Water Furnace
 

A friend wants to heat his rather large house with a geothermal water-source heat pump system. He will have radiant floor heat throughout most of the house and 3 or so air handlers for heating and cooling. For some reason, everybody looking at quoting the job recommends the same manufacturer for the heat pumps and air handlers - Water Furnace. Is that the only game in town for this sort of thing? It sure is pricy.

Is that the arrangement where temps at about 20 to 30 feet underground stay pretty consistent -- 50 to 60 -- and there's a way to use the difference with outside air to generate 70 or 80 degree air?

How's that work exactly?

They work the same way a heat pump works, in Michigan they are the cats meow I know several people that have installed them and will never buy or build another home without it. I know that they say that the savings is about 20% but with larger homes and keeping the temps warmer in the winter and cooler in the summer the bills are about 25% of what a conventional bill. If I stay in my current home I may do it myself. A system in my home is $30,000, if I keep the house above 70 during the winter my bill would be well above $1000 mo and summer would be $500-600, with Geo I would be @ $150 for electricity so you can see I would pay for the system quickly.

It's a heat pump, but it uses ground water instead of outside air as the heat source during winter and heat sink during summer. It is much more efficient than an air source heat pump.

During the summer, when air source heat pumps use 95 degree air to cool the freon in the condenser, the water source heat pump uses 55 degree water.

During summer, when air source heat pumps use the cold outside air to heat the freon in the evaporator (the outside unit operates as a condenser in the summer and an evaporator in the winter), the water source heat pump uses 55 degree water.

The laws of thermodynamics say that the efficiency of the system increases whenever you close the gap between the temperatures of the heat sink and heat source. That's the secret to the water source heat pump. It works especially well with solar hot water.

Great information here, but, as the Brits would say, hold on:

The laws of thermodynamics say that the efficiency of the system increases whenever you close the gap between the temperatures of the heat sink and heat source. That's the secret to the water source heat pump. It works especially well with solar hot water.

I would have thought it would be the opposite, as air way colder than what the condenser needs to bring about condensation would have to work faster than if the temps were close.

I must admit that the thermodynamics of various sorts of refrigeration really gives me a mental workout. I've been trying to get a good understanding of the whole absorptive cycle style (you know, propane refrig.) and it's a challenge. I gather that water at around 85 C can be used for absorptive systems, probably the sort that use lithium bromide instead of ammonia - those systems are supposedly better for above freezing AC whereas ammonia is better for below freezing refrig. Is that what you were referring to with "solar hot water?" I mean using that to run AC in the summer?


I had a small RV propane fridge unit once. No moving parts. No compressor or pump of any kind that I could discern. How they managed to maintain pressure differences in various parts of the system is beyond my grasping. I get the impression that large style absorptive systems use a pump to circulate some of the fluids, which are not nearly as power hungry as compressors if I'm not mistaken.

But when you say groundwater is used as the heat source/heat sink, what, is it just pumped up, circulated over an exchanger, and pumped back down some distance away? I would think that using it and discarding it would not be workable but I'm not familiar with any process for reinserting ground water. No wait, I have read about procedures for pumping ground water up to be filtered (various pollutants) and then re-inserted so I guess it's not an unusual process.

That's incredible. This is serious breakthrough material.

What are you currently using to heat and cool? Savings of that magnitude do not seem possible.....Unless you are using resistance electric heat and window air conditioners.....

The concept is an excellent one but those percentages seem too optomistic, unless your energy source costs are vastly different than they are here in Indiana.

Overall savings of twenty five to forty percent for cooling seem possible.

Savings of thirty to fifty percent for heating seem possible, but again, it depends on the cost of the energy you are burinng currently and the effeciency of the unit being used.

I heat with propane, the real savings comes from a special electrical rate for rural geothermal homes. 50% savings is what is advertised but the reality is that it is much better than that. One thing that I have noticed about people with Geothermal don't monkey around with the thermostat they keep it at 71-74 depending on the season while we propane slaves have 6 different settings for each portion of the day.

This has been done in 'open loop' systems, but as you can see, it is wasteful of water.

So, engineers came up with an indirect system that circulates a fluid, such as water, through a closed loop. In the cooling mode, heat is transferred to a heat sink, and when in the heating mode, heat is absorbed from this same heat sink.

