Sizing a gas boiler

January 03, 2010 | categories: engineering, estimation, heating, energy | View Comments

When you own a house, you are forced into bizarre contortions to reduce the cost of maintenance. For example, our roof needs to be replaced, but part of that is repairing the chimney. We don't really even need the chimney, because we don't have any fireplaces left-- they were all removed decades ago, well before we owned the house. But, until late this fall, when we installed a new boiler and water tank, we still needed a way to vent our ancient and inefficient gas boiler and water heater, or the basement would fill up with carbon dioxide and soot. We realized that if we replace those first, we can buy efficient systems that need only a plastic pipe through a basement wall to vent, rather than a chimney. Our heating cost would drop, and we could just toss the chimney in a dumpster, which is cheaper than repairing it, and need only be done once. Also, when we do replace the roof, we don't have to worry about finding someone with the skills to make a reliable flashing around the chimney.

As a result, we decided to replace our gas boiler and water heater. (The water heater actually had some life left in it, but we just wanted to do this once and be done with it.) An added advantage is that we could tear out the matrix of steel pipes that undergirds our first floor and replace them with PEX tubing, so I no longer have to wear a helmet when I walk around the basement.

Knowing that we're going to replace the boiler and water heater, someone had to figure out how big the replacements should be. The water heater is fairly easy-- we had a 40 gallon tank, which was plenty for the two of us, given that our shower uses around 6 L (1.5 gallons) per minute. Given that we were replacing the boiler and the water heater at the same time, it made sense to install an indirect storage tank, rather than a separate water heater. With an indirect setup, the super-efficient boiler heats water to warm the radiators in the house, but it also has a second loop that runs through a heat exchanger in a water storage tank. The heat exchanger is necessary so you don't end up drinking water filled with radiator rust. The boiler manufacturer we considered most seriously, Viessmann, doesn't make a water storage tank smaller than 42 gallons, so that's what we chose. We also considered getting a boiler with an integral on-demand system (the WB2 6-24C), but the output looked a little shy of what we wanted. (She-who-takes-hot-showers overruled he-who-takes-long-showers on this one.)

Sizing the gas boiler is more difficult. Our old boiler was rated at 48 kW (164,000 BTU/h or 164 MBH). Our old boiler didn't modulate-- it either burned at full power or turned off. Using our gas meter and a stopwatch, I measured the full-power consumption at 3.0 ft3 per minute, which is 53.5 kW for natural gas, slightly above the nominal rating of the boiler. On a cold evening recently, when the temperature was holding constant for several hours around 33° F outside and 66° F inside, and there was no sun and little wind, I measured the duty cycle of our old boiler. Over a 133-minute period, it was burning 26 minutes, which is about 20%. This means that average power going into the boiler with a temperature difference of 33° F between inside and outside was 11 kW. Maybe 8 or 9 kW of that actually makes it into the house, while the rest goes up the chimney.

The choices

I was trying to choose between the Viessmann WB2 6-24 and the larger 8-32. (The parts of the names of the models after the hyphens are close to, but not equal to, their peak output in kW-- perhaps they were the design targets.) Both models are extremely efficient condensing boilers; depending on how they're loaded, they operate in the 90-98% efficient range. As they're loaded closer to peak power, their efficiency drops. It also drops with the returning water temperature-- if the water comes back from the radiators cold, it's draws in heat from the burner more efficiently than if it comes back warm. The radiators in our house were installed before insulation and storm windows were standard, so they're quite effective at distributing heat.

This suggested to me that our new boiler would probably be operating near the high end of its efficiency range; the real danger was that in the fall and spring, when just a little heat was needed, it would inefficiently cycle on and off, because its lowest level of modulation would be too high. The larger of the two models I was choosing between cut off at 11 kW, which, you'll recall, was what we were using at 33° F with a much less efficient boiler, so that made the smaller one seem like a better fit. (For those of you unfamiliar with the weather in Boston, the average daily high temperature in the coldest month is 36° F in January.) My estimation was that on a really cold night, we might have a temperatures outside in the single digits, and we'd keep the house in the upper 50's or low 60's, so call that a delta of 50 degrees, about 1.5 times the 8-9 kW we needed at 33 F-- call it 14 kW to be safe.

The last factor I thought about was wind. Our house is not particularly exposed, but I suspect a cold wind could double the heat loss, which would definitely max out the smaller boiler, which peaked at about 25 kW, allowing for boiler inefficiency. If that were the end of the story, I would have chosen the larger boiler, but there were three other factors. The first was that the new boiler would zone the radiators, so I could turn off less critical rooms if we were short on heat. The second was that I'd rather invest money in insulation than a bigger furnace. Finally, we were thinking about installing a woodstove (for strictly recreational purposes-- Sundays, crossword puzzles, and so forth).

