Geothermal Heat Pumps - What You Need To Know and What I've Learned Living With One
Geothermal heat pumps have a high first cost, are high tech, incredibly energy efficient and can provide your family with tons of hot water that's nearly free. Are they a good idea or not? What are the critical things to keep in mind when buying, sizing, designing and installing one? How important are the sizing and sealing of the air ducts?
In 2010 we built our 2,450 square foot, four star Green rated home on 21 acres about 45 miles north of Austin, Texas. Our builder, David Weekley's Custom Build On Your Lot group was open to working with us on a lot of cutting edge high performance changes. Being a building scientist I knew that there is a logical order to how one spends their money for comfort, health, durability and energy efficiency. I also know that the correct set of priorities between envelope and equipment isn't what most folks think it is. Too many people (especially here in hot and humid Texas) think that having a home with all of the desirable attributes listed above starts with buying the highest efficiency, and largest air conditioner you can fit into the house. Wrong old friend!
The first priority is to ensure that you have built a super efficient, well insulated, air tight envelope. That is actually where comfort, durability and energy efficiency are to be found. After you have done a great job of getting the envelope right, you then buy the best HVAC system that you can afford with your remaining dollars. When you maximize the efficiency and performance of the envelope you can then reduce the size of the HVAC system and use the saved dollars to increase the efficiency of the unit and the all important ducts.
I'll get into how and why details on the envelope in another blog (if that interests you, check the site) but here are the basics. No bat insulation, only total wall fill. I used a damp spray cellulose in the stud bays with a continuous sheathing of rigid foam board around the outside. I then built a sealed, unventilated attic ( yes, they are allowed in the building codes since 2000) with spray in place foam against the underside of the roof deck. I bought high performance windows with excellent SHGC and U factor. I sealed the envelope up so tight that if you threw this house into the lake, it would float for two weeks! I installed a ventilation system to ensure my home would breathe and have very healthy indoor air quality and then I began to consider the HVAC system.
I got a bid from the contractor the builder recommended on a two speed, 16 SEER heat pump with a variable speed (ECM) blower and a thermostat/humidistat control. You may not know this but Austin is an incredibly humid spring, summer and fall climate. It is well within the officially designated "Hot and Humid Climate Zone" as defined by the air conditioning engineers society, ASHRAE. So, just controlling temperature isn't enough, you have to dehumidify that steam sauna environment we call outside air for nine months of the year here.
The bid came in at $12,500 and they proposed a five ton unit for our home. This is in line with the old "rule of thumb" that in the south air conditioners are properly sized when the cool between 400 and 500 square feet of conditioned space per ton of cooling capacity. This bid was at 490 square feet per ton. The contractor explained to me that since our home has ten foot ceilings, 26 windows and most of them were 4 feet by 8 feet, he just couldn't and wouldn't risk a smaller unit and that he would ensure that it would keep us cool.
By the way, you may be asking why our home has so many windows and why they are so large when limiting window area is a key element in energy efficiency and comfort. The simple answer is, I lost the "discussion" about how big and how many windows we would have. I did manage to locate most of them so that they did little harm to our comfort or energy costs and we now have a home with lots of brightly lit rooms and low bills and great comfort. Everybody won.
One of those beloved notions about a/c is that "bigger is better." In air conditioning, it just isn't so. The more you oversize your air conditioner, the worse job it does removing and controlling indoor humidity. It runs in short cycles, the evaporator coil never has time to get really cold, and you control the temperature, but the air is sticky and uncomfortable. The bills are high too. When an air conditioner starts up it is operating way below its rated efficiency. When a 16 SEER starts up, at first it runs at about 8 SEER. Then as it warms up, the efficiency rises. Just like your car. The engine starts off getting poor mileage, but after it warms up and gets out on the highway where it can run at peak efficiency, you get your best mileage. Air conditioners are very similar in this way. It takes a typical air conditioner at least ten or twelve minutes to reach its peak efficiency and to really start to remove humidity the way it should. A few long cycles are far better for comfort and cost than a lot of short cycles.
A right sized unit will run longer, run more efficiently, use less electricity per minute operating because it is a smaller compressor and remove more humidity than a typical oversized unit. Studies have been done by lots of groups and they all find that the average air conditioner is oversized by 50% to 100%. What a deal. You get high bills, high humidity, and low comfort. Who could ask for more?
I knew better than to go with that oversized unit and respectfully went out for a second bid. This time from an old friend, Stan Johnson at Stan's Heating and Air. Stan is pretty knowledgeable when it comes to building science and he was working with another old friend, Tom Romberg, who is certified in geothermal heat pump design. I had them do an ACCA (Air Conditioning Contractors of America) Manual J. That's a computerized heat gain/heat loss calculation that determines exactly how much heating and cooling each room in a home needs. You sum the rooms and that is the size air conditioner and furnace you really need. A manual J includes every detail about your home. Window size, type, orientation, wall R value, orientation, air tightness level, insulation, number of people, city where it is located, and on and on.
