Undersized geothermal system?
I have read a number of the threads on this website and greatly appreciate the expertise of those who share their experience. Here is my problem:
I am planning to install an open loop geothermal system HVAC system in North Carolina on a new home construction. I have already contracted with a local HVAC firm with experience with geothermal systems, but as I learn more about these systems, I am concerned that the HVAC system itself and the size of the loop may be undersized for the home. As a physician, I am a firm believer in the concept of a second opinion, and I was hoping someone who specializes in designing geothermal systems could provide a second opinion about the installation we have planned. I recognize that without floorplans in hand, the opinions provided will be just estimates, but I'd like to know if we are in the right ballpark. Unfortunately, I have very little time to obtain this information (just thought of the idea of a second opinion, though it seems obvious in retrospect). The well has already been dug and the duct work laid. The HVAC system itself has not been installed, however, though I expect it will be within a week or so if I let it proceed. I would be happy to provide whatever information I can to expedite the process:
house size: total 4675 square feet
* 2100 sq ft first floor - half 11 ft ceilings / half 10 ft ceilings
* 1900 sq ft 2nd floor - 9 ft ceilings
* 675 sq ft attic - 9 ft ceilings (attic will be finished; insulated roof deck)
crawl space construction with sealed crawl space
house faces south
* quite a bit of window facing north and south
* windows are Jeld-Wen Premium Wood Siteline EX Low E, argon filled
* U-value = 0.32
* CR = 55
* SHGC = 0.27
insulation - two options being considered
* open cell foam entire house with 5" in roof deck + 3.5" in 2x4 construction
* open cell foam of 3rd floor to include roof deck + batt insulation in remainder of house including R-13 in walls
* 2 adults + 2 children
* largely wooded lot with considerable shade
* 45 degree roof pitch
planned geothermal system
* open loop, vertical well
* 600' column already achieved
* 5-ton total split between crawl space and attic
Thank you for any information you can provide. I would also appreciate any suggestions on whether to use foam insulation throughout or just keep it to the roof deck and 3rd floor and batt elsewhere to save money.
These forums are invaluable to us HOs without your expertise. Thanks.
Any geo system should be sized using the BTU/hour load that is calculated from a Manual J heat load.
If your installer did not do this, he is guessing on the size of your heat pump.
If you can provide the results of the Manual J, then we can better help you with a second opinion.
Thanks. I'm happy to get whatever info will help. Am I just asking him for the BTU/hour load? He assured me he was using manual J calculations - which is how I learned about manual J in the first place - so, assuming he actually did, I do believe the system was based on something more concrete than his guesstimate. Based on other threads where people have mentioned square footage, HVAC tonage, and loop size, I'm concerned that we are trying to skirt by with the bare minimum acceptable size because any larger would make the system cost-prohibitive (2nd well, larger HVAC, etc). I'll see what I can find out.
things to think about with open loop systems
I have been installing and servicing geothermal now for about three years. the company i work for is the biggest geothermal installers in central PA
A few questions i must ask:
Why are you going open loop?
What we have found is that the open loop system needs to have more flow then closed loop system due to the loop water and not being able to take advantage of an antifreeze. which brings up another problem what happens when temp. gets below freezing? even though the loop will be below the frost line (and since you are going with wells it is a better advantage) But pulling up ground water will bring in sediment which is going to plug your coaxle coil quicker then a closed loop would. and another problem with open loops or pump and dump is what happens when north carolina has a drought which occurs often in the southern climates. (i have a sister that lives in creedmore NC which is close tothe durham area.
Another good note to think about is each ton should have around 200 ft per ton or for every 1200 btu
Hmm. Can't remember why we open vs closed loop. I suspect that's just what our contractor is more used to because closed loop wasn't really discussed much.
The 200ft per tone rule of thumb would obviously put us undersized for the well since that rule would suggest 1000' of well vs. the 600' we presently have. This may be a stupid question - I have no engineering background - but would that rule be the same in PA where the ground water may be in the 40's(?) as in NC where our water is in the 50s or 60? That is, if the water is warmer to start, would I need as much well height, or is that just completely unrelated?