The results, in which we learn whether Brandon is an idiot

Against the advice of the heating contractor, we got the smaller boiler. They did a beautiful job installing it in November. Last week, we had a very cold (sub-10° F), very windy night, and the system kept up with demand. We just got our first gas bill covering a month with the new boiler, and our gas usage dropped to 150 therms from around 250 therms for the same time last year, and last year was slightly warmer, so I'm pretty happy. The payback time on the added expense above the cost of an inefficient system will be less than 10 years, which is less than the warranty on the system (except the control electronics). The water storage tank, which is stainless steel, is warranted for the lifetime of the original owner in the original house.

I'd estimate that about half of the savings are from reductions in heated area (spare bedroom, attic) and half are from higher efficiency.


But nothing is perfect. Because we replaced all the large, cast iron pipes in the basement with PEX tubing, the basement hovers in the low 50s in the winter, rather than in the low 60s. This was expected, but I decided I'd rather wear a winter hat than a helmet. Also, because we're not losing heat throughout the system, the boiler runs a little cooler than the old beast, which means that the radiant floor heat in the kitchen has trouble keeping up on the coldest days. I think I can probably fix that by tweaking the boiler target temperature and the circulation pump flow rates.

One other wrinkle is the control scheme used by the new system. Viessmann boilers come with an external temperature sensor, which is mounted outside on the north side of the house. Viessmann's intent (I think) is that the installer calibrates the system to drive the temperature of the hot water supplied to the radiators as a linear function of the outside temperature and the return temperature of the water. The idea is that when the temperature outside changes drastically, the boiler can react earlier than if it had to wait for the effect to be detectable in the radiator water or thermostats. Strangely, our boiler is set up to use the two programmable thermostats we used with the old boiler. The brain of the boiler targets 75° F, but the circulation pumps for the radiators and the floor heat only turn on when the thermostats call for heat. I'm not sure that this is actually worse, but it sure is strange to install an external sensor if you're not planning on using the signal.


Here are the old system and the new system. If you click on the pictures of the new system, you'll get to Flickr, where I've added a few notes about what the different parts in the picture do.

Old system with headbanging pipes

The old gas boiler and the headbanging pipes

New system

The new boiler

Supporting plumbing for new system

Ancillary plumbing

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Building a green server

September 13, 2009 | categories: energy, projects | View Comments

Ever since a brief stint as a network administrator in 2005, I've admired the discipline of data backup with undeserved zeal. A little more than three years ago, I decided that I should probably build my own server as a data repository; it could also serve this blog, store my email, and so forth. At the behest of my friend Mike, who once claimed that the massive data breach that Google will eventually suffer will be worse than September 11th, I also kicked my Gmail habit. (Mike has not kept up his end of the bargain, and still uses Yahoo Messenger. Also, he owes me a comic book about penguins.)

Last spring, as we were getting ready to move to Somerville, I had accumulated most of a server, but I didn't really have a good place to put it. Now that we've moved, I have an insulated attic, a moderately dry basement, and most importantly a killer garage. If the house burns down, the data is safe in the garage!

While I am definitely a sucker for an attractive rackmount server with hot-swap drives, I'm even more of a sucker for energy efficiency. I did some research and built what was, at the time, the greenest server I could build with consumer-level components. With the recent release of solid state hard drives, I don't think I can claim to be setting any records, but I think I'm doing pretty well in terms of required cooling power per processor cycle.

The rig:

  • Dual Xeon L5410 quad core processors
  • 1U rackmount case, RM1002T
  • Sparkle 80plus power supply, FSP400-601UG
  • Western Digital low power drive, WD5000AACS
  • Asus DSBV-DX motherboard
  • Two Corsair 1 GB DDR2 667 MHz FB-DIMM
  • Quiet, low power fans (Antec 40mm Ball Bearing Case Fan from ANTOnline through

Total power consumption started at 131 W with the processors idle, but I was able to reduce it to 102 W by replacing the stock fans with the quieter Antec fans. I haven't actually measured the peak power consumption yet judging by the processor specs, I expect it to be at least 200 W, but probably not more than 250 W.

I tried the server in the poorly-insulated garage this summer, and it overheated pretty quickly. The revised plan is to try the aforementioned moderately dry basement, which affords the additional interesting opportunity of building a geothermal cooling system for it. (Yes, it's a ridiculous idea-- ridiculous and awesome.)

I don't know if I'll ever get around to that, but I'm starting with a water cooling system, which I think is a necessary precursor to a geothermal system. If the water cooling works, I might turn it into a water pre-heater for our domestic hot water heater. I previously calculated our hot water load at around 2 kWh per day, which is an average power of 83 W, less than the average power of the server. We'd certainly lose a lot of the heat along the way, but if I'm going to dump waste heat somewhere, it might as well be into water I want to heat anyway.

(Before you go rushing off to, note that this "heat your water with your PC" plan is not an economically sound proposition-- heat from electricity is about 4 times more expensive than heat from natural gas.)

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2009 woodstove data from the EPA

September 08, 2009 | categories: energy, engineering | View Comments

The EPA regulates woodstove emissions in the US. Each year, they publish a list of the roughly 700 types of woodstoves sold in the US that includes an emissions metric (grams of particulate per hour) and a range of output power (BTU/h) for each stove.

For reasons that I may never understand, the EPA releases this data as a PDF.

