I really only need to extract what the home will lose at the design deltaT .... right??

No, you only need to EXTRACT about 70% - 80% of that -- depending on COP.
The other 20% - 30% comes from the electricity used to run the compressor,
blower, etc.

If your load calc is correct you may want to compare the Geo system vs VRF .Daikin, Mitsubishi,Panasonic,LG are the mfg. that make such a unit.It is just a thought.

For the money involved, I would do my own manual J or hire someone to do it. Or have some experts here review what you already have and see if there are some obvious "assumptions" that were taken that bloat the number.

If the system is too large, you are wasting money, short cycling the equipment and reducing comfort. No need to supply 100% of the load with the Geo. If the temps only dip a few times during the season, the back up heat can cover or you may just flywheel through the event without needing the back up. Be careful what T-stat you get to avoid turning on Aux heat when its not really needed. Longer run times from right sizing will give better buffer tank DHW preheat and help reduce your hot water bills as well.

I never checked my man J calcs. As with yours, they varied quite a bit. I have also added some insulation afterwards, and as a result my system is a bit too big. I could have saved some money if I was better informed upfront. My unit hardly goes to 2nd stage and does cycle a bit more than I would prefer.

@acwizard,
Yes I have checked into the low temp air-source heat pumps and the bid came back almost equal to the installed price of the ground-source heat pump minus the loop field.  Now, I'm not aware of any tax credits for the air-source units, but if not, ground-source is the way to go just because of those incentives.  Thanks for the suggestion however.

@Looby,
That is very interesting that the extraction from the ground-loop only needs to be 70-80% of the building loss, that does make sense, there is 'waste' heat generated in those components that can be utilized.  Great comment, thanks.  Does the loop calculator take that into account?

Forgot to mention that we will also have an ultra-high efficiency wood burning fireplace that kicks out ~50,000 btu/h and covers 2200 sqft. for those occasions that the outdoor temp is below design temp.


So, I think I'm moving toward the 'right-size' loop field:  1500' total depth or 200'/ton.

Of course I don't want to be short looped. 

Posted By therik on 28 Feb 2013 05:53 PM
Does the loop calculator take that into account?

Sorry, I don't know. I don't have a load-to-loop calculator.

I guess it depends on exactly what the program asks for.

Posted By therik on 28 Feb 2013 05:53 PM

.  Does the loop calculator take that into account?

Yes,   the higher the COP the more  heat comes from the ground.  and vice a versa .  Loop programs account for this.

Posted By therik on 28 Feb 2013 02:45 PM
...and ... said 300'/ton sounds safe, and gave me the bid.  ...

This is how it's done in the Dallas area.  Once the number of tons needed for the structure is determined (Manual J + Manual S), it's simply assumed that 300 (vertical) feet per ton is needed, assuming the soil is moderately clay (typical for most Dallas areas, and assuming it's 1 inch HDPE pipe being used, connected in a reverse header manner at the manifold.

Few, if any, of the local geo HVAC professionals, focused on the residential market, use any further analysis, other than the above.

Keep in mind this is an exceptionally warm season climate, where Entering Water Temps in the winter season never get below 60 degrees, and aux heaters are just not ever installed.

Best regards,

Bill

Posted By therik on 28 Feb 2013 05:53 PM

@Looby,
That is very interesting that the extraction from the ground-loop only needs to be 70-80% of the building loss, that does make sense, there is 'waste' heat generated in those components that can be utilized.  Great comment, thanks.  Does the loop calculator take that into account?

Forgot to mention that we will also have an ultra-high efficiency wood burning fireplace that kicks out ~50,000 btu/h and covers 2200 sqft. for those occasions that the outdoor temp is below design temp.


So, I think I'm moving toward the 'right-size' loop field:  1500' total depth or 200'/ton.

Of course I don't want to be short looped. 

Two things affect the size of the ground loop. To a lesser degree your 90KBTU/h peak load, but more so the entire amount heat extracted over the season. In other words, the system which only has a few peaks of 90KBTU/h but otherwise needs 40-60 KBTU/h needs a smaller loop than one which runs at 80 KBTU/H much of the time.

Also of importance for your total costs is wether you can go with a single 6 ton heatpump or if you need two heatpumps. 90 KBTU/H could very well be supported by a single 6 ton heatpump

To give you some more confidence that a smaller loopfield will serve you well, here are a couple examples. Buffalo NY, 6900 heating degree days, we try to size the loopfield for 30F entering water temp (EWT) at the end of the heating season. So all the loops running higher, so they are slightly oversized, all limestone in the Niagara escarpment, with graphs displaying the EWT over the whole year:

http://welserver.com/WEL0478/
Heatloss 45600 BTU/H, 3 ton, 400ft total borehole

http://welserver.com/WEL0336/
81200 BTU/H, 6 ton system, 800ft total boreholes, owner keep it at 67F, that is why the loop is warmer

http://welserver.com/WEL0383/
2900 sqf, 72000 BTU/H, 6 ton radiant, 800ft total boreholes

Hey, thanks for all the help everyone.

