Been lurking around on the forum here for a while, reading and trying to learn as much as i can, and i'm finally wanting to get some feedback on an estimate - if it seems on track/reasonable - for a geothermal retrofit...

Little bit o' background: House is about 17 years old in Upstate South Carolina. Existing unit is Split 3.5 ton AC unit (13 SEER single stage, not high efficiency) and NG Furnace. 2 Zones which we would do away with. Husband is committed to keeping the house 65° and currently the room at the far end of the house from the air handler gets almost no flow and that room stays a solid 10° warmer than the rest of the house. The house will not get cooler than 75 during the heat of the day, so that one room stays about 85°. Either the zone dampers are not working or the ducts are configured poorly at that end of the hosue or the unit is just undersized. Contractor believes it's probably a combination of those 3 factors. Husband also hates the NG heat that is so dry and hot feeling. So we're looking to change. Geothermal seems to make such sense but everything I read her says system design is critical, as well as an experienced contractor, so some feedback would be greatly appreciated.

The first thing the HVAC contractor told us is that systems are not designed to cool to 65°F when the ambient air temp outside is 98°F. He says 20°F is the difference all systems are engineered for, regardless of Geo, Heat Pump, Gas Furnace, whatever. That being the case (true? not true?), he's given us 2 different options for the geothermal.

Some info about the house - about 2400 sq foot ranch relatively few windows but high ceilings in approx. 1000sq feet of the space.

Here's the info from the "GeoLink" analysis. Hopefully I'm putting the right info in here.

Design Data:
Heating Load: 60k Btuh
Heating Temp Diff: 62°F
Cooling Load: 60k Btuh
Cooling Temp Diff: 25°F
Constant Fan: No
Winter Design: 22°F
Summer Design: 93°F
Internal Gains: 9,441 Btuh

Annual Load:
Heating: 55.5 million Btu
Cooling: 42.6 million Btu
Hot Water: 18.1 million Btu
HW Use: 70 Gallons/day (*for some reason assumed 4 people so this is probably at least double for the 2 people)

He is recommending that for normal people (ie people who are ok with 75° in the heat of summer) with this house, they would recommend a 4 ton system. However, due to the increased demand from husband desiring year round cave-like climate, he is recommending a 5 ton system. In either case, they would be removing the damper/zone arrangement from the attic duct work because we just don't really need it. The system would connect to a horizontal loop field, final configuration TBD, but they install, on average, 200 feet per ton of capacity. Does this sound about right? Some of the postings here seem like people have a lot more linear feet of ground loop. Does this vary based on the climate the system is installed in?

The estimates/proposals:

For the 5-ton option: Split WaterFurnace model NDZ064/NAH060 with desuperheater and horizontal loop field. Removal of existing dampers and adding adequate duct capacity to accomodate increased airflow. Installation of 2 dehumidifiers (one in basement, one in crawlspace) to mitigate mold growth potential due to unusually low indoor temperature in the house (this actually struck me as a "limit-liabilty" requirement). Total cost $27,670 (before rebates & incentives)

For the 4-ton option: Split WaterFurnace model NDZ049/NAH048 with desuperheater and horizontal loop field. Removal of existing dampers and reconfiguration of existing duct work as needed to correct airflow problem. Total cost $20,075 (before rebates & incentives)

I don't think you will find geo heat any less dry and hot and I'd look at the cost effectiveness of the entire project. I'd get an energy audit done and see what it would take to get the load down to 3 tons. At that point, you might be happy with the existing system. Or just add a mini-split for some low cost and efficient extra capacity.

So your opinion is that the current system is not necessarily undersized for the size of the house if the house's efficiency were optimized? If that is the case, would it be rational to think the 4-ton geothermal would be adequate if we got it installed with no other changes to the house (other than fixing the duct work problems). But then, by improving the efficiency of the house, it might well achieve husband's low-temperature desires? It's a well-built house and don't think there's a tremendous efficiency problem. I would suspect that in our hot room, except for the fact there's just hardly any airflow in there from the registers. We're really looking to both get the house comfortable and really drastically reduce our monthly cost of utilities. Would installing an additional mini-split system that is using additional energy reduce our costs (i suppose since it is taking some of the burden off the existing system and performing at a more efficient level)?

I've tried to reason with husband about the efficiency of a gas furnace vs heat pump and he cannot be reasoned with. The thing doesn't even run that much with the cool temps we keep the house at. He just has deep hatred toward the NG heat for reasons I don't understand.

Probably, but I won't guess at your numbers. Installers and auditors can roughly model the performance with things including "Manual J". A hot room should be easy to fix - perhaps with a booster fan or just duct checks and adjustments. Also ask about adding a 2 or 3 ton geo system and keeping the existing system for peak loads. Most of the heating and cooling would then be done by the more efficient geo system and you would have lots of excess capacity for deep freeze and quick changes.