Well, after a few days, Tom emailed me the Manual J. It was a little short of 3.0 tons and only hit that mark on the most extreme days. Stan and Tom proposed a two speed Infinity GP-TX geothermal heat pump. The unit would deliver 1.75 tons in low speed and 3.0 tons in high speed with an 31 EER which is equivalent to a 34 SEER. They would drill three 215 foot deep wells for the three ton field. The bid was $22,000. The sizing looked right to me, but just to be sure I reviewed the Manual J and saw that it had been properly done. I told Tom that it looked to me like the system would spend most, if not all of its life in low speed. He agreed. In case you are wondering, the ratios are 1,400 square feet per ton in low speed and 817 square feet per ton in high speed.
That was good enough for me. You see, I knew that the geothermal unit qualified for a 30% Federal tax credit based on turn key installed cost. That means that I would get back $7,260 from Uncle Sam in cash. A tax credit is far better than a tax deduction. One reduces your taxable income, the other comes right off of what you owe, and comes back to you dollar for dollar. This brought the total cost down to $14,740. That left only $2,240 difference between the two systems. If my cheap utility (PEC/LCRA) had offered a rebate, that would have left nada, but they don't. The geothermal system would operate at an efficiency level of more than double that of the air source unit which would reduce our summer cooling bills by one half. I also knew that the Infinity geothermal unit would provide us with lots of hot water that would be almost free in the cooling season and cost less than one fourth what an electric water would cost for the heating season. That meant an annual savings of around $400 for water heating. The numbers were crunched and the answer was that the geothermal unit would recover the added cost in less than 2 years. This was a slam dunk.
They drilled three 215' deep wells spaced 17' apart for the earth coupled field. One for each ton of cooling capacity. Everything was done to the standards of the International Geothermal Heat Pump Association (IGSHPA). You should demand the same. We then had an old friend from Waco, Paul Wieboldt of Tradewinds Appropriate Technology come down and perform the ACCA Quality Installation inspection and testing protocol. Just to be sure the whole system was actually performing at peak efficiency and would deliver real comfort. In building science there is an old saying: "In construction you don't get what you expect, you get what you inspect."
I had to take pictures to remind myself where the field is because it's now covered by our flower gardens of Texas native, water wise, deer resistant, flowering perennials. We have about 2.5 to 3 acres of flower gardens now. If you are interested in what plants we've used and how we have so many birds and butterflies in the yard that every visitor says something about it, look among my other blogs. I'll write one on that topic soon.
The unit lives in our attic, which if you noted (if you were paying attention earlier) is a sealed, unventilated attic with foam insulation on the roof deck. That attic has stayed within three degrees of the house temperature on even the hottest (112ºF a record) afternoons for two summers now. Yes, I actually left one of my digital refrigeration thermometers up there and would go up around 4pm or 5pm and check on the attic temperature and the system temperature split across the evaporator coil. (Geek alert) That allows the air conditioner to operate at full efficiency. It's not sitting in an oven trying to make and distribute cold air!
The two summers we have lived in the house have been the hottest summers in Texas history! Over the last 30 years Austin has averaged 12 days a year over 100º degrees. Most of these days are usually in the 100º to 104º range. In 2011 we had 90 days over 100 and we had stretches several weeks long where it hit 107º to 112º every day. The morning lows were usually about 78º to 80º degrees. The humidity was it's usual unbearable self. The dew point stayed between 70 and 75 degrees. It never got cool.
We kept the house at 74º degrees throughout the summer. Our all electric home electric bill never exceeded $150 and we were watering four hours a day trying to save a new yard so the well pump was running a good bit too. Most of our summer bills over the last two years have been around $90 -$110. The base bill before any usage is $25 and the well pump costs us about $35 a month for a total of $60 before we turn the HVAC on. That means that our cost to cool the house to 74 at 50 % RH is about $30- $50 a month in months of 100º+ degree heat.
The air conditioner stayed in low speed until it went over 108º. It would then go into high speed for a few hours until sundown and then drop back to low speed. Our indoor relative humidity stayed between 45% and 51% all summer. It doesn't get any better than that from either a comfort or a cost perspective.
We had all of the hot water we needed and the unit is so quiet we have to mute the TV to hear it. By the way, since it's a geothermal unit, there is no above ground condenser. It's so quiet outside without that constant drone of the condenser. That's been one of those unexpected benefits. In the winter the system keeps the house at 70º degrees when it's in the teens with no problem. The envelope is so efficient that the house holds it's temperature for a long time after each run cycle. We've found that the house temperature will only rise or fall by a few degrees in a day with the HVAC off and the windows closed.
Just another day in the life of a building science geek...