Monthly Archives: December 2017

Change home air filters 3 times per year

Energy efficient furnaces use a surprisingly large amount of electricity to blow the air around your house. Part of the problem is the pressure drop of the ducts, but quite a lot of energy is lost bowing air through the dust filter. An energy-saving idea: replace the filter on your furnace twice a year or more. Another idea, you don’t have to use the fanciest of filters. Dirty filters provide a lot of back-pressure especially when they are dirty.

I built a water manometer, see diagram below to measure the pressure drop through my furnace filters. The pressure drop is measured from the difference in the height of the water column shown. Each inch of water is 0.04 psi or 275 Pa. Using this pressure difference and the flow rating of the furnace, I calculated the amount of power lost by the following formula:

W = Q ∆P/ µ.

Here W is the amount of power use, Watts, Q is flow rate m3/s, ∆P = the pressure drop in Pa, and µ is the efficiency of the motor and blower, typically about 50%.

With clean filters (two different brands), I measured 1/8″ and 1/4″ of water column, or a pressure drop of 0.005 and 0.01 psi, depending on the filter. The “better the filter”, that is the higher the MERV rating, the higher the pressure drop. I also measured the pressure drop through a 6 month old filter and found it to be 1/2″ of water, or 0.02 psi or 140 Pa. Multiplying this by the amount of air moved, 1000 cfm =  25 m3 per minute or 0.42 m3/s, and dividing by the efficiency, I calculate a power use of 118 W. That is 0.118 kWh/hr. or 2.8 kWh/day.

water manometer used to measure pressure drop through the filter of my furnace. I stuck two copper tubes into the furnace, and attached a plastic hose. Pressure was measured from the difference in the water level in the hose.

The water manometer I used to measure the pressure drop through the filter of my furnace. I stuck two copper tubes into the furnace, and attached a plastic tube half filled with water between the copper tubes. Pressure was measured from the difference in the water level in the plastic tube. Each 1″ of water is 280 Pa or 0.04psi.

At the above rate of power use and a cost of electricity of 11¢/kWhr, I find it would cost me an extra 4 KWhr or about 31¢/day to pump air through my dirty-ish filter; that’s $113/year. The cost through a clean filter would be about half this, suggesting that for every year of filter use I spend an average of $57t where t is the use life of the filter.

To calculate the ideal time to change filters I set up the following formula for the total cost per year $, including cost per year spent on filters (at $5/ filter), and the pressure-induced electric cost:

$ = 5/t + 57 t.

The shorter the life of the filter, t, the more I spend on filters, but the less on electricity. I now use calculus to find the filter life that produces the minimum $, and determine that $ is a minimum at a filter life t = √5/57 = .30 years.  The upshot, then, if you filters are like mine, you should change your three times a year, or so; every 3.6 months to be super-exact. For what it’s worth, I buy MERV 5 filters at Ace or Home Depot. If I bought more expensive filters, the optimal change time would likely be once or twice per year. I figure that, unless you are very allergic or make electronics in your basement you don’t need a filter with MERV rating higher than 8 or so.

I’ve mentioned in a previous essay/post that dust starts out mostly as dead skin cells. Over time dust mites eat the skin, some pretty nasty stuff. Most folks are allergic to the mites, but I’m not convinced that the filter on your furnace dies much to isolate you from them since the mites, etc tend to hang out in your bed and clothes (a charming thought, I know).

Old fashioned, octopus furnace. Free convection.

Old fashioned, octopus furnace. Free convection.

The previous house I had, had no filter on the furnace (and no blower). I noticed no difference in my tendency to cough or itch. That furnace relied for circulation on the tendency for hot air to rise. That is, “free convection” circulated air through the home and furnace by way of “Octopus” ducts. If you wonder what a furnace like that looks like here’s a picture.

I calculate that a 10 foot column of air that is 30°C warmer than that in a house will have a buoyancy of about 0.00055 psi (1/8″ of water). That’s enough pressure to drive circulation through my home, and might have even driven air through a clean, low MERV dust filter. The furnace didn’t use any more gas than a modern furnace would, as best I could tell, since I was able to adjust the damper easily (I could see the flame). It used no electricity except for the thermostat control, and the overall cost was lower than for my current, high-efficiency furnace with its electrical blower and forced convection.

