We moved into our new custom built 2200 sq foot ranch home in early December 2012. North Central Iowa with R23 sidewalls, R48 blown in Attic, spray foam rim joists. The Geo system is a GeoComfort GWT048 4 ton hydronic unit feeding into an HSS 20 Buffer Tank. The Buffer tank feeds a GeoComfort MPH036 3 ton Air Handler for the main floor heat. It also feeds a radiant floor slab manifold for the basement and a manifold for radiant floor heat in the Garage ( about 1000 sq ft). So the HSS buffer tank is feeding a 3 zone system. The ground loop is a horizontal bore 4 loop system with each bore 205 feet for a total of 1600 feet of pipe. Before the house was occupied the slab heat was turned on around early october to heat basement, garage and provide residual heat for the upstairs. Everything appeared to be functiioning properly until we moved in early December and they started the Air Handler. We started to encounter very cold temps for a few days and the system started to stumble. After a visit from the installer it was found that the auxillary heat strips had not been installed. OK - this would explain the problems on the coldest days of 40 mph winds with single digit temps. EWT at 28 degrees and I forget what the said the outgoing temp was but I believe it was 22-25 degrees. Even after the heat strips have been installed it seems the system will not handle running all 3 zones. If we try to keep the garage heated to 60 F or even 50 F the air handler will not maintain temp and the auxilary heat kicks in even on milder days with temps in the 20's. I questioned the EWT temps and asked about the size of the loop field. After pressing the issue the installer brought out the manufacturer rep. They performed the diagnostics and determined the system was working to specs. They addressed the garage issue problem by saying the cold temperature of the incoming water from the garage floor is causing the system to get behind. They say the fix is to put in a slab thermastat that will keep the slab warmer and not shock the system by dumping cold water back in. I questioned this also - the problem is the garage does stay 40 or above on its own even on the coldest days. When we turn off the garage heat the system seems to perform as it should. My thoughts - loop field is too small and or the hydronic unit is too small. Other side notes 1.Annual heating load was calculated at 70.2 MMBtu ( This did not change when I decided to add the garage heat) 2. Design heating load 48696 Btu/hr 3. Indoor design temp - 72 F 4. Outdoor design temp - -10 F 5. The hydronic unit also has a desouper heater (spelling?) 6. Something I just saw on the GeoAnalyst report they gave me when they did the bid had the Trench/Pipe calculated at 1303/2607 ft. The put in 4 205 foot loops which only totals 1600 feet in my math book. Am I reading that correctly? I would appeciate any suggestions. The installer has assured me they will get to the bottom of the problem but I am getting nervous.
I don't see how the calculated load could not go up after adding the garage. The fact that when you turn off the garage, the system works fine, seems to imply that the system was not sized to handle the garage load.
Some thoughts: - that is a pretty small buffer tank for your loads - radiant temperatures and hydronic fan coil temperatures are different beasts. Your HFC generally requires much warmer water than your hydronic floors. So I could see some issues developing with a small buffer tank from this. It may just be a plumbing fix (vs. ground loop issue).
I had a bad relay on a boiler this am so I am late to the party. 4 tons at 12,000 each gives you about 48,000 BTUH to run around the house. Your design load is 48k +/- so I am with Dewayne, the giant garage adds to the load especially depending on insulation of that space and the insulation of the home. I think that Chris is correct. Your tank is too small. I have been using, thanks to the crowd here, at least 10 gallons per ton in a water to water system, even if multiple heat pumps are involved. The issue is in the controls. Mark
Thanks for the input. The manufacturer is rerunning the load calc to see if it is correct. Does the loop field seem large enough for the system? I know there are alot of variables here but there must be some standard loop field sizes for equipment size. Mark That makes sense but doesnt the water unit still need to replace the heat at at least the same rate as what is being used? The larger tank would keep it going longer during peak use but eventually it to would get drained? You are the expert so maybe I need further explanation.
