(This posting is not specific to GSHP systems; just an example of what can go wrong with a two zone HVAC system.)
Our old system had 2 furnaces. One for each floor of my home. With the new system we were given either the option of having two GSHP systems or one system with zones. The two GSHP system would have been much more expensive so we went with the two-zone design. Unfortunately, that design did not work out so well. Here's that story of what happened with the zones and how we fixed it.
The Plan
The plan was to have one 4 ton GSHP with two zones. There is a zone damper on each of the two supply trunks. We have two thermostats. When one thermostat calls for heating/cooling or air circulation, that zone damper opens. If they both call, then both zone dampers open. There are were several problems with this design.
Problem #1 is that the air blower on the GSHP was too big for one trunk when only one zone was calling for air.
Problem #2 is that to handle problem #1, they installed a bypass damper to shunt conditioned air from the supply back to the return. In heating season, the bypass also sends heated air back in to the heat exchanger which reduces the system efficiency. In cooling season, the bypass sends cooled air back to the heat exchanger increasing the probability of freezing up the heat exchanger.
Problem #3 is that the bypass damper did not really work properly. It was handling a huge volume of air and bouncing around as the variable blower changed intensity. We were also getting too much air at the vents around the house when the GSHP was in stage 2 (or aux heat, stage 3) and it tells the blower to blow faster (more CFM).
Basically, it was not a good plan.
The fix: disable the zone dampers
I talked to a lot of people. Posted to various message boards. This is the solution that seems to work for us. I really did not like the idea of sending conditioned air around the bypass as we were. So my goal was to avoid that. Some people said to just cut out the bypass and dump the air in to the basement. But my basement is pretty small and we felt the volume of air was too great.
The solution we chose was to disable the zone dampers. The zone dampers we have are power-open/power-close. That means if you remove the power from the dampers, they stay in whatever position they were in when you removed the power. So that's what we did. We made sure both zones were open, and then disconnected the two power wires for the zone dampers.
So instead of sending the "extra" conditioned air around the bypass damper and through the heat exchanger, we are sending the "extra" air to the other zone. If either thermostat calls for heating or cooling then they both get heating/cooling. I may get a manual damper installed to balance the air flow (or partially close one of the zones) to balance the air flow but for now the system seems to be working very well.
Plan C
My installer had another clever idea that we were going to try if the above did not work out. He designed a little circuit to have the system operate as originally intended in stage 1 with two zones and the zone dampers. But if either thermostat was calling for stage 2 heating, which entails more air movement (higher CFM), then his circuit would open both zones. Then we would avoid sending a huge volume of air around the bypass damper only when using stage 2 heating or cooling.
Friday, June 3, 2011
Fixing the cold basement
In addition to having a GSHP to heat the house and make some hot water, we have an ASHP (air-source heat pump) for finishing the job of heating the hot water to 130 degrees F. We have the GE GeoSpring which is a 50g hybrid hot water heater. When in ASHP-mode, the geospring takes heat energy from the space around it -- in my case the basement -- and uses that energy to heat the hot water to the programmed temperature. My geospring is set for 130 degrees F.
The documentation for the geospring says you only need a 700 square foot room and my small basement is slightly more than double that but I suspect the people at GE assumed the space was heated. My basement is not heated. When we had the oil-fired furnaces operating, they were sufficient to keep the basement warm using just the waste heat radiating off the furnaces and flue pipes. Now that they are gone (yippie!) the basement is fundamentally colder (boo!) and then we have our geospring removing any additional heat it can find.
The basement was getting down to 47-49 degrees and that is just too cold. So my HVAC company cut in a manually variable damper. We open it in the colder months to dump a little heat from one of our two zones in to the basement. That seems to be sufficient to keep the basement comfortable. The vent supplies a little heat energy for the ASHP to scavenge to heat the hot water.
When the GSHP is not operating for heating or cooling, the ASHP appears to cost about $1.10-1.70/day to make the hot water we need. The GSHP is often not required during the "shoulders" seasons. (Although this Spring seemed kind of brief!). Normally the input water is about 50 degrees. When the GSHP is being used, the desuperheater warms the input water to 90-110 degrees. That allows the ASHP to work much less to get the output to 130 degrees.
The documentation for the geospring says you only need a 700 square foot room and my small basement is slightly more than double that but I suspect the people at GE assumed the space was heated. My basement is not heated. When we had the oil-fired furnaces operating, they were sufficient to keep the basement warm using just the waste heat radiating off the furnaces and flue pipes. Now that they are gone (yippie!) the basement is fundamentally colder (boo!) and then we have our geospring removing any additional heat it can find.
