Well pump killing my GSHP efficiency (COP less than 3)
Last Post 14 May 2015 11:29 AM by marmar. 4 Replies.
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NathanUser is Offline
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13 May 2015 04:29 PM
Hi folks, I installed a GSHP four years ago and it has been working great (Nordic 5 ton). I recently setup a energy monitor system to monitor all my electric circuits (GreenEye Monitor -aka GEM) and discovered that my water pump is using 1800watts and Geo in stage 1 is using 2950w (4200w in stage 2). After doing some math my COP is 2.75 and 3, including the well pump. Turns out I oversized my water pump (1hp 20G8) and currently have it set to run at 70psi just to keep it from shutting off while geo is on. Having said that I am now trying to find the most efficient way to bring up water from my well. I don't mind spending the money if ROI is decent (3 yrs). Current setup has me spending appx $420 for electricity for my well pump along ($775 for heat pump), $1200 total per year. Here are some stats: max drawdown/head is 18 feet, with pump drawing 15gpm at 70 psi. According to my charts I could probably do this with a 1/2 HP submersible pump producing 15gpm at 45psi. The single pump/well also supplies water to house (three levels), so need 45psi min for that. The goal is most efficient /cost effective setup. Considering Grundfos 15SQE05-70 1/2hp pump on variable speed controller, or single speed traditional well pump 1/2hp. Also possibly a different style pump(dewatering), with booster for house use. Nathan
jonrUser is Offline
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13 May 2015 06:34 PM
Use the smallest pump possible that will achieve a low pressure (perhaps 15 psi). Then add a booster pump for house water.

In theory, a Grundfos variable speed pump could run at 15 psi most of the time and then kick up to 50 psi when house water was being used. I'm not aware of a controller that will do this.

NathanUser is Offline
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13 May 2015 11:09 PM
According to Blake Clarke in this June 2012 post:
" It is possible to run a grundfos SQE at 10 psi or so using the "resistor trick" and the standard CU301 controller and transducer."
http://www.greenbuildingtalk.com/Forums/tabid/53/afv/topic/aff/13/aft/80263/afpg/1/Default.aspx

If this is the case, and I have found no further description of this resistor trick, I could potentially wire up a relay to turn regular house pressure and switch back to low pressure. Would probably have to run this off a flow sensor to determine if house lines are being used.

As a side note, I did some math on the current pump producing 70PSI at 18ft head and 15gpm that equates to 180ft head and 1800w (which is what I am seeing on meter). If the PSI could be dropped to 10PSI the equivalent head would be 41ft and only require 320w (with a 36% efficient motor like the Grundfor SQE).

Lastly, what about running two pumps off one well? I could keep the existing submersible for house only, and add a small high volume low pressure pump dedicated to GSHP? Well has a 6" casing.
marmarUser is Offline
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14 May 2015 11:04 AM
I have a 2 pump system and it works OK. The deep well 1/2 HP pump is down about 100' for the Geo system and a small jet pump for other water use. Static water is at 14' and my well is endless recharge-- puts out over 75 GPM. Jet pumps typically not to efficient over 25' water lift. Well is also 6" bore and have plenty of room for multiple lines. My deep well cutoff pressure switch is set at 50psi--maybe should be changed to something lower. Works great after 5 years and electric usage is great. Highest bill is usually January each year after heat; Christmas lights and considerable cooking and baking. $250 last couple January's from First Energy (Penelec).
marmarUser is Offline
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14 May 2015 11:29 AM
I forgot to say my Geo is a open system and I have stage 1 Tranquility 27 set at 6GPM. Should have 1/3 HP deep well pump or less would be fine but I had this pump for some time. Would definitely replace with less when I need a new one. I have main valve barely cracked open to get 6GPM or pump would be on/off many times when the Geo is running. Cracked just enough to stay in range of my 30-50 pressure switch. Even though pump is at 100 feet the static water at 14' is really only the lift I need. 1/2HP is way to much and a smaller one might save some electric?
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