Also, what WILL happen during the next drought? I guess I thought that at 600' the well wouldn't run dry, but I guess it will run lower. If the well is undersized to the HVAC system, will we just have to run the system longer to achieve desired temp, or will it just never achieve the set temp?
Thanks for your thoughts.
You might try this company for a second opinion, I think they are in your area. If you paid for a heatload calculation then you should get a copy of it.
Geodean suggested I ask for the BTU/hour load. I requested this information from our contractor who responded:
heat loss: 60269 BTU's
heat gain: 55716 BTU's
He did not provide a per hour figure. These numbers do not mean anything to me. Can anyone explain?
I am trying to arrange a meeting with our contractor ASAP so he can explain the assumptions that went in to his calculation and, hopefully, he can show me how he calculated that the system he plans to install can handle the load. But I'm not sure I can pull it off before the system is installed due to my own out of town work schedule. So any information anyone can suggest from these numbers and the information in the first post would be most appreciated. Our contractor seems very reasonable, and I have no reason to suspect he isn't well-qualified. Based on other threads on this website, I'm just concerned he is using all best-case scenario assumptions.
Also, if it looks like we are undersized and it's too late to change (contract signed, system ordered, partially installed?), would foam insulation (open cell, 2x4 walls + 8" attic) rather than batt (R-13 walls, R-30 attic) make much of a difference in the ability of the HVAC system to handle the load? It means adding a $2500 ERV air exchange system plus the added cost of foam.
It looks like you are OK.
Call me if you have any questions.
My phone number can be found on my website posted at the bottom of this message.
Thank you for looking at the information and providing feedback. I greatly appreciate the second opinion, and it is reassuring to know things don't look dramatically out of line. Our contractor has since provided me with the entire manual J calculation (about 10 pages, more or less). Is there one particular thing I should look for in those calculations? At this point I need to decide if using foam throughout the house - rather than just in the finished 3rd floor attic - is worth the additional $5000 cost for the insulation and air exchange system. If the HVAC is boarderline, I'd spend the money on the foam. If we are well within specs, I would save the money.
Thanks again for your time and knowledge.
Did he do the manual J assuming the foam insulation or batts?
Originally Posted by jab490
I'd say a 5 ton looks right, but be careful about open loop. I ran open loop for awhile, and I clogged my water lines with rust build-up. Also, redmondtech said open loop requires more flow. That's wrong. I believe it's just the opposite. With open loop you can run less flow because you are getting "new" water all the time. In a closed loop you need more flow because you're essentially recycling the water. Without knowing more about your lot it's hard to argue one way or the other (open vs closed). Obviously your contractor was good enough to do a manual J and select the right size unit so he/she seems respectable, but remember that it's pretty important to get your loop right. Geothermal only turns out to be wonderful when your loop is right.
If your loop is undersized of course your system is going to run un-efficiently. poor water flow in the summer months running a/c will over heat the compressor heating the windings in the motor and giving you less compressor life. you would be constantly failing either thermal cut out or high pressure depending on the safty device.
poor water flow in the winter would freeze your water coil and starving the compressor of refrigerant either way is no good. but you have been doing your research and getting good information from everyone good luck ad your doing it right.
Also about the different entering water temp. for different regions in the united states is an answer that i dont have but that is a great suggestion for the loop footage. i will find out and post back
Thanks for that interesting information. As it turns out, we have a surprising 20 gpm flow rate from the well and about 50' of extra well depth. So although the ground water level can certainly drop during a drought, our contractor suggested that if we just cut down on water usage for an hour or so, our well should replenish quickly because of this high flow rate. Hope that's true. My wife and I aren't big risk takers, but we do like the "green" aspects of this. If all works out reasonably, we'll be glad we made the choice. The fact that so many folks who work in this industry day in and day out are willing to help on websites like this only adds to my confidence.
Thanks again to all.
I have been using an open loop geo in Texas for about 15 years and have put in several closed loop and lake loop systems. My groundwater temp is 59F. Rule of thumb is 3 gpm of condenser water per ton. i use my waste water for irrigation on my farm. What are you doing with waste water?
From an energy standpoint, reducing the load and size AC you need is the best approach, but with foam insulation and an oversized unit you will have comfort problems due to lack of dehumidification. The unit won't run long enough to remove the latent heat. You may need to piggyback a dehumidifier on the AC system
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