The basement was getting down to 47-49 degrees and that is just too cold. So my HVAC company cut in a manually variable damper. We open it in the colder months to dump a little heat from one of our two zones in to the basement. That seems to be sufficient to keep the basement comfortable. The vent supplies a little heat energy for the ASHP to scavenge to heat the hot water.
When the GSHP is not operating for heating or cooling, the ASHP appears to cost about $1.10-1.70/day to make the hot water we need. The GSHP is often not required during the "shoulders" seasons. (Although this Spring seemed kind of brief!). Normally the input water is about 50 degrees. When the GSHP is being used, the desuperheater warms the input water to 90-110 degrees. That allows the ASHP to work much less to get the output to 130 degrees.
Winter 2010-2011 Report
We made it through the winter with a few hiccups that I'll attempt to summarize below. Here are my costs for just home heating and hot water for the last few months:
Feb 11 1170 kWh $212
Jan 11 1401 kWh $253
Dec 10 1120 kWh $202
Nov 10 542 kWh $ 98
Oct 10 266 kWh $ 48
Sep 10 254 kWh $ 46
This factors in that I'm paying roughly $0.18 per kWh due to the Nstar 100% Green Energy program surcharges. (It is more like $0.205 now in June).
There were a few days where we used electric heat because of some problems that I'll describe so I'm hoping that the numbers are probably only about $20/month higher than they should have been. I'm still quite happy with this. A six month bill of $859. I'm guessing the next 6 months will be less expensive since the air conditioning is more efficient than the heating. So maybe my annual cost will be in the neighborhood of $1500-$1600. My annual oil bill was typically in the $2400 range + $300 in maintenance. If the savings is only $1000/yr assuming electricity and oil increase in expense roughly similarly, it'll be a while before the system pays for itself. But I'm still quite happy to be off of oil and free frmo the shoddy maintenance of the hurried oil technicians.
Replaced our propylene glycol with ethanol
In March 2011, we flushed out the propylene glycol and replaced it with a 25% ethanol solution called Geosafe. The entering water temp (from the ground back in to the GSHP) -- called the EWT -- was routinely below 40 degrees F. That's okay, except that on the few nights where we were near or below zero, the ground loop froze up and the system reverted to much more expensive electric-only heat. Relative to the propylene glycol, the ethanol solution has better heat transfer characteristics and is easier to pump at very low temperatatures (below 40 degrees). In fact, Climatemaster's flow-center documentation says that they do not recommend propylene glycol for loops below 40 degrees. Propylene glycol gets sludgier and harder to pump the colder it gets (below 40 degrees F.)
I'm told many systems use propylene glycol without problems in New England. I'm guessing that my loop was not sized properly for my home. Another reason installers use propylene glycol is that it is much cheaper than the ethanol (by a factor of 2 roughly, I was told). It is only about 100 gallons of fluid for my wells.
Why was our loop freezing up? Well we are not totally sure. We are extracting more heat from the ground than the loop was designed for. It is supposed to reach a EWT minimum temperature of 30 degrees. But last week (early March) after running for 3 hours in stage 2, it dropped to 29 degrees and showed no sign of stopping. We needed to do a longer test to see where it stabilized -- if at all! -- but given that the system would freeze up after 10-12 hours running on the coldest nights, we think the loop was not able to keep up.
Once the ethanol solution was deployed. I ran a test with the system on its highest setting (stage 2) with the auxillary electric heat disabled. I opened the windows and set the house for 80 degrees and ran it for 7 hours that way. During the test, I plotted the temperature of the entering water temperature -- the water/ethanol solution coming back from under the ground. After about 5 hours, the temperature stabilized at around 35 degrees. That's what you want to see. I believe that I can now run the system indefinitely on stage 2 without the system freezing up. I'm told that GSHP systems are designed to only be able to run in stage 1 indefinitely. Stage one is roughly 2/3s of the total system capacity.
Hot water generator
Other issues: The desuperheater (also called hot-water generator or HWG) was working great all winter. I had a 40gal tank of 110-120 degree water most of the winter. That is, until my HVAC company ran a test, something smoked, and the HWG died. They replaced a card and it started working again. In the process we learned that when the aux electric heat is being called for by the thermostats, the desuperheater disables itself. Usually the technicians say to jack up the thermostats to make the system run, but if that results in a call for the aux electric heat, the desuperheater is disabled. The desuperheater does use some of the heat that would normally go to the house.
Energy Recovery Ventilator
The energy recovery ventilator is working really well. I had some difficulty obtaining filters but my HVAC company has helped out with that. Between the ERV and the ability of the thermostats to circulate the air periodically (without heating or cooling), the air is so very fresh in the house. It is great.
