A customer called me tonight and asked me a question that I’ve never considered before. I’m not in any way comfortable giving advise on Geothermal heating, but I’d like to save him some time before he calls an expert. Can a Ground source geothermal heat pump be used in a multi zone forced air system without having multiple loops? The house he’s building is about 4200 square feet. He’d prefer to use a multi zone system, but he thinks it’ll be too expensive to install multiple loops.. Can it be done on a single loop, and how would it be accomplished? I appreciate any insight you guys can offer me.
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Ground source loops
are sized in most of the continental US for the maximum cooling load. The number and depth of bores for vertical fields or length of the trenching for horizontal fields depends on that cooling load, not zoning. Multiple GSHPs (ground source heat pumps) are typically installed on "single loop" fields. This is the most common type of system that I have seen in commercial and institutional designs. The field and the piping just need to be sized to meet the requirements of the connected equipment.
Just to add to what Tim said.
The zoning of the system is indepentent of the GSHP loop.
I'm building a 4,700 sq. ft. (total including the walkout basement) with a two stage and three zone GSHP. The ground loop is sized for the largest cooling capacity.
The only additional thing he must consider is the added amount duct work needed in a multi-zone system. A good HVAC guy will plan his distribution system to match the loads in each area within each zone. Most often that means additional supply and definitely return ducts. Trying to squeeze extra duct work in a house that is not designed with that in mind creates compromises that have a lot of home owners growling about poor performance from thier multi zone sytem, be it conventional gas, electric, or GSHP.
Need some clarification on terminology. Common terminology is "ground source heat pump" (GSHP) there is no 'geothermal' in this ... which generally refers to high temperature ground water systems.
I'm also confused by "multi-zone" which I think you simply mean multiple zone. In the HVAC industry, those are two very different animals. So are you using multiple heat pumps in separate zones/systems in the house? I think that is doable w/ a single GSHP loop system (although I don't necessarily mean single well hole).
Are you doing wells or slinky's (i.e. buried loop)?
In the HVAC profession
"geothermal", correctly or not, in conversations concerning heat pumps is synonomous with ground source. I realize from many of your posts that you are not involved in the HVAC industry as a professional so the confusion is understandable. The term literally is correct; geo - being earth related and thermal - regarding the movement of heat.
As far as multiple zones, vs multi-zone, again in coversations concerning GSHPs, refers to muliple compressorized units distributed along a single ground water loop. Context taken into account resolves the confusion. While I seldom see the old hot deck/cold deck multi-zone commercial units once common in the non-residential HVAC world, I am familiar with the energy hogs and made the assumption that that was not the subject of the OPs question. Leap of faith maybe?
"Slinky" coils, BTW, are used in surface water features (i.e. ponds or lakes), not buried.
Most GSHP contractors don't use the term geothermal ... even though you are right. It's a bit symantical, but the industry does typically distinguish between the two, although we see the mix a bit. Typical of many HVAC terms. The industry commonly refers to geothermal as higher temperature ground coupled heat source systems.
My profession is fairly heavily involved in the HVAC industry ... which is why I made the 'multi-zone' distinction. I tend to be a bit of a stickler w/ terminology, because it's important in good communication to use the 'right lingo' (even though admitedly, 'right' in the HVAC industry is often a matter of opinion, I suppose).
Slinky's I've seen plenty of times in a buried configuration as well as buried horizontal loops, so there is no 'typical' or standard with respect to that.
I understand and
didn't mean to be so snarky. To me, as design engineer in the HVAC industry, specific terms have specific meanings and I have frankly tired over the past few decades of correcting improper lay use of these terms. Now, I convert what is said into what is meant and move on.
I have seen horizontal bores and horizontal trenches with buried loops, but not "coils". I would challenge the competency of a design(er) that included buried "slinky" coils.
No worries. I've learned over the years that specific terms are often used differently by different professionals, so the way I learned isn't always exactly 'universal'. I've worked enough w/ engineers over the years to know that I generally have my terminology 'right' for a large part of the population ... although it may only be right 'out west' ya know?
I swear I read a compelling argument against geothermal. Basically said you spend a ton up front, and don't save much over a high efficiency modern HVAC system - because you're still pushing air across a coil and the fan motor is the big energy consuming mechanism in both. Pump energy for pushing liquid thru underground loops also consume energy.
I've got my "Googlador retriever" searching for something to substantiate my memory...
I've often been asked ... are GSHP a good thing? I've always responded by saying it depends. The technology is a great technology. It is not particularly cutting edge. GSHP are very efficient in that rather than heating or cooling using a condenser in extreme outdoor temps, it uses the temperature of the ground. This is a MUCH easier task (i.e. more efficient).
However that doesn't mean everyone should (or can) install one. IMO they are generally best used in extreme climates since GENERALLY they can tend to be more expensive (however, that is not always the case). Bigger expense is justified by bigger loads during extreme weather (i.e. when they have the biggest advantage in efficiency).
I've seen GSHP be cheaper than conventional counterparts, but that is not always the case (and usually isn't). So I've always told people to never generalize about whether GSHPs are a good idea or not. They are very site specific (in terms of their cost and what type of system you can/should install).
Using a GSHP in a mild climate where an air source heat pump runs efficiently throughout most of the winter conditions is largely not justified (e.g. Seattle). But you place it in cold winter conditions like Montana or Minnesota, it's a different thing (although site conditions may still affect the choice). IMO - never generalize about the applicability of GSHP systems.
While I agree
with the statements of climate being a deciding factor, the fan energy part is wrong. One could make a compelling argument against a GSHP system, however, if that argument was based on the comparative energy of the fan vs the compressor, it would be flawed.
For instance, a typical 5 ton, 13 SEER system will have a 3/4 hp fan motor and 5 hp compressor. Pump energy required to push the fluid through a 5 ton GS loop is on the order of 1/2 hp. A nominal 5 ton GSHP system, with 55 degF EWT will operate 30 to 40 percent more efficinet in the refrigeration cycle, a 2 hp compressor energy savings. I don't know about you, in my world 2 is bigger than 0.5. High efficiency air based systems are very expensive and produce worse performance in dehumidifcation in many instances.
All factors must be considered to make a solid economic evaluation including the increased cost of a high efficiency air based system, cost of electricity in summer and winter, cost of heating fuel and the cost of installing the loop and all the components and controls. An argument not based on all these factors would not be very compelling.
The supply fan energy is a wash. The loop pumping energy is likely less than the fan energy of the condenser fan in a conventional heat pump, but more than a gas fired furnace, I suppose. The compressor in a GSHP will work less than in a conventional air source heat pump ... 'course that is the nature of GSHP system efficiencies.
If you are referring to multiple heat pumps being served by a common loop, this is fairly common in the industry. Multiple bore holes (if that is what is used) are manifolded together to form essentially one loop. It is quite common for multiple heat pumps to share the same loop.
There are a couple of ways to plumb that loop ... not sure of my terminology here, but essentially a parallel or series type loop serving the equipment are the two basic options.