The standard chilled water systems use water cooled to about 45
degrees going into the heat exchanger and a 10 degree rise in water
temperature in the heat exchanger. I am not sure what the temperature
of the air leaving the heat exchanger, but expect it would be about 58
to 60 degrees.
What I am contemplating doing is using water circulated through
geothermal lines instead of the chilled water. This water would be
about 57 to 60 degrees. If it rises 10 degrees , it would require
about 285 gallons of water per hour for 2 tons of cooling. The air
coming out of the heat exchanger would be about 73 to 75 degrees.
There would not be much, if any dehumidifying. And the house
would likely be about 80 to 85.
I see this as something to do when one was not home. It should reduce
the time for the air conditioner to get the house comfortable. It
would require the addition of a heat exchanger and probably some way
to duct the air either through the regular evaporator or the added
heat exchanger. Maybe the smarter thing would be to have the air
conditioner / heat pump chill or heat water. And not have both an
evaporator and heat exchanger to deal with.
Has anyone seen such a lashup? And are my numbers reasonable? Does
anyone think it would save money over just using a standard geothermal
heat pump with a timer to turn on the air conditioning before one
expects to arrive home?
Note I am not asking for a design. Just wanting the opinions of
practical people with experience. There is likely to be some thing
obvious to you all, that I do not see.
ROT says 4 gpm per ton, 15dF delta T on water in & out of geounit & ground.
Your running 4.75 gpm. Calcs show 2 tons of cooling, but reallife will
prove you wrong.
Unless your water entering the coil is 40-45dF your not going to
also, the size of the coil & flow through it will need to be controlled &
may not match the flow to your loop field.
The key to heat exchange is a difference in the temps of the water & the
Unfortunately, the ground won't recover as fast as you think, & over the
course of the cooling season, your return water temps will climb & your
cooling system will decrease accordingly. It'll probably flat line mid
I've seen Geo loop field returning water temps well over 85dF at the end of
the summer, & well below 32dF in the late winter, these are well designed &
sized fields (poor fields can run from <20dF->100dF)
I have to agree with the decent programmable stat idea, it's much cheaper.
Better yet, put the ground issue away, if you intend to keep an existing
system & replace your conventional A/C with an air source heat pump.
That too will be cheaper that excavating a 2 ton loop field.
If your sold on a ground loop then go all the way with a groundsource heat
Put one of those "hot water heat pumps" onto it....recover what you can for
domestic use then dump the rest into the ground...(or disconnect, rejecting
into the air if the ground loop if so happens the ground loop saturates.
Thanks. Easy to do the calculations and come up with the wrong
answer. I appreciate having the rule of thumb.
I am pretty sold on putting in a ground source heat pump. But will
probably wait until the existing air conditioner dies. My enthusiasm
may wane when I get an idea of the cost of putting in a field.
I saw such a system at "Fort Ritchie Site B", the former home of the
Executive Communication System to keep in touch with the president of
the US when he was traveling. This must have been used up to 1963 or
something like that. Basically, they had a large well pump in a pit and
a big air handler/heat exchanger, feeding air under a raised floor like
a computer room. Obviously tons of vacuum-tube radio gear was in there
when the site was in use. Unfortunately, I wasn't there when the whole
system was running, so I have no idea what temperature they were able to
I've been thinking of trying out such a system, too. I think with a
counterflow heat exchanger with several layers of heat exchanger core,
it ought to give nearly as low a temperature as a normal residential AC
operating under rated load, assuming the geothermal loop was big enough
to not start warming up. That might require a larger than normal,
vertical ground loop, unless you have a lot of moving ground water on
your lot. Or, a pair of true wells rather than a sealed loop. That's
what the Ft. Ritchie system used, although it looked to me like it might
have been a single large-bore well, just returning the warm water down
the same hole.
There is a website http://www.geo4va.vt.edu that has a fair amount of
information on geothermal heat pumps including a map that shows earth
temperatures. It looks to me that Fort Richie has earth temperature
about 52 degrees. Much better for cooling than the 57 degrees I asked
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