Using water at 57 degrees to cool house

Posted by geothermaljones on September 5, 2008, 8:30 pm

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
dehumidify...
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
ground.
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
June...

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
pump.

goodluck
geothermaljones

> 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.
> Dan

Posted by Bipolar Bear on September 5, 2008, 9:14 pm

> 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
> dehumidify...
> 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
> ground.
> 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
> June...
> I've seen Geo loop field returning water temps well over 85dF at the end

> 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
> pump.
> goodluck
> geothermaljones

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.

--

Posted by on September 5, 2008, 9:53 pm

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.

Dan

wrote:

> 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
> dehumidify...
> 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
> ground.
> 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
> June...
> 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
> pump.
> goodluck
> geothermaljones

Posted by Jon Elson on October 10, 2008, 7:04 pm

dcaster@krl.org wrote:

>
> 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.
>
<snip>
> 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?

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
maintain.

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.

Jon

Posted by on October 10, 2008, 10:17 pm

> I've been thinking of trying out such a system, too. =A0I 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. =A0That might require a larger than normal,
> vertical ground loop, unless you have a lot of moving ground water on
> your lot. =A0Or, a pair of true wells rather than a sealed loop. =A0That'=

> 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.
> Jon

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
about.

Dan

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