My installer got another calculation done after I discussed my concerns with him. He may have gotten it from the corporate office of the system manufacturer.

What they said is right in line with what everyone here has been saying.

Total bore depth: 1320'

I guess that solves that puzzle....now I just need to find a driller, anyone know a driller willing to drill near Albuquerque, NM??? (You would be my hero!!)

glad to hear that they came to around. Too bad they got it wrong the first time!!

So, what was the low-temp air source heat pump bid that was almost equal to geothermal?

Don't you think air sourced heatpumps would even loose more performance at that altitude?

If there is not an advertiser here at GBT, you can look up certified vertical loop installers at IGSHPA

So, therik, was it the heat loss calc they redid, the well bore calc, or both? Any numbers to report on what went into the final heat loss total?

I believe my installer had someone else do a loop calculation based on the 90.51 kbtu/h heat loss, and they came up with 1320'.

The heat loss calculation has not changed.  I've spoken with the engineer that did the heat loss calculation and he is very confident that it is on target.  In fact, I also come up with very similar numbers.  (It's not very difficult once all the room sizes and wall/window/ceiling/floor area is known). 

As for the air-source heat pump it was for the Mitsubishi system with 3 MXZ-8B48 outdoor units and 3 3-zone distribution boxes + 8 zones.

In fact, if I went with the series 5 instead of the series 7 the installed system price minus the ground loop would be even lower than the air-source heat pump. 

Not many drillers in New Mexico listed on the IGSHP page. Anyone from Texas or Colorado that might want to come down.  There are another 60+ custom homes to be build in this development. 

Thanks.

What size unit(s) are you going with? Just curious if you are going with one big unit or two. How many zones? I hate to say it, but I think you may find your 90K calculated heat loss number is too large. I am no expert, buit it just feels too big based on your delta T and house size. In any event you do not need to do all that with the Geo. What portion is Geo and what portion will be supplied by Aux? That woodburner will be able to cover you as well for cold snaps.

In any event, you will have a very solid loop if your actual load is lower, but you will be spending extra and short cycling. At least somertime humidy is not likely a concern in your area so long run times may not be needed like a house on the east coast.

Best of luck. Sounds like its going to be a very nice house.

We will go with 2 units that will serve as defacto zones.

Zone 1 heat loss:  48,766 btu/h
Zone 2 heat loss:  41,745 btu/h

Zone 1 will use a Waterfurnace Series 7 060 (51,000 btu/h with 32° EWT)
Zone 2 will use a Waterfurnace Series 7 048 (43,000 btu/h with 32° EWT)

Will will also have (1) 10 kW aux electric strip heater.

Alternatively we could go with...Option 2,

Zone 1 Waterfurnace Series 5 064 (47,100 btu/h with 32° EWT)
Zone 2 Waterfurnace Series 5 049 (37,400 btu/h with 32° EWT)

I guess option 2 would only cover ~93% of the heat loss at design temperature. 

Anyone have any thoughts on the way to go?  (Option 2 would save me 10% on unit cost)

Personnally I would go with the 7 series 048 and a 036, each with 10 Aux. IMO, I think you will prefer the two stage units. Is the bigger one on the first floor?

Set up a proper stat to ensure Aux is only used when actually needed. Easier said than done with some stats.

With that much loop, I am thinking your EWT will likely be closer to the mid to high 30's by the end of winter so your Geo Heat output should be closer 48 + 38 = 86KBTU. I think those size untis will perform better. I bet some experts on here may even go with two 036 units.

Remember, the contractor makes more if he sells more and he will never get calls back due to lack of heat. The downside is units get oversized.

Chris,

Did you mean I would prefer the continuously variable (20%-100%) series 7 units, or the series 5 two stage units?

Either option is multistage. 

I think the series 7 continuously variable units would be very nice, but could be expensive to replace the compressor and loop pumps (both variable speeds, not to mention the blower motor).

Looking at the data again...I think (2) 048 units could be very nice with the upper level using 10 kW electric aux heat.

Generally, the low is around 20° for December, January, February.  The daytime high is ~30-40°, but there are around 10-15 days where the low gets to 0°.

Thoughts?

I may have messed up on the models... I suggest to get the two stage not single stage. The variable looks great, but is pricey.