Robert E. Buxbaum, December 7, 2017. I ran for water commissioner, and post occasional energy-saving or water saving ideas. Another good energy saver is curtains. And here are some ideas on water-saving, and on toilet paper.

Bitcoin risks, uses, and bubble

Bitcoin prices over the last 3 years

Bitcoin prices over the last 3 years

As I write this, the price of a single bitcoin stands are approximately $11,100 yesterday, up some 2000% in the last 6 months suggesting it is a financial bubble. Or maybe it’s not: just a very risky investment suited for inclusion in a regularly balanced portfolio. These are two competing views of bitcoin, and there are two ways to distinguish between them. One is on the basis of technical analysis — does this fast rise look like a bubble (Yes!), and the other is to accept that bitcoin has a fundamental value, one I’ll calculate that below. In either case, the price rise is so fast that it is very difficult to conclude that the rise is not majorly driven by speculation: the belief that someone else will pay more later. The history of many bubbles suggests that all bubbles burst sooner or later, and that everyone holding the item loses when it does. The only winners are the last few who get out just before the burst. The speculator thinks that’s going to be him, while the investor uses rebalancing to get some of benefit and fun, without having to know exactly when to get out.

That bitcoin is a bubble may be seen by comparing the price three years ago. At that point it was $380 and dropping. A year later, it was $360 and rising. One can compare the price rise of the past 2-3 years with that for some famous bubbles and see that bitcoin has risen 30 times approximately, an increase that is on a path to beat them all except, perhaps, the tulip bubble of 1622.

A comparison between Bitcoin prices, and those of tulips, 1929 stocks, and other speculative bubbles; multiple of original price vs year from peak.

A comparison between Bitcoin prices, and those of tulips, 1929 stocks, and other speculative bubbles; multiple of original price vs year from peak.

That its price looks like a bubble is not to deny that bitcoin has a fundamental value. Bitcoin is nearly un-counterfeit-able, and its ownership is nearly untraceable. These are interesting properties that make bitcoin valuable mostly for illegal activity. To calculate the fundamental value of a bitcoin, it is only necessary to know the total value of bitcoin business transactions and the “speed of money.” As a first guess, lets say that all the transactions are illegal and add up to the equivalent of the GDP of Michigan, $400 billion/year. The value of a single bitcoin would be this number divided by the number of bitcoin in circulation, 12,000,000, and by the “speed of money,” the number of business transactions per year per coin. I’ll take this to be 3 per year. It turns out there are 5 bitcoin transactions total per year per coin, but 2/5 of that, I’ll assume, are investment transactions. Based on this, a single bitcoin should be worth about $11,100, exactly its current valuation. The speed number, 3, includes those bitcoins that are held as investments and never traded, and those actively being used in smuggling, drug-deals, etc.

If you assume that the bitcoin trade will grow to $600 billion year in a year or so, the price rise of a single coin will surpass that of Dutch tulip bulbs on fundamentals alone. If you assume it will reach $1,600 billion/year, the GDP of Texas in the semi-near future, before the Feds jump in, the value of a coin could grow to $44,000 or more. There are several problems for bitcoin owners who are betting on this. One is that the Feds are unlikely to tolerate so large an unregulated, illegal economy. Another is that bitcoin are not likely to go legal. It is very hard (near impossible) to connect a bitcoin to its owner. This is great for someone trying to deal in drugs or trying hide profits from the IRS (or his spouse), but a legal merchant will want the protection of courts of law. For this, he or she needs to demonstrate ownership of the item being traded, and that is not available with bitcoin. The lack of good legitimate business suggests to me that the FBI will likely sweep in sooner or later.

Yet another problem is the existence of other cryptocurrencies: Litecoin (LTC), Ethereum (ETH), and Zcash (ZEC) as examples. The existence of these coins increase the divisor I used when calculating bitcoin value above. And even with bitcoins, the total number is not capped at 12,000,000. There are another 12,000,000 coins to be found — or mined, as it were, and these are likely to move faster (assume an average velocity of 4). By my calculations, with 24,000,000 bitcoin and a total use of $1,600 billion/year, the fundamental value of each coin is only $16,000. This is not much higher than its current price. Let the buyer beware.

For an amusing, though not helpful read into the price: here are Bill Gates, Warren Buffet, Charlie Munger, and Noam Chomsky discussing Bitcoin.

Robert Buxbaum, December 3, 2017.