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propaneBeGone
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Posts:50
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| 07/07/2008 9:01 AM |
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Like many others, the high price of (in my case) propane is forcing me to do something I've been thinking about for quite a while...
I'm looking at installing a water to water heat pump to heat my home. We have radiant floor heat. Well insulated 2000' house in NH, from all I've read, I assume we'll be in the 3 - 4 ton range. We have a 400'+ drilled well currently for our domestic water. We use a decent amount of water from it, so new water is always drawn into well.
This will be a heating only system, or at most, a mostly heating system. Is this well a possibility to look at as a useful heat source for the heat pump? |
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engineer
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Posts:396
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| 07/07/2008 8:01 PM |
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Maybe.
You'll likely need, at minimum, 2-3 GPM per ton, almost continuously. Many, if not most, domestic water wells aren't designed to continuously provide that amount of flow. Then you must consider well pump power consumption - it'll probably run continuously whenever the heat pump runs, adding considerably to the power consumption. Water use by an open loop geo system can easily be 10-20 times that used for typical domestic purposes.
You'll need somewhere to put thousands of gallons of water.
Got ductwork? You'll almost certainly need it unless your present system uses warm (~100-120 F) water in radiant floors. Typical oil or propane baseboard hot water system needs 160+ F water, which geo won't provide.
Ductwork for a typical fuel-fired forced air system may not suffice for geo, which may need more airflow and / or different register placement.
Lots of difficult issues. Find a local contractor with good prior geo experience (in other words, happy local customers) willing to do proper Manual J, D, and S calculations.
I like geo, I have it, but there's about to be a headlong rush into it in the northeast as propane and oil homeowners face a $5k heat bill this winter. Many poorly designed and installed systems will result in poor, cold homeowners come January. Try to avoid becoming one of them! |
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Without data, you only have an opinion. |
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propaneBeGone
Registered Users
Posts:50
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| 07/07/2008 8:47 PM |
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thanks for your reply Engineer.
The idea I was pondering was more of a closed loop than an open loop...or at least a circulating loop. Rather than just use the water and dump it (I might run the well dry, I don't know) I was thinking along the lines of a standing water column well deal.
Either insert plastic U tubes into my 400'+ well and circulate heat exchange fluid in that pipe to exchange with the well water, or pull well water out of bottom (?) of well, run it through heat pump and dump it back into top of well after removing some heat from it
So, assuming 2-3 GPM/ton, 3 tons = 6-9 GPM, I guess we'd have to size a pump for that, existing well pump is now providing 6.5 GPM for domestic water. 120 deg water, or in that range would work for my radiant heat!
I guess the question I'm trying to answer is - Is a 400' well (8" diam.?) going to provide enough heat exchange capacity (in heat mode) for 3 - 4 tons?
I do hope to connect with a good contractor here in Southern NH, I have made some requests - we shall see! Thanks again.
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DickRussell
Registered Users
Posts:12
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| 07/08/2008 9:12 AM |
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Your best bet may be the Standing Column Well (SCW) design. A good source of info and perhaps a company to use for the design work is Water and Energy Systems, in Atkinson NH. The web site is:
www.northeastgeo.com
Basically, the water is drawn from the well, heat is extracted from it by the heat pump, and the water returned to the well. In a "pump and dump" configuration, the water would be disposed of somewhere after a single use.
The website, and others, have documented numerous SCW installations, in both commercial and residential situations. From what I could see on the NortheastGeo site, in one tabulation of heat pump sizes, well depths, etc., it seems like around 80 feet of water column provides enough contact area for each ton of heat load. So you would need maybe 240-320 feet of water column, between top of water and the pump, for your 3-4 tons of heat load. So, it may be possible, but....
I gather you will be contacting someone who can get into the details for your situation. You of course will need to have them do a complete heat loss calculate for the house to size things. Since your house apparently is reasonably heat-tight, then your existing well could perhaps work. If it isn't deep enough, perhaps it could be drilled the additional depth required at reasonable cost.
Edit: Radiant heat hot water normally is somewhat hotter than what a heat pump will provide efficiently. I have seen some references to being able to go as high as 145 F. If your system is laid out for using 120 F water you may be ok with a heat pump. Coefficient of performance is better with water-air pumps, which don't have to lift the heat up as far in temperature. |
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propaneBeGone
Registered Users
Posts:50
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| 07/08/2008 10:38 AM |
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Thanks Dick, yes the site you mentioned is helpful. I will probably visit them in several weeks when they have a "homeowner's workshop." It seems that cost wise this may be do-able if we can use the existing well, and that is a good idea, if more capacity needed, maybe they can drill it deeper. I have to try to find the records but I think it's a little more than 400' deep.
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engineer
Registered Users
Posts:396
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| 07/09/2008 12:26 PM |
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| I'm intrigued by the idea of SCWs in areas with high water table. I read somewhere that it can be modified with a slight bleed of water if necessary to source / sink more heat. This would seem to be a kind of hybrid closed / open loop system capturing advantages of both |
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Without data, you only have an opinion. |
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propaneBeGone
Registered Users
Posts:50
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| 07/09/2008 4:16 PM |
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Yes, this should be cost-effective. Since we're just talking ideas...I'm wondering if a slightly different version would have benefits:
A closed loop inside this well, if just one run down the well, and one up, you'd have 800' of pipe in the 400' well. I wonder if pumping costs would be higher or lower than the open system (I know this has been an open issue on this board). The other benefit besides possible pumping cost savings would be to keep the heat pump only exposed to clean water, not the iron and minerals present in my well which might tend to clog things up...