Responsible Installer
My HVAC company had some personnel turn over but they tried real hard to make changes to make my system perform better. They installed the ethanol solution, fixed the HWG card, and added an air vent to my basement -- all for free. (I'll explain the vent in another posting). They had to locate and contract with a more experienced field technician. But that is what you want from a responsible company when you are facing issues with which they do not have enough experience.
Feb 11 1170 kWh $212
Jan 11 1401 kWh $253
Dec 10 1120 kWh $202
Nov 10 542 kWh $ 98
Oct 10 266 kWh $ 48
Sep 10 254 kWh $ 46
This factors in that I'm paying roughly $0.18 per kWh due to the Nstar 100% Green Energy program surcharges. (It is more like $0.205 now in June).
There were a few days where we used electric heat because of some problems that I'll describe so I'm hoping that the numbers are probably only about $20/month higher than they should have been. I'm still quite happy with this. A six month bill of $859. I'm guessing the next 6 months will be less expensive since the air conditioning is more efficient than the heating. So maybe my annual cost will be in the neighborhood of $1500-$1600. My annual oil bill was typically in the $2400 range + $300 in maintenance. If the savings is only $1000/yr assuming electricity and oil increase in expense roughly similarly, it'll be a while before the system pays for itself. But I'm still quite happy to be off of oil and free frmo the shoddy maintenance of the hurried oil technicians.
Replaced our propylene glycol with ethanol
In March 2011, we flushed out the propylene glycol and replaced it with a 25% ethanol solution called Geosafe. The entering water temp (from the ground back in to the GSHP) -- called the EWT -- was routinely below 40 degrees F. That's okay, except that on the few nights where we were near or below zero, the ground loop froze up and the system reverted to much more expensive electric-only heat. Relative to the propylene glycol, the ethanol solution has better heat transfer characteristics and is easier to pump at very low temperatatures (below 40 degrees). In fact, Climatemaster's flow-center documentation says that they do not recommend propylene glycol for loops below 40 degrees. Propylene glycol gets sludgier and harder to pump the colder it gets (below 40 degrees F.)
I'm told many systems use propylene glycol without problems in New England. I'm guessing that my loop was not sized properly for my home. Another reason installers use propylene glycol is that it is much cheaper than the ethanol (by a factor of 2 roughly, I was told). It is only about 100 gallons of fluid for my wells.
Why was our loop freezing up? Well we are not totally sure. We are extracting more heat from the ground than the loop was designed for. It is supposed to reach a EWT minimum temperature of 30 degrees. But last week (early March) after running for 3 hours in stage 2, it dropped to 29 degrees and showed no sign of stopping. We needed to do a longer test to see where it stabilized -- if at all! -- but given that the system would freeze up after 10-12 hours running on the coldest nights, we think the loop was not able to keep up.
Once the ethanol solution was deployed. I ran a test with the system on its highest setting (stage 2) with the auxillary electric heat disabled. I opened the windows and set the house for 80 degrees and ran it for 7 hours that way. During the test, I plotted the temperature of the entering water temperature -- the water/ethanol solution coming back from under the ground. After about 5 hours, the temperature stabilized at around 35 degrees. That's what you want to see. I believe that I can now run the system indefinitely on stage 2 without the system freezing up. I'm told that GSHP systems are designed to only be able to run in stage 1 indefinitely. Stage one is roughly 2/3s of the total system capacity.
Hot water generator
Other issues: The desuperheater (also called hot-water generator or HWG) was working great all winter. I had a 40gal tank of 110-120 degree water most of the winter. That is, until my HVAC company ran a test, something smoked, and the HWG died. They replaced a card and it started working again. In the process we learned that when the aux electric heat is being called for by the thermostats, the desuperheater disables itself. Usually the technicians say to jack up the thermostats to make the system run, but if that results in a call for the aux electric heat, the desuperheater is disabled. The desuperheater does use some of the heat that would normally go to the house.
Energy Recovery Ventilator
The energy recovery ventilator is working really well. I had some difficulty obtaining filters but my HVAC company has helped out with that. Between the ERV and the ability of the thermostats to circulate the air periodically (without heating or cooling), the air is so very fresh in the house. It is great.
Responsible Installer
My HVAC company had some personnel turn over but they tried real hard to make changes to make my system perform better. They installed the ethanol solution, fixed the HWG card, and added an air vent to my basement -- all for free. (I'll explain the vent in another posting). They had to locate and contract with a more experienced field technician. But that is what you want from a responsible company when you are facing issues with which they do not have enough experience.
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