Is our geothermal system not working correctly or is it too small? We have a 2-stage Tranquility 27 size 049 MBtuh (4-ton) installed in Sept. 2010. It has the HWG option. Also has the Stage-3 Aux heating coil. Ground-water closed loop system with four 100 ft trenches 5.5-6ft deep with about 400ft of spiral pipe in each trench; all hooked up via manifolds to supply the heat pump. House is about 2500 sq ft living area with R13 insulation in most walls and R38-R50 in attic areas. Also has about 900 sq ft of basement and 540 sq ft of crawl space. Manual-J design: Outside db Htg-2 deg F., Clg-86 deg F. , Inside db htg-70 deg, Clg-75 deg. Total calculated heating load 78,377. Cooling load 34730. Fan volume 1771 cfm. The installers estimated our annual htg/clg costs to be around $1600 annually. So far this winter, the system does not adequately keep our house warm during the very cold weather. When outside temps are at 20 deg f. Stage-2 htg will not maintain a room temperature of 68 deg. The aux htg comes on at this point and will maintain 68 deg. Then as temp drops, stage 1, stage-2 and finally stg-3 to get heat back to set point. We anticipated that stage-3 would only be used on exceptionally cold days – like below zero F. Actual kwh usage on days with temps averaging around 20 deg f. is about 150-160 kwh for running the heat pump which at 12-cents/kwh equals $560/month. We have had both the installers and the Climate Master reps out to check the unit and they say everything is working correctly. We tend to think the unit may be undersized or it is not working correctly. I measure the EWT at 36 deg.f. and LWT is 33 deg. F. This has been true most of the winter although at one point I measured it at EWT 33 and LWT at 30 degrees. I am very surprised at how cold the water solution is. Even today with outside temps in the 50’s the water in the loop is still very cold. I wonder if the ground will ever get warmer. Another concern we have is the HWG option. Even though the heat pump runs almost continually, it does not always produce adequate hot water. We have the HWG circulating through our new electric 52-gal water heater with the bottom element turned off. I have measured the water temp coming out of the HWG and it is at 120 deg.f. which it should be. It just seems that it doesn’t enough hot water. I would expect at some time that the incoming and outgoing water to be roughly the same temperature. The water coming from the water heater is normally cool even after no use during the day or evening. We would appreciate your comments on whether or not you believe the unit is undersized or if it may not be working correctly. If you require additional information, I’ll try to get it. Thanks in advance.

Could be any number of reasons for the cold house but at first glance I see
78K heat load calculated doesn't match a 4 ton heat pump although 4 ton could be adequate for a well insulated house of that size.   The next thing I see is undersized loops. 400 ft is the bare minimum per ton and that is if it has ideal soil conditions all year and is run as straight horizontal pipe and not a slinky or coiled configuration. Those require more length.
Your balance point is thus higher than a typical design might call for.   Possibly around 25F instead of 10F before AUx kicks in. This will cost you more to operate and probably not worth the cost of installation. The payback will be very long if at all.
Who did the Manual J analysis. What were their credentials? Loop field design?  I will have to think of what I would do to remedy this situation. What is more important to you? Efficiency, cost to remedy, time to payback, cost to operate?

Charlie,

Where is the home located (State, City)? Does the manual J account for the basement, or only living space (2500 ft.) When was the home built?

Answers may shed light on the situation.

Paul
Total Green Geothermal
www.TotalGreenUS.com

Im with Paul first thing I wondered was where.
j

Thanks for the prompt response to my questions. – Additional information:
1. Important considerations now are my cost to operate and ability to keep house warm in coldest winter weather. Although cost to remedy is important, I believe I can get help from the installation companies. It is their design that is not working correctly.
2. The manual-J analysis was done by a local heating contractor using the “Right-J” software. They have been doing this analysis for many years but only recently related to GeoThermal systems. They work with another company experienced with the design and implementation of GeoThermal systems.
3. We are located in Charlotte, Michigan – about 20 miles southwest of Lansing.
4. The Manual-J 78K heat load does account for the basement and crawl spaces. 660 sq ft of basement have heat runs and all heat runs are UN-insulated to aid in keeping the crawl spaces heated. Crawl spaces are only vented into the basement – not to the exterior of home. The floors above crawl spaces are not insulated.
5. The original house was originally built in 1800’s with additions through the years. In 2010, major renovation replace approx. 50% of house with new construction/new roofs, walls, excavated basement, and insulation. Heating system was redesigned with all new duct work as part of the renovation and geo thermal system.

