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General Automotive General automotive discussion. This is intended to be a discussion about other not VW and Diesel cars you may have or interested in.

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Old July 31st, 2017, 10:02   #2806
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They are a better air conditioner than heater though. Only because it works off the basis that the ground temp is around a fixed 58 F or whatever, so if you have that level of temp exchange potential, it means cooler than ideal for humans. They have a tough time, even here, making a house warm and cozy in the winter without at least some extra help. Usually it is an electric "booster" which is just a smallish heating grid like a conventional electric furnace would have. But you can obviously set it up to not make use of that unless the temp gets down to a certain point.
I find this confusing.

Are you saying that the heat exchanger gets sized for cooling and thus is insufficient for the amount of heat needed to heat the house on a cold day?

With outdoor coils I can see why the ambient temperature would play a big part, but if you've got a more or less constant 58 (for instance) ground temperature I had hoped that you'd need no boost heat at all, even in really cold weather.

We run our house around 66 during the winter--maybe it would be okay for that.

I honestly hadn't thought about heat pumps at all since moving from Maryland, so I really haven't looked into this.
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Old July 31st, 2017, 10:11   #2807
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The most basic way to think about this is to think that whatever the temp is in your house, the system can always pull it towards 59 F without any extra help. That is an oversimplification but the essential kernel of how the system works is there.

So, if it is a hot summer day and it is 95 F, then the ability to pull the temp towards 59 F is great! Because you can easily pull it right past the 72 F mark that *most* humans are comfortable with (I prefer it a bit colder).

But if it is 15 F, the system will still pull it towards 59 F, but cannot get it OVER that to 72 F (or even 66 F) because it cannot ever get it warmer than the temp of the ground, which is fixed, without some boosting of some sort.

So while it IS a good heater AND air conditioner, it is still a better (more efficient) air conditioner than heater. And the type installers use varies based on climate due to this fact. In Florida, you won't need the same system you would need in Wisconsin, even if the size of the house in question is identical, simply because of the loads required from the normal swings in climate in those two very different areas.
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Old July 31st, 2017, 10:20   #2808
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But if it is 15 F, the system will still pull it towards 59 F, but cannot get it OVER that to 72 F (or even 66 F) because it cannot ever get it warmer than the temp of the ground, which is fixed, without some boosting of some sort.
This is the only part where you are a little confused. The "boosting" in the GSHP system is the "HP". The heat pump. I've never heard of anyone doing a direct "ground exchange" system although I suppose that someone out there probably is trying it.

My opinion is that the new breed of VRF ASHP's (air source) are a much better option than GSHP's. They are generally slightly less efficient, but they are WAY WAY WAY cheaper than GSHP's.
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Old July 31st, 2017, 10:23   #2809
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The most basic way to think about this is to think that whatever the temp is in your house, the system can always pull it towards 59 F without any extra help. That is an oversimplification but the essential kernel of how the system works is there.
So, if it is a hot summer day and it is 95 F, then the ability to pull the temp towards 59 F is great! Because you can easily pull it right past the 72 F mark that *most* humans are comfortable with (I prefer it a bit colder).
But if it is 15 F, the system will still pull it towards 59 F, but cannot get it OVER that to 72 F (or even 66 F) because it cannot ever get it warmer than the temp of the ground, which is fixed, without some boosting of some sort.
So while it IS a good heater AND air conditioner, it is still a better (more efficient) air conditioner than heater. And the type installers use varies based on climate due to this fact. In Florida, you won't need the same system you would need in Wisconsin, even if the size of the house in question is identical, simply because of the loads required from the normal swings in climate in those two very different areas.
That's not quite right. A ground sourced heat pump can both cool well below the ground temp., and heat well above it. The air source heat pumps that are extremely popular these days do have a hard time heating when ambient temps drop below freezing. They are quite good at cooling though. A ground source system is exceptional though- twice as efficient as air source.

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Old July 31st, 2017, 10:38   #2810
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The air source heat pumps that are extremely popular these days do have a hard time heating when ambient temps drop below freezing. They are quite good at cooling though.
Yes and no. They make extended low range units that heat well down to -13 F (Mitsubishi Hyperheat for example). But that is a significant challenge in northern climates unless you are using them in a modern "low load" type building. As it gets colder and colder outside you need more and more heat and you get less heat.

I think that the majority of people using ASHP's for heat in new england are using them for shoulder season heating or using them with backup heat for periods of extreme cold. But that is changing.
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Old July 31st, 2017, 10:41   #2811
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I think you guys are missing the point. They ALL use a heat pump. It is the efficiency of it that is dictated off of where the temp exchange happens. Since that medium is the Earth, which is at a constant temp below a few inches on down to a few hundred feet, it becomes more efficient at cooling rather than heating. If you lived in a cave, you would never need an air conditioner... you'd need a heater though!

Same way the cooling system in your car is more efficient at removal of heat from the engine on a cold day than it would be on a hot day. If you only ever drove around in 0 F ambient temps, the engine could get by with a much smaller radiator!