Your house needs an energy audit from a professional in order to determine what is happening. You also need to determine if your ducting and zone dampers are, in fact, functional or not.

And, you need to get quotes from more than one contractor.

"The first thing the HVAC contractor told us is that systems are not designed to cool to 65°F when the ambient air temp outside is 98°F. He says 20°F is the difference all systems are engineered for, regardless of Geo, Heat Pump, Gas Furnace, whatever. That being the case (true? not true?), he's given us 2 different options for the geothermal."

That's incorrect, but it is a common error made by contractors in the southeast where it is typical to provide for a 75 interior during 95 exterior temperatures. It is also typical to design a system to cool air by 20 degrees, in other words, return air enters the system at 75 and comes out at 55, which provides good dehumidification and reasonable efficiency. Contractors down here routinely confuse the two entirely separate 20 degree differential goals.

Confused? That's OK - to clear things up, consider this example: Suppose I design an HVAC system to cool a house 20 degrees below outdoor ambient. That may sound good, but suppose I'm an HVAC contractor in Tucson, Phoenix, or Las Vegas. If I tell my customers I'll cool their houses only down to 95 when it is 115 outside I will very quickly need another career.

I and any other competent HVAC contractor can design a system to cool a house to 65 during outdoor weather of 98. It is just a matter of overriding default temps in our load software. That said, executing such a system in the humid southeast would introduce serious mold risk, and would likely entail nearly doubling the size of typical existing ductwork. That the contractor identified the mold risk and proposed mitigation is a huge point in his/her favor.

I often find that customers demanding adherence to lower than average setpoints are in fact overreacting to prior bad experience with poor humidity control. A recent deep energy retrofit customer insisted upon the system being able to maintain 72. I provided that capability using a somewhat oversized 2 stage 4 zone system. Following commissioning, I've toured the house on several occasions and found stats set at 74-78. Apparently, 72 is now way too cold...

Thanks Curt, VERY helpful information! Here's how my thinking is evolving with the additional helpful info... I am thinking we should tackle this in stages by overhauling the ductwork first and getting a properly WORKING 2 zone system in place that would allow us to keep the living/dining/husband's office room at the 65° (or much closer to it) and the bedrooms at maybe 73° during the day and then flip-flop overnight while we are sleeping. That would functionally reduce the burden on the system, correct? Also, we can have the power company come out and do an Energy Audit (looked online and they offer that service) to make sure we're well sealed & insulated, etc... THEN once we know all that is AOK, we can put the geothermal in, maybe next year even and have a much greater expectation of that system functioning properly since we know that not only are the air ducts are properly engineered, that the house is insulated properly. At that point, I feel completely confident that there would be no need for the bigger unit. Reasonable ??

Is the GeoLink load analysis different from the "Manual J" ?

Manual J isn't a load analysis per se, but rather it is a standard to which load analysis software complies.

Geolink helps specify a WaterFurnace geo system. One of its inputs is a Man J compliant load analysis result.

Posted By jonr on 28 Jul 2011 08:54 PM
I don't think you will find geo heat any less dry and hot

Huh?

Geos discharge air temp is not routinely the same as a Nat Gas furnace meaning it is less hot and often eliminates supplemental humidifier requirements by reducing infiltration caused by replacement of combustion air.


wbsgrl,
5 tons strikes me as a bit much I'd want more analysis of the building and a look at envelope improvements as well.

j

often eliminates supplemental humidifier requirements by reducing infiltration caused by replacement of combustion air

Or it makes no difference because the NG furnace is using outside air for combustion. Net result - indoor air (and even duct air) can be the same temperature and humidity when using geo, electric or a NG forced air furnace. They can all be built to achieve exactly the same comfort level.

Let's be clear - a modern and well designed hvac system has some advantages over some older systems. But this is true of NG and other technologies too - so don't let geo salesmen spin that into a geo advantage.

Posted By jonr on 31 Jul 2011 09:08 PM

often eliminates supplemental humidifier requirements by reducing infiltration caused by replacement of combustion air

Or it makes no difference because the NG furnace is using outside air for combustion. Net result - indoor air is the same temperature and humidity when using geo, electric or a NG furnace with an outdoor air intake.

Jon,
I know you are not in the business nor do you have geo, so your research may not have turned up these facts: 
most existing furnaces are not direct vent (do not draw combustion air from outside), furnaces have significantly higher discharge air temps and about half of new geo owners in our kneck of the woods find they do not miss their humidifier when they switch to geo.

Obviously this is not true everywhere, all the time, which is why I chose words such as "often".

There are times when what you suggest is true but comfort is a subjective thing that is based as much on perception as science. When people talk about dry scorched air a contractor in this business knows it has little to do with the reading on a hygrometer and much to do with air 50 degrees hotter than room temp blowing across their noses drying them out.

Since this is the thrust of the original post, I think they will find some sort of heat pump more comfortable regardless of the science of humidity.

j