Datacentric citizen that I am, I have converted the data to spreadsheet format, so you can sort it by emissions rate, if that's your kind of thing. I also removed some of the duplicate data. Some stoves were listed by two manufacturers, where one is a subsidiary of the other, but in some cases, one company or the other no longer exists. Such is the case with CFM Corporation, the bankrupt owner of Vermont Castings and Majestic Products, so I deleted those rows. (It appears that both brands, as well as several others, are now manufactured by a new parent company, Monessen Hearth Systems.)

By the way, as a quick summary of the data: if you want a clean stove, buy one from Vermont Castings. They make 6 of the 10 cleanest stoves, 5 of which emit less than 1 g/h of particulates. For comparison, fireplaces produce around 30-60 g/h.

Here are the files:

And, for those of you using the only operating system where virus scanners are normal: Excel spreadsheet

I should also say that particulate emissions are not the only emissions to think about with a woodstove. According to the Energy Information Administration and Google's sweet unit conversion feature, wood releases around 85 g of CO2 per MJ of heat. That's better than coal, which is up around 90-100 g/MJ, but worse than natural gas, at 50 g/MJ. Electric heat is the worst, at 125 g/MJ, assuming the electricity is coming from a ~40% efficient natural gas power plant like the Exelon Power plant in Everett that supplies Somerville's peak loads.

Of course, you have to heat your house somehow (or move to the tropics). Trees will grow back soon, while coal and natural gas take a little longer.

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Aerial reconnaissance of renewable energy, aided by Google and Bing

September 02, 2009 | categories: energy | View Comments

Here's a story for you, told through links to satellite imagery.

In reading about concentrating solar thermal plants this morning, I learned of a copper electroplating operation in Chandler, AZ, that used 5500 m^2 of solar trough concentrators for process heat. Searching for "copper electroplating chandler arizona" on Google Maps yielded only one address actually in Chandler: Gould Electronics' foil division at 2929 W Chandler Blvd. I see a big lot with a lot of dirt and industrial stuff, but no solar thermal array.

Switch over to for their orthographic "bird's eye" imagery, and search for "2929 W Chandler Blvd, Chandler, AZ". This shows you what I assume is the Gould Electronics building, but I still don't see any solar thermal array.

A search for "gould electronics solar thermal" provides a link to a 1993 paper from NREL that discusses the operation of the plant over the course of 10 years. The NREL paper has a picture that shows the array. But why can't I see that in the aerial shots?

Screenshot-Gould Electronics solar thermal from NREL paper

Larger version

Change to the aerial view, and we have a surprise-- the building is gone! It's strange that Bing has images from two different times, but I suppose some of the images have to be older than others, and there's no point in trying to synchronize them, given how slowly buildings and roads change.

Going back to Google for confirmation, it turns out that all the dirt I noticed earlier was due to the fact that the photo was taken while the building was being demolished. Street view shows nothing but an empty lot.

Oh well-- no copper electroplating solar thermal array for me. As a consolation prize, here are the Kramer Junction plants, SEGS III-VII, built in the 80s. (Looks like about 10 out of their ~8440 troughs are substantially misaligned-- slightly more than 0.1%. I wonder whether we're seeing mechanical failures or controller or sensor errors. (And, no, I didn't count all the troughs. I used multiplication.))

More technical data on the SEGS plants, if you're interested.

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Becoming a renewable energy engineer, part 2 (with videos)

July 26, 2009 | categories: energy, engineering | View Comments

I was invited to speak at Olin College a few months back as part of a seminar series about sustainability and engineering. The talk was indirectly inspired by my previous post on the topic. (By the way, Olin is great. If I were going to engineering school now, I'd definitely apply to Olin.) The folks at Olin recorded the talk, and after some manipulations with mpgtx, it's now up on Youtube.

The video is posted in 6 parts below. If I had to summarize in a few lines what I said, I'd say:

  • Most importantly: answer the right question.
  • If the question is, "How can I use our current energy infrastructure to propel myself in a 4000 pound steel cage?" the right answer is, "Build a hybrid SUV."
  • If the question is, "How can I kill more efficiently?" the right answer is, "Make robots kill on my behalf."
  • These are the wrong questions. Right now, the crucial questions we need to answer involve balancing our desire for the fruits of energy consumption against global heat death.
  • Differential equations: learn to solve them by computer. Learn when to distrust that computer.
  • Take statistics.
  • If a class is boring, it's because it's taught poorly. There are no college classes in boring subjects.
  • Go broad first. Going deep is fine if you have the passion, but as an investment, it's unlikely to pay off.

Direct links: Becoming a renewable energy engineer [1 of 6] Becoming a renewable energy engineer [2 of 6] Becoming a renewable energy engineer [3 of 6] Becoming a renewable energy engineer [4 of 6] Becoming a renewable energy engineer [5 of 6] Becoming a renewable energy engineer [6 of 6]

The videos themselves:

Thanks to Matt Ritter, Elsa, and the rest of the Olin folks who helped with the video taping and mucking around with the video files.

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