Based on the numbers you offered, your balance point would be about 26F. That's a lttle high in my mind.
Definately would've preferred a 5 Ton based on this data.
That said CM software still predicts 1600/yr for heating as you say.
Why that's not happening may be due to a few things. We had an inordinant amount of below 30 weather this year (nearly all of december was below freezing etc.). Lot's of inordinantly windy days (killer in old homes and some new code minimum homes as well).
Your loop field is new (might perform better after some settling particularly if you are in some of that good Mid MI clay).
400' slinky/ton would be uncommon with any of the mid MI contractors I know. The minimum loop with saturated soil would be about 2400' of loop. Our 600' slinkies occupy 110' trenches, is it possible that's what you have?
It is also possible that you have some odds and ends that might reduce the load a little. You'd be suprised, reducing the load 10% might cut your bill by 40%.
Look for poorly or uninsulated bonds atop basement walls for starters, and make sure returns are sealed up and not pulling air from unconditioned spaces.
A few things don't seem quite right here, but I'm not sure we asked the right question yet.
Did they give you the CM geo designer print out?
What does it say for balance point and loops?
j

I was thinking 400' slinky horizontal was a bit light per ton, but EWT in low mid 30s suggests loop is OK. Of course there is the question of whether 33 was really the lowest EWT...it is too late now, but I wonder what EWT was at say 9AM after a night at or below design low temperature.

4T seems light for the heating load of 78k

My gut suggests that the house load is oversized rather than that the system is undersized.

I suspect a blower door and possibly also an IR thermograph test would be good investments - find and fix the sources of inordinately high heat loss. Initial construction in 1800s and a hodgepodge of additions / remodels makes it likely a variety of envelope errors are present but out of view.

@engineer -- thanks for the feedback. Also, thanks to all who have responded so quickly and knowledgeably. EWT 33dF was the lowest I measured; it was a morning when the temp was 7dF, it had been colder during the night. It continues to surprise me how consistently cold the Ewt and Lwt are. According to the information I got from CM manuals, the heating capacity of the HP is reduced to by about 25% when the EWT drops from 50dF to 30dF. I'm hopeful that next winter after all the settling of the loops is complete that it will work better.

With respect to the blower door and IR thermograph test, the GeoThermal consultant has agreed to have the tests done to our home next week. They are really serious about finding the root of the problem because they believe it is sized correctly. The results will be interesting to read. I don't know much about these tests, but I do hope the warmer weather will still provide good results.

I also talked to our htg/clg contractor today and he ran another manual-J with the assumption that we insulated the new basement area. It reduced the total heat load by about 6.5% so based on your earlier comment, that could be a signifcant improvement. We had planned to do that insulation at a later time, but now I'm sure it will be completed by next winter.

Thanks again to all for all great comments and suggestions. - Charlie

I agree with Curt, it is more likely you are mistaken about loop size than loops are undersized.
You should understand that a 30 degree EWT is a design minimum that is accepted in the industry. While 50* is better, it might take 5 miles of loops to achieve that which would be cost prohibitive (say an extra $30K to save $100-$200/yr).
Among things that your installer is inexperienced at is explaining what to expect. It will be much harder to meet your expectations because they weren't sorted out prior to the install.
We'll get ya there.

I have a question on the same lines, but I have narrowed it to the loop, what do you think, NW Ohio 40002ft house blown in insulation, open countryside, 6 ton unit, $300.00 to $400.00 / month cost to operate Dec thru March. loop is 2 trenches 2 foot wide 5 foot deep 20 foot apart 400ft long, 6800 ft pipe in trenches, 4 on bottom 2 on each side about a foot up attached to the side. Back filled with clay soil, also there is a swooshing noise in the lines when started up in the fall, contractor says, lines expanding and contracting.
Any thoughts, Please Bob

I would measure the flow and the temperatures and calculate if the unit is meeting specs in terms of btus extracted. Afterwards you have a good idea if you aren't getting the btus or you are using more of them than expected.

pressures, temp are ok, according to service tecs.

Posted By Bob Kervin on 05 Apr 2011 05:24 PM
I have a question on the same lines, but I have narrowed it to the loop, what do you think, NW Ohio 40002ft house blown in insulation, open countryside, 6 ton unit, $300.00 to $400.00 / month cost to operate Dec thru March. loop is 2 trenches 2 foot wide 5 foot deep 20 foot apart 400ft long, 6800 ft pipe in trenches, 4 on bottom 2 on each side about a foot up attached to the side. Back filled with clay soil, also there is a swooshing noise in the lines when started up in the fall, contractor says, lines expanding and contracting.
Any thoughts, Please Bob

Does not sounds off. We see $300/ton for annual consumption, incl. DSH, A/C and 14 cents/KWH. Geo is the often he cheapest way to go, but it is not free...

