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.
