Geothermal Heat Pumps | Construction and DIY projects | Forums

I'm no expert on Geothermal.  Not sure where you live or what your avg temps and so forth are.  What I will do is just ramble and maybe something I say may help you out.  Geothermal can be a head scratcher for sure and the term is used VERY loosely.

A little insight into operation of a standard hvac system. Same for a Heatpump in cooling mode.

For Air Cond (Cooling) with a standard system. The evaporator coil runs at roughly 35-40 degrees.  It takes this low temp to cool a house.  If your house is 72, then you have 72+/- air entering your 35-40 degree coil, the air should exit 16-20 degrees cooler.  Now you have close to 52 degree air blowing into your house.  That air mixes with your 72 and sooner or later it will drop the room temp to 71.  The temp difference of the air across the coil is called a Delta T or Temp difference.

Now using ground water running through a coil or radiator with a fan blowing across it had better be cold.  Even 55 degree water in the coil will remove some heat from the air traveling across it but not enough to cool a house...depending where you live.  

Now Hot water at 125 from a H20 heater or boiler ran through a coil can be enough to heat your house effectively depending what temp you desire.

Air speed across these coils will directly affect the temps you get....kinda like a hand across a candle.  To fast and no heat transfer to your hand. Works the same with cooling. 

All of this is assuming a ducted system.

This may be of interest to Icanreachit.

2 main ways water is used in a geothermal heat pump application is open and closed water loop system.  

Open systems pull water from a well, river, lake to cool the refrigerant down to be condensed again to be sent back to the evap coil.  Cool water comes in and absorbs heat and is dumped back into the water source.  

Closed loop uses water lines buried in the ground...many feet either vertical or horizontal.  Ground temps will determine how many feet you need to cool the water in the pipes, so it can return and be reused to cool your refrigerant again.  Higher ground temps will require more feet to cool the water traveling at a certain speed.

This principle works in heating or cooling.  These systems still have air handlers and they have condensers..they are just water cooled and usually don't sit outside.  

Water cooled is awesome because it mimics a day the same temp as the water being used.  Standard heat pumps are horrible anytime the outdoor temp is below 35! Which means the Delta T is is 2 low to heat your house, so the electric strip heats kick in.  The refrigerant has a hard time extracting heat from the ambient heat outside below 35.

Here is a example from my house last winter in NC..we had a colder than usual winter.  On days above 35, my heat pump will create 92-94 degree heat without strip heats (2ton 24,000 btu 12 run load amps) With my strip heats (5kw 22 amps approx.) I get 120 degree heat. I'm looking at 34 amps x 240 volt = 8160 watts per hour or 8.1kw used......at 13 cent per kw .  We had multiple days below 19.  Not cool if you have a heat pump!  At one point i took some temp readings and was only getting 7 degrees of heat across my coil. entering air of 67, and exiting into the room at 74. This equals a unit that runs 24/7 without aux heats.  Needless to say my strip heats ran constantly ...sky high power bills.  Hell I went to lowes and bought a Eden pure heater for 400 bucks...plugged it in and within 15 minutes this thing brought the room temp up high enough to shut down my main system @1/10 of the cost to operate. best money I ever spent..lol

That being said, if I was trying to extract heat from 55 degree water (mimicking a 55 degree day) I would have been golden!

Sorry for the FRAT

icanreachit said
Thank you Albinorhino.

My ground temps at 12' are only 72degrees (+- 5 degrees at that depth) so I was thinking of somehow reaching steady state but you have a point, you need the delta T in order to achieve the cooling effect. Another downside of the absence of the larger delta T is that I would not be able to get the coils cool enough to form condensation and dehumidify the air in the house. Even leaving a 135cfm fan on 24/7 year round would only be 289KwH and cost about $15. Couple that with a comparable pump with a flow rate of 3 GPM and we're really only talking a 60w pump and that's another $30 per year. $45 or 867KwH per year would be great.

How does one go about utilizing geothermal in Texas or are they just forced to couple the system with an auxiliary cooler? Heating would be a cinch. I would like to stay with the slinky closed loop system for ease and cost of installation.

Thanks KVR. The only difference between that one and the one I had in mind was ducting and cowling. Amazing what a little galvanized sheet can do for appearances...

Man texas kinda works against you hard for all this.  I would have to think they are forced to couple with aux cooling.  

I remember a thread on another forum last year titled "Free Air Cond!"  Guy did the ol' ice bucket in front of a fan pointing at his chair... The thread turned quickly into...how much energy to make the ice and how much heat from the freezer was putting back into the room.  I think the consensus was that, he was spot cooling, but his system was working against him......was a fun read.

You are correct heating is easy, cooling is much harder.  Dehumification is so important.  I tell my customers we have a HVAC Scale of comfort.  In one hand you have comfort and the other you have efficiency.  they do not move together, always against each other...or so it seems.

Another thing so sadly overlooked is insulation.  I can make all the heat and cooling in the world, its the houses job to hold it.  I use the analogy of...Your house is like a coffee cup, depending on how your house is made will determine how often you need to reheat that coffee.  If your house is built like a glass, you will need to reheat it every 10 mins, if built like a travel mug, then every hour.

Any money spent on my house going forward will be in the form of spray foam insulation!  My god the things i have seem with this stuff.  I have been in attics on 95 degree days and the attic is in the 80's..not 120's