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cnygeo
Registered Users
Posts:68
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| 07/09/2008 9:55 PM |
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| Ignoring the question of whether or not you would get sufficient heat transfer, your pumping costs would be far, far, lower with the closed loop if you size the pipe correctly. You would only have the friction loss in the pipe to overcome rather than the vertical rise of the well. |
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propaneBeGone
Registered Users
Posts:50
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| 07/09/2008 10:18 PM |
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| you'd think over 800' of pipe the temp inside the pipe would equalize with the well water temp, and the well water would exchange heat with the rock same as with an open system. |
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cnygeo
Registered Users
Posts:68
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| 07/10/2008 12:21 AM |
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| Well... IF you assume that the water in the well is completely unaffected by the geo loop (I think the groundwater flow would have to be very high for this to be the case) and assuming a 36,000btu/hr load and 45F water temp, 800 feet of pipe would just about work assuming you wanted to keep your entering temps at 40F to avoid forming ice in the well. However, I doubt very much that the well water will remain at a constant temp once you start pulling a lot of heat out and without that assumption the calculations are much more complicated, and maybe worthless unless you have some idea of how much water naturally percolates through the well. I'm afraid I don't have the brainpower or the time to work through it further tonight. Interesting proposition, though - if you could make it work it would be an excellent solution for existing deep water wells. Pipe is cheap - the best thing would be to try it out and see what happens as long as you were prepared to switch to a more conventional system if it didn't work out. |
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DickRussell
Registered Users
Posts:12
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| 07/10/2008 8:16 AM |
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As to pumping cost, SCW vs. closed loop, the cost for SCW would be determined by two things. The first would be the difference in elevation between top of water column in the well and the heat pump. If the water depth is near the top or the well is artesian, there might be no difference. Perhaps the OP could tell us about the normal height of water, and at what depth the pump is set.
The other thing on pumping cost would be how the pumping to provide both domestic water and circulation for the heat pump is arranged. Some weeks back I posted a question on this. Clearly it wouldn't make sense to pump all the time at a pressure required for domestic water use. I had thought that this would have been worked out by now, and perhaps it has been by the companies doing SCW installations. But the response from this forum was insufficient to address my concern.
On the issue of heat transfer in the SCW design vs. a closed 800' loop dropped into the well, the amount of well depth per ton of heat load for the closed loop sunk into the well water would be somewhere between what is needed for SCW and what is needed for a vertical dry well, with the tube packed in bentonite. The plastic imposes a resistance to heat flow (I suppose copper could be used, perhaps direct exchange or DX). Also, except when the pump for domestic water use is running, the water in the well would be stagnant, so that the heat transfer from rock to water wouldn't be quite as good as when water is in motion down through the bore, picking up heat from the rock.
For southern NH, I would think that the well water temperature would be in the 45-48 F range. I agree that the water in a closed loop probably would have to be something like 40 to the heat pump, 36 return. I don't know if the professionals would find that 36 troublesome for pure water or not, because it means that the refrigerant would be evaporating somewhere below the freezing point of water. Withoug sufficient turbulence in the heat pump, a freezing layer could form in the exchanger surface. Some antifreeze protection would be required, and sinking a loop with antifreeze (propylene glycol) in it into a domestic water supply might raise questions.
Given all the successfull residential SCW installations out there, and the data available from them to confirm designs, I would expect that a company well experienced in SCW systems would be able to set up a system in the existing well, with or perhaps even without deepening the well. In any case, I guess we are at best the cheering section for the OP, who really has to run this by qualified pros to see if this will work for him. Our collective thoughts are "informed speculation."
Propanebegone, please let us know what the pros eventually tell you. Many of us would like to know this, as many are in your situation and want to know if there is hope for cheaper heat. |
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propaneBeGone
Registered Users
Posts:50
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| 07/10/2008 8:35 AM |
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| thank you both for your replies - yes, it is fun to speculate on this, and helping me think it through! Dick, you are correct that the closed loop pipe concept will miss out on the turbulance caused by pumping in the open, and this seems significant. So, SCW goes back to the top in my mind. One well driller, who I hope to have look at the project, told me that one solution he has used to the dual pump mode problem is replacing the existing pump with a Variable Speed Pump and some controller. He will run the pump speed for different pressures depending on if its required by the heat pump or the domestic tank. So this pump goes at the bottom of well and he would return water near the top of well. - I will post more when I learn more! |
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propaneBeGone
Registered Users
Posts:50
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| 07/12/2008 8:35 AM |
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academic research on design and modeling of SCW (Dick) with bleed (Engineer). He concludes SCW is lower life cycle cost than closed loop
http://www.hvac.okstate.edu/research/Documents/Deng_Thesis.pdf |
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