Posted By engineer on 17 May 2012 09:53 PM
... the ground supplies or absorbs the difference.
 
... this is routine in large buildings that always need interior cooling but sometimes need perimeter heating.

Posted By robinnc on 17 May 2012 10:18 PM
Bill, just curious.....do you have a newer house or a very old house? EIGHT tons is ALOT for a 3400sf house!

Answer: About in the middle.

My home (see picture inset, 2nd screen, right side) was built in 1992.  Single story, 3400 sf, brick veneer, big attic with a few wind turbine ventilators along with soffit vents, average attic insulation, average windows and doors with respect to air infiltration, a lot of window (glazing) s.f. area, and a non-edge-insulated on-grade concrete slab.

I also have about 60 recessed light fixtures that are not IC (i.e. they're vented to the attic), a gas-fired hot water heater in the midst of conditioned air space, a fire place, 4 bathroom exhaust fans to the attic, and a stove top exhaust fan to direct outdoors.

Measured at my home (Dallas area), my annual HDD is 2294 and CDD is 3638.  I don't know what typical outdoor air design temps are for my area, but the record low is +1°F and high is 112°.

In '07 I hired an HVAC contractor to remove everything (5-ton 2-stage Lennox A/C and conventional 120 KBTU/hr gas-fired furnace / air-handler zoned into 5 air distribution zones, along with all ductwork), and start over.  The only requirements were: (1) geothermal technology, (2) no air distribution registers placed anywhere except in ceilings (to minimize air leakage), and (3) each of 3 large bedrooms have a separate tstat.  The contractor was given the freedom to optimally determine equipment, and all register sizes/locations even if it called for register addition/removal/relocation.  The contractor was given the freedom to optimally design the ductwork for appropriate air distribution.  Steel pipe except for last 6 feet being flexduct was asked for.

What I got, equipment wise, was 2 WF Envision 2-stage variable speed geo units, 5 ton (family area) and 3 ton (bedrooms), connected to a single approximately 5000' closed 1" water loop of vertical well holes, operating at a 60 psi static pressure.  The contractor said this design was what was required for best efficiency because the units would always stay in first stage (indeed they never run in 2nd stage no matter how hot it might be here).  The contractor further said 2 units were needed to make it possible to have zone control for the bedrooms.  It was also felt that this arrangement provided for insurance on mechanical breakdown.  I.e., if one unit broke, we'd still have the other half of the house being cooled, and thus not requiring emergency repair services.

No Manual J for structure heat/cooling load was shared with me, nor a Manual D for equipment sizing, nor a Manual S for duct design/sizing.  I questioned 8 tons total, but was told this was what the Manual J analysis called for.  So, I ended up with 425 (2 stage) - 516 (1st stage only) sf/ton of cooling.

All equipment and ductwork is up in the hot attic.  Ductwork is insulated to R-8.

Tstats are programmable - daytime: 77° living / 81° bedrooms; night time: 81° living / 74–76° bedrooms.

I have an instrumentation system ( http://www.welserver.com/ ) that a few years of adjusting now tell me everything I'd want to know about structure and system performances.

Past measured EER is 21 (translating to 24 SEER) and COP is 5.0 (meets WF's advertised specs).

Typical system cooling capacity is about 81 KBTU/hr (53 for 5 ton unit + 28 for 3 ton unit).  See chart here.  This can go higher if 2nd stage is used (never is needed).

The structure typically requires about 750 KBTU/day (221 BTU/day/sf) of heat rejection for the hottest Summer days; the structure requires about 250 KBTU/day (74 BTU/day/sf) of heat for the coldest Winter days.  See chart here.

The tstats are set to minimize cycling.  Inside humidity stays between 39 - 49% RH on a rolling 30 day average.  See chart here.

The avg monthly operating energy usage, for the past 12 months, is 629 kWh/mo.  My avg rate for the same period is 9.1 ¢/kWh.  (See 'Electric Rate' chart here.) Thus, the avg monthly operating cost is $57 for the past 12 months (see 'Heating and Cooling Cost' chart here).  This includes last Summer's record breaking heat wave.
 
Geothermal is indeed the real deal for larger and/or average air tightness / average insualted homes.

Finally, to normalize my structure to those in other climate zones and with a different sf amount, my structure's energy efficiency performance is 24 KBTU/CDD (7.0 BTU/CDD/sf) when in cooling mode, and is 8.3 KBTU/HDD (2.4 BTU/HDD/sf) when in heating mode. (See 'Structure Energy Efficiency' chart here.)

Best regards,

Bill

Posted By engineer on 18 May 2012 10:13 PM
You might be an unwitting victim of the myth that natural attic ventilation is good, therefore augmented (mechanical or wind turbine) ventilation must be better....

Posted By engineer on 18 May 2012 10:13 PM
... If you haven't yet done so, work to seal air leaks in ceiling.

Posted By arkie6 on 18 May 2012 10:46 PM

It's hard to argue with an average electric bill of $57/month though.

Posted By engineer on 18 May 2012 10:13 PM
You might be an unwitting victim of the myth that natural attic ventilation is good, therefore augmented (mechanical or wind turbine) ventilation must be better.

In fact, steps to increase attic ventilation work to decrease air pressure in the attic volume. It may seem desireable in that more ventialtion air is drawn through the attic, but in fact ceiling penetrations may cause the attic to draw excessive conditioned air from rooms below. That air is then replaced by outside air infiltrating through exterior wall penetrations, windows and doors. The net effect is to increase, rather than decrease, cooling load on the conditioned space.

...