The buffer tank is to buffer. It will temper the different water temperatures (and flows) it sees and feed the heat pump a nice even flow of constant temperature water. That it can heat up. That's the idea anyways. With an incorrectly sized one you'll have the water temperatures varying quite a bit and lots of starts and stops of your heat pump trying to deal with the fluctuating water temperatures. there must be some standard loop field sizes for equipment size Quick answer to that - nope. We can crunch the numbers and we get familiar with what works in our area. You didn't mention borehole spacing or soil types for example. That has an influence on loop design as well.
Chris, nice explanation. The issue is that the heat loss/gain of the home is a moving target, and it moves because of ambient temperature, wind and sun. The capacity of the heat pump to move BTUH is fixed or at least limited. I do not know what your location is, but I am sure you will not need to supply the design loss all day, every day. That is why the buffing tank helps. It acts to even out the spikes in the load. ie. less loss on a sunny, calm 40* day. Yes you can exceed the ability of the heat pump to re-fill the tank with BTUH based on real time heat loss. Chris and Dewayne bring up good points and I think this system can be improved without starting over. Are you sending the same water temperature to the floors and the air handler? Were the floors insulated? With the piping the way it is, can you "load" the high mass floors on a warmer sunny day? Mark
400' of pipe per ton of load works on rule of thumb. I would want a guru to examine the increased load from the garage spaces impact on the 4 ton house load to make sure I would not "kill" the loops for performance. Eric
Do the math. Your issue appears to be total capacity. A 4 ton Geocomfort W-W has a heat output of 44kBTU at 32F entering water temperature (EWT) and 104F entering load temperature (ELT). At 28F EWT it has about 5% less capacity, it has about 42 KBTU capacity. Then the desuperheater takes away 10% capacity when running, so you are down to about 38kBTU. So yes, on cold nights you do not have enough capacity to keep up with the heatloss in the house. So why do you heat the garage when it is staying at 40F by itself? Now the geo report might say something for trenches and straight pipe, but boreholes are different, they need much lesser length. Your EWT dropping down to 28F is not dramatic, but it shows that you have been pulling a bit more heat out of the boreholes so far, usually I design them for about 30F at the end of the heating season. But again, being off by a couple degrees is not a big deal. Having a larger loopfield would not dramatically change your peak capacity of your system. More importantly, what is the load temperature you are running the buffer tank at? Do you have an outdoor reset?
This being new construction it was not know how warm the garage maight stay on its own in extremely cold temps. It does saty warmer than expected but will drop below 40 on the coldest days. We are 0 today and it is slightly below 40. The T stat bottoms out at 40 I just figured out. The Buffer tank temp is set to run between 85 and 115 degrees. There is no outdoor reset. It is very clear to me the load calc did not take into account the garage area. The system is performing very well without trying to heat the garage slab. I am meeting with the HVAC contractor later this week to determine a fix for the situation as they also have re-run the load calcs and have come up with a new number figuring the garage. Since I already have loops in the floor what other heating options do I have other than increasing the size/capacity of the geo unit? I would like to keep the garage at 50 degrees. Thanks
I am not a pro. After reading the responses: A larger buffer tank has been mentioned a few times. Do you have temp gauges on supply and return for garage zone? If your garage radiant is like mine the return water temp is low, bringing the buffer tank temp down. Maybe more GPM flowing to garage zone would keep a larger buffer tank temp high enough for what the air handlers need w/o aux strips.
After revieiwing the load calcs the HVAC company admitted they made a mistake and did not size the system correctly taking into account the garage. Now the question is how to fix it. There are several options but to stay with a full Geo system they will need to add 2 loops and increase the water to water unit from 4 tons to 6 tons. I think they will also increase the size of the buffer tank. Other options are to install a boiler alone for the garage. I think I want to keep it all geo. I cant think of an advantage to going with any other option.
If they're paying sure. Otherwise keep your existing system, separate the garage and run your 2 new wells to it on a separate geo system. Overall cost will be lower (unless they're going to offer you a refund on the old system?). Or do you exist in some place where more costs less? (A 5 ton unit for the whole home will likely cost you a lot more than just a 2 ton for the garage alone.)