I am in the middle of purchasing a horizontal drilled closed loop system for a WF Envision 4T unit. The dealer has run the loads that were used to size the system and tells me that 5 ea 200' bores located a 14-25 feet deep will satisfy my requirements. I currently use an open loop pumping the 6gpm required for the unit from the home well water supply. With this unit I would have a closed loop. This is with 2 circulating pumps and connections in a trench outside with the lines drilled under the house into the basement to connect to the WF HP.

Is this right or how do you figure this. Total run length out and back in each bore hole is 400' with 5 bore holes = 2000' of ground loop.

Let me know what you guys think. I really need to cut my electricity bill and I'm told this will do it.

Color me skeptical.

I would need to be convinced that power saved by no longer consuming 6 GPM from the well pump is not offset by continuous operation of 2 closed loop circ pumps PLUS any loss of efficiency / COP owing to changeover to closed loop.

Then there's the need to have a reasonable payback / ROI for the closed loop job.

Show me the money...

When you size a ground source heat pump design you should start with two basic bits of data.

1. The heat required by the house. This can be obtained by a thourough app[roximation using manual J. It can also be obtained (for an older house being retrofitted) by observing the amount of time an oil burner runs on the coldest day of the year and estimating the efficiency of the old heating system. (You can get this most simply by hooking an electric clock up to the burner power and recording the time it runs once a day.)

2. The temperature of the ground. This can be obtained from a map of the United States produced by W. D. Collins entitled "Temperature Of Water Available For Industrial Une in the United States", U.S.
Geological Survey Paper 520-Fl. Your location is near the 50 F thermocline on this map. An additional graph, produced by ASHRAE in 1997, entitled "Shallow Ground Temperature Variation with Season" shows that your ground tempeature at a depth of 5 feet will reach a minimum of 40 F on March 1 and a maximum of 60 F on October 1.

Once you know your ground temperature you might assume that calculating the amount of heat you can extract from the ground on any given day would be simple. Unfortunately that is not the case. The TVA finished a research study in 1968 where they monitored 85 test sites for 7 years. Their conclusion was that they could not predict how much heat they could extract from or sink into any given site within +/- 30% because of the month to month variations in the moisture content of the soil. The only real conclusion they put forth was that you would extract or sink more heat if it had rained a lot!

My conclusion is that it always pays to make the ground loop bigger. I don't care where you get your initial guestimate, a heat pump manufacturer, the IGSHPA or a contractor, make it bigger anyway. It's cheap to make a loop bigger when the excavator is on the site, and the additional HDPE pipe cost is inconsequential, so make it bigger. Make it as big as your lot size will permit.

Since your ground temperature is a minimum of 40 F and your water leaves at 30 F and returns at 33 F I suspect that your loop is too small. If you are pumping 30 gpm (or 240 pounds per minute) through the loop and you have a rise of 3 F you are only extractting 720 Btu per minute or 43,200 Btu per hour from the ground.

You can find out how much water you are actually pumping each minute by observing the presure at the inlet and outlet of the pump, then plotting that presure increase on the pump manufacturer's pump curve.

If the loop size is too small the only thing you can do is to start digging again!

Paul Lepkowski, PhD.

Since your ground temperature is a minimum of 40 F

Remember that this is only an undisturbed minimum - the ground can and will get colder as you extract heat. But I agree with Paul - see how many btu you are actually extracting. And soil moisture is critical - which is why a soaker hose is a consideration.

"My conclusion is that it always pays to make the ground loop bigger. I don't care where you get your initial guestimate, a heat pump manufacturer, the IGSHPA or a contractor, make it bigger anyway. It's cheap to make a loop bigger when the excavator is on the site, and the additional HDPE pipe cost is inconsequential, so make it bigger. Make it as big as your lot size will permit."

Sorry doctor, this is an oversimplification which leaves out the following considerations:
Just 'cause HDPE is cheap doesn't mean a geo installer will sell it cheap (diesel and labor are not). Further much more fusion and headers will be required running up t/m costs. More flow center may be required and certainly pumping cost can go up. Depending on the price of electricity, even double the loop field may have very little (i.e. less than $100/year) impact on operating cost.

Plain and simple there are diminishing returns to loop field size.

" And soil moisture is critical - which is why a soaker hose is a consideration."

Jon, soil is a critical factor in design. Size for dry conditions and the soaker hose needn't be a consideration.
Soaker hoses are a product of the DX industry to assisst in compaction. They were never intended for use as a btu booster. Nor would I design one in vs right sizing the field.

j

Way oversizing a loop will drive up water pumping power and cost. Circ pumps are fairly inefficient, so running more than reasonably necessary is a fool's errand.

Will the incremental up front cost of installing an extra ton of loop be returned in the form of reduced operating cost? Perhaps one extra ton will pay off in 10-20 years, subsequent extra tons centuries...