Mechanical Systems for Low-Load Buildings

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Mechanical Systems for Low-Load Buildings

At the Building Science Experts’ Session, Dr. John Straube presented an amazingly comprehensive overview of the challenges posed by high-performance buildings

Posted on Dec 26 2012 by Allison A. Bailes III, PhD, GBA Advisor

Professor John Straube spoke for a whole day at the Building Science Corporation's Experts' Session earlier this month. His topic, a good one for GBAGreenBuildingAdvisor.com readers, was mechanical systems for low-load buildings.

You know that expression about how the information comes at you so fast in some classes that it's like drinking from a firehose? With Professor Straube, it's like trying to drink from a tsunami! The guy has not only a phenomenal knowledge but he's also a fantastic teacher and incredibly witty.

Giving you a good overview of everything Straube talked about would be quite a challenge (and result in an article longer than anything Martin Holladay has written here!), because he really covered the whole gamut of this topic. So, I'll throw out a few highlights of the day here and encourage you to attend one of his workshops if you ever get the chance. If you’re a GBA Pro member, you can watch some of the video clips of his talk when they get posted. (If you’re not a member, sign up today!) ''

You may also get the opportunity some day to buy his book on this topic. As we were talking on the last night, Joe Lstiburek and others strongly encouraged Straube to get this knowledge into a book, and I hope he does.

It's hard to find a furnace rated at less than 40,000 Btu/h

Building enclosures have improved dramatically in the past two decades. We have more insulation, better windows, and sealed air barriers. Many homes have peak heating loads of 15 to 30 thousand BTU per hour (kBTU1,000 Btus/hr) — even in cold climates. This applies to large houses and buildings, too, because high performance enclosures result in zones with low loads. The problem is that most conventional heating equipment available doesn't go that low in capacity. About the smallest furnace you can get is 40 kBTU/hr.


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Professor Straube mentioned that he's been talking with HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment manufacturers about this for about a decade, and they just don't get it. Carrier and Trane, he said, have told him they don't sell low capacity equipment because no one's buying it. They failed to see that if the equipment is not available, no one can buy it so of course they're not buying it because it's not available. Evidently those discussions haven't turned the corner yet.

Specifying low-capacity equipment won't save you any money

Straube covered a lot of ground in his presentation. After introducing the need for low-capacity mechanical systems and the lack of conventional equipment, he dove into the details of the different systems available, how they work, what some of the limitations are, and how difficult it is to do it right.

One of the big obstacles is cost. "We're beyond talking about saving money on equipment for low-load houses," he said, because you won't. If you're a spray foam contractor, for example, trying to get builders to bite on the higher cost of foam because of savings they'll see from smaller mechanical systems, it's not gonna happen.

You can get a 46 kBTU/hr 95 AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. furnace for $697. If you want something with less capacity, you could go for a boiler, tankless water heater, or minisplit heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump., all of which are significantly higher in cost.

The same is true of air conditioners. When you go from a 2-ton system to a 1.5-ton system, it costs more and is less efficient, too. So if you want a low-load building to have a low-capacity mechanical system, get used to higher cost.

Returning water temperature affects efficiency

In the first half of the day, Straube went through some of the basics of efficiency ratings (SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. , EER, and COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. ), details about condensing boilers and furnaces, and combo systems (using one system to provide heat and domestic hot water). He talked about some of the problems with condensing systems and the temperature of the entering water or air.

If you think you have a boiler that’s 95% efficient, for example, you may not be getting that if the water returning in the hydronic distribution system comes back too warm. Condensing boilers and furnaces work by extracting the latent heat from the water vapor in the exhaust gases. They’re able to do so as long as the heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. is below the dew point. If the returning water (or air in an air distribution system) is above the dew point, the water vapor doesn’t condense, the boiler/furnace doesn’t extract as much heat, and the efficiency goes down. The graph of condensation efficiency shows how the efficiency changes with returning water temperature.

Desuperheaters? Thumbs down

After lunch, the discussion turned to heat pumps, both conventional, ground source, and minisplits. He blasted the idea of using desuperheaters on ground-source heat pumps to get hot water because they're usually not cost-effective and don't give you much hot water. Ground-source heat pumps work well in school-sized buildings, he said, but not so well for low-load buildings or really big buildings. He also said that he has not seen any third party data on GSHP efficiency showing a COP higher than about 3. Since the industry doesn't include pump or blower energy in their COP calculations, Straube said, their COP numbers are higher but don't give the complete picture.

The topic was mechanical systems, so there was also talk about water heating. Combo systems give you both heat and hot water, and some people use water heaters just for space heating, too. Straube mentioned that HVAC and DHW need to come together.

Looking back over my notes now, I still can't believe how much material he covered. It really was a tsunami of information. He also spoke briefly about using CO2 as a refrigerant in air-to-water heat pumps, which allows true hot water production because they can get higher temperatures than the standard refrigerants; electric resistance heat, which is OK in some cases for low-load buildings; Legionnaire's disease, which can be a problem if the DHW temperature is too low; heat pump water heaters; distribution of thermal energy (air, water, radiant); and more.

The wit of Professor Straube

As I mentioned at the beginning, Professor Straube is not only incredibly knowledgeable, he’s also a walking quote machine. Here are a few of the many witticisms we were treated to yesterday:

Caveat emptor is Latin for "you're gonna get screwed."

Cats are IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness. problems and you'd have to pass them through the HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. to solve that problem. But they're not really a cat anymore after that.

Recirculating range hoods don't solve the IAQ problem. It's like a recirculating toilet. The stuff just goes round and round and never leaves.

In San Francisco, it never gets really hot or really cold. Buildings are there mainly to keep people away from your stuff.

It's taken America 10 years to realize that tankless water heaters work better with tanks.

If you ever get a chance to go to one of his talks, I guarantee you won't have any trouble staying awake and engaged.

A great reason to be a GBA Pro!

Not everyone can take the time to go off to New England for two days to attend a session like this. The good news is that many people watched the whole thing from their home or office because Green Building Advisor ran a livecast of the sessions.

Anyone could have signed up through Building Science Corporation for the regular fee of $395 per day. GBA Pro members, however, got access to the livecast as part of their membership. What a deal! GBA Pro members will also get to watch some of the recorded videos from the Building Science Experts’ Session once they’re posted. Stay tuned!

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


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