicked.  All her recipes that called for teaspoons, cups, quarts, ounces and pounds would no longer work!

She successfully lobbied to maintain the system of inches, feet, yards, gallons, pounds and slugs (yes, slugs) to remain the official measurements of the land, nevermind that they were arbitrary and incohesive.  The plan to convert to metric was derailed and reversed.

Nancy Reagan was able to procure her measures and ensure that her recipes would always turn out, but it left us as an island speaking an antiquated dialect in a world that had moved on to a more successful way of communicating essential quantities.  Many errors have resulted, one of the most embarrassing was the failure of a Mars mission, where the space probe received last minute instructions for a thruster burn that was based on pounds rather than kilograms.  The orbiter failed to enter orbit and crashed into its target instead, oblivious of the engineering documents that had miscommunicated the course corrections.

Today we cope with the units we live with.  One of them has come to importance as I learn about the requirements of a new heating system for this odd house.

The “British Thermal Unit”, BTU, is at first look, a rather abstract measurement.  It is a measure of energy disguised as a measure of heat.  In fact, as we learned in freshman physics, heat and energy are one and the same, but it’s ok to not recognize this; it is not intuitive, at least not to me.

But once you learn that a BTU is the energy needed to raise one pound of water one Fahrenheit degree, it starts to make sense.  One pound of water.  One Fahrenheit degree. Pure British.

A pound of water is about a pint, an eighth of a gallon.  So your 40 gallon water heater tank can be warmed up by a degree by applying 320 BTUs of energy.  You probably want that 60-degree water from the city to be warmed up to around 120.  That will take 320×60 = 19,200 BTUs.

Oh, and how soon would you like that warm water?  If you can wait an hour, then your water heater needs to deliver 19,200 BTU per hour.  If you are less patient, you will need to boost that rating.

To heat an entire house in a Minnesota winter requires much more.  The old boiler, now removed, was rated at 150,000 BTU/hr.  Not all of it stayed in the house, much of it went up the chimney (now also removed).  The new boiler will be 100,000 BTU/hr, 95% of which will actually do the useful function of keeping the house warm.

I will be asking the gas company to switch me to “high pressure” gas delivery.  High pressure is also called “two pound” gas, meaning that the pressure in the gas line is two pounds per square inch (in familiar British units) above atmosphere.  To do this, they need to know my total worst case gas consumption.  So I considered what it might be.

Here’s the scenario.  I have invited guests to stay over the cold winter holiday, the house is being heated at full capacity.  The guests are taking long hot showers before the big dinner party.  I’m washing and drying clothes while preparing a full dinner with baked potatoes and lutefisk in the oven and all burners going.  And there is a turkey being cooked in the backyard gas grill.  All together, I’m going through 230,000 BTU/hr!  (This is almost 70kW for those of you in the metric world).

This is an obscene amount of power, and it would be unlikely that this scenario would ever actually occur (there would be no lutefisk).  But this is what the gas company needs to plan for, so I will report the numbers.

I will take satisfaction in that the energy being consumed by burning natural gas is being efficiently burned and efficiently consumed.  When winter comes I will be monitoring the duty cycle of the boiler.  It would be nice if it approaches but never reaches that full-on condition.  I’ll be checking the BTUs per hour, and thinking of Nancy Reagan’s recipes as I do so.