Thursday, May 16, 2013

In defense of geothermal HVAC

Over the past few months I have been frustrated with the building science blogosphere and what seems to be a negative bias against GSHP (ground source heat pump) HVAC. This indictment of the entire GSHP industry in my opinion is largely based on a limited sample of problems in GSHP HVAC and there is not enough information on the benefits of GSHPs. The broad brush that is being used to paint all GSHP systems and contractors is more than unfair -- it is inaccurate. Whenever I hear a consumer or builder say that GSHP’s are not as efficient as promised I always say “they are if they’re designed right”. All HVAC systems are subject to design and installation errors and GSHPs are no different.  Equipment sizing, proper duct design and proper well design are all critical components of a GSHP hitting its rated efficiency and capacity. With the exception of the well design, these design issues are just as important for conventional air source heating and cooling systems, including ductless split systems.  As the building science world evolves, bloggers are relied upon as journalists, and as such they have a duty to report as accurately as possible. What we as an industry need to be focusing on is the poor design and installation practices that are rampant in all areas of HVAC.

 Sizing and whole-house perspective matter

One of the most common problems I see in bad GSHP systems is improper sizing. Oversizing HVAC has been a problem for years but it is becoming a much greater issue as the quality of  home construction improves. What was 600 SQFT  per ton in the 1980’s is now 1000-1500 SQFT per ton on a simple code-built house and over 2000 SQFT per ton on an Energy Star  house. Whether it is a new or existing home it is critical to do a “proper” Manual  J heat loss calculation. This should be done with a house-as-a-system approach. I do not care how efficient the HVAC system is, if you have an inefficient house you will never realize the intended efficiency. I often have customers asking me to install a GSHP in a 1985 built home with no energy efficiency improvements; they think that a GSHP will automatically make their utility bills practically free. Unfortunately there are those in my industry that either do not know better or just do not care, and when the consumer does not realize the savings they expect it means that GSHP gets another black eye. I often use the analogy that I cannot take a Toyota Prius engine and drop it in a 1985 Chevy pickup and make it a fuel-efficient hybrid. The HVAC is only part of the equation when it comes to home comfort and efficiency; the shell is just as important and a critical part of the HVAC design process.  There is a misconception that having a GSHP system means not having to properly insulate, and this error has done more damage to the industry than anything having to do with the HVAC system.



Design and commissioning are critical

Airflow matters! Duct design (or lack thereof) is the next biggest issue that saps efficiency in all types of HVAC. I was recently in a session at the Midwest Regional RESNET conference and watched David Richardson of National Comfort Institute  wager the efficiency of a 13 SEER system on a duct system he designed against a 16 SEER system on a code-built duct design; he had no takers. Why? Because every HVAC guy in the room knew that David was right: the ducts can make or break the efficiency of any system.  Excessive static pressure, too little return, right angles and other design elements all dramatically affect the proper operation of a HVAC system. Think of deflating your car tires halfway and going on a long trip --it would kill your gas mileage! This is also a big part of the secret of the efficiency of a ductless mini-split systems as is clearly demonstrated when you compare a ductless split to a ducted split, the efficiency tanks! I have found that the higher the efficiency the HVAC system, the greater the negative impacts of a bad duct design, and GSHP systems are as susceptible to this as any other.

Ask any HVAC guy whether the equipment or the installation is more important and he will tell you that install is everything.  I’ve already discussed the importance of proper duct design, but it’s also critical that the design is followed to the letter. I can’t tell you how many systems I’ve seen with returns not properly cut out or the wrong type of duct used. Most installers do not realize that compressed flex duct has about half the airflow capacity as the same diameter steel pipe and most ductulaters (a duct sizing slide rule) do not differentiate between the two. If the design specs five10-inch returns, the difference between flex and steel is a major impact on performance.  Proper refrigeration charge is also very important:  as little as a 5% over- or under-charge can reduce efficiency by 20%. Add bad charging to bad airflow and you can easily reduce the efficiency of any type system by half.


  How this applies to GSHP

The one unique area to a GSHP is the loop system, and at the risk of being redundant, proper design and installation are what makes or breaks the loop’s relationship to the efficiency of the system. The good news is the industry has a very specific design standard in   ARI 330 / ISO 13256-1. This standard should be used on all GSHP installations to achieve maximum efficiency. There are several ways contractors typically short cut the standard and also short change the efficiency. Well depth on a closed loop system is one way that many contractors short cut the system. On closed loop systems manufacturers will set a minimum well depth for capacity and the HVAC contractor does just that: the minimum. What most contractors do not tell the consumer is the depth of the well will affect the efficiency of the system long before it will affect the capacity of the system. In my area the difference between 150 ft. wells and 200 ft. wells can mean a 20 degree temperature difference in the returning water temperature, which translates to a reduction in EER by almost 10 points, lowering the efficiency of the GSHP down to  levels similar to top-end air-source systems. The size of the water pipe, the speed of water flow, and even the amount of turbulence on the water as it moves through the pipe all affect efficiency more than they do capacity, so many people have systems that can heat and cool just fine but never deliver on the promised efficiency.

So who is to blame for underperforming GSHP systems? The truth is we all are:

The manufacturers need to raise their minimum standards to make the systems as efficient as advertised. They also need to provide and require a greater level of training of the HVAC designers and installers that they allow to sell their product. IGSHPA (International Ground Source Heat Pump Association) offers the best most comprehensive training in GSHP installation and should be the minimum required by manufacturers. Most importantly there needs to be a higher level of accountability from their dealers; if a dealer does not follow the manufacturer’s guidelines, they should no longer be able to sell the product line.

The installers need to educate themselves on best practices and stay current with manufacturer-provided training. They need to get away from rules of thumb and other bad habits. Most importantly they need to quit intentionally cutting corners in order to be the lowest price.

Customers need to do their due diligence and screen the contractor to make sure they choose an experienced contractor. Manual J is a minimum requirement and should be reviewed by a qualified 3rd party, and on an existing home an energy audit is a must. Finally, customers who make their decision based on price alone will always get what the pay for: the cheapest system possible.

At the end of the day a GSHP system is one of the most efficient ways to heat and cool your home. The upfront cost is greater, but the life-span true cost to own will be half that of most conventional systems.  They make sense for most homeowners who are replacing a HVAC system and wanting to upgrade to high efficiency, and I’ve rarely seen a new construction project where you could not justify GSHPs.  They are more technical, and require both a more thorough upfront design and a more educated installer. That being said, GSHPs are no more susceptible to a bad contractor than any other HVAC technology.  It’s my belief that they are just being held to a higher standard, and as building science professionals we should focus more on the fact that HVAC design and install standards are the problem and GSHPs are just one of many systems affected.

1 comment:

  1. Great entry Jamie! As you pointed out, there is a big difference between a laboratory controlled environment and a house that has to deal with the realities of mother nature.

    If the final installation isn't tested and verified then there is no way of knowing the design was achieved.

    There is usually a lack of both of these aspects so the natural reaction from the building science community is to blame the HVAC equipment.

    Since the only performance metric they measure is duct tightness they have no idea how the installed system is truly operating.

    Keep up the great work!