I was on board with geothermal way back before most people had even heard of it. Waterfurnace was the brand of choice back then, although now there are lots of others. My aunt & uncle's new place got a Bosch unit.

The price has come down, but not by a whole lot. So the ROI is really the only thing that would push folks to buy one. And if you are not staying in a house very long (or worse, renting), then it makes little sense. Although when my dad sold his other place, he pretty much got 80% of the cost of the system back because it added that much value to the property... and this was easily 10+ years after the break even point. So long term they are a great investment. I will probably be at a break even point in a few years.
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Old July 31st, 2017, 10:57   #2812
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I think the ROI for a typical geothermal heat pump system is around 5-7 years these days. An adequately sized system can heat a large house to 100+ degrees using ground temps. of 45 degrees. I think a dollar of electricity into a good ground source heat pump system will yield 3 dollars worth of heat from a typical electrical resistance heating system.
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Old July 31st, 2017, 11:04   #2813
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Yes, it just depends on the house, the area, etc. Mine uses an auxiliary heater grid, and it NEEDS that on very cold days without the wood burner going. There is a control panel that keeps track of all that, and you can elect to have it not use the heating element if you wish. It rarely ever comes on unless it dips into single digits overnight, and if I kept the thermostat at a lower setting it likely would not even need that.

The main reason I even wanted that was in case there was a failure of the system, and I was not around, the heating element alone can keep the house from freezing preventing burst pipes, etc. Although the company that installed it said the standard systems they use have them by default anyway, unless someone specifically does not want it. Which I do not know why you wouldn't, as it only adds about $300 to the price IIRC. And of course it needs its own dedicated circuit from the panel, but that was already there from the previous furnace anyway.
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Old July 31st, 2017, 11:31   #2814
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Yup. The problem some folks in colder climates have isn't that the ground source heat pump is any less efficient at heating vs. cooling, it's the differential between indoor and outdoor temps. On a 95* day that differential for cooling is only 25* if the thermostat is set at 70*. On a 20* day that differential is 50*. So a system that's adequate in size for all of the cooling needs might not be adequate for all of the heating needs.
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Old July 31st, 2017, 12:07   #2815
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YES! That "differential" was the wording I was thinking of, thank you. Better explanation than I was giving.
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Old July 31st, 2017, 15:11   #2816
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GSHP's are still very much dependend on the geology and the hydrology of a given area. A very dry bedrock can lead to reduced performance later in a given season (heating or cooling) if there is little hydrology to take the heat away from the loops, then it ends up just storing (or removing) that heat near them as the season goes on. Conversely, a location where there is a lot of water in a relitively high water table, and especially one that has s decent amount of movement, can make for a very efficient system.

Some areas (like where I live) would be best served instead by an evaporative condenser, since the bulk of our cooling season (with the exception of the periodic monsoons) come with very dry weather (dewpoint around 30F). An evaporative condenser can run very efficiently, and the water used MORE than makes up for the electrical savings. This is essentially the same as the large numbers of commercial chillers using cooling towers.

Just like vehicles, it always comes down to which is best for a specific situation, and generalizations just lead to disagreements.
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Old July 31st, 2017, 15:52   #2817
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The most common gshp systems in my area utilize drilled wells or ponds for the outside loop. So there is typically plenty of water to pull the heat/cold away. They do perform best in moderate climates where the temperature "lift" or differential between indoor and outdoor loops is at a minimum.
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Old July 31st, 2017, 16:03   #2818
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The most common gshp systems in my area utilize drilled wells or ponds for the outside loop. So there is typically plenty of water to pull the heat/cold away. They do perform best in moderate climates where the temperature "lift" or differential between indoor and outdoor loops is at a minimum.
That's the case for any heat pump. The "work" the pump is doing is directly related to the difference between evaporator and condenser pressures, which are directly related to indoor and outdoor temperatures. This is not anything specific to ground source. In fact, it is reducing this difference that makes a ground source heat pump work better than an air sourced unit in most situations, especially extreme weather.
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Old July 31st, 2017, 16:06   #2819
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That's the case for any heat pump. The "work" the pump is doing is directly related to the difference between evaporator and condenser pressures, which are directly related to indoor and outdoor temperatures. This is not anything specific to ground source. In fact, it is reducing this difference that makes a ground source heat pump work better than an air sourced unit in most situations, especially extreme weather.
Yes, that's why I mentioned it. I certainly wasn't suggesting that gshp's operate on some different principle of thermodynamics than ashp' s.

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Old July 31st, 2017, 19:00   #2820
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A very dry bedrock can lead to reduced performance later in a given season (heating or cooling) if there is little hydrology to take the heat away from the loops, then it ends up just storing (or removing) that heat near them as the season goes on.
Although storing heat isn't necessarily a bad thing...

I wonder if borehole seasonal thermal energy storage will ever scale down to the individual home level - it appears to work (in Alberta of all places) at the suburb level, although that system isn't using heat pumps AFAIK: https://www.dlsc.ca/borehole.htm
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