The familiar air-source 'air conditioning' unit uses air as the heat sink for cooling, but since there is not much heat in cold air in the dead of winter, an air-source heat pump does not perform well in the heating mode, so most air-source heat pumps have electric resistance heating elements. These work, but kill the unit'sl efficiency in heating mode.

BRIGHT IDEA - Why not use the earth as a heat sink and a heat source! Enter the ground source heat pump, which uses the earth as a heat sink and a heat source.

But wait, there's more... The secret is water. H2O. This miracle liquid can absorb more energy (BTUs) per pound than anything other substance on the planet. The inside story of ground source heat pumps is not the ground, just plain old dirt, but dirt that contains water. When we circulate a heat transfer fluid (water comes to mind again) through wet dirt, it is the water in the dirt that absorbs heat in the cooling mode and provides heat in the heating mode. Brilliant!

OK, so we want to place a heat transfer pipe in the earth and in contact with water. If your backyard is a lake or swamp, you could just run a lot of pipe out into the backyard and hope for the best. Not all of us have a 'water feature' in our yard, and for those of us in the Great White North there is another problem that shows up in the winter - ice. Ice cannot be cooled much further, and so like air, it is a poor source of heat. We really want water! Those of you down south, you can disregard this, and just trench in a lot of pipe and make your backyard a radiator, but make sure the dirt stays wet...

So the engineers hit on another idea - why not dig a well, which we can already do inexpensively, and then drop a loop down the shaft? Better yet, dig deep enough and the far end loop will be in contact with water and do most of the heat transfer work while the rest of the pipe is there to transport the fluid up and down. Bonus - if the end of your loop sits in an aquifer, the heat will be carried away in cooling mode and brought to the loop in heating mode!

So now to the happy ending. A typical ground source heat pump system circulates a heat transfer fluid in a closed loop through the heat exchanger in the heat pump through a field of interconnected wells with heat transfer piping in each. The heat pump then decides whether to use the ground water loop as a heat sink in cooling mode, or as a heat source in heating mode.

That's my understanding...

Geothermal strikes me as a really good idea.
So how long until geothermal heating and cooling gets blamed for some ecologoical disater? I mean we are putting heat into the earth, or taking heat out of the earth. Someone has to figure its somehow bad for the earth. I figure just about the time a substantial number of people make the substantial investment there will be an outcry and the banning of geo thermal.
( Boy, I hope I am only joking!)

It has been many moons since I studied this stuff (almost 30 years :eek:), so I am rusty.

A heat pump, just like a refrigerator or air conditioner, moves heat from a low temperature source to a high temperature sink. As such, it is fighting mother nature the entire time.

In a standard air-to-air air conditioner or heat pump operating in cooling mode, the heat source is room air at about 72 degrees. The heat sink is the outside air, 95 degrees or more on a hot day. In order to pick up the heat from the room air and cause water vapor in the air to condense out, the freon in the evaporator needs to be much colder than 72 degrees, probably something in the low 40s. Like wise the freon in the condenser needs to be well above 95 degrees. So the temperature spread between the evaporator and condenser temperatures is 60 degrees or more.

Compare that to a water source heat pump. The freon in the evaporator is still in the low 40s, but now the condenser is being cooled by 55 degree water instead of 95 degree air. The temperature spread just went from 60 degrees to maybe 20 degrees. (I'm guessing at the temperatures, but I think these are in the range.)

The laws of thermodynamics say that whenever you bring the temperatures of the heat source and heat sink closer, you increase the efficiency of the system (all else being equal).Absorption AC has always baffled me.AFAIK, they are all using closed loops around these days. They dig a well and drop a loop down in the ground water and/or they run the closed loop through trenches. It ain't cheap.

In Michigan pump and dump (pond) is the most common.

This is the device I plan to add to an existing heatpump for geothermal use. It will be added just upstream of the outdoor condensor coil.

It's original use is for heat recovery from the hi side of the AC system for domestic hot water.

http://www.acoverstock.com/ecu-heat-recovery-unit.html


Here in north Texas you need approx 400 feet/ton of horizontal pipe buried six feet deep where the temp is about 68f all year. A "slinky" of 3/4" black poly laid in a 3' wide trench is reccomended.

It was, but with the new emphasis on Green buildings and LEED certification, existing open-loop pump-and-dump systems may remain, but new installations will be closed-loop and well fields.